This single-celled pathogen infects over half the world’s population, including an estimated 50 million Americans. Each of Toxoplasma’s victims carries thousands of the parasites, many residing in the brain.
For the vast majority of people, Toxoplasma causes no serious effects. It manages this feat by hijacking our cells and immune system, and establishing a careful harmony between parasite and host. “Once you get infected with Toxoplasma, you’re infected for life,” Dr. Kasper said.
Toxoplasma can, however, cause serious brain damage in those with weak immune systems, like fetuses and adults with AIDS.
Keep in mind that “serious side effects” is an entirely subjective term. When investigating the effects on rats:
Toxoplasma forms cysts throughout its intermediate host’s body, including the brain. And yet a Toxoplasma-ridden rat is perfectly healthy. That makes good sense for the parasite, since a cat would not be particularly interested in eating a dead rat. But scientists at Oxford discovered that the parasite changes the rats in one subtle but vital way.
The scientists studied the rats in a six-foot by six-foot outdoor enclosure. They used bricks to turn it into a maze of paths and cells. In each corner of the enclosure they put a nest box along with a bowl of food and water. On each the nests they added a few drops of a particular odor. On one they added the scent of fresh straw bedding, on another the bedding from a rat’s nests, on another the scent of rabbit urine, on another, the urine of a cat. When they set healthy rats loose in the enclosure, the animals rooted around curiously and investigated the nests. But when they came across the cat odor, they shied away and never returned to that corner. This was no surprise: the odor of a cat triggers a sudden shift in the chemistry of rat brains that brings on intense anxiety. (When researchers test anti-anxiety drugs on rats, they use a whiff of cat urine to make them panic.) The anxiety attack made the healthy rats shy away from the odor and in general makes them leery of investigating new things. Better to lie low and stay alive.
Then the researchers put Toxoplasma-carrying rats in the enclosure. Rats carrying the parasite are for the most part indistinguishable from healthy ones. They can compete for mates just as well and have no trouble feeding themselves. The only difference, the researchers found, is that they are more likely to get themselves killed. The scent of a cat in the enclosure didn’t make them anxious, and they went about their business as if nothing was bothering them. They would explore around the odor at least as often as they did anywhere else in the enclosure. In some cases, they even took a special interest in the spot and came back to it over and over again.
And on humans:
Some scientists believe that Toxoplasma changes the personality of its human hosts, bringing different shifts to men and women. Parasitologist Jaroslav Flegr of Charles University in Prague administered psychological questionnaires to people infected with Toxoplasma and controls. Those infected, he found, show a small, but statistically significant, tendency to be more self-reproaching and insecure. Paradoxically, infected women, on average, tend to be more outgoing and warmhearted than controls, while infected men tend to be more jealous and suspicious.
It’s controversial work, disputed by many. But it attracted the attention of E. Fuller Torrey of the Stanley Medical Research Institute in Bethesda, Maryland. Torrey and his colleagues had noticed some intriguing links between Toxoplasma and schizophrenia. Infection with the parasite has been associated with damage to a certain class of neurons (astrocytes). So has schizophrenia. Pregnant women with high levels of Toxoplasma antibodies in their blood were more likely to give birth to children who would later develop schizophrenia. Torrey lays out more links in this 2003 paper. While none is a smoking gun, they are certainly food for thought. It’s conceivable that exposure to Toxoplasma causes subtle changes in most people’s personality, but in a small minority, it has more devastating effects.
A year later, Torrey and his colleagues discovered one more fascinating link. They raised human cells in Petri dishes and infected them with Toxoplasma. Then they dosed the cells with a variety of drugs used to treat schizophrenia. Several of the drugs–most notably haloperidol–blocked the growth of the parasite.
So Fuller and the Oxford scientists joined forces to find an answer to the next logical question: can drugs used to treat schizophrenia help a parasite-crazed rat? They now report their results in the Proceedings of the Royal Society of London (press release). They ran the original tests on 49 more rats. Once again, parasitized rats lost their healthy fear of cats. Then the researchers treated the rats with haloperidol and several other anti-psychotic drugs. They found that the drugs made the rats more scared. They also found that the antipsychotics were as effective as pyrimethamine, a drug that is specifically used to eliminate Toxoplasma.
From another source:
“Interestingly, the effect of infection is different between men and women,” Dr Boulter writes in the latest issue of Australasian Science magazine.
“Infected men have lower IQs, achieve a lower level of education and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more anti-social, suspicious, jealous and morose, and are deemed less attractive to women.
“On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.
The New York Times, A Common Parasite Reveals Its Strongest Asset: Stealth, By CARL ZIMMER, June 20, 2006
Corante, The Return of the Puppet Masters, Carl Zimmer, January 17, 2006
CDC, Toxoplasma gondii and Schizophrenia, E. Fuller Torrey* and Robert H. Yolken†
*Stanley Medical Research Institute, Bethesda, Maryland, USA; and †Johns Hopkins University Medical Center, Baltimore, Maryland, USA
Imperial College, Scientists find stronger evidence for link between cat faeces and schizophrenia, Wednesday 18 January 2006
news.com.au, Parasite ‘turns women into sex kittens’, Jane Bunce,December 26, 2006 05:27pm