The North American Opossum (Didelphis virginiana) is a familiar sight in backyards across the continent, yet it is often misunderstood, particularly regarding its role in disease transmission. As the only marsupial found north of Mexico, this adaptable creature interacts frequently with human environments, leading to public concern about the health risks it may pose. While opossums are remarkably resistant to some diseases that affect other wildlife, they are carriers and definitive hosts for several other pathogens that can be transmitted to humans, domestic pets, and livestock. Understanding the specific diseases they carry and the mechanisms of transmission is the most effective way to mitigate potential health risks.
The Rabies Paradox
A common misconception is that opossums are significant carriers of the rabies virus, but scientific evidence indicates they have an exceptionally low incidence of the disease. This resistance is rooted in a unique biological trait: their body temperature. The average core body temperature of the North American opossum ranges between 94 and 97 degrees Fahrenheit, which is several degrees cooler than most placental mammals. The rabies virus requires a higher body temperature to successfully replicate and establish a full infection. The opossum’s naturally low temperature creates an unfavorable environment, making it difficult for the virus to thrive. This contrasts sharply with other common wildlife species such as raccoons, bats, and skunks, which are the primary reservoirs for rabies in North America. Though any mammal can theoretically contract rabies, the physiological barrier in the opossum means clinical cases are rare.
Internal Bacterial and Protozoan Threats
While opossums are largely resistant to rabies, they act as hosts and carriers for various internal pathogens, including bacteria and protozoans, which pose specific zoonotic risks. These threats are often transmitted through the contamination of food, water, or soil with the animal’s urine or feces. The protozoan parasite Toxoplasma gondii, which causes toxoplasmosis, is one such concern. Opossums can carry the parasite and may contribute to the wild cycle of infection through contact with their feces, though felids are the definitive host.
Another significant threat is the bacterium Leptospira, which causes the disease leptospirosis. This bacterial infection is transmitted when humans or pets come into contact with water or soil contaminated by the urine of infected animals. Leptospirosis can lead to flu-like symptoms in humans and severe kidney or liver complications in dogs.
Equine Protozoal Myeloencephalitis (EPM)
For horse owners, the opossum presents a specific risk as the definitive host for Sarcocystis neurona, the protozoan responsible for Equine Protozoal Myeloencephalitis (EPM). Opossums shed the infective sporocysts of S. neurona in their feces, which can contaminate horse feed or water sources. Ingesting these sporocysts can lead to a debilitating neurological disease in horses. Opossums can also carry bacteria like Salmonella, which is shed through their feces, and the protozoan Coccidia, which can cause gastrointestinal illness in other animals.
Possums as Carriers of External Parasites
Opossums also serve as hosts for various ectoparasites, such as fleas, ticks, and mites, which can act as vectors for secondary diseases. The opossum’s potential role in tick control has been a subject of research, with some older studies suggesting they are prolific groomers that consume thousands of ticks per season. However, more recent analyses of wild opossum stomach contents have failed to find evidence of ingested ticks, suggesting the “tick vacuum” theory may be overstated. Opossums are warm-blooded mammals that encounter these parasites in the environment and can carry them into yards and homes. Ticks, which can hitch a ride on an opossum, are known to transmit serious illnesses like Lyme disease and Ehrlichiosis to humans and pets.
Coexisting Safely
Mitigating the risks associated with opossums involves minimizing the chances of contact with their waste and the parasites they carry. Since the primary internal threats are spread through contamination, it is prudent to secure all potential food and water sources that attract wildlife. Pet food and water bowls should be brought inside at night, and garbage cans must have tightly secured lids.
Care should be taken when cleaning up any wild animal droppings, particularly in areas where pets or livestock feed. Contaminated bedding, soil, or feed should be removed carefully to avoid ingestion of protozoan cysts or bacterial spores. Preventing direct contact is also a simple and effective measure; opossums should always be observed from a distance, and never handled. Ensuring that domestic pets are regularly treated with veterinarian-recommended parasite control products and are current on vaccines, such as the one for leptospirosis, provides protection against these common environmental pathogens.