What defines an epizootic?

An epizootic is defined by a sudden outbreak of disease that affects a specific population of animals within a particular geographic area. This term is analogous to the term "epidemic" used in human medicine. The focal point of an epizootic is the rapid increase in the incidence of disease, which can lead to significant impact on animal health and welfare, as well as economic consequences in agriculture.

The other options do not accurately capture this definition. For example, a common illness in domestic animals suggests a steady, ongoing presence rather than a sudden outbreak. The occurrence of multiple species of animals may describe a situation involving various wildlife or livestock but does not specifically emphasize the rapid rise of disease impacting those animals. Regular vaccination schedules pertain to preventive measures against diseases but do not relate to the occurrence or definition of an epizootic event itself. Thus, the answer focusing on the sudden outbreak is aligned with the true nature of an epizootic.