Under study. One of the Senegalese villages where the researchers caught and tested mosquitoes.

Brian Foy

Antiparasitic Drug Has Bonus Effect on Mosquitoes

A drug already widely used in massive campaigns to help control two parasitic diseases, river blindness and elephantiasis, could also drive down malaria, a new study has found. The compound, called ivermectin, shortens mosquitoes' life spans and makes them less likely to transmit the malaria parasite.

Ivermectin is known in the developed world primarily as a drug to treat head lice in children and heartworm in animals. But it also kills several tropical parasites, including the mosquito-borne ones that cause lymphatic filariasis—a disease also called elephantiasis for the horribly swollen limbs suffered by patients—and those carried by black flies that cause onchocerciasis, or river blindness.

Many countries use ivermectin in a strategy called mass drug administration (MDA), which means that the entire population in an affected area is given the drug once or twice a year. Merck, which produces ivermectin under the brand name Mectizan, provides it for free to treat river blindness and elephantiasis; the company has pledged to do so for as long as it's needed.

But Brian Foy, who studies insect vectors at Colorado State University in Fort Collins (and who became a minor internet celebrity last year after sexually transmitting the Zika virus to his wife and then publishing a study about it), is interested in another effect of ivermectin: It kills mosquitoes. Lab studies have shown that when mosquitoes bite a person recently treated with the drug, they swallow a dose that is sometimes lethal to them. To find out whether the drug could thwart malaria transmission, Foy and his colleagues have been studying mosquito populations in an area in southeastern Senegal where ivermectin is used in annual MDAs to stop river blindness. In a study published last year, the team showed that an MDA there sharply curtailed the life span of mosquitoes that fed on treated people.

In the new study, Foy and colleagues wanted to know whether ivermectin's insecticidal effect translated into fewer infectious mosquitoes, which in turn could slow the spread of malaria. In theory, it should, because Plasmodium falciparum, the malaria parasite, takes about 2 weeks to develop inside the mosquito body, which is most of the insect's lifetime. So the team caught mosquitoes in three villages that took part in an MDA and three others nearby that did not. Sure enough, they found that in the treated villages, the proportion of mosquitoes with fully developed P. falciparum in their saliva dropped by 79% in 2 weeks. But in the control villages, the proportion rose by 246%, the researchers report online today in The American Journal of Tropical Medicine and Hygiene.

The finding suggests mass treatment with ivermectin would be a new weapon against malaria, complementing insecticide spraying, bed nets, and other drugs, Foy says. But ivermectin would have to be given much more often than once a year, he says—perhaps even monthly—because the drug's effect on mosquito populations won't last long. Whether Merck would donate the extra amount of ivermectin needed is unclear.

Brian Greenwood of the London School of Hygiene and Tropical Medicine calls the paper an "interesting pilot study," but he is "not very impressed" by the evidence in the paper. He says that the number of mosquitoes tested by the researchers was small, and the paper leaves some questions unanswered—such as why mosquitoes in the treatment villages had much higher malaria rates at the start of the study, or why rates soared in control villages.

Peter Hotez, head of the Sabin Vaccine Institute in Washington, D.C., and the current president of the American Society of Tropical Medicine and Hygiene in Deerfield, Illinois, agrees that much more study of ivermectin's antimalarial activity is needed, but he calls Foy's idea "exciting" and "potentially very important." Hotez helped create the Global Network for Neglected Tropical Diseases, which advocates the widespread use of a package of cheap drugs, including ivermectin, to target seven lesser-known tropical diseases. "Now we have possible evidence that this package may also have an impact on other infections, including malaria," Hotez says.

Hotez sees an emerging pattern. In 2009, a study published in the Journal of the American Medical Association showed that mass administration of antibiotics in Ethiopia to treat trachoma, a bacterial eye infection, had huge added benefits. For reasons still being debated, it cut childhood mortality in half. "We're only beginning to understand the enormous potential impact of MDAs on diseases for which they weren't intended," Hotez says.

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