Evidence-Based Reviews

Cats, toxoplasmosis, and psychosis: Understanding the risks

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Accumulating evidence has linked Toxoplasma gondii infection with schizophrenia.



It has been clearly established that most human infectious diseases are caused by infectious agents that have been transmitted from animals to humans.1 Based on published estimates from the 2000s, 60% to 76% of emerging infectious disease events are transmitted from animals to humans.2

When we consider animals that cause human diseases, we usually think of rats and bats. We rarely think of the 90 million cats owned as pets in the United States, or the approximately 30 to 80 million feral cats. Many consider cats as family members, and three-fourths of cats owned in the United States are allowed to sleep on the beds of their owners.1 These cats may be a substantial source of human disease. Researchers at the University of Liverpool have identified 273 infectious agents carried by cats, of which 151 are known to be shared with humans.1 The most widely known of these agents are Lyssavirus, the virus that causes rabies; Bartonella henselae, the bacteria that causes cat scratch disease; and Toxoplasma gondii (T. gondii), the parasite that causes toxoplasmosis.

In my new open-access book Parasites, Pussycats and Psychosis (available at https://link.springer.com/book/10.1007/978-3-030-86811-6), I describe the relationship between cats, T. gondii, and toxoplasmosis, and detail the evidence linking T. gondii to some cases of schizophrenia, bipolar disorder, and other diseases.1 Though human T. gondii infection is typically asymptomatic or produces minor, flu-like symptoms, there are a few important exceptions. This article outlines those exceptions, and investigates evidence that implicates a link between T. gondii and psychosis.

How T. gondii can be transmitted

T. gondii has been called “one of the most successful parasites on earth.”3 Globally, approximately one-third of the human population is infected with T. gondii, though this varies widely by country and is dependent on dietary habits and exposure to cats. A 2014 survey reported that 11% of Americans—approximately 40 million people—have been infected, as evidenced by the presence of antibodies in their blood.1

T. gondii begins its life cycle when a cat becomes infected, usually as a kitten. Most infected cats are asymptomatic, but for approximately 8 days they excrete up to 50 million infectious oocysts in their feces daily. Depending on the temperature, these oocysts can live for 2 years or longer.It is thought that a single oocyst can cause human infection.1 Since cats like loose soil for defecation, the infective oocysts commonly end up in gardens, uncovered sandboxes, or animal feed piles in barns. After 24 hours, the oocysts dry out and may become aerosolized. For this reason, cat owners are advised to change their cat’s litter daily.

The number of ways T. gondii can be transmitted to humans is extensive. Farm animals can become infected from contaminated feed; this causes T. gondii oocysts in animals’ muscles, which later may cause human infection if eaten as undercooked meat. Many such family outbreaks of toxoplasmosis have been described.1

If infective oocysts get into the water supply, they may also cause outbreaks of disease. More than 200 such outbreaks have been described, including an instance in Victoria, British Columbia, in which 100 people became clinically infected.4

Continue to: Family outbreaks...


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