Epizootic

In epizoology, an epizootic (from Greek epi- upon + zoion 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). (An epidemic is the same term applied to human populations.) High population density is a major contributing factor to epizootics, with aquaculture being an example of 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." 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. 

An example of an epizootic would be the 1990 outbreak of Newcastle disease virus in double-crested cormorant colonies on the Great Lakes that resulted in the death of some 10000 birds. 

Zambia EUS outbreak
On September 14, 2007, epizootic ulcerative sydrome (EUS), a mysterious disease killed hundreds of sore-covered fish in River Zambezi. Zambia Agriculture Minister Ben Kapita asked experts to investigate the outbreak to probe the cause to find out if the disease can be transmitted to humans.