Epizootic
In epizoology, an epizootic (from Greek: "epi-" upon + "zoon" animal) is a disease event in a nonhuman animal population analogous to an epidemic in humans. An epizootic may be restricted to a specific locale (an "outbreak"), general (an "epizootic"), or widespread ("panzootic"). 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; it is based upon the number of new cases in a given animal population, during a given period, and must  be judged to be a rate that substantially exceeds what is expected based on recent experience ("i.e." a sharp elevation in the incidence rate). Because it 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 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 viral hemorrhagic septicemia 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 10,000 birds.