- Healthy landscapes that are rich in biodiversity and clean of plastic, rather than widespread spraying of pesticides, is the key to controlling mosquito-borne diseases like malaria and Zika virus, according to a leading epidemiologist.
- When present in healthy numbers, beneficial species such as bats, birds and geckos will consume vast quantities of adult mosquitoes, while birds, fish, dragonfly nymphs and diving beetles devour the larvae.
- People are working on solutions, and Cathy Watson runs down some of the most promising.
Sitting on his verandah in Laikipia, northern Kenya, Dr. Dino Martins, 38, a Kenyan entomologist with experience with malaria, tick fever, liverfluke and other vector-borne diseases, talks about the need to create healthy landscapes that are rich in biodiversity and clean of plastic.
This, rather than widespread spraying of pesticides, is the key to controlling mosquito-borne diseases like malaria and Zika virus, just declared a global emergency by the World Health Organization, according to Dr. Martins.
When present in healthy numbers, beneficial species such as bats, birds and geckos will consume vast quantities of adult mosquitoes, while birds, fish, dragonfly nymphs and diving beetles devour the larvae.
People are working on solutions. For the last three years, for instance, a project in El Salvador, spurred by the threat of dengue fever, has had some success reducing mosquitoes by placing fish in water tanks in homes. But much more will be needed to shift global thinking towards more ecological vector control.
Pesticides are not the solution
Recently, Bill Gates and the UK government pledged $4 billion for new insecticides against malaria, largely for new insecticides.
“This investment in malaria is fantastic,” says Dr. Martins. “Without foundations and governments, we would be facing a far greater epidemic. But the fact is that we are only able to knock malaria back to a certain level and then it either resurges or changes its biology.”
Pesticides alone are not the solution, Dr. Martins added. “With mosquitoes, and this applies to Zika, chemicals are just one tool. We’ve not invested in the broad research that will address this problem sustainably or in plastic recycling. Plastic bags and bottles have become a ubiquitous feature of life across East Africa.”
Spraying with pesticides is useful for removing flying adult mosquitoes that bite people, but will fail to deal with the epidemic of Zika virus. Martins makes an exception for indoor residual spraying, because when pesticide is applied to walls where mosquitoes rest after feeding, it can be highly effective. But spraying in landscapes is “extremely dangerous,” he said.
“It is a quick fix but you pay for it. You kill other species that would have predated on the mosquitoes, and you create a mosaic of the sprayed and unsprayed and low densities of chemicals that foster the rapid evolution of resistance.”
Mosquitoes have life cycles of a week or less, and each generation is an opportunity for random mutations to occur that might predispose a group of mosquitoes not to succumb to pesticides. “In addition, when you use chemicals, you are actually applying a selection pressure on mosquito populations that will drive them to become resistant,” said the scientist, who studied for his PhD at Harvard in the department of renowned naturalist and ant researcher EO Wilson.
“We are basically fighting an arms race with mosquitoes rather than cleverly understanding their life cycle and solving the problem there,” said Martins, who now runs the Mpala Research Centre, a field station affiliated with Princeton, the Smithsonian Institute, the Museums of Kenya and the Kenya Wildlife Service. “Resistance can never evolve to getting rid of the breeding sites. But resistance will always evolve to the use of pesticides.”
Two causes of Zika outbreak in Latin America – And tried and tested solutions from Africa
The explosion of mosquitoes in urban areas that has driven the Zika crisis in Latin America has at least two causes. “One is the lack of natural diversity to control them. We have dangerously simplified landscapes. The other is lack of waste disposal and the proliferation of plastic.”
Photos from Brazil and elsewhere in Latin America show public health workers “fogging” streets, bush and piles of refuse. Martins said that “it is impossible to fumigate every corner of a habitat where mosquitoes might possibly breed.”
“Furthermore, in towns we multiply breeding sites exponentially. Aedes aegypti, the vector of Zika virus, would have typically bred in rock pools. I have reared it in spoonfuls of water in a plastic bag. Mosquitoes are hard wired to put their eggs somewhere safe. In urban areas, its competence as a vector improves.”
A quick way to solve the Zika problem is to clean up these areas and regulate plastics, Martins argues.
“It might be a little bit more effort and perhaps even a bit more expensive,” the scientist said. “And it might seem easier to just use pesticide. But pesticides will not work long term.”
Martins stressed that what we need to be asking is, what is the weakest point in the life cycle of the mosquitoes spreading the disease?
“For me, it is the larvae because they are fixed and findable,” he said. “You can destroy them right there. Once the mosquitoes fly, it is far harder. You have to use greater quantities of chemical more frequently with very complex consequences for public health and the environment. We need more investment in mosquito control at early rather than late stages.”
Martins successfully controlled malaria in part of Turkana, a Kenyan county of extreme aridity where he works with paleontologist Richard Leakey. “We knew that mosquitoes breed in standing water, yet there was almost none around,” Martins explained. It took the scientists some time to figure out that the mosquitoes were breeding in the traditional shallow wells that local women dig and then abandon once the quality of water has declined.
Martins and Leakey worked with locals to fill in the wells, eliminating “almost 100% of breeding sites,” he said. “This, combined with treatment of active cases and indoor residual spraying using WHO-approved long-acting chemicals, ended a severe localized epidemic.”
But even there plastic bags made things worse. “We found that, if people threw them into the well, it increased the survival of mosquito larvae. The plastic slowed down the drying up of the well. And larvae could hide from predators underneath the plastic or in its folds. It also increased temperatures slightly in the well, which seemed to speed up larval development.”
Martins speculates that the ban on plastic bags in Rwanda under President Kagame might account for much of the decline in malaria in the small Great Lakes country.
Mosquitoes and garbage (and land degradation and global warming)
Martins is not the only person to worry about mosquitoes and garbage. In an opinion piece in The New York Times, Professor Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said, “The world has changed dramatically in the past 40 years with regard to increasing the habitat for Aedes aegypti breeding. An explosion of plastic and rubber solid waste now litters virtually all parts of the globe, particularly in the developing world. We must clean up the garbage to have any hope of reducing Zika infections in humans.”
Martins is also not alone in regarding land degradation as key to increasing mosquito populations, largely because of its impact on biodiversity. Tor Vagen is a geoscientist who works at the World Agroforestry Centre and grew up on the Ethiopian border. Like Martins, Vagen has dedicated his life to the geology and biology of the region.
“When you have erosion on upper catchments,” Vagen said, “you tend to see heavy sediment loading into wetlands, which can cause them to silt up completely. Instead of shallow but clear water where fish and insects swim eating mosquito larvae, you get stagnant water. With degradation of wetlands, you knock out a lot of predators and the mosquito population explodes.”
Global warming is unhelpful too. Mosquito species that typically occurred in lower-altitude, warmer areas are able now to survive in high-altitude areas, leading to malaria and other diseases. A 1935 British government guide instructed English settlers to sleep under a mosquito net only below 5,000 feet. Yet Martins has found the mosquito which transmits malaria at 6,400 feet.
The same guide recommends “the correct siting of servants quarters and labour lines.” Colonial medicine was primarily interested in European health, often seeing local people as hosts of disease. But with no pesticides at their disposal, Colonial Malaria Research Officers were at least acutely aware of the significance of breeding sites. In the 1930s in Zanzibar, a Dr. McCarthy instructed that no bottles, tins, broken pieces of earthenware, coconut shell, gourds or scrap metal be left lying around, and he introduced larvivorous fish into mosque tanks and “other places where water was kept permanently,” writes historian Amina Issa.
“Colonial mosquito control was top down and often aggressive,” says Martins. “That is not what we need. But what we can take from it is the ground monitoring by people who knew how to identify mosquito larvae. As scientists we have to share how to do this. Then we use technology to crowd source and make mosquito surveillance citizen science.”
Citizen science
Martins has already used citizen science to reduce copious and indiscriminate use of chemicals against ticks. “In northern Kenya, ranchers and pastoralists must control ticks because they are vectors for East Coast Fever in livestock and tick fevers in humans. So the minute they see any tick, they douse the cows, burn the grasslands. This is a huge expense for them and has a huge impact on birds.”
But Martins and colleagues discovered that only a very small number of tick species are major transmitters of diseases. “Now the ranchers and pastoralists know this, they only dip and burn when disease-bearing ticks are present. We have limited the evolution of resistance, reduced the amount of chemicals being used and the costs to the ranchers and pastoralists and gained a better ecosystem with tick eating birds and other species that naturally help control them.”
The Zika virus was first detected in Uganda in 1947. Its exact origins are unknown. But Martins said, “It is almost certainly African — not because Africa is a scary place but because humans evolved here. We have had a long evolutionary exposure, including to mosquitoes and other pathogens. There are just as many pathogens in South America or Asia but the likelihood there of crossing into our species is less.”
This is where good surveillance, archiving of data, and public commons scientific spaces are important, he concluded. “It is highly unlikely that the blood samples from when it was discovered have survived. Uganda has gone through problems and science has not been well supported. Yet this is a public health crisis. Having historical information and supporting scientists in developing countries now will solve a lot of problems in future.”
Cathy Watson is Chief of Programme Development at the World Agroforestry Centre in Nairobi, which is collaborating with Dr. Martins on the links between landscapes and human health. She can be reached at c.watson@cgiar.org. A shorter version of this article appeared in The Guardian.