A zoonotic disease is defined as any disease that can be transmitted directly from animals to humans. Zoonoses are distinctly different from diseases affecting both humans and animals, such as West Nile virus or Lyme disease, which are transmitted by a mosquito carrying the West Nile virus or a tick carrying Lyme disease, respectively. For instance, an infected horse cannot pass West Nile virus to a human, but the horse and the human might be infected with the virus by the same mosquito. The term “zoonosis” may sometimes refer to a disease transmitted from humans to animals but is sometimes specified as “reverse zoonosis.”
How are Zoonotic Diseases Spread?
True zoonotic diseases can be transmitted by several routes. The most common routes of transmission include fecal-oral (when feces are handled and then hands are placed in or near the mouth), skin contact (especially with open wounds), blood- or tissue-borne, entry through a mucous membranes or via saliva (either through bite wounds or licking).
Diseases spread via the fecal-oral route include intestinal parasites such as roundworms, toxoplasmosis, giardia and cryptosporidium. Zoonotic skin diseases include ringworm (which is actually caused by a fungus, not a parasite as the name might have one believe), scabies (sarcoptic mange) and hookworm.
Blood- and tissue-borne illnesses are rare. However, brucellosis (which causes undulant fever and abortions) and leptospirosis (a bacterial infection affecting the liver and kidneys) can be transmitted to humans if the aborted fetus or the placenta of an infected animal comes in contact with open wounds or mucous membranes. Leptospirosis may also be transmitted by urine from an infected animal, whether ingested, via mucous membrane contact or even by inhaling aerosolized particles, such as when hoses are used to flush urine from cement flooring. Cat-scratch disease (bartonellosis) is transmitted to humans from cats that are infected by the bacteria.
Rabies, which is very rare in humans in the U.S. but fatal in nearly all cases if left untreated, is spread via saliva. Most commonly this occurs when an infected animal bites a human, but there have been cases of rabies transmitted when saliva was in contact with open wounds or mucous membranes.
It is not always easy to determine whether an animal is infected with a zoonotic disease because they may not show any signed of being infected. There are, however, some clinical signs to watch out for in animals. Any animal with hair loss or crusty or reddened patches of skin could be infected with mange or ringworm. An animal that is acting unusually, whether wobbling or drooling, might be a suspect for rabies. Animals with diarrhea could be harboring parasites.
Veterinarians are often asked whether some illnesses such as the common cold virus and gastrointestinal “stomach flu” viruses can be passed between a client and their pets. These particular diseases are species-specific, meaning that a person cannot catch them from their animal companions.
Preventing a Zoonotic Disease
The best ways to avoid contracting a zoonotic disease include being sure that your animals are vaccinated against diseases such as rabies and leptospirosis and that they receive medicine to prevent or treat intestinal parasites and ectoparasites such as fleas and mites. Frequent hand-washing after cleaning litter boxes or picking up feces is highly recommended. People, especially children, should not have extensive contact with animals unknown to them. Additionally, pregnant women and people with compromised immune systems are more susceptible to contracting zoonotic diseases and should exercise caution accordingly.
Hillary Olin Smith, DVM is a graduate of Texas A&M College of Veterinary Medicine and lives in Navasota, Texas. She works for three different practices: Hempstead Veterinary Clinic in Hempstead, Texas; Waller Veterinary Clinic in Waller, Texas; and Beard-Navasota Veterinary Hospital.