Toxoplasma gondii

Toxoplasma gondii is a parasitic microbe that causes the disease toxoplasmosis. It is found all over the world and can infect nearly any warm-blooded animal, but only cats—both wild and domestic—serve as its definitive hosts, meaning the parasite can only undergo sexual reproduction inside them. This makes T. gondii one of the most widespread parasites in developed nations; blood tests suggest that as many as half of all people globally have been exposed to it and may carry a chronic infection.

In rodents, the parasite alters behavior in ways that make them more likely to be caught by cats. Infected rats show less fear of cat urine, while infected mice become less anxious, more exploratory, and less wary of predators in general. Since cats are the only animals where T. gondii can reproduce sexually, these behavioral changes are thought to be evolutionary tricks that boost the parasite’s reproductive success—rodents that don’t avoid cat territory are more likely to become cat prey. The main way the parasite causes these changes is by remodeling the epigenetics of neurons that control the relevant behaviors.

In humans, infection is usually symptom-free, but it can become serious in infants and people with weakened immune systems. During the first few weeks after exposure, there may be mild, flu-like symptoms, but otherwise healthy adults typically show no signs. This quiet, latent infection has been linked to subtle shifts in behavior, personality, and mental health. Compared to uninfected people, those with T. gondii may have a reduced aversion to cat urine (though the effect differs between men and women) and a higher risk of schizophrenia and suicidal thoughts. Early evidence hints that the parasite might cause some of the same brain changes in humans as it does in rodents, though many of these links are debated, and a 2016 study found them to be weak.

The parasite has special organelles called rhoptries and micronemes, which contain proteins for invading cells and manipulating the host’s immune response. To inject these proteins, T. gondii uses an apical complex at its tip to puncture the host cell membrane and release the organelle contents. Two microtubulins and their associated proteins inside the conoid of this complex help organize and dock the rhoptries, though the exact mechanism is not yet fully understood.

The life cycle has two parts: a sexual stage that happens only in cats, and an asexual stage that can occur in almost any warm-blooded animal, including humans, cats, and birds. Because only cats allow sexual reproduction, they are the definitive host; all other animals are intermediate hosts. When a cat eats an infected mouse carrying tissue cysts, the parasite survives the stomach and infects cells in the cat’s small intestine. There, it undergoes sexual development and produces millions of thick-walled oocysts. Cats are the only definitive host because they lack the enzyme delta-6-desaturase (D6D) in their gut, which normally converts linoleic acid; without this enzyme, linoleic acid builds up in the body, and this excess is essential for the parasite’s sexual reproduction.

Infected intestinal cells eventually burst, releasing oocysts into the cat’s feces. These oocysts can contaminate soil, water, food, or anything touched by the feces. They are very tough and can stay infectious for months in cold, dry conditions. People or other animals can get infected by swallowing oocysts—for example, by eating unwashed vegetables, drinking contaminated water, or handling cat litter. Cats can also be infected by ingesting oocysts, but they are much less sensitive to this route than intermediate hosts are.

Intermediate hosts include pigs, chickens, goats, sheep, and red kangaroos (as shown in a 2010 study). Cattle and horses are resistant and are thought not to become significantly infected. The parasite has three life stages: the fast-dividing tachyzoite stage, the slow-dividing bradyzoite stage inside tissue cysts, and the oocyst stage in the environment. When a person or animal eats an oocyst or tissue cyst, the cyst wall is dissolved by digestive enzymes in the stomach and small intestine, freeing sporozoites. These parasites then invade cells in and around the intestinal lining.