Epizootic

In epizoology, an epizootic is a disease event in a nonhuman animal population analogous to an epidemic in humans. An epizootic disease may occur in a specific locale, more generally, or become widespread. 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. Because the judgement is based on what is "expected" or thought normal, a few cases of a very rare disease might be classified as an "epizootic", while many cases of a common disease would not.
Common diseases that occur at a constant but relatively high rate in the population class as "enzootic". An example of an enzootic disease would be the influenza virus in some bird populations or, at a lower incidence, the Type IVb strain of viral hemorrhagic septicemia in certain Atlantic fish populations.
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.