The world’s felines have something in common, a trait shared by both domestic housecats and lions in the African savannah: they carry the parasite Toxoplasma gondii, a highly infectious protozoan that can spread to most warm-blooded mammals, including humans.
Felines are very special to T. gondii; although the parasite exists in many different animal species, cat intestines provide the only place where it can breed. Evidently, cat guts have the perfect mixture of chemicals and nutrients to induce T. gondii to mate. For thousands of years the parasite has carried out a reproductive cycle that begins and ends with cats. Here’s an example.
After infecting a lion, T. gondii makes a home in the lion’s gut until its eggs are released in feces. A gazelle eats a plant contaminated by the feces. The eggs hatch in the gazelle’s bloodstream and begin multiplying, eventually forming small cysts in muscle and brain tissue. Another lion hunts and kills the gazelle. That lion eats the raw meat, and the genes of two unique strains of T. gondii mingle, leading to genetic diversity and a more resilient parasite.
Now imagine the same process, but replace the lion with a housecat and the gazelle with a human. Even though most cats will not end up eating their owners, the parasite is blind to the complexities of human-cat relationships. T. gondii will still infect people who, for instance, aren’t careful to wash their hands after emptying their cat’s litter box. Given the opportunity, the parasite will form cysts in the brain, where it usually stays dormant for the rest of that person’s life, waiting to be devoured by a cat so it can reproduce in the pet’s intestines, thus completing the cycle. Scientists have known about these cysts since the 1930s, but until recently they thought they were mostly harmless.
New studies show correlations between the cysts and serious health problems in humans. Some of the first people infected with AIDS died from toxoplasmosis because their immune systems were too weak to hold the parasitic infection back. When the AIDS epidemic began, many doctors believed T. gondii had evolved into a deadly strain. Unencumbered by a healthy immune system, T. gondii can continue growing until it destroys the brain. Now scientists warn people with AIDS to stay away from litter boxes; the same guidance is given to pregnant women, who can pass the parasite to unborn children. When it comes to T. gondii, the unborn face the same challenges as AIDS patients: an immune system unable to keep the infection at bay. T. gondii can cause miscarriages, premature births, and defects, including brain and eye damage.
Scientists studying rodents found a connection between T. gondii and behavior, something that might be relevant to humans infected with the parasite (although no one yet knows for sure). Rats infected with T. gondii are sexually aroused by the smell of cat urine, making them more likely to get caught and eaten. Infected rodents also show less aversion to risk in general because the T. gondii cysts induce their brains into releasing more dopamine. That might explain why some people collect so many cats and why researchers may have found an odd correlation between people who die from risky behavior, such as riding a motorcycle, and T. gondii cysts.
If you’re a dog person like me, you might be thinking you have nothing to worry about. Unfortunately, T. gondii can be transmitted to humans the same way it returns to cats: undercooked meat. You can also be infected through water, although the only recorded example of that method of transmission happened in 1995 in Victoria, British Columbia, when a population of cougars and cats were found to be shedding and excreting into a reservoir.
T. gondii is one of the world’s more successful parasites. Scientists estimate that about a third of all humans are infected; that number could rise once data are available from all countries around the world. That said, we are far from an epidemic of toxoplasmosis because T. gondii is only deadly to small subsets of the population. Additionally, a recent study has suggested that the link between behavior and the cysts is tenuous at best. The advice experts give is to wash your hands, cook your meat, and stay away from the litter box if you’re pregnant. Or, you know, get a dog.