Scientists from the London School of Hygiene and Tropical Medicine, have discovered a potentially dangerous new malaria-transmitting mosquito in Kenya.

The 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.

The scientists warn of risk that bed nets and insecticides could fail to fully protect the population.

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,” she said.

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 percent of mosquitoes caught were outdoors, the majority before 2230 hours.

Three hundred and forty-eight mosquitoes were identified using DNA sequencing techniques of which over 40 percent 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 Kenya, 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 had so far been seen only in one location in Kenya, it was essential that tourists still protected 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.

The London School of Hygiene & Tropical Medicine is a world-leading centre for research and postgraduate education in public and global health, with 4000 students and more than 1300 staff working in over 100 countries.

The School is one of the highest-rated research institutions in the UK, and was recently cited as one of the world's top universities for collaborative research. The School's mission is to improve health and health equity in the UK and worldwide; working in partnership to achieve excellence in public and global health research, education and translation of knowledge into policy and practice.