Squirrels can transmit a surprisingly varied list of diseases to humans, ranging from bacterial infections like tularemia and leptospirosis to parasitic gut infections and, in rare cases, prion diseases. The good news is that most people who see squirrels in their yard or local park face very low risk. Transmission usually requires a specific event, such as a bite, exposure to droppings in an enclosed space, or a tick that fed on an infected squirrel before biting a person. The risk profile shifts depending on the species of squirrel, where you live, and whether you handle or eat them.
Tularemia from Bites and Handling
Tularemia is a bacterial infection caused by Francisella tularensis that can be contracted through the bite of an infected animal, handling of carcasses, or contact with contaminated water. Squirrels are among the known carriers. In a documented case from Germany, a patient developed the ulceroglandular form of tularemia after being bitten by a squirrel. The wound became infected and swollen, and it took two weeks before laboratory testing confirmed the diagnosis.
That case was considered the first report of tularemia from a squirrel bite in Germany, which tells you how uncommon direct squirrel-to-human transmission is in many regions.1PubMed Central. Ulceroglandular form of tularemia after squirrel bite: a case report In North America, tularemia cases tied to squirrels tend to happen among hunters who skin and clean the animals without gloves. The bacteria can enter through small cuts in the skin and cause fever, swollen lymph nodes, and ulcers at the site of contact. It responds to antibiotics when caught early, but delayed diagnosis can lead to serious complications.
Squirrels as Reservoir Hosts for Lyme Disease
Squirrels do not give you Lyme disease directly, but certain species serve as a hidden link in the chain that keeps the Lyme disease bacterium, Borrelia burgdorferi, circulating in nature. The western gray squirrel in California is one of the clearest examples. A survey of 222 western gray squirrels found that about 30% were infected with B. burgdorferi, with infection rates in some counties reaching 50%. In laboratory trials, infected squirrels passed the bacterium to up to 86% of feeding tick larvae, and infections in the squirrels lasted as long as 14 months, meaning they can maintain the pathogen across seasons.2PubMed Central. Identifying the reservoir hosts of the Lyme disease spirochete Borrelia burgdorferi in California: the role of the western gray squirrel (Sciurus griseus)
Follow-up research confirmed that the bacteria tend to stay concentrated in the squirrels’ skin and lymph nodes rather than spreading widely through their organs, which may explain why the squirrels themselves seem unbothered by the infection. Spirochetes were detectable 7 to 21 months after infection, reinforcing the idea that these animals serve as long-term reservoirs.3PubMed Central. Distribution of the Lyme disease spirochete Borrelia burgdorferi in naturally and experimentally infected western gray squirrels (Sciurus griseus)
This reservoir role is not limited to the American West. In the United Kingdom, researchers demonstrated that grey squirrels can amplify and maintain Borrelia burgdorferi as well. A wild-caught grey squirrel in one study showed a disseminated infection that persisted for at least 11 weeks, and ticks that fed on it became infected and successfully transmitted the spirochete to an uninfected hamster in the lab.4Folia Parasitologica. Role of grey squirrels and pheasants in the transmission of Borrelia burgdorferi sensu lato, the Lyme disease spirochaete, in the U.K. The practical takeaway here is that a yard full of squirrels can indirectly raise your Lyme risk by keeping local tick populations infected, even if you never touch the squirrels themselves.
Epidemic Typhus and Flying Squirrels
One of the more unexpected disease links involves southern flying squirrels and Rickettsia prowazekii, the bacterium that causes epidemic typhus. Classic epidemic typhus is associated with body lice in overcrowded, unsanitary conditions, but in the eastern United States, the pathogen circulates quietly among flying squirrels. In a cluster of human typhus cases investigated between 2004 and 2006, researchers trapped 14 flying squirrels in the woodlands around a cabin where patients had stayed. About 71% of those squirrels showed evidence of R. prowazekii infection, based on blood and tissue analysis.5PubMed Central. Cluster of sylvatic epidemic typhus cases associated with flying squirrels, 2004-2006
People typically get infected when flying squirrels nest in attics or wall voids and their fleas or lice, or aerosolized feces, reach the human occupants. Symptoms include high fever, headache, and rash, and they can be severe if untreated. The cases are rare enough that many doctors do not think to test for typhus, which can lead to delayed diagnosis. If you have flying squirrels in your attic and develop an unexplained fever with headache and body aches, this is worth mentioning to your doctor.
Diseases Spread Through Squirrel Droppings
Squirrel feces and urine can harbor organisms that cause gastrointestinal and systemic infections. This matters most when squirrels nest in or near human living spaces, particularly attics, sheds, and crawlspaces where droppings accumulate in enclosed areas.
Leptospirosis is one concern. Caused by Leptospira bacteria shed in urine, it can infect people through skin abrasions or mucous membranes exposed to contaminated water or surfaces. A study of captive flying squirrels in China found that about 4% carried Leptospira in their urine. The same study detected the intestinal parasite Blastocystis in roughly 30% of the animals. The researchers flagged a specific cultural practice as a risk factor: in parts of China, fresh flying squirrel excrement is used in traditional medicine, creating a direct route for pathogen exposure.6PubMed. First record of Leptospira and Blastocystis infections in captive flying squirrels (Trogopterus xanthipes) from Enshi County, China
Intestinal parasites beyond Blastocystis are also a concern. A large study of Pallas’s squirrels in Japan tested 423 fecal samples and found Cryptosporidium in about 4%, the microsporidian parasite Enterocytozoon bieneusi in 13%, and Blastocystis in 44%. All three organisms are known to cause diarrheal illness in people. The Cryptosporidium species identified was Cryptosporidium ubiquitum, a type known to infect humans. Because Pallas’s squirrels live in urban areas close to people, the researchers noted they could serve as a source of infection in human populations.7PubMed Central. Prevalence and Molecular Characterization of the Zoonotic Enteric Protozoans Cryptosporidium spp., Enterocytozoon bieneusi, and Blastocystis from Pallas’s Squirrels (Callosciurus erythraeus) in Kanagawa Prefecture, Japan
For the average homeowner, the practical risk comes from cleaning up accumulated squirrel droppings without protection. Wearing gloves, a mask, and dampening the material before sweeping are straightforward steps that reduce exposure to any pathogens present in dried feces.
Tick-Borne Viruses, Including Powassan
Beyond Lyme disease, squirrels play a role in the ecology of tick-borne viruses. Powassan virus is a rare but serious cause of encephalitis in North America. Two lineages of the virus circulate in nature: lineage I is transmitted primarily by the woodchuck tick (Ixodes cookei) and the squirrel tick (I. marxi), while lineage II, sometimes called deer tick virus, is carried by the more common deer tick (I. scapularis). Both lineages can cause human disease and are indistinguishable by standard blood tests.8Journal of Maine Medical Center. An Overview of the History, Ecology, and Phylogeny of Powassan Virus in Maine
Powassan virus infections are still uncommon, with roughly a few dozen cases reported in the United States each year, but they can be devastating. Unlike Lyme disease, which requires a tick to be attached for a day or more, Powassan virus can be transmitted within 15 minutes of a tick bite. Symptoms range from mild fever to severe brain inflammation. There is no specific treatment, and about 10% of encephalitis cases are fatal. Squirrels do not transmit Powassan directly to people, but by supporting tick populations and serving as hosts for the virus, they are part of the transmission cycle.
Plague
The historical connection between squirrels and plague is often forgotten, but it was once a major public health concern in the western United States. Ground squirrels in particular can carry Yersinia pestis, the bacterium that causes plague, and their fleas readily bite humans. In the early twentieth century, California ran organized campaigns to control squirrel populations around populated areas specifically because of the plague risk.9JAMA. THE PLAGUE ERADICATIVE MEASURES (SQUIRREL CAMPAIGN) IN CONTRA COSTA COUNTY, CALIFORNIA
Plague still occurs in the United States today, mostly in rural parts of the Southwest. Ground squirrels, prairie dogs, and other burrowing rodents maintain the bacterium in wild populations, and human cases pop up when people handle sick or dead animals or are bitten by their fleas. Die-offs in local ground squirrel colonies can actually serve as a warning sign that plague is circulating in an area. If you encounter multiple dead squirrels clustered together, particularly in the western U.S., keeping your distance and notifying local health authorities is a reasonable precaution.
Prion Disease and Eating Squirrel Brains
This one sits in unusual territory. In certain parts of the rural American South, eating squirrel brains has been a traditional practice, and some case reports have linked it to Creutzfeldt-Jakob disease (CJD), a fatal degenerative brain disorder caused by misfolded proteins called prions. A report in The Lancet flagged squirrel brain consumption as a potential risk factor for CJD.10PubMed. Creutzfeldt-Jakob disease and eating squirrel brains Separately, a report in The American Journal of Medicine described four CJD cases in patients who had a history of eating the brains of wild animals, including squirrels, and concluded that the cases supported the hypothesis that CJD could be acquired through ingestion of the infectious agent from wild animal nervous tissue.11The American Journal of Medicine. Creutzfeldt-Jakob disease: Possible transmission to humans by consumption of wild animal brains
The evidence here is suggestive rather than definitive. CJD is extremely rare, and the case reports could not prove that the squirrel brains were the source of the prion agent. Still, prion diseases are untreatable and invariably fatal, so even a plausible risk carries weight. If you hunt and eat squirrels, avoiding the brain and spinal cord is a simple way to reduce whatever risk exists. Cooking does not destroy prions the way it destroys bacteria and viruses.
Fungal Pathogens in Squirrels
Squirrels can also carry Cryptococcus neoformans, a yeast-like fungus that causes cryptococcosis in humans, particularly in people with weakened immune systems. A study of Finlayson’s squirrels found Cryptococcus neoformans in the nostrils and lungs of about 6% of the animals examined. Two molecular types were identified, with one variety predominating. The fungus was found in especially high numbers in the nostrils, suggesting that squirrels could shed it into the environment through respiratory secretions.12Oxford Academic. Cryptococcus neoformans in the respiratory tract of squirrels, Callosciurus finlaysonii (Rodentia, Sciuridae)
Cryptococcosis in humans typically starts as a lung infection after inhaling the fungal spores and can spread to the brain, causing meningitis. In healthy individuals, the immune system usually keeps the fungus in check, and infection may go unnoticed. The real danger is for immunocompromised people, including organ transplant recipients and those with HIV/AIDS. Squirrels are not the primary environmental source of Cryptococcus (pigeon droppings and soil are far more important), but in areas where squirrels nest in close quarters with people, they could contribute to local fungal loads.
Wound Infections and Rat Bite Fever
Any animal bite carries a risk of secondary bacterial infection, and squirrel bites are no exception. The mouth of a squirrel harbors a mix of bacteria that can cause cellulitis, abscesses, or more serious wound infections if a bite is not properly cleaned and treated. Tetanus is another standard concern with any puncture wound from an animal.
Rat bite fever, caused by Streptobacillus or Spirillum bacteria found in the mouths and respiratory tracts of rodents, is typically associated with rats. Squirrels are less commonly implicated, but as rodents in the same broader order, they can theoretically carry the same organisms. Symptoms of rat bite fever include relapsing fevers, joint pain, and rash, and it can become serious without antibiotic treatment. Anyone bitten by a squirrel who develops fever or worsening redness around the wound in the days following should seek medical attention promptly.
Who Faces the Most Exposure
For the vast majority of people, the risk of catching a disease from a squirrel is extremely low. Casual encounters in parks and backyards almost never lead to illness. The risk concentrates in a few groups:
- Wildlife professionals: A survey of wildlife health workers found that rodent bites were among the more common animal-related injuries, reported by about 16% of those who had contact with rodents.13PubMed Central. Occupational Risks and Exposures Among Wildlife Health Professionals Rehabilitation workers who handle squirrels regularly face cumulative exposure to bites, scratches, and ectoparasites.
- Hunters and trappers: People who skin, gut, and consume squirrels have direct contact with blood, tissue, and internal organs. Tularemia and, in certain cultural contexts, prion disease are specific concerns for this group.
- Homeowners with nesting squirrels: When squirrels take up residence in attics or wall cavities, the accumulation of droppings and urine in enclosed spaces creates ongoing exposure. Flying squirrels in attics bring the additional risk of typhus. Ectoparasites like fleas and mites from squirrel nests can also migrate into living spaces.
- Immunocompromised individuals: People with weakened immune systems face higher risk from organisms like Cryptosporidium and Cryptococcus that healthy people typically shrug off.
Children who try to feed or handle wild squirrels in parks are occasionally bitten, but serious disease from these encounters is rare. The most common outcome is a painful bite wound that needs cleaning and possibly a tetanus booster rather than a zoonotic infection.
Why the Risk Feels Bigger Than It Is
Squirrels are among the most visible wildlife in urban and suburban settings, which creates a perception gap. People see them constantly, sometimes at arm’s length, and assume that frequent contact translates to frequent disease transmission. In reality, the diseases squirrels carry are overwhelmingly transmitted through indirect routes, particularly through ticks and fleas, or through circumstances that most people never encounter, such as handling carcasses or cleaning up large accumulations of droppings in enclosed spaces.
Rabies is a good example of misplaced worry. Many people assume squirrels are a rabies risk, but small rodents including squirrels, chipmunks, and rats are almost never found to be rabid. Public health agencies generally do not recommend rabies post-exposure treatment after a squirrel bite unless the animal was behaving very abnormally. The diseases that squirrels actually do carry tend to be less dramatic than rabies but more varied, and the routes of transmission are often less obvious. A tick that fed on a squirrel last week and lands on your ankle today is a far more likely pathway for disease than a squirrel biting you in the park.