Women hold mosquito nets after receiving them at a distribution point in Sesheke, Zambia. Researchers say malaria may have rebounded in some parts of Zambia and Senegal because of resistance to the insecticide-treated nets.
Women hold mosquito nets after receiving them at a distribution point in Sesheke, Zambia. Researchers say malaria may have rebounded in some parts of Zambia and Senegal because of resistance to the insecticide-treated nets. ASSOCIATED PRESS
New cases of malaria plummeted by 13-fold in the village of Dielmo, in the central highlands of Senegal, after residents started sleeping under insecticide-treated bed nets and people with malaria were treated with a drug combo that clears their blood of the parasites. That was three years ago.
But more recently, malaria's been on the rebound in Dielmo. Some scientists think it's because of two factors:
- The Anophelese gambiae mosquitoes that transmit malaria in Africa are becoming widely resistant to the insecticide used on bed nets, and
- Teens and adults in the village are losing the partial immunity they used to have to the malaria parasite.
If this is true – and there are prominent skeptics – the implications are profound. It suggests that the principal strategies behind the global campaign to eliminate malaria are fundamentally flawed, and that progress may be fragile and short-lived.
Moreover, if the reasons for resurgence are correct, it's not at all clear what might be done. There isn't a ready supply of alternative insecticides that are effective, cheap and safe. And if waning immunity is a factor, that's a real catch-22 – no one would want to abandon effective drug treatments in order to boost whatever natural immunity people develop to malaria, since that defeats the whole purpose of preventing infection and treating the disease in the first place.
Dielmo, which sits on the marshy bank of a stream where Anopheles mosquitoes breed year-round, isn't the only place researchers have noticed a recent resurgence of malaria after chalking up earlier successes in malaria control. Rebounds have been recorded since 2009 in Rwanda, the islands of Sao Torme and Principe off west central Africa, and in Zambia.
"A number of groups are seeing resurgence," says Dyann Wirth, a well-known malaria expert who's chair of immunology and infectious diseases at the Harvard School of Public Health.
But the Senegalese village may be particularly important in the effort to figure out what's behind malaria rebound, and how big a problem it might become. That's because the village has been the focus of a 21-year research project to track malaria – a uniquely long-lived effort.
The Senegalese group is known as a particularly rigorous research team. They document the rebound effect in Dielmo in Lancet Infectious Diseases.
Here's what happened: In 2007 and early 2008, malaria afflicted 5.5 people out of every 100 per month. Then villagers got bed nets impregnated with a long-lasting insecticide. The malaria rate dropped to 41 per 1,000 people per month between August 2008 and August 2010.
But then, malaria incidence shot back up to 4.6 cases per 100 per month between September and December of last year. It was even higher among adults and children age 10 or older.
Meanwhile, the researchers documented a striking increase in Anopheles mosquitoes unfazed by the deltamethrin insecticides coating the bed nets. Before the nets were distributed, 8 percent of the mosquitoes carried a gene that confers resistance to the chemical, but a little more than two years later, 48 percent were resistant.
The evidence for declining immunity to malaria among the residents of Dielmo (at least those over 10 years old) is more speculative. The researchers, led by Jean-Francoise Trape of the Senegalese Institute for Research for Development, say the increase in malaria incidence could only be explained by greater exposure to mosquitoes in adolescents and adults "or a decrease in protective immunity." And their data don't support an increase in mosquito exposure.
By the way, when it comes to malaria, "immunity" is a slippery concept. Basically, people who have been repeatedly exposed to the malaria parasite over years – if they survive – develop some immunity. Not enough to prevent reinfection, but enough to reduce the severity of the disease.
"We don't really know what we mean when we talk about immunity to malaria, the way you can with measles," Harvard's Wirth told Shots. "So it's hard to measure."
Another explanation might be that when malaria rates dropped, people stopped seeking care when they got a fever that they used to assume was due to malaria. Wirth says "that then creates a reservoir of infected people" on whom mosquitoes can feed, starting another cycle of transmission.
Joseph Keating and Thomas Eisele of Tulane University also advise taking a grain of salt with the reports of malaria resurgence. In a commentary that accompanies the new report, they write: "This study lacks sufficient external validity to allow results to be robustly generalized across other regions of Africa...."
It should also be noted that malaria experts around the world fervently hope the Senegalese researchers are wrong.