1.2. applications of epidem
TRANSCRIPT
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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