First things first: a host is the home of a parasite– it generally does not want to kill it. This parasite does. And it does so by modifying the brain and the behavior of the host.
Toxoplasma Gondii is a fascinating parasite with an even more fascinating life cycle. This parasite can be found in many mammals, including humans. In fact, in humans, it is one of the most common parasites. Studies estimate that up to a third of the global population has been exposed to and may be chronically infected with T. gondii, although infection rates differ significantly from country to country.
Now here is the really freaky part. It has been observed that the parasite can alter the behavior of mammals’ brains so that they become more susceptible to falling prey to their natural (food-chain) enemies. Mice and rats have been observed to completely ignore their natural instinct to avoid cats, and commit suicide by actively seeking them out.
The T. Gondii parasite can only sexually reproduce in the intestines of the cat family (although it can asexually reproduce inside other warm blooded animals (including humans)), so its main function is to get consumed by a cat.
Here’s how it works: an infected feline excretes feces that contains the parasite, which, when consumed by a rat or mouse, cause it to ignore its natural instinct to avoid cats. The mouse gets eaten by the cat, and the parasite enters the cat’s intestines to reproduce. Interestingly enough, even when the parasites have been removed from mice, the mouse will still exhibit this bizarre behavior, as if the parasite has permanently rewired the brain. There is no cure.
A number of studies have suggested that behavioral or personality changes may occur in infected humans,and infection with the parasite has recently been associated with a number of neurological disorders, including schizophrenia and suicide.
Scientific America writes about a recent study done in Denmark in which 45,000 new mothers were screened to better understand how the parasite is transmitted from mother to child and the presence of related antibodies:
The results were clear. Women with Toxoplasma infections were 54% more likely to attempt suicide – and twice as likely to succeed. In particular, these women were more likely to attempt violent suicides (using a knife or gun, for example, instead of overdosing on pills). But even more disturbing: suicide attempt risk was positively correlated with the level of infection. Those with the highest levels of antibodies were 91% more likely to attempt suicide than uninfected women. The connection between parasite and suicide held even for women who had no history of mental illness: among them, infected women were 56% more likely to commit self-directed violence. LINK
Even more fascinating might be Kevin D Lafferty’s (University of Santa Barbara) conjecture that whole cultures may be affected at the population level because of the parasite Toxoplasma Gondii, making them more aggressive, and frighteningly, more neurotic.