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New Mosquito Identified in Africa

In Malaria on February 7, 2011 at 7:01 am

With distinct genetics and behavior, a novel subgroup of mosquitoes encourages scientists to rethink the fight against malaria

A new genetically distinct subgroup of mosquitoes has been identified in sub-Saharan Africa that displays different behaviors and has a higher susceptibility to the malaria parasite than the traditionally-studied type.

The finding, published online today (February 3) in Science, may provide a clue as to why malaria eradication in the dry sub-Sahara has proven so difficult.

Anopheles gambiae, the mosquito species studied
Image: Wikimedia commons, James D. Gathany

“This is a very thought-provoking paper,” said vector transmission biologist Carolina Barillas Mury of the National Institutes of Health, who was not involved in the research. “If this turns out to be true, it would mean that there might be other mosquitoes in this area — a subpopulation that has very different behavior — and we’re not finding them.”

“This is an area that has been worked relatively well by some of the most influential and excellent people in the field,” added population geneticist Tovi Lehmann of National Institute of Allergy and Infectious Diseases‘s Laboratory of Malaria and Vector Research, who was not involved in the study. “One possibility is, of course, that this subpopulation recently arrived to this area, but I’m unable to answer how it could have been missed altogether.”

Malaria is a devastating disease that kills more than 750,000 people each year, most of whom are children in sub-Saharan Africa. It is caused by Plasmodium protists, carried from person-to-person by female mosquitoes. After a mosquito takes a full meal, it rests for a few days while digesting and allowing its eggs to develop. During this stage, researchers or health workers employ insecticides to kill the mosquitoes to study malaria transmission in the lab or to try to eradicate the disease locally.

Insecticide spraying is almost exclusively done inside of houses, as mosquitoes that feed indoors are more likely to have fed on humans and thus to carry malaria. It’s also much easier to collect them indoors: Just spray the room, throw down a sheet, and stand back to watch them fall. But previous eradication attempts failed despite intensive indoor spraying, leaving researchers wondering how the parasite could be so resilient.

While working on a project to map genes related to malaria susceptibility and resistance, vector biologist Ken Vernick of the Institut Pasteur and his team collected mosquito larvae from pools at 3 village sites in Burkina Faso, a small land-locked country in west Africa, as well as adult mosquitoes from village houses.

Once they started looking at the genotypes, they noticed a curious pattern: While some of the mosquitoes raised from the larval pools had identical genotypes to the adults collected indoors, others formed a distinct genetic group that was not found in the indoor samples. This novel subgroup, which presumably rests outdoors after feeding and avoids traditional indoor pesticide sprays, was not a rare find: 57 percent of the total pool-collected larvae shared this genotype.

Since water is a limiting resource in the arid sub-Sahara, the larvae in the pool should represent the total population of mosquitoes in the area, Vernick said. Thus, the new subgroup of previously undescribed mosquitoes appears to be more abundant than the indoor variety, at least in the larval stage.

To measure the relative susceptibility to the malaria infection, Vernick’s team raised adult mosquitoes from larvae of both subgroups on blood donated by malaria patients. While 35 percent of the indoor group picked up the infection, 58 percent of the outdoor population became infected, indicating that this population is more susceptible to the Plasmodium parasite.

The finding raises concerns about how scientists study and control malaria in this part of Africa, said Vernick. “We need to rethink how we apply vector control in the fight against malaria because depending on the amount of contribution of this new form to the actual malaria transmission and on the abundance of this new form in different places, it may partly explain why we see variable results in different places.”

How much these more susceptible mosquitoes actually contribute to the spread of malaria, however, remains unclear. “I am convinced that there is some part of the population of the larvae that produces mosquitoes that never enter the houses,” said Marcelo Jacobs-Lorena, a vector biologist at John Hopkins’s Malaria Research Institute not involved in the research. “But even though their ability to be infected is high, the actual proportion that are actually infected in the field is unknown.”

Riehle, M.M. et al., “A Cryptic Subgroup of Anopheles gambiae Is Highly Susceptible to Human Malaria Parasite,” Science, DOI: 10.1126/science.1196759, 2011

Read more: New mosquito identified – The Scientist – Magazine of the Life Sciences http://www.the-scientist.com/news/display/57974/#ixzz1DH1uHIcL
Read more: New mosquito identified – The Scientist – Magazine of the Life Sciences http://www.the-scientist.com/news/display/57974/#ixzz1DH1no4Dh

 

 

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  1. from a well qualified entomoligist: “What is being discussed in the article and the discussion points relates to mosquitoes being endophagic (enter houses to feed) and others being exophagic (do not enter houses). We have long known about such distinct behaviors and those behaviors are basic descriptors of malaria vector efficiency. The whole discussion is not new, although the excitement of researchers might suggest that this is new.

    If you go back to the era of the Garki Project (a WHO project conducted in the late 1960s), you find that researchers decided to use a non DDT insecticide in the project because they found that malaria vectors, in presence of DDT, became much more exophagic. Their interpretation was that a dominance of exophagic behavior detracted from DDT’s efficacy. As it turned out, the substitute chemical proved highly ineffective in the control of malaria in the Garki Project. It was a major project that was supposedly designed to validate the utility of spraying insecticides to control malaria. Unfortunately the project used the wrong insecticide and allowed mosquitoes to enter houses and transmit malaria inside the houses. That demonstrated failure of spraying to control malaria was the basis for many people to begin campaigning against spraying insecticides for control of malaria (WHO’s Najera was one such person).

    The project was a huge fiasco based on an erroneous understanding of what actually happens when mosquitoes are denied access to hosts inside houses, which DDT did. There was never a finding that spraying DDT in the Garki region failed to exert control over malaria–it did but not to the level of malaria eradication. The yardstick for effectiveness in that era was total interdiction of malaria, and DDT could not perform at that level in that environment. However, there was clear evidence that exophagic behavior was more common when DDT was used. In a nutshell, exophagic behavior is a characteristic of an inefficient vector of malaria. Yet, it was certainly effective in control of the disease–and that is no small accomplishment.

    This whole issue of exophagic behavior was the fundamental reason that house spraying with DDT was never enough to effectively stop malaria in Central America. So, while malaria could not be stopped, it was greatly reduced by spraying. The two major species of malaria (falciparum versus vivax malaria) in Central America respond to control efforts differently. Under conditions of DDT spraying the more dangerous form of malaria essentially disappeared from Central America. In contrast, the less dangerous form which is efficiently transmitted by the exophagic vector (An. albimanus), persisted. However, through all those years of spraying the malaria problems were not great. Again, this is because exophagic transmission of malaria is not highly efficient.

    In summation, finding a new subpopulation that is primarily exophagic and slightly more susceptible to malaria than the endophagic populations does not translate into a new and alarming situation. First and foremost, we have decades of information attesting to the presence of exophagic behaviors of malaria vectors in Africa. The exophagic sub-population is not new, it is just newly discovered using new technologies. Based on old survey methodologies, we have known about those sub-populations for decades. Additionally, the finding does not signal any type of new and growing threat. Again, exophagic transmission of malaria is not highly efficient.”

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