Meet Toxo

Toxo is short for Toxoplasma gondii, a parasitic protozoan with a dependence on cats, both wild and domestic. Toxo is a sporozoan, meaning a protozoan that forms spores.

The life cycle of Toxo involves a definitive host, the cat, where sexual reproduction occurs, and an intermediate host, a warm-blooded animal like a rodent, where asexual reproduction occurs. The life cycle is complete when the end products of the asexual reproduction are ingested by the definitive host, thus setting the stage for the cycle to begin again. (Finding a starting point for a detailed description is a bit like the chicken or egg dilemma.)

Toxo enters the definitive host in the form of tissue cysts which are present in the body of a prey animal (dinner). When the tissue cysts are ingested, they travel to the cat's stomach and intestines. Once Toxoplasma cysts reach the intestines, they settle into the epithelial cells of the small intestinal lining. There the tissue cysts undergo sexual reproduction, which leads to the formation of oocysts. Within 3–10 days of ingestion of the tissue cysts, the cat host sheds the oocysts in its feces; rarely do cats shed oocysts more than once.

Oocysts, after being shed and through the process of sporulation (which takes 1–5 days), form sporocysts and sporozoites. When sporulated oocysts are ingested by an intermediate host, the sporozoites are released during digestion. The sporozoites differentiate into tachyzoites which then travel to other parts of the host's body via the blood circulation, penetrate cells, enter the bradyzoite stage, and form tissue cysts (where the bradyzoites multiply).

In most cases, people become infected via one of two routes:

Other routes of infection are less commonly implicated but include:

Toxo causes a disease called toxoplasmosis. It is largely asymptomatic in cats and other hosts. In humans with compromised immune systems it can cause severe, even life-threatening, illness. In humans Toxo can be acquired congenitally: unborn children can be infected via the placenta; the infection is found in the amniotic fluid, cord blood, and placenta. Congenital toxoplasmosis results from an acute primary infection acquired by the mother during pregnancy. It is possible for a mother to transmit Toxo to her fetus when she acquired it before the pregnancy, but this is most likely when her immune system is tanked. Congenital toxoplasmosis can be a severe, even fatal, illness.

The risk of congenital taxoplasmosis is the reason pregnant women are warned to avoid cat litter and gardening without gloves.

Once in a host Toxo remains for life. In humans it can cause severe illness at a later date when the person's immune system becomes damaged, as with a viral infection, HIV, cancer, or some drugs.

One fascinating aspect to Toxo's life cycle is its ability to change the brain and behavior of infected rodents. Toxo-infected rodents are less fearful of cats, even finding the smell of cat urine attractive, and thus are more likely to become a cat's dinner. This is clearly advantageous to the life cycle of Toxo. Toxo has been found to be able to change the amygdala, where normal predator aversion resides. As Stanford researcher Robert M. Sapolsky stated, "the Toxo genome has the mammalian gene for making [dopamine]. It's got a little tail on the gene that targets, specifies, that when this is turned into the actual enzyme, it gets secreted out of the Toxo and into neurons. This parasite doesn't need to learn how to make neurons act as if they are pleasurably anticipatory; it takes over the brain chemistry of it all on its own."

In people Toxo seems to overcome inhibitions. Sometimes this results in accidental deaths in cars and motorcycles. There is a strong correlation between high Toxo levels and schizophrenia.

I wondered if humans' preference for cats might be caused by Toxo. Robert Sapolsky wrote in response to my question: "There's every reason to think that . . . there are unintentional behavioral changes in humans, and that this might have produced the unintentional outcome of domestication." Cool—a parasite that brings cats and people together.

Revision: 10-20-2010.