Toxic House Cats?

Research Blogging / by Dave Munger /

Up to half of all humans are infected by a cat-borne parasite that can cause stillbirth, brain damage, and a host of other subtle neurological effects. Is vaccination the solution?

Credit: Flickr user kevindooley

Toxoplasmosis is a serious disease when transmitted from a pregnant mother to the unborn fetus, potentially resulting in stillbirth, brain damage, or long-term eye damage that can lead to blindness. Even worse, it’s extremely common in a most common animal, the house cat—and it’s easily transmissible from cats to humans. The parasite that causes it, Toxoplasma gondii, is found in one-third to one-half of all humans—over two billion individuals! This potential killer likes to take up residence inside your brain.

So why don’t we hear more about toxoplasmosis? There are a couple reasons. First, it’s a relatively benign disease for most people, comparable to a moderate flu. Second, it’s much less common in its most dangerous form, when transmitted from a pregnant woman to the fetus. This is because it’s only likely to be transmitted if the mother was recently infected herself. Still, it’s estimated that the disease costs as much as eight billion dollars a year in the US; it affects over 1 in 15,000 childbirths in the US and as many as 1 in 500 in parts of the world where the disease is more common.

British science writer Ed Yong gives a good introduction to T. gondii in this post: The parasite is primarily transmitted by cats, through their feces. But cats don’t normally eat cat feces, so path of transmission is often more complicated. In the ideal scenario (for the parasite), rats eat the stuff. Then when a cat preys on an infected rat, the parasite infects the cat, starting the cycle anew. Amazingly, the parasite appears to have evolved to attack the brains of rats, making them easier for cats to catch by suppressing the rats’ fear of cats and motivating them to move around more often so they are easier to spot.

Since few cats prey on humans, the ability of T. gondii to infect people is likely just an unhappy accident—both for us, who can get sick from it, and for the parasite, which is unlikely to extend its life-cycle by being transmitted any further. Humans acquire the parasite by eating raw or undercooked meat from infected animals, or by eating items that have been contaminated with cat feces (let this stand as a reminder to wash your hands after cleaning the litter box!). The paper Yong cites offers controversial speculation that the T. gondii may be responsible for a number of subtle and not-so-subtle mental differences in individuals, ranging from neuroticism to stereotyped gender roles. The research, by Kevin Lafferty and published in Proceedings of the Royal Society B: Biological Sciences, cannot establish causality—it only finds that these conditions are more common in regions of the world where the parasite is more common.

Last week, the amateur scientist and science-themed musician DJ Busby discussed another report that analyzed 11 different studies, nine of which found correlations between toxoplasma infection and several different personality traits. So there seems to be converging evidence—albeit only correlations—that T. gondii isn’t just a benign resident of human brains after the initial toxoplasmosis infection is beaten back by the immune system.

Fortunately, primarily motivated by the more severe effects of toxoplasmosis in fetuses and infants, researchers are making significant progress developing vaccines against T. gondii, Rob Mitchum, a science reporter for the University of Chicago Medical Center, writes on the center’s blog. A team led by University of Chicago researcher Samuel Hutson has created a deactivated form of T. gondii that functions effectively as a vaccine against wild T. gondii in laboratory mice. Like other vaccines, it works by harmlessly activating the animal’s natural immune system, preparing it to defend against the dangerous parasite. But since it’s conceivable that the inactive form of the parasite could also be harmful, most people prefer not to risk being injected with a life vaccine.

In a separate study from the same laboratory and led by Hua Cong, researchers identified key bits of proteins from the parasite that were likely to activate the immune system response, then inoculated mice with only those bits before injecting them with live T. gondii. The vaccine was 80 percent effective in preventing infection. The research was published in Vaccine. There are many steps before this work might result in a practical vaccine, but this are encouraging initial results.

So given the widespread but (possibly) non-harmful nature of T. gondii, would it make sense to vaccinate everyone? The laboratory director, Rima McLeod, thinks so. In addition to the possibility of real danger in transmission from mother to child, people with immune deficiencies such as AIDS are also susceptible to the parasite’s more severe impacts on eyesight and the brain. And since the research isn’t yet conclusive on whether T. gondii is responsible for personality effects and mental disorders, it certainly makes sense to pursue additional research on vaccines that might benefit even more people.

Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

 

Originally published December 15, 2010

Tags behavior health medicine

Share this Stumbleupon Reddit Email + More

Now on SEEDMAGAZINE.COM

  • Ideas

    I Tried Almost Everything Else

    John Rinn, snowboarder, skateboarder, and “genomic origamist,” on why we should dumpster-dive in our genomes and the inspiration of a middle-distance runner.

  • Ideas

    Going, Going, Gone

    The second most common element in the universe is increasingly rare on Earth—except, for now, in America.

  • Ideas

    Earth-like Planets Aren’t Rare

    Renowned planetary scientist James Kasting on the odds of finding another Earth-like planet and the power of science fiction.

The Seed Salon

Video: conversations with leading scientists and thinkers on fundamental issues and ideas at the edge of science and culture.

Are We Beyond the Two Cultures?

Video: Seed revisits the questions C.P. Snow raised about science and the humanities 50 years by asking six great thinkers, Where are we now?

Saved by Science

Audio slideshow: Justine Cooper's large-format photographs of the collections behind the walls of the American Museum of Natural History.

The Universe in 2009

In 2009, we are celebrating curiosity and creativity with a dynamic look at the very best ideas that give us reason for optimism.

Revolutionary Minds
The Interpreters

In this installment of Revolutionary Minds, five people who use the new tools of science to educate, illuminate, and engage.

The Seed Design Series

Leading scientists, designers, and architects on ideas like the personal genome, brain visualization, generative architecture, and collective design.

The Seed State of Science

Seed examines the radical changes within science itself by assessing the evolving role of scientists and the shifting dimensions of scientific practice.

A Place for Science

On the trail of the haunts, homes, and posts of knowledge, from the laboratory to the field.

Portfolio

Witness the science. Stunning photographic portfolios from the pages of Seed magazine.

SEEDMAGAZINE.COM by Seed Media Group. ©2005-2015 Seed Media Group LLC. All Rights Reserved.

Sites by Seed Media Group: Seed Media Group | ScienceBlogs | Research Blogging | SEEDMAGAZINE.COM