Infection and Epizootiology

7/28/2019 Infection and Epizootiology

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Discussed by: Adrian S. Mausig Master of Science in Entomology Insect Pathology

INFECTION AND EPIZOOTIOLOGY

THEORIES OF INFECTION AND DISEASE

Phytogenic Theory held that disease originated from decomposition or filth.

Germ Theory affirms the definite and specific relationship of microorganisms to infectious disease.

INFECTION

Infection a biological relationship, resulting in disease, in which an invading microorganism settles,grows, and multiplies in the host organism.

Contamination harboring of or contact with microorganisms.

Infestation a situation in which considerable numbers of a parasite enter or attach themselves to theirhost.

Intoxication manifested by the presence of symptoms caused of symptoms caused by the activity ofmicroorganisms.

Classifications of Infection1. The extent of the infectious process in the host (local, focal, and general or systemic)

2. The site of the infection (intestinal, fat body, blood)3. The course of the disease (acute, subacute, chronic, latent)4. The source of the infecting agent (exogenous, endogenus, and idiopathic, or hidden)5. The type of etiological agent (bacterial, protozoan, fungal, viral)6. The distribution or extent of the infection in the insect population (sporadic, enzootic, epizootic)7. The mode of transmission (food-borne, water-borne, direct contact, fomites)8. The basis of sequence (primary, secondary, mixed or multiple, terminal)

Virulence the disease-producing intensity or power of a microorganism.

Immunization process of increasing the resistance of the host.

Attenuation decreasing the virulence of a microorganism.

Virulence of a microorganism may be increased through:1. Passing it through insects;

2. Causing it to dissociate into its more virulent or less virulent strains;3. Introducing, together with the microorganism, substances that may aid in increasing its invasive

powers;4. Associating it in a mutualistic relationship with other microorganisms.

Virulence of a microorganism may frequently be decreased by:1. Passing it through insects or animals unfavorable for its growth and development;2. Causing it to dissociate into strains of low and high virulence;3. Cultivating it at abnormally high temperatures;4. Cultivating it under abnormal nutrient conditions.

Opportunist pathogens those which live in constant association with the host, such as those in thealimentary tract.

True pathogens capable of invading under normal conditions of host resistance and rarely live in close

association with the insect without producing disease. Portal of entry the vulnerable point on which microbe enters the body of the insect.

Incubation period time elapsing between the entrance or introduction of the microorganisms into theinsect body and the development of symptoms.

Beginning of disease the period from the first appearance of symptoms until they are fully developed.

Height of disease the period from the time symptoms attain their full height until they begin to fall.

Bacteremia when bacteria are present in the hemolymph of an insect, but are producing no harmful tonsor other deleterious effects.

Septicemia the microorganisms actually multiply in the blood and thereby bring about harmful reactions.


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Toxemia a condition produced by the dissemination of bacterial toxins or other poisonous substances inthe blood.

Kochs Postulates:1. The microorganism must be present in every case of the disease.2. The microorganism must be isolated in pure culture.3. The microorganism in pure culture must, when inoculated into a susceptible animal, give rise to the

disease.4. The same microorganism must be present in, and recoverable from, the experimentally diseased

animal. Mechanisms of producing infection:

1. Production of chemical or toxic substances.2. Causation of mechanical destruction.

Types of toxic substances produced by bacteria:1. Catabolic substances the results of decomposition of the bodies of microorganisms.2. Anabolic substances toxic or destructive substances synthesized by the bacteria.

Exotoxins poisonous substances produced by the microbial cell and liberated into the surroundingmedium outside the cell.

Endotoxins not secreted into the surrounding medium but are confined within the microbial cell.

EPIZOOTIOLOGY OF INSECT DISEASES

Epizootiology the science that seeks to explain infectious diseases of animals on the basis of massphenomena.

Epizootic disease one that of high morbidity and is irregularly present in clinically recognizable form.

Enzootic disease has a low incidence but is constantly present in a population.

Morbidity refers to sickness or disease; Morbidity statistics include cases that recover as well as thosethat die.

Mortality refers to death; Mortality statistics are concerned with fatal cases only.

Rates expression of epizootilogical statistics.

Proportionate or percentage mortality used to express the proportion that the deaths from any givendisease bear to the total deaths from all causes.

Crude rates expressed in terms of all stages of all the insects in a population.

Specific rates expressed in terms of the proportion of cases or deaths in a particular instar or stage ofthe insects concerned.

Standardized rates expressed in terms of a standard population of the insect in question in a definitearea.

Case fatality rate refers to the percentage of deaths.

Incidence the degree of occurrence of a disease in a particular population; the ratio of diseasedindividual to healthy ones in a given population.

Prevalence a function of incidence of the disease times its duration.

Primary factors in Epizootics:1. The infectious agent with its variable virulence and infectivity;2. The susceptibility or resistance of the individuals that compose the population at risk;

3. An efficient means of transmission.

Population susceptibility and immunity in relation to Epizootics

Theoretical categories of insect hosts:1. Typically diseased insects2. Atypically diseased insects3. Uninfected immune4. Uninfected susceptible


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Infectivity or capacity to spread

One of the most important factors concerned in epizootic within an insect population.

Direct contact an important means of spread in the case of fungal disease.

Aggregation the closeness and extent to which insects come together in the course of their activities.

The maximum opportunity for the spread of infection will occur when a center of close aggregation isassociated with marked dispersal.

Critical dose the quantity of infective entities necessary to overcome the resistances offered by aparticular insect to a particular microorganism.

The Epizootic Wave

Types of curve:1. With a longer descending than ascending limb;2. Symmetrical or nearly symmetrical;3. With a longer ascending than descending limb.

Preepizootic phase the period just preceding the increased prevalence of the disease underconsideration.

Epizootic phase the period when the disease attained the climactic level.

Postepizootic phase the end of a single epizootic wave.

Variations in Disease Prevalence

Short-time irregular fluctuations due to variations in temperature, humidity, nourishment of host,population densities, intercurrent infection and parasitism, and man-made factors.

Annual seasonal variations dependent more or less directly upon the seasonal meteorological changes,but also dependent to some extent upon the effect of seasonal changes in the behavior of the host.

Cyclic or intrinsic periodicities of duration in months and years but not coinciding with the annual solarcycle.

Secular variations in prevalence such as might be observed from one century to the next.

Infection and Epizootiology

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