Epidimiology Chapter 4

7/23/2019 Epidimiology Chapter 4

1/24

Chapter 1

What is epidemiology?

Key messages

Epidemiology is a fundamental science of public health.

Epidemiology has made major contributions to improving population health. Epidemiology is essential to the process of identifying and mapping emerging

diseases.

There is often a frustrating delay between acquiring epidemiological evidence

and applying this evidence to health policy.

The historical context

Origins

Epidemiology originates from Hippocrates observation more than 2000 years ago that environmental

factors influence the occurrence of disease. However, it was not until the nineteenth century that the

distribution of disease in specific human population groups was measured to any large extent. This

work marked not only the formal beginnings of epidemiology but also some of its most spectacular

achievements.

1 The finding by John Snow that the risk of cholera in London was related to the drinking of water

supplied by a particular company provides a well-known example; the map (see Figure 4.1)

highlights the clustering of cases. Snows epidemiological studies were one aspect of a wide-

ranging series of investigations that examined related physical, chemical, biological, sociological

and political processes.

2Comparing rates of disease in subgroups of the human population became common practice in the

late nineteenth and early twentieth centuries. This approach was initially applied to the control of

communicable diseases (see Chapter 7), but proved to be a useful way of linking environmental

conditions or agents to specific diseases. In the second half of the twentieth century, these methods

were applied to chronic noncommunicable diseases such as heart disease and cancer, especially inmiddleand high-income countries.

Recent developments in epidemiology

Epidemiology in its modern form is a relatively new discipline1 and uses quantitative methods to

study diseases in human populations to inform prevention and control efforts. For example, Richard

Doll and Andrew Hill studied the relationship between tobacco use and lung cancer, beginning in

the 1950s.4 Their work was preceded by experimental studies on the carcinogenicity of tobacco tars

and by clinical observations linking tobacco use and other possible factors to lung cancer. By using

longterm cohort studies, they were able to establish the association between smoking and lung

cancer (Figure 1.1).

Definition, scope, and uses of epidemiology

Definition

Epidemiology as defined by Last9 is the study of the distribution and determ inants of health-related

states or events in specified populations, and the application of this study to the prevention and

control of health problems (see Box 1.2). Epidemiologists are concerned not only with death,

illness and disability, but also with more positive health states and, most importantly, with the


2/24

means to improve health. The term disease encompasses all unfavourable health changes,

including injuries and mental health.

Definition of epidemiology

Epidemiology is the study of disease in populations and of factors that determine its

occurrence; the key word being populations. Veterinary epidemiology additionally includes

investigation and assessment of other health-related events, notably productivity. All of these

investigations involve observing animal populations and making inferences from the

observations.

A literal translation of the word 'epidemiology', based on itsGreek roots e- (epi-) = upon,

o- (demo-) =people,and o'o- (logo-) = discoursing, is 'the study of that which is upon

the people' or, in modern parlance, 'the study of disease in populations'. Traditionally,

epidemiology' related to studies of human populations, and epizootiology', from the Greek

wo- (zoo-) = , tothe studies of animal (excluding human)

populations (e.g., Karstad, 1962). Outbreaks of disease in human populations were called'epidemics', in animal populations were called 'epizootics', and in avian populations were

called 'epornitics', from the Greek opvt-(ornith-) = bird (e.g., Montgomery et al., 1979).

Other derivatives, such as 'epidemein' ('to visit a community'), give hints of the early

association between epidemiology and infections that periodically entered a community, in

contrast to other diseases which were usually present in the population.

The uses of epidemiology

There are five objectives of epidemiology:

I . determination of the origin of a disease whose cause is known;

2. investigation and control of a disease whose cause is

either unknown or poorly understood;

3. acquisition of information on the ecology and natural

history of a disease;

4. planning and monitoring of disease control programmes;

5. assessment of the economic effects of a disease and analysis of the costs and economic

benefits of alternative control programmes

Determination of the origin of a disease whose cause is known

Many diseases with a known cause can be diagnosed precisely by the signs exhibited by the

affected animals, by appropriate laboratory tests and by other clinical procedures such as

radiological investigation. For instance, the diagnosis of salmonellosis in a group of calves

is relatively straightforward (the infection frequently produces distinct clinical signs).

However, determining why an outbreak occurred and using the correct procedures to prevent

recurrence can be difficult. For example, the outbreak may have been caused either by the

purchase of infected animals or by contaminated food. Further investigations are required to

identify the source of infection. When the food is suspected, the ration may consist of several

components. Even if a sample of each component is still available, it would be expensive and

l


3/24

possibly uneconomic to submit all of the samples for laboratory examination. Consideration

of the risk associated with the consumption of each component of the ration may narrow the

field of investigation to only one or two items.

There are many examples of the investigation of diseases with known causes that involve

answering the questions `Why has an outbreak occurred?' or `Why has the number of casesincreased?'. For instance, an increased number of actinobacillosis cases in a group of cattle

might be associated with grazing a particular pasture of `burnt off' stubble. Such an

occurrence could be associated with an increase in abrasions of the buccal mucosae which

could increase the animals' susceptibility to infection with Actinobacillus lignieresi. An

increased number of cases of bone defects in puppies might be due to local publicity given to

the use of vitamin supplements, resulting in their administration to animals that were

already fed a balanced diet, with consequent hypervitaminosis D and osteodystrophy (Jubb and

Kennedy, 1971). An increase in the number of lamb carcasses with high ultimate pH values

could be associated with excessive washing of the animals prior to slaughter (Petersen,

1983). These possible explanations can be verified only by epidemiological investigations.

Investigation and control of a disease whose cause is either unknown or poorly understood

There are many instances of disease control based on epidemiological observations before a

cause was identified. Contagious bovine pleuropneumonia was eradicated from the US by an

appreciation of the infectious nature of the disease before the causal agent, Mycoplasma

mycoides, was isolated (Schwabe, 1984). Lancisi's slaughter policy to control rinderpest,

mentioned in Chapter 1, was based on the assumption that the disease was infectious, even

though the causal agent had not been discovered. Edward Jenner's classical observations on the

protective effects of cowpox virus against human small pox infection in the 18th century

(Fisk, 1959), before viruses were isolated, laid the foundations for the global eradication of

smallpox.More recently, epidemiological studies in the UK suggested that cattle develop bovine spongiform

encephalopathy (BSE) following consumption of feedstuffs containing meat and bone meal

contaminated with a scrapie-like agent (Wilesmith et al., 1988). This was sufficient to

introduce legislation prohibiting the feeding of ruminant derived protein, although the causal

agent had not been identified.

Although the exact cause of `blood splashing' (eccymoses in muscle) in carcasses is still not

known, observations have shown that there is a correlation between this defect and

electrical stunning by a `head only' method (Blackmore, 1983); and the occurrence of this

condition can be reduced by adopting a short 'stun-to-stick' interval, stunning animals with a

captive bolt, or using a method of electrical stunning that causes concurrent cardiac dysfunction

(Gracey, 1986). Similarly, there is a strong correlation between grass sickness and grazing,

and the disease can be almost totally prevented by stabling horses continuously during spring

and summer, although the cause of the disease is unknown (Gilmour, 1989).

The cause of squamous cell carcinoma of the eye in Hereford cattle ('cancer eye') is not known.

Epidemiological studies have shown that animals with unpigmented eyelids are much more likely

to develop the condition than those with pigment (Anderson et al., 1957). This


4/24

information can be utilized by cattle breeders to select animals with a low susceptibility to this neoplasm.

Epidemiological studies are also used to identify causes of disease (many of which are multifactorial and initial

poorly understood) so that the most appropriate disease control techniques can be applied. Thus, the identification low levels of water intake as an important component of the cause of feline urolithiasis (Willeberg, 1981) facilitate

control of this syndrome by dietary modification. Investigations can also be used to identify characteristics

animals that increase the risk of disease. For example, entire bitches with a history of oestrus irregularity a

pseudopregnancy are particularly at risk of developing pyometra (Fidler et al., 1966); this information is

diagnostic value to the clinician, and is of assistance when advising owners on breeding policy.

Acquisition of information on the ecology and natural history of a disease

An animal that can become infected with an infectious agent is a host of that agent. Hosts and agents exist

communities that include other organisms, all of which live in particular environments. The aggregate of all fac

relating to animals and plants is their natural history. Related communities and their environments are term

ecosystems. The study of ecosystems is ecology.

A comprehensive understanding of the natural history of infectious agents is possible only when they are studied in t

context of their hosts' ecosystems. Similarly, an improved knowledge of non-infectious diseases can be obtaine

by studying the ecosystems and the associated physical features with which affected animals are related. T

geological structure of an ecosystem, for example, can affect the mineral content of plants and therefore can be

important factor in the occurrence of mineral deficiencies and excesses in animals.

The environment of an ecosystem affects the survival rate of infectious agents and of their hosts. Thus, infection w

the helminthFasciola hepatica is a serious problem only in poorly drained areas, because the parasite spends part

its life-cycle in a snail which requires moist surroundings.

Each of the 200 antigenic types (serovars) ofLeptospira interrogans is maintained in one or more species of hos

Serovar copenhageni, for instance, is maintained primarily in rats (Babudieri, 1958). Thus, if this serovar associated with leptospirosis in either man or domestic stock, then part of a disease control programme must invol

anecological study of rat populations and control of infected rats. Similarly, in Africa, a herpesvirus that

produces infections without signs in wildebeeste is responsible for malignant catarrhal fever of cattle (Plowright

al., 1960). Wildebeeste populations, therefore, must be investigated when attempting to control the disease

cattle.

An ecosystem's climate also is important because it limits the geographical distribution of infectious agents th

are transmitted by arthropods by limiting the distribution of the arthropods. For example, the tsetse fly, whic

transmits trypanosomiasis, is restricted to the humid parts of Sub-Saharan Africa (Ford, 1971).

Infectious agents may extend beyond the ecosystems of their traditional hosts. This has occurred in bovin

tuberculosis in the UK, where the badger population appears to be an alternative host for Mycobacteriu

tuberculosis (Little et al., 1982; Wilesmith et al., 1982). Similarly, in certain areas of New Zealand, wi

opossums are infected with this bacterium and can therefore be a source of infection to cattle (Thorns an

Morris, 1983). Purposeful routine observation of such infections provides valuable information on changes in t

amount of disease and relevant ecological factors and may therefore indicate necessary changes in contr


5/24

strategies.

Infectious diseases that are transmitted by insects, ticks and other arthropods, and which may be maintained

wildlife, present complex ecological relationships and even more complex problems relating to their contro

Comprehensive epidemiological studies of these diseases help to unravel their life-cycles, and can indicate suitab

methods of control.

Planning and monitoring of disease control programmes

The institution of a programme to either control or eradicate a disease in an animal population must bbased on a knowledge of the amount of the disease in that population, the factors associated with

occurrence, the facilities required to control the disease, and the costs and benefits involved. Th

information is equally important for a mastitis control programme on a single dairy farm and for a nation

brucellosis eradication scheme involving all the herds in a country. The epidemiological techniques th

are employed include the routine collection of data on disease in populations (monitoring a

surveillance) to decide if the various strategies are being successful.

Surveillance is also required to determine whether the occurrence of a disease is being affected by new factor

For example, during the eradication scheme for bovine


6/24

18 The scope of epidemiology

tuberculosis in New Zealand, opossums became infected in certain areas. New strategies had to be

introduced to control this problem (Julian, 1981). During the foot-andmouth disease epidemic

in the UK in 1967 and 1968, surveillance programmes indicated the importance of wind-borne

virus particles in the transmission of the disease (Smith and Hugh-Jones, 1969). This

additional knowledge was relevant to the establishment of areas within which there was arestriction of animal movement, thus facilitating eradication of the disease.

Assessing the economic effects of a disease and of its control

The cost of the control of disease in the livestock industry must be balanced against the economic

loss attributable to the disease. Economic analysis therefore is required. This is an essential part

of most modern planned animal health programmes. Although it may be economic to reduce a

high level of disease in a herd or flock, it may be uneconomic to reduce even further the

level of a disease that is present at only a very low level. If 15 % of the cows in a herd were

affected by mastitis, productivity would be severely affected and a control programme would

be likely to reap financial benefit. On the other hand, if less than I % of the herd were

affected, the cost of further reduction of the disease might not result in a sufficient increase in

productivity to pay for the control programme.

This introduction to the uses of epidemiology indicates that the subject is relevant to many areas

of veterinary science. The general agricultural practitioner is becoming increasingly concerned

with herd health. The companion animal practitioner is faced with chronic refractory diseases,

such as the idiopathic dermatoses, which may be understood better by an investigation of the

factors that are common to all cases. The state veterinarian cannot perform his routine duties

without reference to disease in the national animal population. The diagnostic pathologist

investigates the associations between causes and effects (i.e., lesions); this approach is

epidemiological when inferences are made from groups of animals. The veterinarian in

abattoirs and meat-processing plants attempts to reduce the occurrence of defects and contami-

nation by understanding and eliminating their causes. Similarly, industrial veterinarians,

concerned with the design of clinical trials, compare disease rates and response to treatment

in groups of animals to which different prophylactic and therapeutic compounds are

administered.

Types of epidemiological investigation

There are four approaches to epidemiological investigation that traditionally have been called

`types' of epidemiology. These types are descriptive, analytical, experimental and theoretical

epidemiology.


7/24

18 The scope of epidemiology

Descriptive epidemiology

Descriptive epidemiology involves observing and recording diseases and possible causal factors.

It is usually the first part of an investigation. The observations are sometimes partially

subjective, but, in common with observations in other scientific disciplines, may generate

hypotheses that can be tested more rigorously later. Darwin's theory of evolution, forexample, was derived mainly from subjective observations, but with slight modification it has

withstood rigorous testing by plant and animal scientists.

Analytical epidemiology

Analytical epidemiology is the analysis of observations using suitable diagnostic and statistical

tests.

Experimental epidemiology

The experimental epidemiologist observes and analyses data from groups of animals from which

he can select, and in which he can alter, the factors associated with the groups. An important

component of the experimental approach is the control of the groups. Experimental

epidemiology developed in the 1920s and 1930s, and utilized laboratory animals whose short

lifespans enabled events to be observed more rapidly than in humans (see Chapter 18). A

notable example is the work of Topley (1942) who infected colonies of mice with ectromelia

virus and Pasteurella spp. The effects of varying the rate of exposure of mice maintained in

groups of various sizes provided insights into the behaviour of human epidemic diseases such as

measles, scarlet fever, whooping cough and diptheria which followed similar patterns to the

experimental infections (MRC, 1938). This work demonstrated the importance of the

proportion of susceptible individuals in the population in determining the progress of epidemics

(see Chapter 8); hitherto, changes in the virulence of a microorganism were thought to be the

most important factor affecting epidemic patterns.

Rarely, a `natural' experiment can be conducted when the naturally occurring disease or other

fortuitous circumstance approximates closely to the ideally designed experiment. For instance,when BSE occurred in the UK, outbreaks of the disease on the Channel Islands (Jersey


8/24

Types of epidemiological investigation

and Guernsey), which maintain isolated populations of cattle, provided an ideal situation in which to study the disea

uncomplicated by the possibility of transmission by contact with infected animals (Wilesmith, 1993). This add

credence to the hypothesis that the disease was transmitted in contaminated feedstuffs.

Theoretical epidemiologyTheoretical epidemiology consists of the representation of disease using mathematical 'models' that attempt

simulate natural patterns of disease occurrence.

Epidemiological subdisciplines

Various epidemiological subdisciplines are now recognized. These reflect different areas of interest, rather th

fundamentally different techniques. They all apply the four types of epidemiology described above, and c

overlap, but their separate identities are considered by some to be justifiable.

Clinical epidemiology

Clinical epidemiology is the use of epidemiological principles, methods and findings in the care of individuals, w

particular reference to diagnosis and prognosis (Last, 1988), and therefore brings a numerate approachtraditional clinical medicine, which has tended to be anecdotal and subjective (Grufferman and Kimm, 1984)

is concerned with the frequency and cause of disease, the factors that affect prognosis, the validity

diagnostic tests, and the effectiveness of therapeutic and preventive techniques (Fletcheret al., 1988).

Computational epidemiology

Computational epidemiology involves the application of computer science to epidemiological studies (Habtemariam

al., 1988). This includes the representation of disease by mathematical models (see 'Quantitative investigatio

below) and the use of expert systems. These systems are commonly applied to disease diagnosis where th

incorporate a set of rules for solving problems, details of clinical signs, lesions, laboratory results, and

opinions of experts; examples are the identification of the cause of coughing in dogs (Roudebush, 1984), and tdiagnosis of bovine mastitis (Hogeveen et a!., 1993). Expert systems are also employed in formulating disea

control strategies (e.g., for East coast fever: Gettinby and Byrom, 1989), predicting animal productivity (e

reproductive performance in dairy herds: McKay e t al., 1988), and

supporting management decisions (e.g., decisions on replacing sows: Huirne et al., 1991).

Genetic epidemiology

Genetic epidemiology is the study of the cause, distribution and control of disease in related individuals, and

inherited defects in populations (Morton, 1982; Roberts, 1985). It indicates that the disciplinary boundary betwe

genetics and epidemiology is blurred. Many diseases involve both genetic and non-genetic factors (see Chapter

and genes are increasingly incriminated in diseases of all organ systems (Figure 1.3). Thus, the geneticist a

epidemiologist are both concerned with interactions between genetic and non-genetic factors - only the frequen

indistinct time of interaction may be used to classify an investigation as genetic or epidemiological.Molecular epidemiology

New biochemical techniques now enable microbiologists and molecular biologists to study small genetic a

antigenic differences between viruses and other microorganisms at a higher level of discrimination than has be


9/24

possible using conventional serological techniques. The methods include peptide mapping, nucleic acid 'fing

printing' and hybridization (Keller and Manak, 1989; Kricka, 1992), restriction enzyme analysis, monoclo

antibodies (Oxford, 1985; Goldspink and Gerlach, 1990; Goldspink, 1993) and the polymerase chain react

(Belak and Ballagi-Pordany, 1993). For example, nucleotide sequencing of European foot-and-mouth disease vi

has indicated that recent outbreaks of the disease involved vaccinal strains, suggesting that improper inactivation

escape of virus from vaccine production plants may have been responsible for the outbreaks (Beck and Strohma

1987). Similarly, infections that hitherto have been difficult to identify are now readily distinguished using the

new molecular techniques; examples are infection

withMycobacterium paratuberculosis (the cause of

Johne's disease) (Murray et al., 1989) and latent infection with Aujeszky's disease virus (Belak et al., 1989). T

application of these new diagnostic techniques constitutes molecular epidemiology. A general description of

methods is given by Persing et al. (1993).

Molecular epidemiology is part of the wider use of biological markers (Hulka et al., 1990). These are cellu

biochemical or molecular alterations that are measurable in biological media such as tissues, cells or fluids. Th

may indicate susceptibility to a causal factor, or a biological response, suggesting a sequence of events fr

exposure to disease (Perera and Weinstein, 1982). Some have been used by veterinarians for many years, for


10/24

instance, serum magnesium levels as indicators of susceptibility to clinical hypocalcaemia

(Whitaker and Kelly, 1982; Van de Braak et al., 1987), serum transaminase levels as markersfor liver disease, and antibodies as indicators of exposure to infectious agents (see Chapter 17).

Other subdisciplines

Several other epidemiological subdisciplines have also been defined. Chronic disease

epidemiology is involved with diseases of long duration (e.g., cancers), many of which are non-

infectious. Environmental epidemiology is concerned with the relationship between disease

and environmental factors such as industrial pollution and, in human medicine, occupational

hazards. Domestic animals can act as monitors of environmental hazards and can provide

early warning of disease in man (see Chapter 18). Micro-epidemiology is the study of disease

in a small group of individuals with respect to factors that influence its occurrence in larger

segments of the population. For example, studies of feline acquired immunodeficiency

syndrome (FAIDS) in groups of kittens have provided insights into the widespread human

disease, AIDS (Torres-Anjel and Tshikuka, 1988). Micro-epidemiology, which frequently

uses animal biological models of disease, therefore is closely related to comparative

epidemiology

(Chapter18). In contrast, macroepidemiology is the study of national patterns of disease, and the

social, economic and political factors that influence them (Hueston and Walker, 1993). Other

subdisciplines, such as nutritional epidemiology (Willett, 1990) and subclinical epidemiology

(Evans, 1987), can also be identified to reflect particular areas of interest.

The components of epidemiology are summarized in Figure 2.1. The first stage in any

investigation is the collection of relevant data. The main sources of information are

outlined in Chapter 10. Methods of storing, retrieving and disseminating information arediscussed in Chapter 11. Investigations can be either qualitative or quantitative or a

combination of these two approaches.

Qualitative investigations The

natural history of disease

The ecology of diseases, including the distribution, mode of transmission and maintenance of

infectious diseases, is

investigated by field observation. Ecological principles are outlined in Chapter 7. Methods of

transmission and maintenance are described in Chapter 6, and patterns of disease occurrence

are described in Chapter 8. Field observations also may reveal information about factors that

may directly or indirectly cause disease. The various factors that act to produce disease are

described in Chapter 5.

mponents of epidemiology


11/24

Causal hypothesis testing

If field observations suggest that certain factors may be causally associated with a disease, then

the association must be assessed by formulating a causal hypothesis. Causality (the relating of

causes to effects) and hypothesis formulation are described in Chapter 3.

Qualitative investigations were the mainstay of epidemiologists before the Second World War.

These epidemiologists were concerned largely with the identification of unknown causes of

infectious disease and sources of infection. Some interesting examples of the epidemiologist

acting as a medical `detective' are described by Roueche (1991).

Quantitative investigations

Quantitative investigations involve measurement (e.g., the number of cases of disease), and

therefore expression and analysis of numerical values. Basic methods of expressing these

values are outlined in Chapters 4 and 12. The types of measurement that are encountered in

veterinary medicine are described in Chapter 9. Quantitative investigations include surveys,

monitoring and surveillance, studies, modelling, and the biological and economic evaluation of

disease control.

Surveys

A survey is an examination of an aggregate of units (Kendall and Buckland, 1982). A group of

animals is an example of an aggregate. The examination usually involves counting membersof the aggregate and characteristics of the members. In epidemiological surveys, characteristics

might include the presence of particular diseases, weight, and milk yield. Surveys can be

undertaken on a sample of the population. Less commonly, a census, which examines the total

animal population, can be undertaken (e.g., tuberculin testing). A cross-sectional survey records

events occurring at a particular point in time. A longitudinal survey records events over a period

of time. These latter events may be recorded prospectively from the present into the future; or

may be a retrospective record of past event


12/24

Scope

A focus of an epidemiological study is the population defined in geographical or other

terms; for example, a specific group of hospital patients or factory workers could be

the unit of study. A common population used in epidemiology is one selected from

a specific area or country at a specific time. This forms the base for defining subgroups

with respect to sex, age group or ethnicity. The structures of populations vary between

geographical areas and time periods. Epidemiological analyses must take such

variation into account.

The scope of epidemiology

A particular type of diagnostic survey is screening. This is the identification of undiagnosed cases of

disease using rapid tests or examinations. The aim is to separate individuals that probably have a

disease from those that probably do not. Screening tests are not intended to be definitive; individuals

with positive test results (i.e., that are classified as diseased by the screening test) require further

investigation for definite diagnosis.

Diagnostic tests, including serological surveys and screening, are considered in Chapter 17. The

design of surveys in general is described in Chapter 13.

Monitoring and surveillance

Monitoring is the making of routine observations on health, productivity and environmental factors and the

recording and transmission of these observations. Thus, the regular recording of milk yields is

monitoring, as is the routine recording of meat inspection findings at abattoirs. The identity of

individual diseased animals usually is not recorded.

Surveillance is a more intensive form of data recording than monitoring. Originally, surveillance was

used to describe the tracing and observation of people who were in contact with cases of infectious

disease. It is now used in a much wider sense (Langmuir, 1965) to include all types of disease -

infectious and non-infectious - and involves the collation and interpretation of data collected during

monitoring programmes, usually with the recording of the identity of diseased individuals, with a view

to detecting changes in a population's health. It is normally part of control programmes for specific

diseases. The recording of tuberculosis lesions at an abattoir, followed by tracing of infected animals

from the abattoir back to their farms of origin, is an example of surveillance. The terms `monitoring'

and `surveillance' have previously been used synonymously, but the distinction between them is now

generally accepted. The national and international aspects of surveillance are reviewed by Blajan

(1979), Davies (1980, 1993), Ellis (1980) and Blajan and Welte (1988), and some animal disease

information systems are described in Chapter 11.

Monitoring and surveillance can include all of the national herd. Alternatively, a few farms, abattoirs,

veterinary practices or laboratories may be selected; these are then referred to as `sentinel' units,

because they are designed to `keep watch' on a disease. Similarly, horses can be used as sentinelsfor Venezeulan equine encephalitis virus infection (Dickerman and Scherer, 1983), and stray dogs as

sentinels for canine parvovirus infection (Gordon and Angrick, 1985), the infections being identified

serologically. Other species of animals that also


13/24

are susceptible to an infectious agent can be used as sentinels for the infection in the main animal

population. For example, wild birds can be used to monitor the activity of St Louis encephalitis

virus, providing early information on the activity of the virus at a time when avian infection rates are

still too low to pose an immediate threat to man (Lord et al., 1974). Domestic animals can also be used

as sentinels of human environmental health hazards such as carcinogens and insecticides; this topic is

discussed in detail in Chapter 18.

Studies

`Study' is a general term that refers to any type of investigation. However, in epidemiology, a study

usually involves comparison of groups of animals, for example, a comparison of the weights of

animals that are fed different diets. Thus, although a survey generally could be classified as a

study, it is excluded from epidemiological studies because it involves only description rather than

comparison and the analysis that the comparison requires. There are four main types of

epidemiological study:

1. experimental studies; 2. cross-sectional studies; 3. case-control studies; 4.

cohort studies.

In an experimental study the investigator has the ability to allocate animals to various groups,according to factors which the investigator can randomly assign to animals (e.g., treatment

regimen, preventive technique); such studies are therefore part of experimental epidemiology. An

important example is the clinical trial. In a clinical trial, the investigator assigns animals either to

a group to which a prophylactic or therapeutic procedure is applied, or to a control group. It is then

possible to evaluate the efficacy of the procedure by comparing the two groups. Clinical trials

are discussed in Chapter 16.

The other types of study - cross-sectional, case-control and cohort - are observational. An observational

study is similar to an experimental study: animals are allocated to groups with respect to certain

characteristics that they possess (trait, disease, etc.). However, in observational studies, it is not

possible to assign animals to groups randomly because the investigator has little control over the

factors that are being studied; the characteristics are inherent (e.g., sex, weight or normal diet).

A cross-sectional study investigates relationships between disease (or other health-related factors) and


14/24

hypothesized causal factors in a specified population. Animals are categorized according to presence and ab

of disease and hypothesized causal factors; inferences can then be made about associations between disease a

hypothesized causal factors, for example, between heart valve incompetence (the disease) and breed (the

hypothesized causal factor).

A case-control study compares a group of diseased animals with a group of healthy animals with respect to exp

to hypothesized causal factors. For example, a group of cats with urolithiasis (the disease) can be compare

group of cats without urolithiasis with respect to consumption of dry cat food (the factor) to determine whet

type of food has an effect on the pathogenesis of the disease.

In a cohort study, a group exposed to factors is compared with a group not exposed to the factors with respect

development of a disease. It is then possible to calculate a level of risk of developing the disease in relati

exposure to the hypothesized causal factors.

Case-control and cohort studies have often been applied in human medicine in which experimental investiga

cause are usually unethical. For example, it would not be possible to investigate the suspected toxicity of a d

intentionally administering the drug to a group of people in order to s tudy possible sideeffects. However,

symptoms of toxicity have occurred, then a case-control study could be used to evaluate the association betw

symptoms and the drug suspected of causing the toxicity. There are fewer ethical restraints on experimental

investigation in veterinary medicine than in human medicine and so experimental investigation of serious condimore tenable. However, observational studies have a role in veterinary epidemiology; for example, when

investigating diseases in farm and companion animal populations. The increasing concern for animal welfare is

these techniques even more useful than previously.

Basic methods of assessing association between disease and hypothesized causal factors in observational studies

described in Chapters 14 and 15.

Observational studies form the majority of epidemiological studies. Observational and experimental science hav

own strengths and weaknesses which are discussed in detail by Trotter (1930). A major advantage of an

observational investigation is that it studies the natural occurrence of disease. Experimentation may separa

factors associated with disease from other factors that may have important interactions with them in natural outbr

Modelling

Disease dynamics and the effects of different control strategies can be represented using mathematical equations.

representation is `modelling'. Many modern methods rely heavily on computers. Another type of modelling is

biological simulation using experimental animals (frequently laboratory animals) to simulate the pathogenesis

diseases that occur naturally in animals and man. Additionally, the spontaneous occurrence of disease in animals

studied in the field (e.g., using observational studies) to increase understanding of human diseases. Mathematical

modelling is outlined in Chapter 19, and spontaneous disease models are described in Chapter 18.

Disease control

The goal of epidemiology is to improve the veterinarian's knowledge so that diseases can be controlled effectiv

productivity thereby optimized. This can be fulfilled by treatment, prevention or eradication. The economicevaluation of disease and its control is discussed in Chapter 20. Health schemes are described in Chapter 2

Finally, the principles of disease control are outlined in Chapter 22.

The different components of epidemiology apply the four epidemiological approaches to varying degrees. Surve

studies, for example, consist of a descriptive and an analytical part. Modelling additionally may include a

theoretical approach


15/24

Definition of epidemiology9

The word epidemiology is derived from the Greek words: epi upon, demos people and logos study.

This broad definition of epidemiology can be further elaborated as follows:

Term Explanation

Study includes : surveillance, observation, hypothesis testing, analytic researchand experiments.

Distribution : refers to analysis of: times, persons, places and classes of people affected.

Determinants : include factors that influence health: biological, chemical, physical, social, cultural, econ

genetic and behavioural.

Health-related states and events : refer to: diseases, causes of death, behaviours such as use of tobacco,positive hea

states, reactions to preventive regimes and provision anduse of health s

Specified populations : include those with identifiable characteristics, such as occupational groups.

Application to prevention and control : the aims of public healthto promote, protect, and restore health

Epidemiology and public health

Public health, broadly speaking, refers to collective actions to improve populationhealth.1 Epidemiology, one of the tools for improving public health, is used in several

ways (Figures 1.31.6). Early studies in epidemiology were concerned with the causes(etiology) of communicable di

and such work continues to be essential sinceit can lead to the identification of preventive methods. In this sense,

epidemiology isa basic medical science with the goal of improving the health of populations, andespecially the hea

the disadvantaged.

Causation of disease

Although some diseases are caused solely by genetic factors, most result from an

interaction between genetic and environmental factors. Diabetes, for example, has

both genetic and environmental components. We define environment broadly to

include any biological, chemical, physical, psychological, economic or cultural factors that can affect health (see Cha

Personal behaviours

affect this interplay, and epidemiology is used to study

their influence and the effects of preventive interventions

through health promotion (Figure 1.3).

Morbidity

Death rates are particularly useful for investigating diseases with a high case-fatality.

However, many diseases have low case-fatality, for example, most mental illnesses,

musculoskeletal diseases, rheumatoid arthritis, chickenpox and mumps. In this situation,

data on morbidity (illness) are more useful than mortality rates.

Morbidity data are often helpful in clarifying the reasons for particular trends inmortality. Changes in death rates could be due to changes in morbidity rates or in

case-fatality. For example, the recent decline in cardiovascular disease mortality rates

in many developed countries could be due to a fall in either incidence (suggesting

improvements in primary prevention) or in case-fatality (suggesting improvements in

treatment). Because population age structures change with time, time-trend analyses

should be based on age-standardized morbidity and mortality rates.

Other sources of morbidity data include:


16/24

hospital admissions and discharges

outpatient and primary health care consultations

specialist services (such as injury treatment)

registers of disease events (such as cancer and congenital malformations).

To be useful for epidemiological studies, the data must be relevant and easily accessible.

In some countries, the confidential nature of patient medical records may make

hospital data inaccessible for epidemiological studies. A recording system focusing

on administrative or financial data, rather than on diagnostic and individual characteristics

may diminish the epidemiological value of routine health service data.

Hospital admission rates are influenced by factors other than the morbidity of the population,such as the availability o

hospital admission policies and social factors.

Because of the numerous limitations of routinely recorded morbidity data, many

epidemiological studies of morbidity rely on the collection of new data using specially designed questionnaires and s

methods. This enables investigators to have more confidence in the data and the rates calculated from them.

Mortality

Epidemiologists often investigate the health status of a

population by starting with information that is routinelycollected. In many high-income countries the fact and

cause of death are recorded on a standard death certificate,

which also carries information on age, sex, and place of

residence. The International Statistical Classification of

Diseases and Related Health Problems (ICD) provides

guidelines on classifying deaths.14 The procedures are revised

periodically to account for new diseases and changes

in case-definitions, and are used for coding causes of death

(see Box 2.2). The International Classification of Diseases

is now in its 10th revision, so it is called the ICD-10.

Limitations of death certificates

Data derived from death statistics are prone to various sources of error but, from an

epidemiological perspective, often provide invaluable information on trends in a populations

health status. The usefulness of the data depends on many factors, including

the completeness of records and the accuracy in assigning the underlying causes of

deathespecially in elderly people for whom autopsy rates are often low.

Epidemiologists rely heavily on death statistics for assessing the burden of disease,

as well as for tracking changes in diseases over time. However, in many countries

basic mortality statistics are not available, usually because of a lack of resources to

establish routine vital registration systems. The provision of accurate cause-of-death

information is a priority for health services.15

Death rates

The death rate (or crude mortality rate) for all deaths or a specific cause of death is

calculated as follows:

Crude mortal it y rate =

Number of deaths during a specified period ( 10n )


17/24

Number of persons at risk of dying during

the same period

The main disadvantage of the crude mortality rate is that it does not take into

account the fact that the chance of dying varies according to age, sex, race, socioeconomic

class and other factors. It is not usually appropriate to use it for comparing

different time periods or geographical areas. For example, patterns of death in newly

occupied urban developments with many young families are likely to be very different

from those in seaside resorts, where retired people may choose to live. Comparisons

of mortality rates between groups of diverse age structure are usually based on agestandardized

rates.

Age-specif ic death rates

Death rates can be expressed for specific groups in a population which are definedby age, race, sex, occupation or

geographical location, or for specific causes of death.

For example, an age- and sex-specific death rate is defined as:

Total number of deaths occurring in a specific age and sex group

of the population in a defined area during a specified period

Estimated total population of the same age and sex group of thepopulation in the same area during the same period

(10n )

Proporti onate mortali ty

Occasionally the mortality in a population is described by using proportionate

mortality, which is actually a ratio: the number of deaths from a given cause per 100

or 1000 total deaths in the same period. Proportionate mortality does not express the

risk of members of a population contracting or dying from a disease.

Comparisons of proportionate rates between groups may show interesting

differences. However, unless the crude or age-group-specific mortality rates are

known, it may not be clear whether a difference between groups relates to variations

in the numerators or the denominators. For example, proportionate mortality ratesfor cancer would be much greater in high-income countries with many old people

than in low- and middle-income countries with few old people, even if the actual

lifetime risk of cancer is the same.

Infant mortality

The infant mortality rate is commonly used as an indicator of the level of health in a

community. It measures the rate of death in children during the first year of life, the

denominator being the number of live births in the same year.

The infant mortality rate is calculated as follows:

Infant mortality rate =Number of deaths in a year of children

less than 1 year of age

Number of live births in the same year

1000

The use of infant mortality rates as a measure of overall health status for a given

population is based on the assumption that it is particularly sensitive to socioeconomic

changes and to health care interventions. Infant mortality has declined in all


18/24

regions of the world, but wide differences persist between and within countries

Child mortality rate

The child mortality rate (under-5 mortality rate) is based on deaths of children aged

14 years, and is frequently used as a basic health indicator. Injuries, malnutrition

and infectious diseases are common causes of death in this age group. The under-5

mortality rate describes the probability (expressed per 1000 live births) of a child dying

before reaching 5 years of age. Table 2.5 shows the mortality rates for countries

representing a range of income categories. The areas of uncertainty around the estimates

for middle-income and low-income countries are shown in parentheses.

Data in Table 2.5 have been calculated so that the information can be compared

between countries. Mortality rates per 1000 live births vary from as low as 4 for

Japan (based on precise data) to 297 for males in Sierra Leone (with a wide range ofuncertainty: between 250 and 340

1000 live births).23 Gathering accurate data is not easy and alternative approaches have been developed

Maternal mortality rate

The maternal mortality rate refers to the risk of mothers dying from causes associated

with delivering babies, complications of pregnancy or childbirth. This importantstatistic is often neglected because it is difficult to calculate accurately. The maternal

mortality rate is given by:

Maternal mortal it y rate=

Number of maternal deaths in a given

geographic area in a given year

Number of live births that occurred

among the population of the given

geographic area during the same year

(10n )

The maternal mortality rate ranges from about 3 per 100 000 live births in high-incomecountries to over 1500 per 100

births in low-income countries.23 However,even this comparison does not adequately reflect the much greater liferisk of

dying from pregnancy-related causes in poorer countries.

Adult mortality rate

The adult mortality rate is defined as the probability ofdying between the ages of 15 and 60 years per 1000 population

The adult mortality rate offers a way to analysehealth gaps between countries in the main working agegroups.24 The

probability of dying in adulthood is greaterfor men than for women in almost all countries, but thevariation betwe

countries is very large. In Japan, lessthan 1 in 10 men (and 1 in 20 women) die in these productiveage groups, com

with almost 2 in 3 men (and1 in 2 women) in Angola

Life expectancy

Life expectancy is another summary measure of the health status of a population. It is defined as the average number

an individual of a given age is expected to live if

current mortality rates continue. It is not always easy to interpret the reasons for the differences in life expectancy bet

countries; different patterns may emerge according

to the measures that are used. For the world as a whole, life expectancy at birth has


19/24

increased from 46.5 years during the period 19501955 to 65.0 years during the period 19952000 (see Figure 2.5).

Reversals in life expectancy have occurred in some sub-Saharan countries largely due to AIDS. Similar reversals i

expectancy have also occurred in middle-aged men in the former Soviet Union, where almost 1 in 2 men die betw

ages of 15 and 60 years, largely due to changes in the use of alcohol and tobacco.26

Life expectancy at birth, as an overall measure of health status, attaches greater

importance to deaths in infancy than to deaths later in life. Table 2.7 gives data for selected countries. As the data are

on existing age-specific death rates, additional calculation is necessary to allow comparability between countries;

uncertainty of the estimates are

shown in parentheses. Confidence intervals can be quite

largeas in Zimbabwebut quite precise in countries like Japan which has complete vital registration. These data sh

large variations in life expectancies

between countries. For example, a girl born in Japan

in 2004 can expect to live 86 years, whereas a girl born in

Zimbabwe at the same time will live between 30 and 38

years. In almost all countries, women live longer than

men.27


Although some diseases are caused solely by genetic factors, most result from an

interaction between genetic and environmental factors. Diabetes, for example, has

both genetic and environmental components. We define environment broadly to

include any biological, chemical, physical, psychological, economic or cultural factors

that can affect health (see Chapter 9). Personal behaviours

affect this interplay, and epidemiology is used to study

their influence and the effects of preventive interventions

through health promotion


Genetic factors

Environmental factors

(including behaviours)

Good health Ill health


20/24

Screening

Screening people for diseaseor risk factors which predict diseaseis motivated by

the potential benefits of secondary prevention through early detection and treatment.

Definition

Screening is the process of using tests on a large scale to identify the presence

of disease in apparently healthy people. Screening tests do not usually establish a

diagnosis, but rather the presence or absence of an identified risk factor, and thus

require individual follow-up and treatment. As the recipients of screening are usually

people who have no illness it is important that the screening test itself is very unlikely

to cause harm.26 Screening can also be used to identify high exposure to risk factors.

For instance, childrens blood samples can be screened for lead in areas of high use

of lead in paint.

Types of screening

There are different types of screening, each with specific aims:

mass screening aims to screen the whole population (or

subset);

multiple or multiphasic screening uses several screening tests at the same time; targeted screening of groups with specific exposures, e.g. workers in lead battery factories, is often used in

environmental and occupational health (Box 6.5); and

case-finding or opportunistic screening is aimed at patients who consult a health practitioner for some other purp

Criteria for screening

Table 6.4 lists the main criteria for establishing a screening programme.27 These relate

to the characteristics of the disorder or disease, its treatment and the screening test.

Table 6.4. Requirements for instituting a medical screening programme

Disorder Well-defined

Prevalence Known

Natural history Long period between first signs and overt disease; medically

important disorder for which there is an effective remedy

Test choice Simple and safe

Test performance Distributions of test values in affected and unaffected individuals

known

Financial Cost-effective

Facilities Available or easily provided

Acceptability Procedures following a positive result are generally agreed upon and

acceptable to both the screening authorities and to those screened.

Equity Equity of access to screening services; effective, acceptable and safe

treatment available

In addition, several issues need to be addressed before establishing a screening

programme.

Costs

The costs of a screening programme must be balanced against the number of cases


21/24

detected and the consequences of not screening. Generally, the prevalence of the

preclinical stage of the disease should be high in the population screened, but occasionally

it may be worthwhile to screen even for diseases of low prevalence which

have serious consequences, such as phenylketonuria. If children with phenylketonuria

are identified at birth, they can be given a special diet that will allow them to develop

normally. If they are not given the diet, they become mentally retarded and require

special care throughout life. In spite of the low incidence of this metabolic disease

(24 per 100 000 births), secondary prevention screening programmes are highly

cost-effective.

Lead time

The disease must have a reasonably long lead time; that is, the interval between the

time when the disease can be first diagnosed by screening and when it is usually

diagnosed in patients presenting with symptoms. Noise-induced hearing loss has a

very long lead time; pancreatic cancer usually has a short one. A short lead time

implies a rapidly progressing disease, and treatment initiated after screening is unlikely

to be more effective than that begun after the more usual diagnostic procedures.

Length biasEarly treatment should be more effective in reducing mortality or morbidity than

treatment begun after the development of overt disease, as, for example, in the treatment

of cervical cancer in situ. A treatment must be effective and acceptable to people

who are asymptomatic. If treatment is ineffective, earlier diagnosis only increases the

Table 6.4. Requirements for instituting a medical screening programme

Disorder Well-defined

Prevalence Known

Natural history Long period between first signs and overt disease; medically

important disorder for which there is an effective remedy

Test choice Simple and safe

Test performance Distributions of test values in affected and unaffected individualsknown

Financial Cost-effective

Facilities Available or easily provided

Acceptability Procedures following a positive result are generally agreed upon and

acceptable to both the screening authorities and to those screened.

Equity Equity of access to screening services; effective, acceptable and safe

treatment available

Epidemiology and prevention: chronic noncommunicable diseases 111

time period during which the participant is aware of the disease; this effect is known

as length bias or length/time bias.Screening test

The screening test itself must be cheap, easy to apply, acceptable to the public, reliable

and valid. A test is reliable if it provides consistent results, and valid if it correctly

categorizes people into groups with and without disease, as measured by its sensitivity

and specificity.

Sensitivity is the proportion of people with the disease in the screened population

who are identified as ill by the screening test. (When the disease is


22/24

present, how often does the test detect it?)

Specificity is the proportion of disease-free people who are so identified by the

screening test. (When the disease is absent, how often does the test provide

a negative result?)

The methods for calculating these measures and the positive and negative predictive

values are given in Table 6.5.

Although a screening test ideally is both highly sensitive and highly specific, we

need to strike a balance between these characteristics, because most tests cannot do

both. We determine this balance by an arbitrary cut-off point between normal and

abnormal. If we want to increase sensitivity and to include all true positives, we are

obliged to increase the number of false positives, which means decreasing specificity.

Reducing the strictness of the criteria for a positive test can increase sensitivity, but

by doing this the tests specificity is reduced. Likewise, increasing the strictness of

the criteria increases specificity but decreases sensitivity. We also need to account

for predictive value when interpreting the results of screening tests

Surveillance and response

Definition

Health surveillance is the ongoing systematic collection, analysis and interpretation

of health data essential for planning, implementing and evaluating public health

activities. Surveillance needs to be linked to timely dissemination of the data, so that

effective action can be taken to prevent disease. Surveillance mechanisms include

compulsory notification regarding specific diseases, specific disease registries

(population-based or hospital-based), continuous or repeated population surveys and

aggregate data that show trends of consumption patterns and economic activity.

Box 7.6. Uses of surveillance

Surveillance is an essential feature of epidemiologic practice and may be used to: recognize isolated or clustered cases;

assess the public health impact of events and assess trends;

measure the causal factors of disease;

monitor effectiveness and evaluate the impact of prevention and control measures, intervention

strategies and health policy changes; and

plan and provide care. In addition to estimating the magnitude of an epidemic

and monitoring its trends, data can also be used to:

strengthen commitment,

mobilize communities, and

advocate for sufficient resources.19128Ch

The scope of survei l lance

The scope of surveillance is broad, from early warning systems for rapid response in

the case of communicable diseases, to planned response in the case of chronic diseases

which generally have a longer lag time between exposure and disease. Most

countries have regulations for mandatory reporting of a list of diseases. These lists

of notifiable diseases often include vaccine-preventable diseases such as polio,


23/24

measles, tetanus and diphtheria as well as other communicable diseases such as

tuberculosis, hepatitis, meningitis and leprosy. Reporting may be required also for

Box 7.5. Immunization: key to prevention and

control

Immunization is a powerful tool in the management and control of infectious diseases. Systematic immunization

programmes can be very effective. For example, by the

late 1980s, most countries in South and Latin America had incorporated measles vaccination into routine immuniz

programs and many had done follow-up

immunization campaigns to reach all children and interrupt measles transmission.17 non-communicable condition

as maternal deaths, injuries and occupational

and environmental diseases such as pesticide poisoning Mandatory reporting of specific conditions is a subset of

surveillance. There are many other uses of surveillance

Pri nciples of surveil lance

A key principle is to include only conditions for which surveillance can effectively lead to prevention. Another im

principle is that surveillance systems should reflect

the overall disease burden of the community. Other criteria for selecting diseases include:

incidence and prevalence indices of severity (case-fatality ratio)

mortality rate and premature mortality

an index of lost productivity (bed-disability days)

medical costs

preventability

epidemic potential

information gaps on new diseases.

Sources of data

Sources of data may be general or disease-specific, and include the following:

mortality and morbidity reports

hospital records

laboratory diagnoses

outbreak reports

vaccine utilization

sickness absence records

biological changes in agent, vectors, or reservoirs

blood banks.

Surveillance can collect data on any element of the causal chain of disease

behavioural risk factors, preventive actions, cases and program or treatment costs.

The scope of a surveillance system is constrained by human and financial resources.

Surveil lance in practiceSurveillance relies upon a routine system of reporting suspected cases within the

health system, followed by validation and confirmation. Active and appropriate

responses ranging from local containment measures to investigation and containment

by a highly specialized team, are then put in place.

Surveillance requires continuing scrutiny of all aspects of the occurrence and

spread of disease, generally using methods distinguished by their practicability, uniformity

and, frequently, their rapidity, rather than by complete accuracy. The analysis


24/24

of data from a surveillance system indicates whether there has been a significant

increase in the reported number of cases. In many countries, unfortunately, surveillance

systems are inadequate, particularly if they depend on voluntary notification.

A wide range of networks, including nongovernmental organizations, electronic discussion

groups, search engines on the World Wide Web, and laboratory and training

networks, offer powerful ways of obtaining information that leads to a coordinated

international response.

Sentinel health information systems, in which a limited number of general practitioners report on a defined list of

carefully chosen topics that may be changed from time to time, are increasingly used to provide supplementary

information for the surveillance of both communicable and chronic diseases. Surveillance of chronic disease risk f

discussed in Chapter 2. A sentinel network keeps a watchful eye on a sample of the population by

supplying regular, standardized reports on specific diseases and procedures in primary health care. Regular feedba

information occurs and the participants usually have a permanent link with researchers.

Analysis and interpretation of surveil lance data

Surveillance is not only a matter of collecting data, as the

analysis, dissemination and use of the data for prevention

and control are equally important. Many public healthprograms have far more data than they can presently analyse

Table 7.3 outlines Millennium Development Goal 6,

which focuses on HIV/AIDS, malaria and other diseases,

which are largely interpreted as communicable diseases.

Non-communicable diseaseswhich account for the bulk

of death and disability in most countrieshave been

omitted.

The indicators, operational definitions and overall objectives to be met for

tuberculosis (target 8) are also shown in Table 7.3; all require detailed surveillance.

Epidimiology Chapter 4

Documents

Transcript of Epidimiology Chapter 4