Preventing Disease Spread Between Humans and Companion Animals

Some zoonotic diseases—sicknesses caused by pathogens transmitted from animals—are well known, such as Lyme disease, malaria, and West Nile virus. Sudden outbreaks of lesser known zoonotic illnesses, such as avian flu, can cause public alarm. There has been less focus on the risk of zoonotic infection from companion dogs and cats. While no significant disease eruption has been triggered by a dog-to-human or cat-to-human transfer, pets can still be a source of human infection.

The sheer number of companion animals on the planet warrants attention. Dog and cat populations continue to increase: they are present in more than 50% of U.S. and European households, with recent increases in many Asian countries as well. And because pets are in close, affectionate contact with people, it makes sense to be aware of known zoonoses to which dogs and cats are vulnerable.

This study lays out the current status and risk factors of emerging and re-emerging zoonoses that can sicken pets and be transmitted to humans. Three categories of zoonoses are covered: viral, bacterial, and parasitic/fungal. Among viral diseases, dog and cats are responsible for almost all human cases of rabies and a significant portion of human cases of cowpox. Evidence is not as clear when it comes to gastrointestinal viruses and avian influenza, but in both cases there is confirmation of pet infection or pet transmittal between humans.

Among bacterial infections, there is increasing evidence suggesting that pets may play a role in household transmission of MRSA (Methicillin-Resistant Staphylococcus Aureus). Other pathogens can cause severe disease among those with weak immune systems, elderly people, organ transplant recipients, or cancer patients. Some of these pathogens are re-emerging in developing parts of the world, associated with both climatic changes and the rise of new strains of bacteria.

Parasitic and fungal zoonoses have taken on new patterns as well. For example, Echinococcus multilocularis (tapeworm) is most prevalent among foxes and their rodent prey. But as foxes and coyotes have increasingly encroached on suburban and urban areas, domestic dogs and cats may become infected when they eat infected wild rodents, and the companion animals can pass the parasite to humans. Sporotrichosis, a fungal disease, can be contracted by outdoor cats and transferred to humans via a scratch or bite.

The risk of zoonoses from companion dogs and cats is still low, but caution is warranted, especially by those with weak or compromised immune systems. Global transfer of animals, climate change, and increasing interaction between humans and wild animals have all affected the spread and potential of pathogens to infect companion animals. Some human behaviors with pets, such as kissing, being licked, sharing food or utensils with and sleeping close to pets, also serve to increase risk. Especially for people with pets who spend time roaming freely outdoors, caution may be warranted and pet hygiene is advisable.