Manual of Diagnostic Tests for Aquatic Animals (2003)

DEFINITIONS

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The Aquatic Animal Health Code (companion volume to this Aquatic Manual) contains a list of definitions that may be consulted for the meaning of terms used in this Aquatic Manual. Some terms that are not used in the Aquatic Code but that appear in the Aquatic Manual, are defined below:
 

Confidence	In the context of demonstrating freedom from infection (in which the null hypothesis is that infection is present), the confidence is the probability that a surveillance system or combination of surveillance systems would detect the presence of infection if the population were infected. The confidence depends on the design prevalence, or the assumed level of infection in an infected population. Confidence therefore refers to our confidence in the ability of a surveillance system to detect disease, and is equal to the sensitivity of the system. This is distinct from (but may be used to calculate) the probability that a given population is free from infection, based on the results of one or more surveillance systems.
Early detection system	A system for the timely detection and identification of the incursion or emergence of infection or disease in a country, zone or aquaculture establishment. An early detection system must be under the control of the Competent Authorities and must include the following characteristics:
	a)	representative coverage of target animal populations by field services;
	b)	ability to undertake effective disease investigation and reporting;
	c)	access to laboratories capable of diagnosing and differentiating relevant diseases;
	d)	a training programme for veterinarians or fish health specialists for detecting and reporting unusual disease occurrence.
Epidemiological unit	A group of animals that share approximately the same risk of exposure to a disease agent with a defined location. This may be because they share a common aquatic environment (e.g. fish in a pond, caged fish in a lake), or because management practices make it likely that a disease agent in one group of animals would quickly spread to other animals (e.g. all the ponds on a farm, all the ponds in a village system).
Fry	Newly hatched fish larvae.
Population	A group of units sharing a common defined characteristic.
Prescribed biosecurity	A set of conditions applying to a particular disease or infection, and a particular
conditions	zone or country, required to ensure adequate biosecurity, namely:
	a)	the disease is legally notifiable to the Competent Authority;
	b)	an early detection system is in place within the zone or country;
	c)	no vaccination against the disease is carried out;
	d)	infection is not known to be established in wild populations;
	e)	import requirements to prevent the introduction of disease or infection into the country or zone, as outlined in the Aquatic Code, are in place.
Probability sampling	A sampling strategy in which every unit has a known non-zero probability of inclusion in the sample.
Sensitivity	the proportion of true positive tests given in a diagnostic test, i.e. the number of true positive results divided by the number of true positive and false negative results.
Specificity	the probability that absence of infection will be correctly identified by a diagnostic test, i.e. the number of true negative results divided by the number of true negative and false positive results.
Study population	The population from which evidence of freedom from infection is derived. This may be the same as the target population or a subset of it.
Surveillance system	A method of surveillance that generates a source of information on the animal health status of populations.
Target population	For the purposes of demonstrating freedom from infection, the population of interest, usually made up of all aquatic animals of species susceptible to a specified disease agent in a defined country, zone or aquaculture establishment
Targeted surveillance	Surveillance targeted at a specific disease or infection.
Test	A procedure used to classify a unit as either positive or negative with respect to an infection or disease. Tests may be classified as:
	a)	diagnostic, when applied to clinically diseased individuals;
	b)	screening, when applied to apparently healthy individuals; or
	c)	confirmatory, when applied to confirm the result of a previous test.
Test system	A combination of multiple tests and rules of interpretation that are used for the same purpose as a test.
Units	Individually identifiable elements. This is a generic concept used to describe, for example, the members of a population, or the elements selected when sampling. In these contexts, examples of units include individual animals, to ponds, nets, cages, farms, villages, districts, etc.

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