Anti-mosquito drives stumble at final ten per cent

[DAR ES SALAAM] Eliminating malaria by controlling mosquitoes may prove impossible unless control programmes consider that mosquitoes reproduce faster as their numbers dwindle, says a study.

Insecticide-treated nets (ITNs) have reduced mosquito populations by 90 per cent in countries like Tanzania, said Gerry Killeen, a researcher at the country’s Ifakara Health Institute.

But this is probably the limit of what ITNs can achieve, he added — and achieving the last ten per cent of control may be as difficult as the first 90 per cent because sparse mosquito populations have higher individual fitness.

Killeen conducted field studies in Tanzania — with colleagues from Australia and the United Kingdom — and found that, as mosquito numbers go down, their ability to reproduce faster increases. Mosquito larvae experience less competition when population size is low, enabling each individual to grow larger in size and become more fertile.

The findings reinforce the need to recognise the limitations of current control methods, said Killeen.

“Researchers and field workers need to start working on killing mosquitoes that feed or rest outdoors or, even better, to kill them in the aquatic breeding sites they originate from,” Killeen told SciDev.Net.

Many of these complementary strategies to supplement ITNs in mosquito control are being pioneered by young African innovators, he said, adding that the Ifakara Health Institute is working on a follow-up to the study.

But Killeen cautioned that their paper applies only to the last push to eliminate mosquitoes — conventional control methods like ITNs are essential to reduce numbers in the first place.

“This study has little influence on immediate implementation priorities and strategies for ITNs or indoor residual spraying — these are extremely effective and have delivered huge gains, and will remain a priority for many years to come,” he said.

Nick Brown, cell team leader at the National Malaria Control Programme, Tanzania, said that the key objective for malaria control, and eventually elimination, should be to reduce, and eventually halt, transmission of the parasite, rather than eliminating the vector.

“If mosquitoes are prevented from securing a blood meal from an infected person or transmitting the parasite further, because most people are protected by nets, then this represents a significant reduction in the possibility of transmission,” he said.

The study was published in Proceedings of the Royal Society B earlier this month (9 March).

Link to full paper in the Proceedings of the Royal Society B

Source – SciDev.Net – Masembe Tambwe – 25 Mar 2011