08
Jul
With 400,000 Malaria Deaths Worldwide, Insect Resistance to Mosquito Pesticides Calls for Urgent Need to Shift to Alternative Management Strategies
(Beyond Pesticides, July 8, 2020) Efforts to control the transmission of malaria are encountering a big, though predictable, problem: the mosquitoes that transmit malaria are developing resistance to at least five of the insecticides that have been central to limiting transmission of the disease. A study released in late June reveals a dramatic increase in resistance to pyrethroid insecticides and DDT across sub-Saharan Africa. This signals the failure of a mainstay chemical approach to the spread of malarial mosquitoes; this same problem — resistance — is happening with chemical management of agricultural pests and weeds, and with antibiotics to treat human bacterial infections. This study underscores a point Beyond Pesticides has made repeatedly: resistance to pesticides (whether insecticides, herbicides, biocides, fungicides, or medical antibiotics) is nearly inevitable. The solution to containing the spread of malaria lies not in the use of more and different chemicals, but in nontoxic approaches that respect nature and ecological balance.
Malaria is a sometimes deadly disease caused by female Anopheles mosquitoes infected with any of four varieties of the Plasmodium parasite. The disease kills roughly 400,000 people annually, with half that mortality in sub-Saharan Africa. The U.S. sees approximately 2,000 cases of malaria annually, primarily in people returning from countries in south Asia and sub-Saharan Africa that deal with consistent malaria threats.
In such regions, primary control strategies for these mosquito vectors during the past couple of decades have been the insecticidal treatment of bed nets (known as ITNs), and indoor residual spraying (IRS) of insecticides on walls, floors, ceilings, and eaves prior to the intensive malaria transmission season. The development of mosquito resistance to these insecticides means that existing control programs, which promote ITNs and/or IRS, are becoming far less effective. Over the course of the last two decades, deltamethrin and λ-cyhalothrin (synthetic pyrethroids), and DDT have been used for IRS, but the authors note that other classes of insecticides, such as carbamates and organophosphates, are increasingly being used for IRS.
It is noteworthy that DDT (dichlorodiphenyltrichloroethane) was used intensively as a malarial control (and for other purposes) from the 1940s through the 1960s. In the U.S., the compound was banned in 1972 because of its extreme persistence in and harm to the environment, and because it accumulates in fatty tissue in humans. Exposure to DDT and its breakdown products is linked with harms to the human reproductive, endocrine, and neurological systems, as well as to development of cancer, diabetes, and obesity. Although DDT is no longer manufactured or used in much of the world (China is currently the largest manufacturer), its use continues in 19 countries, and much of that is for mosquito control.
In 2018, Beyond Pesticides reported that “Rampant overuse [of DDT], both to control disease vectors and in agriculture, resulted in the development of significant resistance to the compound. Today, DDT resistance is widespread in Anopheles mosquitoes.” In 2017, Pesticide Action Network North America (PANNA) noted, “Of the 73 countries that provided monitoring data to WHO [the United Nations World Health Organization] from 2010 onward, 60 countries reported insect resistance to at least one insecticide and 50 reported resistance to 2 or more insecticides. This highlights the problem of relying on insecticide-based strategies for vector control. . . . Ultimately, disease vectors and parasites develop resistance to the insecticide and it becomes almost ineffective in the long run.”
The subject study comes out of the University of Oxford, and was published by PLOS Biology. Researchers and co-authors Catherine Moyes, PhD and Penelope Hancock, PhD analyzed a database of information on mosquitoes across the sub-Saharan region of Africa, mapping dates and locations of the rise of insecticide resistance in Anopheles gambiae mosquitoes. They then created modeling to quantify temporal and spatial trends in eastern and western regions of the continent. During the period from 2005 to 2017, their modeling found, mosquito mortality after exposure to pesticides dropped from nearly 100% to under 30% in some regions, and the geographic spread of such resistance grew.
West Africa showed drastic increases in resistance to all synthetic pyrethroids. For example, in 2005, mean mortality to deltamethrin was below the WHO (World Health Organization) threshold for confirmed resistance across 15% of the region; by 2017, that figure rose to 98%. East Africa has seen a real, though somewhat less dramatic, increase in resistance to pyrethroids, with an analogous rise in spread during the same period from 9% to 45% of the region. DDT resistance was more widespread in 2005 than was resistance to pyrethroids, but it, too, showed progression. In the west in 2005, confirmed resistance (as defined by mortality below the WHO threshold) to DDT was found in 53% of the territory, and rose to a spread across 97% of the area by 2017.
The researchers assert that, although their modeling included more than 100 predictor variables that might influence selection for resistance, “it is unlikely that we have captured the full set of causal variables underlying selection. In particular, data on the quantities of insecticides used in agriculture, and where they were applied, were not available. Such information would better inform predictive relationships between resistance and agricultural insecticide use. We note that the relationships between insecticide resistance and the predictor variables represented in our models do not prove causality. Each variable interacts with other variables . . . and possibly with variables not included in our analysis.” This acknowledgment points to a potential role for agricultural use of pesticides in the resistance scenario for malarial mosquitoes.
The issue of resistance is growing in agriculture, in medicine, and in other sectors (such as mosquito control) in which humans hope to quell the advance of organisms that harm people and critical supports for human life, such as food and medical care. In all these areas, the “fixes” on which people have come to depend, whether antibiotics for bacterial infections, or pesticides to beat back weed and insect pests, or insecticides to try to prevent vector-borne diseases, are increasingly failing as organisms develop resistance to compounds that would thwart them. PANNA notes, “The World Health Organization underscored the problem in their 2012 guidance on policy making for Integrated Vector Management (IVM): ‘Resistance to insecticides is an increasing problem in vector control because of the reliance on chemical control and expanding operations . . . Furthermore, the chemical insecticides used can have adverse effects on health and the environment.’”
Development of resistance is an entirely normal, adaptive phenomenon: organisms evolve, “exploiting” beneficial genetic mutations that give them survival advantage. For nearly a century, human response to this has been primarily a chemical “chasing” of such evolutionary changes — developing a compound that kills the offending organism (whether pest or weed or bacterium or fungus) for a while. Organisms nearly inevitably change to become resistant to that particular chemical assault, whereupon people — the chemical industry, researchers, applicators, farmers, public health workers, clinicians, et al. — have typically moved on to the next chemical “solution.”
Beyond Pesticides has written extensively on the issue of resistance, particularly as it relates to the use of agricultural and other land-management pesticides, with the central message: resistance is a symptom of the ineffectiveness of chemical-intensive agriculture, and leads to increased use of more, and more-toxic, pesticides. In addition, resistance in one of the “sectors” mentioned above can “cross over” to become problematic in another. Agricultural and veterinary uses of antibiotics, for example, have contributed very significantly to the problem of resistance of certain bacteria or fungi to antibiotics that have historically knocked down such infections in humans. Examples include familiar drug names: penicillin, vancomycin, azithromycin, and fluconazole — all of which have become less and less effective as pathogens have developed resistance to them.
Transmission of malaria is a problem that needs solutions far less reliant on intensive chemical treatments, especially noxious DDT. PANNA has endorsed the WHO concept of Integrated Vector Management (IVM), writing: “Vector control relying on a community-based, least-toxic version of Integrated Vector Management (IVM) has proved to be much more effective in the long run in controlling mosquito populations and the diseases they transmit. . . . When communities are at the forefront of active mosquito control and malaria management efforts — using the least toxic, yet very effective IVM methods — significant reductions in malaria incidences can be observed. . . . Given the effective alternatives to using hazardous pesticides, it is essential that governments around the world focus on supporting IVM strategies that are least toxic and can be sustained over the long term through community leadership and participation.”
A decade ago, Pesticide Action Network Germany published on its website, “Control malaria without DDT! There are more options than currently used” — a call to using no- or low-risk approaches. Among the alternative strategies it identifies are:
• prevention measures that eliminate mosquito breeding sites (any standing water), such as leveling land to eliminate water-catching depressions, clearing vegetation, removing trash, and planting trees; use of ecosystem-compatible predators and nematodes; use of bacterial and/or botanical larvicides; improvements in home/building sanitation and structural integrity; personal protection, such as long-sleeved shirts, botanical repellants, and mosquito nets and screens
• pathogen control, including medicinal herbs, chemoprophylaxis, and vaccination when available
• vector controls, such as mosquito traps and targets; pyrethroids if necessary; and use of fungi
Research published in 2018 demonstrated yet another potential approach to controlling malaria transmission: exposing A. gambiae to specific antimalarial compounds via treated surfaces (such as bed nets). When the mosquitoes take up low concentrations of an antimalarial drug prior to or shortly after infection by the Plasmodium parasite, the drug causes “full parasite arrest in the midgut, and prevents transmission of infection,” the research found. Of course, antimalarial drugs are subject to the same resistance development as any other chemical approach.
Even if malaria is not a local concern, most people are concerned about the diseases mosquitoes can transmit, including West Nile virus, Eastern Equine Encephalitis, and Zika fever. Beyond Pesticides provides useful information on mosquito management and insect-borne diseases on a section of its website devoted to these issues.
Beyond Pesticides advocates alternatives to chemical approaches. The most successful malaria control programs combine a variety of strategies with community education, and require government commitment and political will. For example, Vietnam reduced malaria deaths by 97% and malaria cases by 59% when it switched in 1991 from trying to eradicate malaria using DDT to a DDT-free malaria control program involving distribution of drugs and mosquito nets, along with widespread health education organized with village leaders. A program in central Kenya focuses on reducing malaria by working with the rice-growing community to improve water management. The program also involves using livestock as bait, introducing biological controls, and distributing mosquito nets in affected areas.
Beyond Pesticides maintains that management strategies to combat insect-borne malaria cannot be successful if they are based on chemical-intensive strategies that ignore the underlying conditions that exacerbate the spread of the disease. Jay Feldman, executive director of Beyond Pesticides, has noted, “We should be advocating for a just world where we no longer treat poverty and development with poisonous band-aids, but join together to address the root causes of insect-borne disease, because the chemical-dependent alternatives are ultimately deadly for everyone.” He also said, “We should focus on the deplorable living conditions, and inequitable distribution of wealth and resources worldwide that give rise to squalor, inhumane living conditions, and the poor state of development that, together, breed insect-borne diseases like malaria.”
Even if malaria is not a local concern, most people are concerned about the diseases mosquitoes can transmit, including West Nile virus, Eastern Equine Encephalitis, and Zika fever. Beyond Pesticides provides useful information on mosquito management and insect-borne diseases on a section of its website devoted to these issues.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources: https://www.courthousenews.com/scientists-track-pesticide-resistance-in-malaria-carrying-mosquitoes/amp/; https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3000633#sec001
