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In epizoology , an epizootic (or epizootic , from Greek : epi- "upon" + zoon "animal") is a disease event in a nonhuman animal population analogous to an epidemic in humans. An epizootic disease (or epizooty ) may occur in a specific locale (an " outbreak "), more generally (an "epizootic"), or become widespread (" panzootic "). High population density is a major contributing factor to epizootics. The aquaculture industry is sometimes plagued by disease because of the large number of fish confined to a small area.

Defining and declaring an epizootic can be subjective; health authorities evaluate the number of new cases in a given animal population during a given period, and estimate a rate of spread that substantially exceeds what they might expect based on recent experience ( i.e. a sharp elevation in the incidence rate). Because the judgement is based on what is "expected" or thought normal, a few cases of a very rare disease (like a transmissible spongiform encephalopathy outbreak in a cervid population) might be classified as an "epizootic", while many cases of a common disease (like lymphocystis in esocids ) would not.

Common diseases that occur at a constant but relatively high rate in the population class as " enzootic " (compare the epidemiological meaning of "endemic" for human diseases). An example of an enzootic disease would be the influenza virus in some bird populations ^[1] or, at a lower incidence, the Type IVb strain of viral hemorrhagic septicemia in certain Atlantic fish populations. ^{[2] [3]}

An example of an epizootic was the 1990 outbreak of Newcastle disease virus in double-crested cormorant colonies on the Great Lakes that resulted in the death of some 10,000 birds. ^{[4] [5]}

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