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a widely spreading contagious disease (infectious or infestatant) of animals, with morbidity far in excess of that ordinarily (sporadically) observed in a particular locality. The study of epizootics is called epizootiology.
An epizootic is characterized by the steady spread of an infectious disease and a microbe-carrier state among animals. An epizootic can occur only in the presence of a number of interdependent elements that constitute an epizootic chain: the source of the causative agent (a diseased animal or animal microbe-carrier), live transmitters or environmental factors conducive to infection, and susceptible animals. The outbreak and spread of an epizootic are influenced by geographic, climatic, soil, and other environmental conditions and economic factors, including agricultural conditions, as well as social upheavals, such as war or economic crisis. The nature and duration of an epizootic vary depending on the means of transmission of the causative agent, the length of the incubation period, the ratio of diseased to susceptible animals, the conditions under which animals are maintained, and the effectiveness of the countermeasures.
Epizootics of certain diseases may occur periodically, usually every few years. They tend to break out seasonally and to have specific stages of development. All of these characteristics are most evident when the epizootic progresses spontaneously. Intervention by man, specifically the use of countermeasures, as in the USSR, to a considerable degree prevents the spread of epizootics.