1.2. Applications of Epidem

7/27/2019 1.2. Applications of Epidem

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APPLICATIONS OF EPIDEMIOLOGYIN VETERINARY PRACTICE


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Epidemiologic methods can

be used in a number ofdistinct purposes. In thislecture, you will beacquainted with some areas

of application ofepidemiology in veterinarypractice.

The content of this lecture is cited from AVET

manual (2008).


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Investigating causes of disease &

understanding the means to control them

What could cause a disease such as this?Epidemiologists trail the events resulting from

exposure of certain types of individuals in certain

types of environment, and identify those exposuresassociated with the disease of interest.

Identification of herd-level and individual-level risk

factors associated with important infections isneeded to develop appropriate & cost-effective

disease prevention and control programs.


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BOX 1. Identification of risk factors for caprine brucellosis

infection (hypothetical data)

B. melitensis is the most-frequent cause of human brucellosis.

To identify risk factors associated with B. melitensis

seropositivity in goats, a questionnaire-based cross-sectional

cluster study in 79 randomly selected goat herds was

conducted in Laguna. Farmers were interviewed using astructured questionnaire. In addition, blood samples collected

from 5114 animals were tested for B. melitensis antibodies

using the RBPT & the CFT. The potential risk factors assessed

for herd-level brucellosis seropositivity were locality, herd size,stock density, duration of lactation, abortion last year, cleaning

up the corral, nursing of other kids, sharing of sires, have their

own sire and culling of seropositive animals.


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BOX 1. Identification of risk factors for caprine brucellosis

infection (continuation)

Results showed that animals in large herds (>34 animals), inherds with high stock density (>3.5 animals/m2) or animals >24

months old had higher odds of seropositivity (2.0, 1.7 and 1.8,

respectively) than those in small herds, in herds with low stock

density or animals


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Epidemiologic principles and methods

guide veterinary practitioners, disease

control authorities, diagnostic

laboratories or farmers in test selection &

Test Selection and Interpretation

Tests of high accuracy are essential in prevalence

estimation, risk factor identification, disease

surveillance, health certification schemes and in

substantiating freedom from disease.

interpretation, resulting in increase probability of

correct diagnosis or prognoses and in timelyapplication of appropriate intervention when

warranted.


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To estimate the true animal, within-herd, and between-herdprevalences ofM. avium-paratuberculosis (MAP) antibodies in

Isabela Boer goats, a cross-sectional study was conducted in

25 herds in the province. Only goats 24 months old were

tested by a commercial ELISA kit.

Herds were declared positive for MAP if >1 goats in the herd

tested positive. The apparent animal, within-herd, & between-

herd prevalences & 95% confidence intervals were calculated.In all calculations, the apparent sensitivity and specificity of the

ELISA were assumed to be 65% & 99%, respectively, based on

the manufacturers report.

BOX 2. Seroprevalence of Caprine Paratuberculosis in Boer

Goat Herds Using ELISA (hypothetical data)


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True animal, within-herd, and between-herd prevalences in

the sampled herds were 4% (95% CI = 0.1 to 5.6%), 7% (95%

CI = 0 to 10%), and 64.7% (95% CI = 28.2 to 86.2%),

respectively.

The prevalence estimate obtained in the study cannot be

extrapolated to the national prevalence of caprine PTB and to

other breeds of goats reared in the US due to the smallsample size and the low ELISA sensitivity.

BOX 2. Seroprevalence of Caprine Paratuberculosis in Boer

Goat Herds (continuation)


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Selection of Prophylactic/TherapeuticProducts and Animal Health Strategies

The clinical efficacy of new vaccines and drugs orother interventions must be determined prior totheir integration into main-stream veterinarypractice. The effect of an animal health intervention

must also be assessed for an optimum delivery ofveterinary service.

The standard approach is the use of randomized

controlled trial (RCT). Epidemiologic principles andtechniques guide the design, conduct and analysisof RCTs.


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BOX 3. Designing a randomized controlled field trial of

two commercial modified- live (MLV) vaccines

To assess the effects of 2 commercial MLV vaccines againstPRRS in breeding pigs on viral replication, reproductive

performance and transplacental infection in gilts exposed to

vaccine strains, a single-blind RCT will be conducted in a

Bulacan farm in which 120 Landrace x Large White pregnant

gilts (70 days of gestation) with no detectable PRRSV serum

antibodies will be randomly assigned equally into four groups:

Treatment 1 (T1; Control) = field strain of PRRSV isolated fromPRRSV outbreak in a piggery in the province; T2- European-

type MLV PRRSV commercial vaccine and T3 American-type

MLV PRRSV commercial vaccine. All inoculations will be done

on 90 days of gestation &given intranasally.


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BOX 3. Designing a RCT of two MLV vaccines (contn)

Vaccine dispenser and vaccine vials will be covered. & the farm

staff blinded to treatment assignment until the end of the trial

period. The gilts in treatment groups will be separately housed

until the end of the trial.

A PRRSV-infected pig is defined as either dead-born or liveborn pig that is PCR-positive for the virus to at least one of the

tissue samples taken at necropsy. The % of live-born/dead-

born/born weak/weaned piglets positive for PRRSV in each

group will be calculated & compared between treatment groups.The mean survival rate and mean piglet weight at birth & day 2,

the total no. of pigs/fetuses dead at late term and the total no.

of stillbirths 1will be separately calculated and compared

among groups.


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Appraisal and Evaluation of Veterinary Services

Epidemiologic studies and techniquesprovide disease control authorities with

evidence-based information to appraise

and evaluate veterinary services andconsequently, make decisions that will

lead to improvements in veterinary

services, animal health, productivity andwelfare and farm profitability.


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A DA director in one region wants to implement an integrated

dairy farmers mastitis education program and monitoring

(MEM). He wants to know whether this new mastitis control

program will work.

As the epidemiologist hired to do the study, you need toconsider the following: 1) choice of study design; 2) study

groups; 3) sample size per group to show significant difference

in outcome between groups; 3) criteria for inclusion in the

study; 4) data to be collected; 5) tests to be used; 6) design &pre-testing of a structured questionnaire; 7) outcome variables

& their measurement; 8) methods to reduce bias such as

blinding & masking; and 8) methods for data analysis.

BOX 4. Assessment of a new veterinary service

k


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Risk assessmentRisk assessment is a scientific evaluation of likelihood of

adverse effects, considering the release and exposure topotential hazards, including their effects. It provides evidence-

based information to decision-makers in support of policy or

regulatory change, where change may introduce an undefined

risk, or an identified potential risk.Import risk assessment has been used by many developed countries to

support decisions on importation of animals & animal products. Animals

and animal products may carry unwanted pathogens and as such, there is

always a risk of introducing disease agents into the country. Such riskmust be deemed acceptable to justify importation. Epidemiology

provides crucial inputs to import risk assessment and contributes to the

formulation of science-based animal quarantine policies and protocols for

imported animals and animal products specific to a country of origin.


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BOX 5. Risk assessment of fresh frozen deboned, deglanded

buffalo meat imported from India

Since August, 1993, the country imports fresh frozen deboned, deglanded

matured buffalo meat from FMD-free zones without vaccination in India.

The imported carabeef is a cheap source of raw material for the local

manufacture of buffalo meat products. The continuation of this import

policy needs to be reviewed in the light of the recognition of the country by

OIE as FMD-free without vaccination.

Risk assessment involves: 1) identification of possible hazards

that may enter the country through this imported commodity &

possibly quantification of the risk; 2) Assessment of release of

these hazards into the country; 3) Assessment of exposure of

domestic animals to the identified hazards and 4) Assessment of

consequences of these hazards. Epidemiologists should also

describe the measures to manage the risk.


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Provides the Data and Tools for Informing andGuiding Policy Makers on Selection of AppropriateDisease Control Strategies Before Any Cases Occur

Epidemiologists use models to describe diseasedynamics, forecast epidemics, predict disease

spread, or examine potential consequences of awide range of control interventions underdifferent potential scenarios. Such modelsenhance the understanding of complex problems

and determine the best control strategy in real-life situations so that adequate logistics can bemade available.


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A simple epidemiologic model for FMD in the Philippines was made to predict the future incidence of FMD

outbreaks in various animal species (backyard cattle/carabaos/swine and commercial swine) in three FMD

control zones, given the current control strategies for the period 1996-2001 and the actual disease dynamics of

past outbreaks for the period 1996-1999. These zones were: Zone 1 = Infected and Buffer zones; Zone 2 =

Surveillance zone and Zone 3 = FMD-free zone without vaccination) The model assumed that (1) only the

current strain of swine FMD virus remains preset in the country and that no new strains are introduced; (2) FMD

control will continue at the current levels, but without successfully achieving eradication, i.e., consistent with thetrends observed from 1996-1999.

The first model used the following parameters: probability of outbreak, the average number of outbreaks if an

outbreak occurs in a given year, number of animals affected per outbreak and average case fatality from each

species and zone, for the different levels of vaccination intensity and the other current control strategies. The

overall scale and duration of the outbreak, and incidence occurring with time and place were not determined

because of data limitations.

Another epidemiologic model for FMD was made to determine the impact of strengthened disease control

activities (i.e., monitoring and surveillance, movement controls, vaccinations, outbreak response and public

awareness) on FMD incidence given four scenarios: Baseline, Eradication Scenario 1 (FMD eradicated by

2003), Eradication Scenario 2 (eradication achieved by 2005) and Eradication Scenario 3 (eradication by 2009).

The parameters used were the average number of outbreaks, average number of animals affected per outbreak

and case fatality rates computed from historical outbreak data from 1996-2000, and aggregate changes inmortality and morbidity by species. The model assumed that effective implementation of the eradication

strategies reduces incidence projected under the Baseline Scenario by 75% in the year immediately preceding

that in which the disease is eradicated. The accuracy of model prediction depends on the accuracy of

epidemiologic data collected in the field.

Randolph TF, Perry BD, Benigno CC, Santos IJ, Agbayani AL, Coleman P, Webb R & Gleeson LJ. 2002. The

economic impact of foot and mouth disease control and eradication in the Philippines. Rev. sci. tech. Off. int.Epiz 21 (3): 645-661.


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Provides the data needed in determining the economic

cost of animal disease and in assessment of cost-

effective disease control strategies

The options for the producer to invest in animal health care

are often driven by economics. Government resources are

scarce and economics partly governs allocation of valuable

resources to various programs and projects.

Epidemiology provides data needed in estimating the cost of

diseases and in guiding decision-makers, disease control

authorities and animal producers in the selection of the best,most cost-effective animal health management options to

solve problems among a range of prevention and control

measures or in support ofproducers policies with respect to

animal production, health and welfare.


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Provides evidence-based evidence to improve

farm performance and profitability

Epidemiologic techniques provide data to

assist producers on effectiveness of herd

health programs.


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BOX 7. Association of farmers attitudes and farm

performance

Farmers attitudes are partly responsible for the

farms performance, as these determine adoption of

management procedures & good management.

You are to design a study

An example of such a design is described in the next

slide.


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Study design & sample size: Questionnaire cross-sectionalstudy in 220 randomly selected dairy cattle farms in

Batangas.

Study variables:

Farm performance variables: rate of retained placenta, metritisrate, culling rate & herd breed class average (BCA) for milk

Explanatory variables

Socio-demographic - no. years of farming experience,

educational level, no. farm dependents, days off-farmwork/year, land area, total no. dairy females in herd, no.

employees, average work hours

Psychological Satisfaction with farming, value

orientations, aspirations, basic needs

BOX 7. Association of farmers attitudes and

farm performance (continuation)


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Plans for data analysis: scatterplot to examine relationship

between quantitative variables & assess linearity; multiple/

polynomial regression analysis to identify socio-demographicvariables associated with farm performance; coefficient of

multiple determination to assess the importance of the socio-

psychological variables among the factors affecting farm

performance that were identified by regression analysis.

BOX 7. Association of farmers attitudes and

farm performance (continuation)


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Monitoring and veterinary surveillance ofdiseases and disease agents

Epidemiologic techniques and tools are used in monitoring and

veterinary surveillance of diseases and disease agents to detect

sudden changes in disease incidence, new or reemerging

etiologic agents or abnormal time, population and spatial patternsof disease occurrence.

Abattoir, veterinary clinics, farms, diagnostic laboratories, and

susceptible animal species can be used as sentinels to serve asindicators or early warning of emerging problems to animal

health regulatory agencies. An increase in frequency of diseases

or disease agents would indicate that someone should start to

look into the factors behind it.


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BOX 8. Monitoring and Surveillance of Classical

Swine Fever (CSF) in the Philippines

CSF is one of the first priority diseases for control inthe country. Pre-border, border and postborder

surveillance is one of the components of CSF control

program. The objectives are to study the disease

pattern/behavior in the field, identify factors whichinfluence its occurrence & spread, quantify economic

losses due to CSF & get inputs in making feasible

disease control strategies. The activities include

gathering disease information from other countries;

import ban from countries with CSF; border &

slaughterhouse checks & outbreak investigation.


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Critical Appraisal of Veterinary Medical Literature

The current state of knowledge about diseases and disease

processes is fast increasing. Veterinarians search books or journalarticles for new information that will be applied in their practice.

Publication of research is not an assurance that the study

conclusions are valid and/or relevant to veterinary

practice. Epidemiology trains veterinarians to critically evaluateveterinary and epidemiological literature, particularly the

strengths of scientific evidence and relevance of research

findings to their veterinary practice.

Critical appraisal involves evaluation of all aspects of pertinentliterature in a systematic manner & interpret the findings

accordingly. Guides in evaluating studies on therapeutic and

prophylactic interventions (Begg et al., 1996) and diagnostic

tests are available (OConnorand Evans, 2007).

BOX 9 U id f i i t di ti


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BOX 9. Users guide for appraising studies reporting

evaluation of diagnostic tests(OConnor and Evans, 2007)

1. Was the spectrum of patients representative of the patients

that will receive the new test? Consider if age, breed, sex,species, disease severity, concurrent disease, geographic

area, local diseases, etc. are similar for your patients. If not,

then the results are not relevant to your veterinary practice.

2. Were the selection criteria clearly described? The patient

characteristics, exclusion and inclusion criteria for selection

of patients in the study, including the start and end dates of

study, the study setting and location, must be welldescribed.

3. Is the current gold standard likely to correctly diagnose the

disease of interest?


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BOX 9. Guide in evaluation of articles on diagnostic tests

4.Is the time period between the application of gold standard

and the new test short enough to be certain that thedisease of interest did not change between the two tests?

5.Did the whole sample or a random sample of the sample

receive verification using the gold standard?

6. Did the patients receive the same gold standard

regardless of the new test used?

7. Was the gold standard independent of the new test (i.e.,

the new test did not form part of the gold standard?8. Was the execution of the gold standard described in

sufficient detail to permit replication of the test?

BOX 9 Guide for the evaluation of articles on diagnostic


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BOX 9. Guide for the evaluation of articles on diagnostic

tests (continuation)

9. Was the execution of the gold standard described in

sufficient detail to permit replication of the test?10. Were the new test results interpreted without

knowledge of the results of the gold standard

(blinding)?

11.Was the gold standard results interpreted without

knowledge of the results of the new test (blinding)?

12. Were the same clinical data available when test

results were interpreted as would be available whenthe test is used in practice?

13. Were uninterpretable / intermediate results reported?

14. Were withdrawals from the study explained?


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Provides the Tools for Evidence-BasedVeterinary Medicine (EBVM)

Veterinarians must weigh potential risks and benefits of an

intervention, choose and interpret diagnostic test and make

decisions about the care of their patients based on clear, honest

and careful examination of best available current scientific

evidence. Information from veterinary and epidemiologicalliterature may yield conflicting results.

Epidemiology provides the tools for careful, methodical, precise

and critical examination of scientific evidence for its validity and

usefulness and in evaluating the outcomes should anintervention or diagnostic test be integrated into the clinical

practice. The most compelling supporting evidence comes from

systematic reviews, followed by meta-analyses and blinded

RCTs.

i i f i O b k


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Investigation of Disease OutbreaksEpidemiologic techniques are used to help identify the source of

ongoing outbreaks, prevent additional cases from occurring and

increase our knowledge of a given disease.

The epidemiologic approach may use molecular techniques to

supplement field investigation of outbreaks. Strain typing to

discriminate isolates from outbreaks, in combination withepidemiological data, improve our understanding regarding the

sources and routes/modes of infection and transmission;

geographical distribution & temporal variation of strains; identify

the natural reservoirs, mode of transmission & spread in natureand in the animal or human population; & to keep track of the

distribution & movement of various clones of these organisms in

the environment, the food chain, and the animal/human

o ulation.


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In an investigation of PRRSV-suspected outbreaks in pig farms

in Bulacan, Nueva Ecija and Batangas, clinical and

epidemiological data were collected through a structured

questionnaire accomplished by the farm veterinarian. Definition

of a case was made using the standard criteria. Data collected

were farm location, production type, herd size, animal flow,animal sources, including introductions, herd vaccination

program for PRRSV, the suspected cause of contamination for

new outbreaks and the distance from neighboring farms.

Further, samples of blood, lung and lymph nodes were obtainedfrom farms experiencing reproductive & respiratory disease and

in those farms without any clinical signs. Samples were taken

from nursery and grower pigs with clinical signs, and in

breeding

BOX 10. Molecular epidemiological investigation of a

disease outbreak (hypothetical example)


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herds from suckling piglets w/ respiratory problems

or from weak-born piglets aged


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Identification of Prognostic FactorsKnowing the factors that predict a favorable

or unfavorable clinical outcome of acondition (e.g., recovery, survival, treatmentsuccess or failure) is helpful to the veterinarypractitioner.Epidemiologists identify the prognostic

factors that affect the likelihood of survival of

a patient. These include animal

characteristics, clinical signs and symptoms

at presentation of illness and laboratory test

results.


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Establishing the standards & ranges for the

Reference values are used to diagnose or screen for

disease, laboratory test interpretation and health care.

Epidemiology has been useful in the laboratory in

studying the distribution of biological characteristics

such as body weight, body temperature, hematological

values, serum enzyme levels, and serum electrolytes inorder to determine the normal characteristics of

populations.

normal values of biological measures

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