London School of Hygiene & Tropical Medicine

A potentially dangerous new malaria-transmitting mosquito has been discovered in Kenya by scientists from the London School of Hygiene & Tropical Medicine. This species, which has never been implicated in the transmission of malaria before, poses a threat because it bites humans at times when they are not protected by current malaria control techniques.

The commonly caught Anopheles mosquitoes that transmit malaria in Africa generally prefer to rest indoors and feed on humans at night. This led to the development of programmes to stop the spread of malaria such as spraying insecticide in homes and issuing bed nets for people to sleep under. However, this mosquito was found to be active outdoors and bite people earlier in the evening soon after sunset.

Lead author Jennifer Stevenson, Research Fellow at the London School of Hygiene & Tropical Medicine, said: “We observed that many mosquitoes we caught, including those infected with malaria, did not physically resemble other known malaria mosquitoes. Analysis indicated that their DNA differed from sequences available for known malaria-transmitting mosquitoes in Africa. 

"These unidentified mosquitoes are potentially dangerous because they are outdoor-active and early-biting, and so may evade the current indoor-based interventions to control mosquitoes. In this way, they may prevent the complete suppression of malaria transmission in the area.”

The study outlines how researchers set up indoor and outdoor mosquito traps in a village in Kisii in the highlands of western Kenya, an area with seasonal and unstable malaria transmission. Over 65% of mosquitoes caught were outdoors, the majority before 10.30pm. 348 mosquitoes were identified using DNA sequencing techniques of which over 40% were found to be of this unidentified species. Five mosquitoes of this species were carrying malaria parasites and two had fed on humans.  

The researchers are now calling for increased entomological surveillance and a focus on integrating a wider range of malaria control tools to deal with the threat of outdoor transmission. Malaria is the leading cause of death in the country, with 25 million out of a population of 34 million Kenyans at risk of the disease.*

Jo Lines, Reader in Malaria Control and Vector Biology at the London School of Hygiene & Tropical Medicine and former coordinator for the World Health Organization’s Global Malaria Programme, said: “These findings remind us that the basic biology of malaria transmission is subtle and complex: there are still plenty of gaps in our knowledge, and local variations that we do not understand. We do not yet know what these unidentified specimens are, or whether they are acting as vectors on a wider scale, but in the study area they are clearly playing a major and previously unsuspected role.    

“The practical implication for malaria control programmes is that there is no substitute for careful monitoring of mosquito populations.  In order to be effective, such monitoring must be carried out by specialist experts who have the skills to recognise and investigate unexpected entomological observations.”

The researchers added that as these mosquitoes have so far been seen only in one location in Kenya, it is essential that tourists still protect themselves with a mosquito net treated with a long-lasting insecticidal treatment whilst travelling.

The research was carried out in collaboration with the Kenya Medical Research Institute under the Malaria Transmission Consortium, funded by the Bill and Melinda Gates Foundation. The findings are reported in Emerging Infectious Diseases.

• Novel Vectors of Malaria Parasites in the Western Highlands of Kenya. Jennifer Stevenson, Mary K. Cooke, Jonathan Cox, Chris Drakeley (London School of Hygiene & Tropical Medicine); Brandyce St. Laurent, Neil F. Lobo (University of Notre Dame); Samuel C. Kahindi, Robin M. Oriango (Kenya Medical Research Institute, Centre for Global Health Research, Kisumu); Ralph E. Harbach (Natural History Museum, London).  Emerging Infectious Diseases. DOI: http://dx.doi.org/10.3201/eid1809.120283

* Statistics from Kemri.org