The term zoonosis (plural zoonoses) means a disease that can be transmitted from animals to humans. The name is derived from the Greek “zoi” or “zoe,” meaning life, plus “nosos,” meaning disease. An equine zoonosis is a disease that humans can contract from members of the equid family.

While there are a number of zoonoses, the good news is that horses spread relatively few diseases to humans. The majority of zoonoses involve other domestic species or wildlife. The bad news is that one or two of the diseases that we can contract from horses can be serious, even life-threatening.

An infectious disease is caused by a living organism or pathogen that invades and colonizes the host. Equine zoonoses are caused by a variety of pathogens–viral, bacterial, rickettsial, fungal, or parasitic (see “Equine Zoonoses on page 58).

Equine diseases are usually solely of veterinary importance. When equine diseases affect humans, those diseases also become a public health concern. From a public health standpoint, “significance” has a particular meaning. It reflects a combination of the clinical importance of a disease with the ease and extent to which it can be spread. For example, acute equine respiratory syndrome (caused by Hendra virus), first diagnosed in Brisbane, Australia, in 1994 has “considerable zoonotic significance.” Although it is rarely transmitted to man, the case fatality rate is very high in humans who get the disease.

The Office International des Epizooties (www.oie.int), the animal health equivalent of the World Health Organization, categorizes animal diseases as List A or List B, depending on their significance. List A diseases are “Transmissible diseases that have the potential for very serious and rapid spread, irrespective of national borders; that are of serious socio-economic or public health consequence; and that are of major importance with respect to the international trade of animals and animal products