Cats, Toxoplasmosis, and Human Health
Toxoplasmosis is a disease caused by infection with the parasite Toxoplasma gondii that depends on cats to complete its life cycle (although it can infect all warm-blooded species, including humans).
Toxoplasmosis can have severe health consequences and even be fatal, and as many as 74 percent of all domestic cats in the United States will be infected with T. gondii during their lifetimes. Due to the potentially severe consequences of infection and potentially high levels of environmental contamination, toxoplasmosis is a serious concern and a neglected public health threat that impacts approximately one-third of humans globally.
People infected with toxoplasmosis may experience flu-like symptoms or no symptoms at all. But it can lead to severe health problems, especially during pregnancy and in people with weakened immune systems. Consequences of infection may also include deafness, seizures, intellectual disabilities, blindness, schizophrenia, Alzheimer’s Disease, obsessive-compulsive disorder, memory loss, multi-organ failure, and even death. In the United States, as many as 1.26 million people have lesions on their eyes associated with toxoplasmosis, and more than 40 million people are infected with toxoplasmosis, according to the Centers for Disease Control and Prevention.
Pregnant women and individuals with compromised immune systems are particularly susceptible to toxoplasmosis, and doctors have long advised pregnant women against cleaning cat litter boxes for this reason. New research, however, suggests that even adults with healthy immune systems are at risk.
People may become infected by T. gondii in several ways: ingestion or inhalation of infectious eggs (called oocysts, distributed only via cat feces), eating undercooked and infected meat, transmission from mother to child during pregnancy, blood transfusions, and organ transplants.
In the United States, infection via cat-excreted oocysts is likely the most common route. In fact, a study by Boyer et al. (2011) found that 78 percent of mothers of infants infected during pregnancy were exposed to this disease from oocysts. A cat infected with T. gondii may expel hundreds of millions of infectious oocysts into the environment through its feces. These oocysts are incredibly resistant to degradation and may remain viable for years in soil or water.
With nearly three-fourths of all domestic cats in the U.S. potentially infected with toxoplasmosis and an estimated 60 to 160 million free-roaming domestic cats in the United States, the potential exists for large-scale environmental contamination. As one study concluded, cats (and their feces) are everywhere, in parks, playgrounds, and gardens. “Indeed, as cats increasingly contaminate public areas with T. gondii oocysts, it will become progressively more difficult to avoid exposure.”
By keeping cats indoors, people can substantially reduce their own and their community’s risk of exposure to this potentially fatal disease. They can also protect the many wild species that live around our homes and are equally at risk from toxoplasmosis.
Cats & Rabies
In the United States, cats are the top carrier of rabies among domestic animals, far outpacing dogs for decades. Rabies is a viral disease impacting mammals that is almost always fatal if left untreated. The rabies virus may be transmitted through a bite or saliva that comes into contact with an open wound or mucous membrane.
In the United States, wildlife species account for the majority of reported rabid animals, but domestic cats represent a disproportionate risk for potential human exposures, in part because people are more likely to interact with them. While required mass vaccination, aggressive control of strays, and restrictions against running at large have significantly reduced rabies prevalence in dogs, these policies are scarce for cats.
Treatment for rabies — post-exposure prophylaxis, or PEP — is only administered after an exposure to rabies has been identified. PEP has been critical in preventing human deaths from rabies, but it’s only as effective as the identification process. It’s also expensive, typically in excess of $1,000 per exposure. More than 6,000 people in the U.S. receive PEP for potential rabies exposure from cats each year. These costs add up — PEP for a single New Hampshire mass exposure event in 1994, caused by a single rabid kitten, cost over $1.1 million.
The surest and cheapest way of protecting cats and people from rabies is to prevent exposures from occurring. This means making sure that all cats have current vaccinations and are kept safely separated from wildlife, whether on a leash, in an enclosure, or kept exclusively indoors.
Scientific Literature on Toxoplasmosis & Rabies
Scientific Literature on Toxoplasmosis & Rabies
- Aguirre et al. 2019: The One Health approach to toxoplasmosis: epidemiology, control, and prevention strategies
- Boyer et al. 2011: Unrecognized ingestion of T. gondii oocysts
- Chadwick et al. 2013: Seroprevalence of T. gondii in Eurasian otter in UK
- Conrad et al. 2005: Transmission of Toxoplasma – clues from the study of sea otters
- Dabritz and Conrad 2009: Cats and Toxoplasma – implications for public health
- Gajewski et al. 2014: T. gondii impairs memory in infected seniors
- Gerhold and Jessup 2013: Zoonotic diseases and free-roaming cats
- Torrey and Yolken 2013: Toxoplasma oocysts as a public health problem
- Gerhold and Jessup 2013: Zoonotic diseases associated with free-roaming cats
- Roebling et al. 2014: Rabies prevention and management of cats in the context of Trap-Neuter-Vaccinate-Release Programmes
