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Common Parasite Can Manipulate Our Behavior

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Gustavo Arrizabalaga and Bill Sullivan report at Scientific American:

Imagine a world without fear. It might be empowering to go about your daily life uninhibited by everyday distresses. You could cross highways with confidence, take on all kinds of daredevilry and watch horror flicks without flinching. Yet consider the prospect a little more deeply, and the possibilities become darker, even deadly. Our fears, after all, can protect us.

The basic aversion that a mouse has for a cat, for instance, keeps the rodent out of death’s jaws. But unfortunately for mice everywhere, there is a second enemy with which to contend, one that may prevent them from experiencing that fear in the first place. A unicellular organism (a protozoan), Toxoplasma gondii, can override a rodent’s most basic survival instincts. The result is a rodent that does not race away from a cat but is instead strangely attracted to it.

Toxoplasma‘s reach extends far beyond the world of cat and mouse. It may have a special relationship with rodent and feline hosts, but this parasite also infects the brains of billions of animals on land, at sea and in the air. Humans are no exception. Worldwide, scientists estimate that as many as three billion people may be carrying Toxoplasma. In the U.S., there is a one-in-five chance that Toxoplasma parasites are lodged in your neural circuits, and infection rates are as high as 95 percent in other countries.

For most people, this infection appears asymptomatic, but recent evidence shows that Toxoplasma actively remodels the molecular landscape of mammalian brain cells. Now some researchers have begun to speculate that this tiny single-celled organism may be tweaking human health and personalities in stealthy, subtle ways.

What the cat dragged in
Researchers first discovered T. gondii in 1908, and by the end of the 20th century they had a good grasp on how people could pick up this parasite. The story starts with cats: for reasons that scientists have yet to unravel, Toxoplasma can sexually reproduce only in the feline gut. The parasite breeds within its feline host and is released from the feline’s tail end. Cats are such obsessive groomers that it is rarely found in their fur. Instead people can become infected from kitty litter or by ingesting it in contaminated water or food.

Within a new host the parasite begins dividing asexually and spreading throughout the host’s body. During this initial stage of the infection, Toxoplasma can cause the disease toxoplasmosis in immunocompromised or otherwise susceptible hosts, leading to extensive tissue damage. Pregnant women are particularly at risk. If a woman is infected with Toxoplasma for the first time during pregnancy, the parasite may invade the developing fetus, cutting through tissues and organs as it spreads from cell to cell. Infection early in pregnancy can result in miscarriage or birth defects.

In otherwise healthy individuals, however, the only symptoms during this period are brief, flulike discomforts such as chills, fever and body ache. Within days the immune system gets the parasite under control, and Toxoplasma retreats into a dormant state. It conceals itself within a hardened wall in the host’s cells, a structure called a tissue cyst.

This stage of the infection has no other discernible symptoms, but individuals with dormant infections who develop compromised immune systems—because of AIDS, an organ transplant or chemotherapy—may experience severe complications. With the body’s defense systems weakened, Toxoplasma can reactivate and grow uncontrollably.

Once infected, a person will remain a carrier for life. Our immune system is apparently incapable of eliminating the tissue cysts, nor can any known drug. Nevertheless, the infection, detectable with a blood test, has long been viewed as relatively benign. After all, many people carry this parasite with no obvious ill effects. Only recently have scientists begun reexamining this belief.

Eat me, Mr. Kitty
In the 1980s researchers noticed unusual behaviors in Toxoplasma-infected mice. The rodents became hyperactive and groomed less. In 1994 epidemiologist Joanne Webster, then at the University of Oxford, observed that rats harboring tissue cysts behaved differently from their uninfected counterparts. Instead of fleeing from cats, the infected rodents moved toward them—making them easier prey.

Webster suspected that this “fatal feline attraction,” as she called it, was a crafty way for the parasite to get back into a cat’s belly to complete the sexual stage of its life cycle. In the years to follow, this idea gained ground: a large body of work now shows that the parasite can indeed manipulate rodents’ behavior by altering neural activity and gene expression.

Several well-controlled experiments have shown that although uninfected rodents avoid areas that have been infused with cat stench, infected rodents do not seem to mind. Even more bizarre, in 2011 neuroendocrinologist Robert Sapolsky of Stanford University, molecular biologist Ajai Vyas of Nanyang Technological University in Singapore and their colleagues found that—at least in terms of neural activity—infected rats appeared to be sexually attracted to cat scent.

In the mammalian brain, the “defensive” and “reproductive” neuronal pathways run in parallel. These pathways start at the olfactory bulb, involved in odor detection, and ter-minate at the limbic system, an area critical to basic reactions such as fear and arousal. Their proximity may partially explain how the parasite manipulates rodent behavior. . .

Continue reading.

Written by LeisureGuy

27 March 2015 at 2:26 pm

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