Surveillance System Standards

Size of sample 

service area  

  Number of reporting units, (e.g., labs, clinics, slaughter plants.)   

  Should include geographic area serviced per unit sampled. 

  Number of eligible units served by reporting unit (per unit of 

geographic area being serviced). 

Animal or group 

type 

  The species, breed, and type (if applicable) of animals should be 

evident; include breeds and crosses. 

  Define the animal by appropriate production phase concept. 

  Age categories should include all appropriate categories pertinent 

to the surveillance objectives. 

Date 

  Population description should include applicable date. 

  All date entries are as accurate as possible.  At the minimum, 

include the date of sample collection and date of lab diagnosis. 

  Report results are consistent using the defined date, (i.e., positive 

scrapie date is NVSL confirmed date.)  

Sampling process 

  Refer to standard 1.9, sampling methods. 

Risk and 

exposure factors 

  Population risk factors that may influence the outcome of the 

study. Confounders should be included in the description of the 

population (e.g., waste feeder hog operations). 

  Risk factors need to be identified for statistical analysis.  

Anecdotal descriptions should not be used. 

  When populations under consideration have unique risk factors 

or exposure to disease agents, the risk and exposure factors 

under study (independent variables) should be carefully defined 

to clearly differentiate the sectors of the population. If the 

surveillance applies to waste feeder, transitional, and commercial 

herds, risk/exposure factors must be clearly described prior to 

sampling. 

 

1.7 Case Definition 

Standard: 

The surveillance system has clear and understandable case definitions that 

include criteria for diagnosis, laboratory criteria for confirmation, any restriction or 

application of the case definition to specific geographical or demographic 

characteristics, and descriptions of case classification categories. 

 

a.  The working case definition is clear and understandable by the individuals 

who use it to identify and report cases. 

b.  Case definitions are consistent between all documents relating to a 

surveillance system or program, (e.g., CFR, UM&R, VS policy memos, etc.) 

c.  Case definitions are specific enough to avoid counting cases in more than 

one category. 

d.  Methods used to verify reported cases are clearly described. 

e.  The process is documented for handling data and information when case 

definitions change. 

 

 

Supporting Information: 

 

A surveillance system depends on clear case definitions for animal health-related events under 

surveillance. The case definition may include clinical manifestations (i.e., clinical signs); 

laboratory test results; necropsy findings; epidemiologic information such as subject, place, and 

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time; and/or specified behaviors, as well as levels of certainty including confirmed/definite, 

probable/presumptive, or possible/suspected).  

 

The use of a standard case definition increases the specificity of reporting and improves the 

comparability of animal health-related events reported from different sources of data including 

geographic areas, and minimizes inappropriate regulatory actions, (i.e., movement restrictions, 

by placing cases in mutually exclusive categories.) Case definitions might exist for a variety of 

animal health-related events under surveillance, including diseases, injuries, adverse exposures, 

and risk factor or protective behaviors. 

 

The following table provides guidelines on these classes of a case definition: 

 

Class Guidelines 

Clinical 

description 

  Brief one- to two-paragraph synopsis of clinical signs, history, and 

presentation.  

  Acute, chronic, and late onset forms of disease should be described. 

  Consideration should be given to asymptomatic or inapparent carrier 

reservoirs that could play a role in disease transmission. 

Clinical case 

definition 

  A clinical case definition can be used to broaden or restrict the 

sensitivity of a surveillance system by designating the species of 

animal(s) under surveillance and inclusion or exclusion of clinical 

signs or lesions for the disease or condition under investigation. 

Animals meeting a clinical case definition may be used to screen for 

inclusion of further testing. 

Epidemiologic 

criteria and 

restrictions 

  Criteria may restrict case definition to individual animals, herds, 

flocks, or premises that possess specific epidemiological 

characteristics. 

  Criteria and restrictions may relate to the geographic location of an 

animal, farm, or premises; a particular point in time or season of the 

year; or a particular behavior associated with disease transmission or 

risk factor. 

  Surveillance may be compartmentalized within a segment of a 

vertically integrated industry, (e.g., genetic grandparent stock, 

multipliers, commercial production); age group, (e.g., nursery vs. 

weaners); or commodity type, (e.g., meat type chickens vs. layers.)   

  Criteria and restrictions should be used to clearly define population of 

interest under surveillance.  See standard 1.6. 

  May also include variables related to habitat, environmental 

conditions, seasonality, climate, etc. 

Laboratory criteria 

  May vary depending on the level of certainty required for surveillance. 

  Screening tests are generally performed rapidly, are usually widely 

available within a laboratory system, and are relatively inexpensive.  

They typically trade lower specificity for higher sensitivity, which 

results in some level of false positive results.  False negatives are 

undesirable but may occur.   

  Confirmatory tests are typically less rapid and more difficult to 

perform, are less readily available within a laboratory system because 

of additional expertise needed to perform the test, and are more 

expensive than more commonly used screening tests.  The ideal 

confirmatory test should be highly specific.  

   Laboratory tests may not always serve as the gold standard for 

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