Led by disease ecologist Frédéric Thomas of MIVEGEC and parasite ecologist Kevin Lafferty of WERC and their colleagues, the study analyzed 37 countries for the incidence of adult brain cancers and the percent of people infected with the parasite Toxoplasma gondii — a single-celled organism found worldwide in at least one-third of the human population.

Their analysis showed that countries where Toxoplasma gondii is common also had higher incidences of adult brain cancers, than in those countries where the organism is not common.

The study does not prove that Toxoplasma gondii directly causes cancer in humans, and the study does not imply that an infected person automatically has high cancer risk," says Lafferty, who is based at the University of California, Santa Barbara. "However, we do know that Toxoplasma gondii behaves in ways that could stimulate cells towards cancerous states, so the discovery of this correlation offers a new hypothesis for an infectious link to cancer."

Toxoplasma gondii is well-known to science. To ecologists who study zoonotic diseases -- pathogens that can infect both humans and wildlife -- T. gondii is known to be one of the most widespread and successful parasites in the world.

"It can infect almost any type of warm blooded animal," says Lafferty. "It's been found in rodents to birds -- even in sperm whales." Furthermore, infectious stages of this protozoan parasite can only be transmitted via cat species, including bobcats, mountain lions and the domestic cat.

However, the main risk for exposure to Toxoplasma gondii is poor hygiene and consumption of undercooked meats. Medical researchers and health professionals are familiar with the parasite because it can cause harm in immunosuppresed persons (e.g. HIV patients) and fetuses.

Both the U.S. Center for Disease Control (CDC) and the National Institutes of Health (NIH) already list the disease toxoplasmosis in their online index, with suggested hygiene and prevention practices to limit infection.

Sources/Usage: Public Domain. View Media Details

But how might this parasite induce cancerous brain cells? Lafferty says once Toxoplasma gondii enters an animal host, its “bradyzoite” cyst stage can latently persist for a host’s lifetime in the host’s brain and other tissues. According to past studies on infected cells of laboratory mice, these cysts can provoke cell inflammation and inhibit natural programmed cell death — both of which are conditions that can stimulate host cells towards cancerous states.

Brain cancers as a whole are rare — annual risks are only a few individuals per 100,000 persons, even in persons infected with Toxoplasma gondii. "It is important to emphasize that we are hypothesizing Toxoplasma infection as a risk factor -- not a sole cause," says Lafferty. "Also, brain cancers are a broad category and likely have multiple causes and risk factors."

“Nevertheless, given how common toxoplasmosis is in the global human population and how its biology may be associated with tumor formation, we were curious if national rates of brain cancers were linked to the parasite,” says MIVGEC's Thomas. “Our results suggest that Toxoplasma gondii potentially increases the risk of brain cancers in humans, and we hope this hypothesis stimulates further research on individual risk of cancers and of toxoplasmosis.”

-- Ben Young Landis