Africa: More Resistance to the Treated Mosquito Nets

Kakaire Kirunda | 11 Jun 2008
Monitor (Kampala)

As the fight against malaria gains ground, studies are showing that mosquitoes that transmit the disease are beginning to gain resistance against the commonly used pyrethroid insecticides. "Generally, the results showed a trend of increase in mosquito resistance status with cross-resistance against all the three pyrethroid insecticides.

This study reveals for the first time the development of pyrethroid resistance in malaria vector Anopheles gambiae s.l. in Western Uganda," according to the findings published in the Malaria Journal on May 26.

The study was conducted by three scientists from the Fort Portal-based Mountains of the Moon University, the Medical Department of Kabarole District and the Medical Research Council in Entebbe.

The trio embarked on the study at a time when Pyrethroid insecticide-treated mosquito nets were massively being scaled-up for malaria prevention particularly in children under five years of age and pregnant mothers in Sub-Saharan Africa.

But this is amid serious concern of the likely evolution of widespread pyrethroid resistance in the malaria transmitting Anopheles gambiae s.l. due to the extensive use of pyrethroid insecticide-treated mosquito nets.

It was against this backdrop that researchers John Rubaihayo, Ephraim Tukesiga and Andrew Abaasa sought to ascertain the status of pyrethroid resistance in malaria causing mosquitoes in western Uganda. Being cost-effective, highly insecticidal at low dosage and highly biodegradable with low mammalian toxicity, pyrethroid insecticides were recommended by WHO Pesticides Evaluation Scheme (Whopes) for large-scale use on mosquito bed nets for malaria prevention.

For purposes of the western Ugandan study in Kamwenge District, wild mosquitoes (1-2 days old) were exposed in 10 replicates to new nets impregnated with K-othrine, Solfac EW50 and Fendona 6SC and observed under normal room temperature and humidity.

A similar set of mosquitoes collected from the control area 80 km away were exposed to a deltamethrin 25mg/m2 impregnated net at the same time and under the same conditions. The 10-year mean knockdown time and mortality rates for each of the three pyrethroid insecticides were then compared.


The study findings indicated that it was taking longer than it should for the mosquitoes to be killed after coming into contact with the insecticide treated nets. The overall results showed that it was now taking four times longer for the insects to succumb, an indication of reduced susceptibility to pyrethroid insecticides.

Following the findings, the study authors are now recommending that the impact of these developments on malaria control efforts be closely monitored and alternative fabric treatments considered before this problem curtails community wide implementation of this malaria control strategy in Uganda.

The findings come at a time when malaria is still the leading cause of childhood and maternal ill health and death in the country, according to WHO.

And statistics from the Ministry of Health show that the burden of malaria is unacceptably high, accounting for 20-40 percent of outpatient attendance at health facilities, 14 percent of in-patient deaths and 20-23 percent of childhood mortality.

It is further estimated that there are 70,000-110,000 malaria-specific deaths every year and $24.7 is being spent on each individual per annum to treat malaria. Matters have not been helped by the evolution of both drug and insecticide resistance to the commonly used anti-malarials and insecticides respectively.

Citing the study Malaria: Current and future prospects for control, the authors of the western Uganda research noted that a number of novel technologies have been developed to combat malaria including insecticide treated nets and malaria vaccines but the vaccines have shown limited efficacy and need further development.