Tuesday, September 06, 2005

Bizarre parasite manipulation

Via the NYTimes comes news of a parasitic worm that grows inside grasshoppers and manipulates its host's behavior: when it comes time for the worm to move to fresh water, it actually induces the grasshopper to jump into water (a lethal activity for the grasshopper, which cannot swim). (Paper here.)
Biologists have discovered and hope to decipher a deadly cross talk between the genomes of a grasshopper and a parasitic worm that infects it. The interaction occurs as the worm induces the grasshopper to seek out a large body of water and then leap into it.

The parasite, known as a hairworm, lives and breeds in fresh water. But it spends the early part of its life cycle eating away the innards of the grasshoppers and crickets it infects.

When it is fully grown, it faces a difficult problem, that of returning to water. So it has evolved a clever way of influencing its host to deliver just one further service - the stricken grasshopper looks for water and dives in. ...

"We found the parasite produces and injects proteins into the brain of its host," Dr. Thomas said.

Two of the proteins belonged to a well-known family of signaling agents known as the Wnt family that are deployed in developing the cells of the nervous system.

Though produced by the worm, the two proteins seemed similar to insect-type proteins and perhaps developed so as to mimic them.
Lest you think that only lowly creatures such as grasshoppers are subject to such behavior manipulation, human behavior seems to be manipulated by the parasite Toxoplasma gondii, a relative of the malarial parasite Plasmodium. Humans are not the target host of Toxoplasma (it reproduces in cats but infects many other hosts indiscriminately) but it's estimated that up to one billion people are infected. Toxoplasma grows inside cells but usually lies dormant inside cysts, anywhere in the body but most notably in the brain. It usually has no effects unless the immune system is compromised (toxoplasmosis is a common cause of death from AIDS, and is dangerous to unborn fetuses, whose immune systems are not yet active). However, as Chris of Mixing Memory summarizes here (with references), Toxoplasma infection seems to change the personality of the host: "Men become dirty, dogmatic recluses, and women become naive, outgoing, and promiscuous." It also slows reaction times and makes the host 2.65 times more likely to get into car accidents. It may even increase the risk of schizophrenia.

Why would Toxoplasma do this? In its natural life cycle, it reproduces in cats, releases oocysts in the cat's feces, which may be eaten by a prey animal, where it grow; if this prey is then eaten by a cat, it's back to the cat. The behavior manipulation comes in at the stage where the prey is eaten by the cat: rodents infected with Toxoplasma lose their fear of cat odor, have defective learning and memory, and show (like humans) slow reaction times: all things that would help them get eaten by a cat.

Since rodent and human nervous systems have a great deal of homology, it seems likely that the "strategy" that Toxoplasma uses on rodents also goes to work on humans. The going hypothesis is that Toxoplasma infection changes neurotransmitter levels, for example increasing dopamine levels. This could explain the increased risk of schizophrenia (which is thought to be related to increased dopamine levels) and the personality changes (increased background dopamine levels are correlated to reduced novelty seeking in humans). Dopamine is also involved in motor coordination (Parkinson's disease is caused by the degeneration of dopaminergic neurons in the basal ganglia), so slow reaction times could also be related to dopamine levels. It could be that the infection induces an inflammatory response by the immune system (this response is what holds the infection in check in cysts), and some of the inflammatory chemicals (called interleukins) could influence levels of neurotransmitters, including dopamine.

All this just goes to show how effective parasites can be in sneakily manipulating the behavior of their hosts. We saw earlier that malaria parasites make their human hosts smell more attractive to mosquitoes: I honestly wouldn't be surprised if it turns out that malaria also makes people want to spend more time outside or hang around stagnant pools of water.

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