Suggestions that giving people drugs against parasitic worms could help control malaria should be treated with caution, according to research published this month.
Researchers led by Michel Cot of the French Faculté de Pharmacie have shown that one worm, Ascaris lumbricoides, appears to offer some protection against malaria.
Previous studies of interactions between parasitic worms and the malaria parasite Plasmodium falciparum have come to varying conclusions. Some suggested that the worms suppress the malaria parasite; others hinted that they help it.
Last year, Pierre Druilhe of the Pasteur Institute in France and his colleagues considered recent evidence from Africa and Asia that worms worsen malaria infections.
If confirmed, they wrote, the effect of worm infections on malaria could have profound implications for public health. Treating parasitic worms could offer "an affordable, strongly effective and novel means to roll back malaria", they added.
But Cot and his colleagues, who published their study in the American Journal of Tropical Medicine and Hygiene this month, say they are in "complete disagreement" in the case of Ascaris lumbricoides and Plasmodium falciparum.
Most studies of the interactions between parasites that infect humans have been observational, but the current study used a medical intervention to better pinpoint cause and effect.
It involved 300 villagers in an area of Madagascar where both Ascaris and Plasmodium are prevalent. Roughly half of the participants received a drug to eliminate any Ascaris worms from their bodies.
Cot's team compared the number of malaria parasites in the blood of participants who had received treatment with those who had not. They found that treatment worsened malaria infections in people over five years of age.
The authors warn that providing anti-worm drugs at the community level could have unexpected consequences, such as an increase in malaria attacks.
"What the authors are saying is that when you are making a public health statement with implications for treatment, the evidence for them has to be strong," says Sanjeev Krishna, a parasitologist at St. George's in the United Kingdom.
"If there is dispute there needs to be more evidence. They're making a point of caution."
Krishna says Cot's conclusion that Ascaris lumbricoides can help to control the malaria parasite seems strong "for this area and these parasites".
Reference: American Journal of Tropical Medicine and Hygiene 75, 194 (2006)