New mosquito identified

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 linkurl:finding,;http://www.sciencemag.org/content/331/6017/596 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 linkurl:Carolina Barillas Mury;http://www.niaid.nih.gov/labsandresources/labs/aboutlabs/lmvr/mosquitoimmunityvectorcompetenceunit/Pages/barillasMury.aspx 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 linkurl:Tovi Lehmann;http://www.niaid.nih.gov/labsandresources/labs/aboutlabs/lmvr/internationalstudiesmalariaentomologysection/Pages/gwadz.aspx 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 linkurl:Ken Vernick;http://f1000.com/thefaculty/member/3869956824422204 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 linkurl:Marcelo Jacobs-Lorena,;http://faculty.jhsph.edu/default.cfm?faculty_id=659 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
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[20th March 2007]