Veterianry Epidemiology

Recommended Books

1. Veterinary Epidemiology by Michael Thurshfield2. Veterinary Epidemiology: Principles and Methods by Martin SW, Meek AH and Willegerg P

Health: Health is a condition of physical, mental and social well-being and the absence of disease and infirmity.

Disease: A disease is a particular abnormal condition, a disorder of a structure or function that affects part or all of an organism. The study of disease is called pathology which includes the causal study of etiology. It also can be defined as disturbances of physiological imbalance between individual and its environment.

Medicine: The term medicine used in four senses -i) profession ii) subject iii) degree and iv) drug. Medicine is a branch of biological science which deals with diagnosis, treatment, prevention and control of disease.

Clinical Veterinary Medicine: It is a branch of medicine which deals with diagnosis, treatment, prognosis and observation of individual animals.

Preventive Veterinary Medicine: Branch of medicine which deals with prevention and control of disease in population of animals and birds.

Epidemiology: It deals with the study of frequency, distribution and determinants of health and disease in population.

# Frequency: Number of occurrence of disease in population

#Distribution: Distribution of an event by individual, place and time. Epidemiology studies distribution of diseases

#Determinants: Factors the presence/absence of which affect the occurrence and level of an event. Epidemiology studies what determines health events.

# Health and disease: The focus of epidemiology is not only patients. It studies all health related conditions

# Application: Epidemiological studies have direct and practical applications for prevention of diseases & promotion of health.

#Epizootology: Study on disease in animal population.

#Epiecthyology: Study on diseases in fish population

#Epiornithology: Study on disease in bird population

#Epiphytology: Study on disease in plant population.

Veterinary Epidemiology: It deals with the study of the frequency, distribution and determinants of disease, health and related productivity in a population and thereby the optimizations of disease control and improvement of productivity.

Population

The number of animal in a groups being studied who are biologically at risk under study.

Or, To refer the larger number of individual of a particular type or species about being inference (draw a conclusion) are made based on information from sample. e.g cattle of Bangladesh.

Herd: A group of animals that is epidemically distinct from others.

Herd health: A planned animal health and production program.

Two veterinary disciplines

a) Holistic discipline: From individual to the herd. e.g veterinary public health, epidemiology and animal health, management sciences etc.

b) Reductionistic discipline: From individual to cells (clinical medicine). e.g. basic vet science, clinical vet science, paraclinical vet science etc.

Aims and Objectives of Veterinary Epidemiology

.Aims: To introduce veterinary epidemiology and its role in informing animal and human health.

Ojectives:

1. Epidemiological determination and uses (diagnosis of disease on the basis of history and clinical findings)

2. Investigation and control of diseases whose cause is poorly understood.3. Acquisition of information on the basis of ecology and natural history of disease4. Planning and monitoring of disease control program.5. Assessing economical significance of a disease and its control program.

Principles of epidemiology:

1. To study the health and diseases in population2. To measure the frequency, distribution and determinants of health and diseases in population3. To assess the economic significance of diseases in population4. To study the prevention and control programs of diseases

Types of Epidemiology Four categories of epidemiology

a) Descriptive Epidemiology

Defines frequency and distribution of diseases and other health related events. Observing and recording of diseases and their causal agent and possible causal factor.

b) Analytic EpidemiologyAnalyses determinants of health problems. It analyzes the observation using suitable diagnostic tools.

c) Experimental EpidemiologyExperimental epidemiologists observe and analyze data from groups of animals from which they can select, and in which they can alter, the factors associated with the groups. An important component of the experimental approach is the control of the groups.

d) Theoretical epidemiologyTheoretical epidemiology consists of the representation of disease using mathematical 'models' that attempt to simulate natural patterns of disease occurrence

Sub disciplines of epidemiology

a) Clinical epidemiology: Use of epidemiological principles, methods and findings in the care of individuals, with particular reference to diagnosis and prognosis.

b) Computational epidemiology: It involves the application of computer science to epidemiological studies .This includes the representation of disease by mathematical models.

c) Genetic epidemiology: Genetic epidemiology is the study of the cause, distribution and control of disease in related individuals, and of inherited defects in populations.

d) Field epidemiology: Practice of epidemiology in response to problems of a magnitude significant enough to require a rapid or immediate action. For example, when outbreaks of foot-and-mouth disease occur, field epidemiologists promptly trace potential sources of infection in an attempt to limit spread of the disease.

e) Participatory epidemiology: The techniques that are employed evolved in the social sciences, and consist of simple visual methods and interviews to generate qualitative data. This approach known as 'participatory appraisal' and its application in veterinary medicine is now termed 'participatory epidemiology'.

f) Molecular epidemiology: New biochemical techniques now enable microbiologists and molecular biologists to study small genetic and antigenic differences between viruses and other microorganisms at a higher level of discrimination than has been possible using conventional serological techniques. e.g. Nucleotide sequencing of foot-and-mouth disease virus has indicated that some outbreaks of the disease involved vaccinal strains, suggesting that improper inactivation or escape of virus from vaccine production plants may have been responsible for the outbreaks.

Other subdisciplines of epidemiology are-a) Chronic disease epidemiologyb) Environmental epidemiologyc) Microepidemiologyd) Macroepidemiology

History of Epidemiology

Seven land marks in the history of Epidemiology1. Hippocrates (460BC): Environment & human behaviors affects health2. John Graunt (1662): Quantified births, deaths and diseases 3. Lind (1747): Scurvy could be treated with fresh fruit4. William Farr (1839): Established application of vital statistics for the evaluation of health

problems5. John Snow (1854): tested a hypothesis on the origin of epidemic of cholera

6. Alexander Louis (1872): Systematized application of numerical thinking (quantitative reasoning)7. Bradford Hill (1937): Suggested criteria for establishing causation Epidemiological thought emerged in 460 BC Epidemiology flourished as a discipline in 1940s

Causation of diseases in population:

1. Supernatural theory of diseaseLong time before it has been believed that disease come from god curse or evil forces. Around 10% people in developed country and 30% in developing country believe the supernatural origin of diseases. Long time before people used to please the god by prayers or sacrifice something for the satisfaction of god or evil forces.

2. Ecological theory of disease463 BC, Hippocrates advised to search the environment for the causation of disease. Ecological determinants of health play an important role for the progression of diseases. This theory interacts among human, animals, microbes, plants, ecosystems, climate, geography and topography.

3. Germ theory of diseaseMicrobes were found to be cause for many diseases. Pasteur, Henle, Koch were the strong proponent of microbial theory following the discovery of microbes in the patient’s secretions or excretions.Henle-Koch postulates:Henle and Koch postulated that—a. Each disease will be caused by a germb. Without that germ, individual disease will not be causedc. By introducing that germ, the disease can be caused in animals experimentallyd. The germ can again be isolated from that sick animal experimented with

4. Multifactorial causation theory of disease (Evan’s postulate theory)Pettenkoffer stated that agent, host and environmental factors will act and interact synergistically and act as a joint independent partner for the causation of disease.

Sources of animal health related data collection

1. Field veterinary practices2. Computerized data (veterinary hospital/clinic)3. Abattoir4. Poultry processing plant5. Zoological garden6. Farm record7. Veterinary school and college8. Research laboratory9. Pet food manufacturer10. Pet shop11. Animal breeder organization

Patterns of occurrences of disease

a) Sporadic disease: Occurring occasionally, singly or in irregular or scattered instances. Example: bloat, encephalitis, acidosis, abomasal displacement etc.

b) Endemic disease: The habitual presence of a disease within a given area. Example: Foot and mouth disease, anthrax, black quarter, hemorrhagic septicemia etc.

c) Epidemic disease: The occurrence of disease in a community or region of a group of illness of similar nature, clearly in excess of normal expectancy. Example: Appearance of new antigenic type of FMD, outbreak of anthrax after flooding, Cholera etc.

d) Pandemic disease: Large-scale epidemic is called pandemic. Example: Avian influenza, SARS viral disease, Ebola viral disease etc.

e) Exotic disease: The diseases which are introduced from other country. Example: Avian influenza, blue tongue in sheep, SARS viral disease, Ebola viral disease etc.

Determinants or risk factor: The factors which increase the susceptibility of disease.

There are three components of determinants of disease 1) Host II) Environment and management, and III) Agent

I) HostAge: The newborn animals which are deprived with colostrum increase the susceptibility of infection.

Size of herd: Animals get more contact in large size of herd as compared with small size herd. Infectious and contagious diseases are easily transmitted in large size herd of animals.

Nutritional status: Deficiency of zinc, copper, cobalt, iron, selenium etc. increase the susceptibility of deficiency in animals

Immunity: Immunocompromised animals are highly susceptible to disease than immunocompetant individuals.

Pre existing disease in herd: Pre existing diseased animals are highly affected than disease free animals. Example: parasitic infestation, any disease in mammary gland increase the susceptibility of mastitis, calcium deficiency etc.

Stage of pregnancy or production: The animals which are pregnant or in peak lactating condition are more susceptible to production disease than non pregnant or non lactating cow.

Breed: Exotic breed cattle are more susceptible to disease than indigenous (zebu) cattle.

Type: Milking type cattle are high risk to mastitis than beef cattle. On the other hand beef type cattle are more prone to musculoskeletal disorder.

Sex: Usually female animals are more susceptible to production or reproductive tract related diseases than male animals.

II) Environment and management risk factor Poor housing environment, inadequate ventilation of animal house and high stocking density

increase the susceptibility of infection Unhygienic condition of farm Improper isolation and quarantine of diseased animals Climate change factor Use poor bedding material for animal shed, Floor design, drainage system, surrounding environment of farmIII) Pathogen risk factor Virulence or reservoir Concurrent infection (Diarrhoea- colibacillosis, rotavirus and cryptosporidiosis mixed infection, Respiratory infection-Pasteurellosis, and Mycoplasmosis mixed infection ).

Transmission of infection

Transmission of infectious agents may be either A) Horizontal (lateral) or B) Vertical

A) Horizontal transmission: Transmission of one animal or group of animal to another. e.g. FMD virus transmitted from one cattle to herdmate. Transmission of infectious agent performed by directly or indirectly (vector).

B) Vertical transmission: Transmitted from one generation to the next usually from the dam to the newborn through embryonic or fetal infection. There are two types of vertical transmission of infectious agents a) Hereditary (carried within the genome of either patient) and b) Congenital (the disease present at birth). The vertically transmitted causal agents produce abortion or teratological defects in newborne.

Germinative transmission : Infection of ovum-Avian leukosis virus, Avian salmonellosis Transplacentally : Feline panleukopenia virus, Bovine viral diarrhea virus, Toxoplasmosis Ascending infection: Staphylococcus, Streptococcus infection Transovarian transmission: Babesiosis Trans-stadial transmission: Theileriosis Both transovarian and transstadial transmission: Nairobi sheep disease virus Transovarian, transstadial and sexual transmission: African swine fever virus

Route of transmission of infectious agents

a) Oral/Ingestion: Contaminated feed & water (feco-oral). e.g. GI parasites, Rotavirus, Salmonella sp, Escherichia coli etc.

b) Aerosol/aerial/respiratory: Poor ventilation, high population densities and air borne particles increase the susceptibility of respiratory infection. e.g. Influenza virus, respiratory syncytial virus, rhino virus etc.

c) Percutaneous: Through unbroken skin. e.g. Arthropod borne infection, hook worm, blood fluke (schistosmoa), Leptospira and Brucella.

d) Cornea: Localized infection. eg. Keratoconjunctivitis, Newcastle disease e) Coitus: Sexual intercourse. e.g. Syphilis, gonorrhea, tritrichomoniasis, campylobacteriasis,

trypanosomiasis etc.f) Iatrogenic transmission: Gr.Iatro-physician-the disease created by doctor using dirty instruments,

contaminated prophylactic & therapeutic preparations.

Principles of control of disease in population

1. Environment and management control: Ecological imbalance between host and environment produce disease. So, it is recommended that addition of clean litter daily to poultry house, removal of feces and soiled litter daily

2. Reduce infection pressure and prevent new infection: Maintenance of a clean environment to prevent contamination of feed and water supplies.. For this reason, feeder and water trough should be placed above the ground

3. Quarantine: Quarantine is place where restriction of movement of animals and animal should be kept for fixed period of isolation. Enforced physical separation should be performed for healthy animals from diseased individuals. Compulsory quarantine for imported food and zoo animals should be maintained in national level. It may be voluntary or required legislation at regional and herd level.

4. Chmoprophylaxis: Use of chemotherapeutic drug as prophylactic measure to control the animal diseases. Chemotherapeutic durugs can be administered by following ways- Antimicrobial to high risk calves immediately after arrival from market place Mass mediation wih antimicrobial to check the onset of clinical disease Addition of antibiotics to animal & poultry feed to promote growth and post-operative usages

to prevent bacterial infection Disadvantages of chemoprophylaxis

Obtain variable result Develop new bacterial and viral strain Resistant to antibiotics

5. Control of vector Vector habitat can be destroyed by using insecticides Remove snail from moist land (physical or biological means-duck) Strict disinfection of veterinarian or animal caretaker, who act as mechanical vector Vector control by irradiation: Use of irradiated sterile male flies to control screw-worm disease.

Since the female mate only once, mating with an irradiated male leads to no offspring and subsequent reduction of flies.

6. Use of disinfectant Formaldehyde fumigation between batches of egg in hatcheries (35 ml formalin to 10 gram

potassium permanganate/mm3) Pasteurization of milk to destroy microorganism Iatrogenic transmission-surgical instruments, farm equipments, contaminated syringe and needle

7. Genetic improvement Some infectious diseases can be reduced by selective breeding-Marek’s disease Canine hip dysplasia: early detection by radiography to detect the affected animals should not be

used for breeding Genetic screening to identify the genetic disease-modern transgenic biology

8. Niche filling The non pathogenic microbes usually present in intestine can prevent its occupation by

pathogenic organism which is termed as competitive exclusion. Endogenous intestinal microbes have been fed to day-old chicks prevent the colonization of

pathogenic organism. e.g. Salmonella sp, E. coli, Camphylobacter jejuni etc.9. Biosecurity: Greek word bios means life and security means to save/guard. It may be defined as the

technology for the process through which all the possible channels of entrance and spread out of pathogens are efficiently and conveniently prevented in farm.Types of practice

a) Conceptual & structural (site selection and construction of farm) Away from locality Proper ventilation Proper curtains Feeding and watering systems Sign board sited‘No entrance’

b) Regulation concerning movement: Importation of animals and birds and their products must be controlled by regulations of veterinary services

c) Traffic control Controlling human traffic before entering the farm-protective clothing and disinfectant Restriction of motor vehicle entering the farm

d) Operational: Management and routine procedures Sanitation Disinfection Vaccination Medication Parasitic control Good nutrition and environment

10. Immunoprophylaxis/ VaccinationVaccine: Vaccine is a biological agent, which provides active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease causing microorganism and is often made from weaken or killed forms of the microbe, toxin or its surface antigen.

Components of epidemiology

The first stage in any investigation is the collection of relevant data. Investigations can be either qualitative or quantitative or a combination of these two approaches.

1) Qualitative epidemiological investigations: Qualitative epidemiology deals with the details of the transmission, maintenance and ecology of infections and infestations.

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

infectious disease, is investigated by field observation Field observations may reveal information about factors that may directly or indirectly cause

disease Causal hypothesis testing

If field observations suggest that certain factors may be casually associated with disease, then formulating a causal hypothesis must assess the association.

The epidemiologist is concerned with the identification of unknown causes of infectious disease and source of infection.

2) Quantitative epidemiological investigations Quantitative epidemiology requires an understanding of basic observational techniques, such as

sampling and measurement of association Quantitative investigations involve measurement like the number of cases of disease and therefore

expression and analysis of numerical values, which include the followings-a) Survey

An epidemiological survey is an examination of a group of animal and characteristics might include the presence of particular diseases, weight and milk yield. Survey can be undertaken on a sample of the population

A cross –sectional survey records events occurring at a particular point of time A longitudinal survey records events over a period of time Screening is a particular type of diagnostic survey, which is the diagnosis of undiagnosed cases

of disease using rapid test or examinations like serological tests.b) Monitoring

Monitoring is the making of routine observations on health, productivity and environmental factors and the recording and transmission of these observations.

The regular recording of milk yields is monitoring and routine recording of meat inspection findings at abattoirs

c) Surveillance Surveillance is a more intensive form of data recording than monitoring which include all types

of disease –infections and non-infections It is normally part of control programs for specific disease, 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.

d) Studies Study is a general term that refers to any type of investigation and there are four main types of

epidemiological study i) Experimental studies, ii) Cohort studies , iii) Case-control studies and iv) Cross-sectional studies,

i) Experimental studies When the investigator is to allocate animals to various groups according to testing factors

like clinical trial of drugs with keeping a control group It is then possible to evaluate the efficacy of the procedure by comparing the two groups. The cross-sectional, case-control and cohort types of studies are called observational

studiesii) Cohort studies

A cohort study select groups according to presence or absence of exposure to hypothesized to the factors and then look forward to the development of disease.

A group (cohort) of animals exposed to risk factors is compared with a group not exposed to the factors with respect to the development of a disease.

For example, if castration are considered to be a risk factor for development of urolithiasis in male goats; a suitable cohort study would comprise a group of castrated and a group of uncastrated goats, each of which would be monitored for the development of urolithiasis.

Therefore incidence is measured and a+b and c+d in Table 215 are predetermined.

Table: A The 2×2 contingency table constructed in observational studies

S/N Hypothesized risk factors Diseased Non diseased Total

animals animals 1 Present a b a+b2 Absent c d c+d3 Total a+c b+d a+b+c+d=n

iii) Case-control studies A case control study compares diseased animals (cases) with non-diseased animals

(controls) and therefore has variously been called a case-control study A group of diseased animals (cases) and a group of healthy animals (controls) are

selected and compared with respect to presence of the hypothesized risk factor For example, a group of male goats and urolithiasis can be compared with a group of

male goats without urolithiasis with respect to determine whether that type of stall-feeding has an effect on the pathogenesis of the disease.

Therefore, a+c and b+d are predeterminediv) Cross-sectional studies (Synonym=prevalence)

A cross-sectional study investigates relationship between diseases (or other health related problem) and hypothesized causal factors in a specified population

The study involves the selection of a sample of n individual from a larger population, and then determination, for each individual, of the simultaneous presence or absence of disease and hypothesized risk factor, prevalence is therefore recorded.

At the beginning of the cross-sectional study only the total number of animal n in Table A is predetermined.

v) Longitudinal study Both the case-control and cohort studies are considered two events-exposures to a

hypothesized causal factor or factors and development of disease that are separated by a period of time.

Because of this temporal separation of the two events, each of these studies is sometimes called longitudinal study.

vi) Retrospective study It refers to any study that records data from the past and prospective to any study designed

to collect future data Therefore, a non-current cohort (prospective in the causal sense) study may alternatively

be termed a retrospective (in the temporal sense) cohort study.vii) Modeling

Disease dynamics and the effects of different control strategies can be represented using mathematical equations are called ‘modeling’. Some examples of modeling are given below:

Biological simulation using experimental animals (frequently lab. animals) to stimulate the pathogenesis of diseases.

Spontaneous occurrence of disease in animals can be studied in the field e.g. using observational studies.

viii) Disease control The goal of epidemiology is to improve the veterinarian’s knowledge so that diseases can

be controlled effectively

Treatment, prevention and eradication of diseases can fulfill the control of diseases and optimum productivity.

Levels of disease prevention

Three major levels of disease prevention

1) Primary prevention :Targeted at healthy people

Objectives: i) Promotion of health ii) Prevention of exposure iii) Prevention of disease

2) Secondary prevention: Targeted at sick individuals

Objectives: i) Stop or slow the progression of disease ii) Prevent or limit permanent damage through early detection & treatment

3) Tertiary prevention: Targeted at animals with chronic diseases & disabilities that can’t be cured

Measures of disease occurrence

What are the measures of disease occurrence?

Frequency/magnitude/amount of disease are the measures of disease in populations

How do we measure diseases?

Four quantitative descriptors a) Numbers b) Ratios c) Proportions d) Rates

Descriptors Numbers: Use of actual number of events. e.g 100 cases of TB in community A Ratios: Quantifies the magnitude of one occurrence X, in relation to another event Y as X/Y e.g

Ratio of TB cases in community A to B is 1:10 Descriptors Proportions: A ratio in which the numerator is included in the denominator e.g

proportion of TB cases in community A is 10% Rates: A proportion with time element. It measure the occurrence of an event overtime e.g U5

measles cases in 2000/U5 population in 2000

Ratio: When we call a measure a ratio, we mean a non proportional ratio

Proportion: When we call a measure a proportion, we mean a proportional ratio that doesn’t measure event overtime

Rate: When we call a rate, we mean a proportional ratio that does measure an event in population overtime

Types of rates

• Crude rates: Apply to the total population in a given area

• Specific rates: Apply to specific subgroups in the population (age, sex etc) or specific diseases

Standardized rates: used to permit comparisons of rates in population which differ in structure (e.g age structure)

Morbidity rates

# Morbidity rates are rates that are used to quantify the magnitude/frequency of diseases

# Morbidity refers to the proportion of animals, which become infected with a given disease.

# Calculation of morbidity rate is done by counting the animals that develop an illness over a period of time (cases) and dividing the number by the total number of animals in the group at the start of the time period.

Morbidity rate (%)= No. of animals manifesting the disease/ no. of animals at risk × 100.

Two common morbidity rates

Incidence rates(Cumulative incidence, incidence density) Prevalence (Period prevalence, point prevalence)

Incidence rate

# The proportion of a population that develops a disease overtime

# The risk/probability of an individual developing a disease overtime

#The rapidity with which new cases of a disease develop overtime

#The proportion of unaffected individuals who on average will contract the disease overtime

Number of new cases of a disease during a specified period

Cumulative incidence = Population at risk in the same period

Incidence: Incidence is the number of new cases that occur in a known population over a specified period of time. It consists of two essential components. a) The number of new cases and b) The period of time over which the new cases occur.

Incidence rate can be calculated as number of new case of disease, divided by population at risk and multiply by 100.

Practical challenges in measuring incidence rate

1. Identification of population at risk 2. Population at risk constitutes all those free of the disease and susceptible to it3. Population is not static/it fluctuates/as a result of births, deaths and migration

4. People are at risk only until they get the disease and then no more at risk

Practical solution to the challenges

1. Use the total population as a denominator 2. This gives an estimate of the incidence rate and not the actual incidence rate

Prevalence rate

# It measures the proportion of a population with a disease during a specified period or at a point in time.

# Prevalence refers to all cases of disease observed at a given moment within the population at risk.

Two types 1. Point prevalence rate 2. Period prevalence rate

1. Point prevalence: Measures the proportion of a population with a disease at a point in time. 2. Period prevalence: Refers to the number of cases that are known to have occurred during a

specified period of time. e.g. a year (annual prevalence). Life time prevalence is the number of individuals known to have had disease for at least part of their life.

Prevalence rate can be calculated as number of cases of disease divided by population at risk and multiply by 100.

Prevalence rate (%)= No. of cases of disease/ population at risk × 100Example: If 200 broiler birds in a flock of 1000 birds are affected with coccidiosis, then the prevalence rate of coccidiosis is 20%

Incidence Vs prevalence

Incidence rate considers only new cases of a disease Prevalence rate considers all (new + old) cases of a disease Incidence rate considers population at risk as a denominator Prevalence rate considers total population as a denominator Incidence & period prevalence rates require follow up studies Point prevalence rate requires cross- sectional study

Mortality: Mortality is a measure of the number of deaths in a population. Mortality rate is calculated as similar to morbidity rate but deaths are counted as cases.

Mortality rate (%)= Total death in time period/ Av. Population at risk in time period ×100.

Case fatality: Case fatality is the proportion of diseased animals that die. It is the proportion of the death of affected animals in a specified time.

Case fatality rate (%)= No. of death/ no. of diseased animals ×100.

Culling rate: Culling rate is the percentage of animals culled because of chronic illness or inferior performance during the month.

Culling rate (%) = No. of animal culled during the month/ Av. No. of animals risk during the month ×100.

Table: A The 2×2 contingency table constructed in observational studies

S/N Hypothesized risk factors Diseased animals

Non diseased animals

Total

1 Present a b a+b2 Absent c d c+d3 Total a+c b+d a+b+c+d=n

Chi-square statistics

Chi-square tests whether there is an association between two categorical variables. If the calculated chi-square value is greater than the critical or P<0.05 we say that there is association.

Relative risk (RR)

Expresses risk of developing a diseases in exposed group (a + b) as compared to non-exposed group (c + d)

Incidence (risk) among exposed

RR=

Incidence (risk) among non-exposed

RR= a/(a+b)

c/(c+d)

Interpretation of relative risk

What does a RR of 2 mean?

Risk in exposed =RR X Risk in non-exposed

RR of 2 means Risk in exposed=2X Risk in non-exposed

#Thus a relative risk of 2 means the exposed group is two times at a higher risk when compared to non-exposed.

Strength of association

In general strength of association can be considered as:

High if RR>3, Moderate if RR is between 1.5 & 2.9 and Weak if RR is between 1.2 & 1.4

Odds ratio (OR)

Odds ratio is the ratio of odds of exposure among diseased to odds of exposure among non-diseased.Odds ratio : Odds of exposure among exposed=a/c Odds of exposure among non-diseased=b/d# OR = Odds of exposure among diseased Odds of exposure among non-diseased# OR= (a/c)/(b/d) OR= ad/bc (it is also called cross-product ratio) Interpretation of OR is the same as that of RR.

Attributable Risk (AR)

AR indicates how much of the risk is due to /attributable/ to the exposure Quantifies the excess risk in the exposed that can be attributable to the exposure by removing the risk of the disease occurred due to other causes AR= Risk (incidence) in exposed- Risk (incidence) in non exposed AR= {a/(a+b)} / {c/(c+d)} # Attributable risk is also called risk difference

Interpreting AR

What does attributable risk of 10 mean?

#10 of the exposed cases is attributable to the exposure

# By removing the exposure one can prevent 10 cases from getting the disease

Population Attributable Risk (PAR)

Estimates the rate of disease in total population that is attributable to the exposure

# PAR = Risk in population – Risk in unexposed

# PAR = AR X prevalence rate of exposure

Possible outcomes in studying the relationship between exposure & disease

1. No association RR=1, AR=0 2. Positive association RR>1, AR>0 3. Negative association RR<1 (fraction) and AR<0 (Negative)

Ecology of disease

The term ecology is derived from Greek word ‘oikos’ means house and ‘logos’ means study. The study of animals and plants in relation to their habits and habitation is ecology.

Ecology developed as a discipline relating to animals and plants, but has been extended to include microorganisms.

The study of a disease’s ecology (also termed natural history) is frequently a part of epidemiological investigations. This has two objectives:

a) An increase in the understanding of the pathogenesis, maintenance for infectious agents and transmission of disease.

b) The use of knowledge of a disease’s ecology to predict the sources and occurrences of diseases which enable the development suitable control techniques.

Sampling:  Sampling is concerned with the selection of a subset of individuals from within a statistical population to estimate characteristics of the whole population. Two advantages of sampling are that the cost is lower and data collection is faster than measuring the entire population.

Type of sampling

1. Probability sampling2. Non-probability sampling

1. Probability sampling: The selection of the sample is made using unbiased process. So that each sampling unit in a group has an equal probability of being selected.

a) Simple random sampling

In a simple random sample (SRS) of a given size, all such subsets of the frame are given an equal probability. Furthermore, any given pair of elements has the same chance of selection as any other such pair (and similarly for triples, and so on). This minimizes bias and simplifies analysis of results. In particular, the variance between individual results within the sample is a good indicator of variance in the overall population.

b) Systematic sampling

Systematic sampling (also known as interval sampling) relies on arranging the study population according to some ordering scheme and then selecting elements at regular intervals through that ordered list. A simple example would be to select every 10th name from the telephone directory.

c) Stratified sampling

When the population embraces a number of distinct categories, the frame can be organized by these categories into separate "strata." Each stratum is then sampled as an independent sub-population, out of which individual elements can be randomly selected. Assume that we need to estimate average number of

votes for each candidate in an election. Assume that country has 3 towns: Town A has 1 million factory workers, Town B has 2 million office workers and Town C has 3 million retirees. We can choose to get a random sample of size 60 over entire population but there is some chance that the random sample turns out to be not well balanced across these towns. Instead if we choose to take a random sample of 10, 20 and 30 from Town A, B and C respectively then we can produce a smaller error in estimation for the same total size of sample.

d) Cluster sampling

The total population is divided into groups (known as clusters) and a simple random sample of the groups is selected. The elements in each cluster are then sampled. If all elements in each sampled cluster are

sampled, then this is referred to as a "one-stage" cluster sampling plan. If a simple random subsample of elements is selected within each of these groups, this is referred to as a "two-stage" cluster sampling plan.

2. Non probability sampling

Non-probability sampling is any sampling method where some elements of the population have no chance of selection (these are sometimes referred to as 'out of coverage'/'undercovered'), or where the probability of selection can't be accurately determined. It involves the selection of elements based on assumptions regarding the population of interest, which forms the criteria for selection. Example: We visit every household in a given street, and interview the first person to answer the door. In any household with more than one occupant, this is a non-probability sample because some people are more likely to answer the door (e.g. an unemployed person who spends most of their time at home is more likely to answer than an employed housemate who might be at work when the interviewer calls).

a) Convenience sampling: A convenience sample is a type of non-probability sampling method where the sample is taken from a group of people easy to contact or to reach. For example, standing at a mall or a grocery store and asking people to answer questions would be an example of a convenience sample. This type of sampling is also known as grab sampling or availability sampling.

b) Judgmental sampling or purposive sampling: The researcher chooses the sample based on who they think would be appropriate for the study. This is used primarily when there is a limited number of people that have expertise in the area being researched, or when the interest of the research is on a specific field or a small group.

# Sensitivity (also called the true positive rate, the recall, or probability of detection in some fields) measures the proportion of positives that are correctly identified as such (e.g. the percentage of sick people who are correctly identified as having the condition).

# Specificity (also called the true negative rate) measures the proportion of negatives that are correctly identified as such (e.g. the percentage of healthy people who are correctly identified as not having the condition).

Number of true positiviesNumber of

true positives+ Nuber of

false negatives

Sensitivity =

Number of true negativesNumber of true

negatives+ Number of false

positives

Specificity =