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In epizoology, an epizootic (from Greek epi- upon + zoon animal) is a disease that appears as new cases in a given animal population, during a given period, at a rate that substantially exceeds what is "expected" based on recent experience (i.e. a sharp elevation in the incidence rate). Epidemic is the analogous term applied to human populations. High population density is a major contributing factor to epizootics. Aquaculture is an industry sometimes plagued by disease because of the large number of fish confined to a small area.
Defining an epizootic can be subjective, depending in part on what is "expected". An epizootic may be: a) restricted to a specific locale (an outbreak), b) general (an "epizootic") or c) widespread (panzootic). Because it is based on what is "expected" or thought normal, a few cases of a very rare disease (like a TSE 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 are said to be "enzootic" (cf. the epidemiological meaning of "endemic" for human diseases). 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 VHS in certain Atlantic fish populations.