Do feral cats pose a risk to human health? While possible, the rate of free-roaming domestic cats spreading disease to humans is often misrepresented.
Managing and mitigating public health risks
As the American Association of Feline Practitioners explains, “transmission of zoonotic agents from animals to people can potentially occur by direct contact with the animal, indirect contact with secretions or excretions from the animal, and contact with vehicles like water, food or fomites that were contaminated by the animal” [1]. The degree of risk is a function of multiple factors (e.g., duration of exposure, geographical distribution of disease and carriers).
Managing public health risks involves numerous tradeoffs. Mandating annual flu shots, for example, would almost certainly have considerable benefits to public health — but not without significant tradeoffs (e.g., economic costs, concerns over privacy, legal challenges).
Mitigating the public health risks associated with free-roaming cats can be thought of similarly: a law prohibiting cats to roam freely would — if it could be enforced — virtually eliminate the risk of rabies transmission from these cats. But such measures are impractical due to various factors (e.g., costs of enforcement and impoundment of cats) and might also give the public a false sense of security. Indeed, such a policy would likely backfire by driving residents who voluntarily sterilize and vaccinate unowned cats “underground” out of fear of possible violations.
The Centers for Disease Control and Prevention (CDC) considers tradeoffs critical to the “systematic assessment of the impact of public health policies, programs, and practices on health outcomes” [2].
What the science tells us about cats spreading disease
Among the zoonotic agents most often associated with free-roaming cats are the rabies virus and Toxoplasma gondii parasite. As the following analysis reveals, the extent to which free-roaming cats contribute to each of these public health threats is often misunderstood.
Risks of rabies virus transmission from cats to humans
Rabies in domestic animals was once relatively common; however, data compiled by the CDC show that nearly 93% of rabies cases in the U.S. occur in wildlife [3]. Annual rabies surveillance data reveals distinct geographic trends, with 183 of the 241 cats (76%) testing positive in the U.S. during 2018 originating from just nine states (Pennsylvania, Maryland, New York, Virginia, Florida, Georgia, New Jersey, South Carolina, and North Carolina) where the raccoon variant is enzootic [3].
Although the number of cats testing positive for the rabies virus annually in the U.S. exceeds the number of dogs testing positive, human cases are more likely to originate from exposure to dogs and wildlife. Between 1975 and 2018 (the most recent data available), the CDC has documented 115 cases of human rabies in the U.S., most of which were attributed to contact with wildlife. Of the 26 cases attributed to domestic animals, 25 were attributed to dogs (nearly all exposures occurred outside the U.S.). Just one case, which occurred in Minnesota, was attributed to contact with a cat [4]. (Seven cases were the result of organ and arterial transplants.).
Between 1975 and 2018, the CDC has documented 115 cases of human rabies in the U.S. [...] Just one case was attributed to contact with a cat.
A study of patients seeking post-exposure treatment at 11 “university-affiliated, urban emergency departments” found that 81% of 2,030 exposures were attributed to dogs while 13% were attributed to cats [5]. Moreover, 499 of 1,499 dog exposures (33%) occurred in the public street or park, compared to 29 of 248 cat exposures (12%); by contrast, 358 of 1,499 dog exposures (24%) occurred in the home, compared to 132 of 248 cat exposures (53%).
These researchers also reported that most exposures, for both dogs and cats, were provoked: 1,277 of 1,499 dog exposures (85%) and 227 of 248 (92%) of cat exposures [6]. Such findings challenge common misperceptions about the rabies risk posed by free-roaming cats.
Risk of spreading the T. gondii parasite from cats to humans
Toxoplasma gondii is parasite found worldwide and capable of infecting virtually all warm-blooded mammals [7]. Cats — both domestic and wild felids (e.g., bobcats and mountain lions) — are the only known “definitive host,” as the parasite is able to reproduce sexually and form egg-like spores (called oocysts) in these animals.
According to the CDC, humans can become infected by consuming the undercooked meat of an infected animal or by the accidental ingestion of oocysts found in the environment (e.g., garden vegetables from contaminated soil). However, infected cats typically shed these oocysts in their feces only once, for a one-week period [7].
Infection rates vary widely around the world [8]. Data from the large-scale National Health and Nutrition Examination Survey reveal a significant decrease between 1988 and 2010 in the age-adjusted infection rate of toxoplasmosis among people 12–49 years of age born in the U.S., from 14.1% to 6.6% [9]. Most people never exhibit symptoms; for those who do, illness is generally mild [10].
Prevention methods recommended by the CDC include freezing and/or thoroughly cooking meat and washing hands with soap and warm water after handling meat (or avoiding meat altogether). Gloves should be worn while gardening or otherwise in contact with soil or sand, and cat owners should change litter boxes daily, washing hands afterward [10].
T. gondii is often portrayed as a “mind-controlling” parasite, making stories about its connection to domestic cats attractive to the mainstream media [11–13]. Adding to the media frenzy is some published research suggesting that simple contact with cats might increase one’s risk of mental illness. These studies rarely stand up to careful scrutiny, however [14,15].
T. gondii is often portrayed as a "mind-controlling" parasite, making stories about its connection to domestic cats attractive to the mainstream media.
In fact, recently published studies have challenged the findings of earlier research. One large-scale longitudinal study found, for example, no evidence of a link between cat ownership in early childhood and “psychotic disorders” later in life [15]. Another found, “little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment” [16].
Managing public health risks associated with feral cats
Trap-neuter-return (TNR) is a non-lethal technique for managing unowned, free-roaming cats. The cats are humanely trapped, spayed or neutered by a licensed veterinarian, ear-tipped (the universal sign that they have been sterilized), and returned to where they were trapped.
Vaccination against rabies is common practice for TNR programs in the U.S., especially in parts of the country where rabies in cats occurs most frequently (e.g., Mid-Atlantic and Southeastern states). These programs are therefore often referred to as trap-neuter-vaccinate-return, or TNVR. Such programs protect public health by vaccinating a population of cats that is otherwise ignored. These cats also form a powerful barrier against disease transmission between wildlife and humans by eliminating an important transmission path (i.e., from wildlife to domestic animals to humans). And not every cat needs to be vaccinated to achieve a significant level of “herd immunity” [17].
Global efforts to eliminate canine rabies have also made enormous strides in recent years, in large part due to the recognition that sterilization and vaccination campaigns are more likely to be successful than traditional culling for controlling the spread of rabies [18].
TNR can also reduce the threat of toxoplasmosis, most directly by reducing local populations of free-roaming cats. Studies show that kittens and young cats are more likely than older cats to shed oocytes if exposed to T. gondii [19–21], but these individuals are routinely removed for adoption as part of TNR programs, providing an additional public health benefit. (The corollary — that older cats are less likely to shed oocysts — suggests that removal of older cats, especially those already sterilized and vaccinated, does relatively little to mitigate the threat of toxoplasmosis.)
TNR can reduce the threat of toxoplasmosis, most directly by reducing local populations of free-roaming cats.
And finally, research has shown that cats living in close proximity to humans often take advantage of the various food sources provided rather than hunt [22,23], and that such cats are therefore much less likely to be exposed to the T. gondii parasite than “solitary, feral domestic cats living in undeveloped landscapes” [24].
A study of free-roaming cats in Rome, Italy, for example, found a significant decrease of T. gondii seroprevalence between 1991 (50%) and 2013 (28%), “mainly attributable to the common practice of feeding cats (unowned and/or pets) with industrial food rather than with home leftovers and/or meat remnants from butchers” [25]. Feeding free-roaming cats, a common practice in the U.S. [26–28], therefore helps mitigate the spread of toxoplasmosis in cats, humans, and wildlife.
Effective population management improves public health
Like all misinformation in the public health domain, misinformation about the public health risks associated with free-roaming cats is likely to be counterproductive. Exaggerating the threat of getting rabies or toxoplasmosis from ‘stray’ cats, for example, not only creates confusion for policy makers and the general public, but it can also undermine efforts to reduce the actual risk through TNR vaccination programs.
Related resources
References
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