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A fight between gut parasites means a win for people

Worms and Giardia can antagonize each other in the human intestinal tract

Giardia lamblia, a protozoan parasite, infects people who drink contaminated water. Researchers have found that infection with parasitic worms decreases a person's odds of being infected with G. lamblia.

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Parasitic worms may have their perks. For indigenous people at the Amazon’s southern edge, infection with the slithering gut hitchhikers may protect against other parasites, a new study shows.

The finding calls into question whether parasitic infections should always receive treatment. “By treating one thing, we may increase susceptibility to something else,” says anthropologist Aaron Blackwell of the University of California, Santa Barbara, who led the study.

While interactions between worms and other parasites have been studied in animals, this is the first large-scale study in humans, says Maria Yazdanbakhsh, an immunologist from Leiden University in the Netherlands.

To look at such interactions, Blackwell and his team turned to the Tsimane people of northern Bolivia, who subsist on hunting, foraging and staple crops they plant along the Maniqui River. The Tsimane are continually exposed to soil-dwelling parasitic worms called helminths, such as hookworm and roundworm. They also frequently contract waterborne protozoan parasites such as Giardia lamblia.

Over the course of six years, the researchers collected fecal samples from more than 3,000 Tsimane villagers. Of those samples, 56 percent contained at least one species of hookworm, 30 percent contained G. lamblia, and 15 percent harbored the eggs of the giant roundworm Ascaris lumbricoides.

The researchers screened some of the villagers more than once to see how the parasitic infections changed over time. Infection with one worm increased the odds of contracting another. But people infected with worms were less than half as likely to contract G. lamblia as their worm-free counterparts, the researchers report August 28 in the Proceedings of the Royal Society B. To some extent, the protection seems to go both ways: Compared to people without G. lamblia, those infected with the protozoan were 70 percent as likely to contract a parasitic worm.

The researchers don’t yet know how the parasites antagonize each other. But Blackwell speculates that the parasites compete for resources or affect the immune responses in the small intestine, where the parasites live.

The antagonism between parasites adds to arguments against using drugs in people who are continually exposed to parasites, Blackwell contends. “There's a tendency for people to want to swoop in and give everybody treatment, but that sort of one-shot approach doesn't really work.”

Helminths can cause anemia, delay neuronal development and stunt children’s growth, and G. lamblia can cause diarrhea. But a majority of infected Tsimane people have no symptoms. Besides, Blackwell says, people tend to get rapidly reinfected following treatment because the parasitic eggs and larvae linger in soil and water.

The study should be repeated with other people in different locations, says Yazdanbakhsh. But the data clearly point to the importance of considering the gut’s menagerie of parasites, rather than focusing on one at a time.

Editor's note: The photo caption was changed on Octobter 11, 2013 to correct an error.

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