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 …
strange behaviors 
kristina nadreau said
I really have never been inspired to eat cat feces, thus I have no fear of toxoplasmosis
Richard Conniff said
No doubt the 60 million Americans who now carry the toxoplasmosis parasite felt the same way: http://www.cdc.gov/parasites/toxoplasmosis/
JJ McKibbin said
Obviously the vast majority of people don’t intentionally eat cat feces. And yet 60 million Americans have been infected. Here’s how that happens. First of all, Toxoplasma gondii can only complete its life cycle in the intestine of a cat, and only cats shed the parasite’s oocysts (eggs) in their feces. A very high percentage of outdoor and indoor/outdoor cats are infected. Each infected cat sheds the microscopic oocysts in its feces by the hundreds of millions per cat. The oocysts can remain viable in soil or water for 18 months or longer. The oocysts can be transferred from feces to food sources by rodents or insects, or even the wind. Rain can wash the oocysts into water sources. Do you garden or work in your yard? All it takes is one swipe of your hand (gloved or not) across a sweaty brow and you could end up ingesting an oocyst. It only takes one oocyst to become infected. And you don’t even have to touch your face. In dry conditions the oocysts can become aerosolized and then inhaled. Simply breathing in dry dusty areas while gardening or mowing could result in infection. And small children are especially at risk as they are often playing on the ground and constantly put their fingers in their mouths. Where large numbers of cats are concentrated such as in feral cat colonies the oocyst contamination in the environment may become so great that it could be difficult to avoid infection. Keeping your pet cat cat indoors all the time and properly disposing of cat litter presents very little risk of infection from your own cat.
Richard Conniff said
Your reply is much more informative than my own. Thank you.
JJ McKibbin said
Thank you, Mr. Conniff for bringing attention to this important health issue. If you haven’t already read it, I highly recommend Torrey and Yolken’s 2013 research paper published in “Trends in Parasitology” entitled “Toxoplasma oocysts as a public health problem”: http://www.cell.com/trends/parasitology/abstract/S1471-4922%2813%2900090-1
Kim said
I Have been infected with the Toxoplasmosis virus for more than 20 years its dormant in my right eye, I’ve had two episodes and now have constant black areas in my vision. They say it’s a male and will stay in cyst form? It causes some nausea. Is there anything I can do to be rid of it? My family doctor has never heard of it. My parasite is located upper superior nasal. Any information would be helpful
Richard Conniff said
Very sorry to hear it. This is a 2012 study of a treatment in mice: http://www.ncbi.nlm.nih.gov/pubmed/22271863
And this is what the CDC says: http://www.cdc.gov/parasites/toxoplasmosis/health_professionals/
Good luck with it.
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