A panzootic is an epizootic or an outbreak of an infectious disease of animals, that spreads across a large region, for example a continent, or even worldwide. The equivalent in human populations is called a pandemic. A panzootic can start when three conditions have been met: the emergence of a disease new to the population; the agent infects a species and causes serious illness; and the agent spreads easily and sustainably among animals.

A disease or condition is not a panzootic merely because it is widespread or kills a large number of animals; it must also be infectious. For example, cancer is responsible for a large number of deaths but is not considered a panzootic because the disease is, generally speaking, not infectious.

Cattle plague is a panzootic that recurred throughout history, often accompanying wars and military campaigns. Cattle plague affected Europe especially in the 18th century, with three long panzootics from 1709–1720, 1742–1760, and 1768–1786.

There was a major outbreak covering the whole of Britain in 1865/66. Later in history, an outbreak in the 1890s killed 80 to 90% of all cattle in southern Africa, as well as in the Horn of Africa. More recently, a rinderpest outbreak raged across much of Africa in 1982–1984, costing US$500 million in losses.

Avian flu is another zoonotic than can become panzootic. It is feared that if the avian influenza virus combines with a human influenza virus (in a bird or a human), the new subtype created could be both highly contagious and highly lethal.

In epizoology, an epizootic 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. That is, an epizootic represents a sharp elevation in the incidence rate.

In contrast to an epizootic, common diseases that occur at a constant but relatively high rate in the population are said to be "enzootic,” for example, influenza virus in some bird populations. High population density is a major contributing factor to epizootics. Epidemic is the analogous term applied to human populations.

In 1996, the UK culled 4.4 million cattle to eradicate mad cow disease, while 400,000 were killed in 2001 in Germany. In 2009, Egypt ordered the cull of all pig herds, over 400,000 pigs, to avoid swine flu. In 2014 in the US, 7,000,000 piglets, or 10% of piglets born, died due to Porcine epidemic diarrhea virus.

In 2014 alone, concerns over avian influenza in South Korea led to 14 million birds being culled, 324,000 in China, 46,000 in North Korea, 112,000 in Japan, 64,000 in Vietnam, 40,000 in Holland, 38,000 in Germany, 20,000 in Hong Kong, and thousands more in Nepal. In northeast China, after 18,000 geese died from H5N6 bird flu, 69,000 were culled.

In Beijing, 20,000 ducks died suddenly due to avian influenza, while 10,000 chickens died in Malaysia. In Sweden, 24,000 chickens were culled due to outbreak of Paramyxovirus type 1 disease. Also, in 2014, thousands of chickens died in Indonesia from Boyolali coli disease, and farmers suspect weather anomalies make chickens susceptible to the disease.i

i Mass Animal Deaths for 2014