Toxoplasmosis continued: Cats can be source of human disease | Columnists


For the past two weeks, I have been telling you about the possible effects of toxoplasmosis on eyes and appearance. There is also accumulating evidence linking Toxoplasma gondii infection with schizophrenia.

While there are other sources of Toxoplasmosis, it is most commonly associated with cats. So, I will start with some information about cats.

There are about 90 million cats owned as pets in the US with three-fourths of them allowed to sleep on the beds of their owners. 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. 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, the parasite that causes toxoplasmosis.

Though human Toxoplasma infection is typically asymptomatic or produces minor, flu-like symptoms, there is some evidence that implicates a link between Toxoplasma and psychosis.

Globally, approximately one-third of the human population is infected with Toxoplasma, 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.

Toxoplasma begins its life cycle when a cat becomes infected, usually as a kitten. Most infected cats have no symptoms, but for approximately eight days they excrete up to 50 million infectious oocysts in their feces daily. Depending on the temperature, these oocysts can live for two years or longer. It is thought that a single oocyst can cause human infection. 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.

Farm animals can become infected from contaminated feed, which in turn may cause human infection if meat is undercooked. This and infective oocysts occasionally getting into the water supply may also cause outbreaks of Toxoplasmosis. More than 200 such outbreaks have been described. Family outbreaks have also been described that involve multiple children who played in an infected sandbox or dirt pile. Multiple other possible modes of transmission are being investigated, including sexual transmission.

Until recently, cerebral infections, congenital infections, and eye disease were thought to be the main clinical problems associated with toxoplasmosis. However, accumulating evidence suggests that psychosis should be added to this list.

Five lines of evidence support this:

• Toxoplasma can cause delusions, auditory hallucinations, and other psychotic symptoms.

• People with schizophrenia who are infected with Toxoplasma have more severe psychotic symptoms according to at least seven studies.

• Compared with controls, patients with psychosis are significantly more likely to have antibodies against Toxoplasma, indicating previous infection.

• Compared with controls, individuals with schizophrenia or bipolar disorder are significantly more likely as a child to have lived in a home with a cat.

• Epidemiologically, there is a close time correlation between the rise of cats as pets and the rise of psychosis.

In 2014, a researcher concluded that 21% of cases of schizophrenia might have been caused by Toxoplasma. In the US, this would mean more than 10,000 new cases of schizophrenia each year might be attributable to this parasite.

Some researchers have also suggested links between Toxoplasma and epilepsy, brain cancer, rheumatoid arthritis, and possibly even motor vehicle accidents (since Toxoplasma is known to decrease motor reaction times in humans).

Cats that are kept exclusively indoors are relatively safe pets. However, cats that are allowed to go outdoors may not be safe, especially for children and young adults.

What is needed is an effective vaccine that could be given to newborn kittens to prevent infection, but development of this type of vaccine has never been prioritized.

On a personal level, we can decrease Toxoplasma infections by not eating undercooked meat. Pregnant women and individuals who are immunocompromised should not change cat litter. When gardening, we should wear gloves because cats favor loose soil for depositing their feces. We should also protect children by covering sandboxes when not in use and by not allowing children to play in uncovered public sandboxes.

Toxoplasmosis typically is treated with pyrimethamine, usually in combination with a sulfa drug. However, pyrimethamine does not cross the blood brain barrier and thus is ineffective when Toxoplasma infects the brain. So, a drug that will effectively treat Toxoplasma in the brain is still needed.

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