Turning to Greener Weapons In the Battle Against Malaria
Yale e360, May 3, 2010
Insecticides such as DDT have long been used to combat the scourge of malaria in the developing world. But with the disease parasite becoming increasingly adept at resisting the chemical onslaught, some countries are achieving striking success by eliminating the environmental conditions that give rise to malarial mosquitoes.
For over half a century, the battle against malaria has been waged with powerful anti-malarial drugs and potent mosquito-killing insecticides, weapons born from the wonders of synthetic chemistry. In recent years, however, fed up with the financial and ecological drawbacks of chemical warfare, malarious communities from China to Tanzania to Mexico have been forging a new way to fight the scourge, one that draws inspiration from the lessons of ecology more than chemistry. Rather than attempt to destroy mosquitoes and parasites outright, these new methods call for subtle manipulations of human habitats and the draining of local water bodies — from puddles to irrigation canals — where malarial mosquitoes hatch.
The most striking example comes from Mexico, which has completely abandoned its previously lavish use of DDT in malaria control for insecticide-free methods and has seen malaria cases plummet.
Like many countries, Mexico for decades relied upon insecticides to fight the disease, by spraying mosquito-killing chemicals on the interior walls of homes where blood-feeding mosquitoes rest, among other methods. Between 1957 and 1999, taming Mexico’s malaria required 70,000 tons of DDT.
New, environmentally-sensitive methods, such as clearing vegetation along waterways and around homes, were introduced in Oaxaca, the country’s most malarious region, in 1998. By 2002, malaria cases had fallen from more than 17,500 to just 254, and Mexico incorporated the new methods into its national anti-malaria program. By 2000, Mexico had completely phased out use of DDT in malaria control; by 2002, it had phased out all other insecticides in malaria control as well, while simultaneously keeping malaria in check. No deaths from malaria were reported in Mexico in 2008, the most recent year of data available from the World Health Organization.
Similarly, in Sichuan, China, new, non-chemical methods involving the manipulation of water flow in irrigation canals have led to the near cessation of malaria, with malaria rates plummeting from 4 per 10,000 in 1993, to less than 1 per 10,000 by 2004. In several counties of the province, no malaria cases were reported at all between 2001 and 2004. Similar non-chemical gains against the disease have been achieved in Dar es Salaam, Tanzania, as well.
Malaria currently infects 300 million people a year and kills nearly one million, and though the incidence of malaria is decreasing in some countries, it still rages in many others.
The new, greener methods of control rely upon insights into the exacting set of local environmental conditions that malaria transmission requires. While it is commonly considered a disease of poverty, malaria is just as much a disease of the environment. In part, that’s because both malaria parasites and the mosquitoes that carry them thrive in warm, humid conditions.
But it is also because malaria-carrying mosquitoes, all of which hail from the genus Anopheles, don’t generally venture far from where they hatch, and each species tends to lay its eggs in a specific kind of water body. Some prefer shady, flowing waters; others require sunlit puddles. Some can tolerate brackish water, while others must have clear water. That means that if people’s exposure to the habitats of local malarial mosquitoes can be reduced, they will get fewer bites, and thus less malaria.
Malaria transmission is also critically dependent on the life span of the mosquito. The malaria parasite won’t become infective inside the insect until it completes a 7-12 day cycle of development. That means that anything that decreases mosquito longevity — a dearth of useful places to hide from predators, say, or excessively dry conditions — can also effectively squelch malaria.
In Oaxaca, Mexico, malariologists found that the local malaria vector, Anopheles pseudopunctipennis, hatches from the still, algae-choked waters on the edges of streams, rarely flying more than 2 kilometers from its birthplace. And so, starting in 1999, they recruited volunteers in malarious communities to remove green algae and trash from the rivers and streams near their settlements.
“As a gift,” says Jorge Mendez, former chief of Mexico’s anti-malaria agency in the Ministry of Health, “we gave house paint to the local residents, to motivate community participation.” The density of Anopheles larvae dropped by 90 percent within three years. Mexican health officials made life more dangerous for the surviving insects, too, by clearing the vegetation around domiciles, where Anopheles mosquitoes hid from both predators and the desiccating sunshine. They also provided prophylactic anti-malarial drugs. The program ultimately cost 75 percent less than the insecticide-reliant one it replaced.
In Sichuan, China, Anopheles hyrcanus prefers the standing water found in rice paddies, traditionally kept permanently flooded. A water-saving “wet/dry” irrigation scheme introduced in 1994 called for periodically drying out the rice fields. “The Chinese have fine-tuned this to an art,” says Princeton University malariologist Burton Singer. The result was the destruction of Anopheles’ larval habitats, a four-fold reduction in malaria, and increased harvests to boot.
In Dar es Salaam, Tanzania, Anopheles gambiae lays its eggs in trash-blocked sewer drains, and so community workers there began a program of clearing drains and spreading the microbial insecticide Bacillus thuringiensis into sewers. “This was the lowest-hanging fruit of them all,” says Gerry Killeen of the Ifakara Health Institute in Tanzania, “the most basic and undramatic environmental management.” It led to a 30 percent drop in A. gambiae’s transmission of malaria.
Other techniques, useful in areas where destroying or minimizing mosquito habitats is untenable, can be as simple as making sure people close the eaves of their houses. More capital-intensive methods include leveling roads to avoid the formation of puddles, and installing running water and sanitation systems so that homes are less likely to be near stagnant water.
These nuanced — but decidedly low-tech — programs recall an earlier, pre-chemical era, when malaria control workers made similar gains against the disease by tinkering with the local environment, mostly because they had few other options. In the copper mines of Zambia during the 1930s, for example, malariologists significantly reduced malaria by clearing vegetation, removing obstructions from local waterways, and draining flooded areas. In Panama, during the building of the canal in the early 1900s, anti-malaria workers drained swamps and coated puddles with a thin skin of larvae-suffocating oil, part of a multi-pronged anti-malaria strategy that enabled the canal to be built. Similar measures helped eradicate malaria in the U.S. South.
Environmental management methods fell into disuse after World War II, with the development of a string of synthetic insecticides and drugs, led by DDT and chloroquine. Powerful and highly effective, modern insecticides and anti-malarial drugs can kill malaria mosquitoes and parasites quickly and cheaply, wherever they are used, regardless of local conditions. They can be implemented in even the most remote locales, with minimal infrastructure.
Managing the local ecosystem to minimize malaria vectors, in contrast, requires concerted effort from local communities, and expertise not just from health officials, but from ecologists, farmers, and engineers as well. It is labor-intensive. Ditches must be dug, drains cleared, vegetation removed. And what might be the perfect salve in one place could be the worst possible thing to do in another. “The details of what you need depend on the local ecological conditions,” says Singer. “You can’t mastermind it with a master plan.” And while insecticide-spraying campaigns, drug distribution, and the doling out of insecticide-treated bednets can bring malaria mortality down rapidly, reaping the benefits of environmental tinkering takes years.
And so today, while the preferred chemicals have changed — instead of DDT and chloroquine common in the postwar era, the chemicals of choice are now primarily pyrethroid insecticides, impregnated in bednets, and anti-malarial drugs based on artemisinin, an extract from the sweet wormwood tree — the emphasis on chemical control has not.
The current war against malaria in sub-Saharan Africa, for which financing from governments and NGOs increased tenfold between 1998 and 2008, calls for 730 million bednets doused with insecticides, 172 million homes sprayed annually with insecticides, 228 million drug treatments for malaria patients, and 25 million preventive drug treatments for pregnant women, to be blanketed across Africa’s malarial heartland, as the inter-agency Roll Back Malaria Partnership has outlined. Today, 11 countries are conducting formal campaigns to eradicate the disease, and malaria declines in the wake of chemical-based anti-malaria campaigns have been reported in Equatorial Guinea, Zanzibar, Sao Tome and Principe, Rwanda, and Ethiopia.
And yet, as dramatically effective and universally applicable chemical methods may be, they cannot provide the long-lasting sustainability of environmental management methods. None of the chemical methods of malaria control last longer than a handful of years. Insecticide-treated bednets must be replaced or re-treated every three to four years. Drugs must be continually administered. Interior walls must be re-sprayed with insecticide every six to 12 months.
With sustained funding and political commitment, insecticidal and pharmaceutical treatments for malaria could, in theory, go on indefinitely. The trouble is that in the meantime, the malaria parasite and the mosquitoes that carry it become increasingly adept at resisting the chemical onslaught. Plasmodium parasites that could circumvent the killing action of artemisinin drugs have already emerged in parts of Southeast Asia. By 2007, artemisinin drugs were failing in up to 30 percent of malaria cases in parts of Thailand and Cambodia, and by 2009, those drug-resistant parasites had spread deeper into southern Cambodia. Experts worry that it is only a matter of time before these drug-resistant malarias spread into the malarial heartland in sub-Saharan Africa.
Similarly, malaria-carrying mosquitoes that can resist the pyrethroid insecticides commonly used to treat bednets were first reported in 1993, and have since turned up across sub-Saharan Africa. In a 2005 study in Cameroon, just as many kids using treated nets came down with malaria infections as those using untreated ones.
While DDT is still used in indoor spray campaigns against malaria, resistance to the chemical — and related insecticides — is widespread.
“Our enthusiastic programs are again going to founder in the swamp of biological resistance,” warned malariologist William Jobin, on the scientific website MalariaWorld in April.
Finally, as the chemical war against malaria intensifies, so, too, do fears of toxicity. While the volume of DDT and other insecticides used in spray campaigns against malarial mosquitoes is miniscule compared to agricultural use, environmentalists and farmers worry that the increasing availability of DDT for malaria control could result in surreptitious diversion onto farms. Concerns simmer, too, about the understudied problem of disposal of the insecticide-treated bednets.
The environmental management programs in Mexico, China, and Tanzania all arose in the wake of just such concerns. Mexico’s program, for example, was implemented after a 1996 agreement with the United States and Canada to phase out all uses of DDT. The irrigation program in Sichuan, China was implemented after the cost of running a 1986-1993 program of insecticide-treated bednet distribution became unmanageable. In Dar es Salam, the local malaria vectors had adapted to the widespread presence of bednets by biting outdoors instead.
The benefits of environmental management techniques — their longer-term sustainability, ability to harness community participation, and lower overall costs — may tip the balance in their favor in other fronts in the war on malaria, too. Health officials from Ecuador and Nicaragua, for example, have been flocking to Mexico to learn about their malaria program, Mendez says.
Many experts hope these techniques — still limited to just a handful of countries — will become more widespread, not to replace chemical methods entirely, but as coYale e360mplementary alternatives that will reduce the use of insecticides and drugs.
“Current methods are good at dramatic reductions, but the resilience and long-term sustainability are open questions,” says Robert Bos, a senior scientist for the World Health Organization. Adds Mendez, “We need to put on the table a new model, in order to get an enduring control.”
Courtesy of Yale e360