Insecticidal nets do not always work against diseases transmitted by insect bites

Longlasting insecticidal nets yielded an important breakthrough in malaria prevention, but this does not automatically mean they always work against diseases transmitted by insect bites. Against the transmission of kala azar disease in India and Nepal they did not have an effect. This was reported by an international group of researchers, led by Marleen Boelaert of the Institute of Tropical Medicine Antwerp, in the reputed British Medical Journal.

Kala-azar, or visceral leishmaniasis in doctor's speak, affects half a million people annually. The Leshmania parasite, in fact a group of related parasites, is transmitted by sand flies. The parasite destroys your blood cells, leading to an enlarged spleen, inflammation and progressive wasting. If left untreated, the outcome is fatal.

Until now, in India and Nepal sand flies are controlled by indoor spraying of DDT or other insecticides; some families use classical mosquito nets, which are not treated with longlasting insecticides. Spraying happens local and irregular, which means at any moment sufficient sand flies and prey remain to continue the disease. As an alternative for DDT, a large scale campaign was proposed, providing everyone in a region with a mosquito net treated with insecticide that remains active for several years.

Such a campaign needs a large amount of money and effort, so the researchers of the Institute of Tropical medicine, together with colleagues from England, Switzerland, India and Nepal, first checked if it made sense. The prospects were good: in Sudan the approach had worked - but there the disease was transmitted by a different sand fly. And in Syria and Iran, the treated nets had helped against cutaneous leishmaniasis, caused by other species of the genus Leishmania. It was also known that the sand fly transmitting the disease on the Indian continent, often bites indoor and mostly at night. So sleeping under a net had to make sense.

However. The scientists followed twenty thousand people during two years, in 26 hamlets with a high incidence of visceral leishmaniasis, in India and Nepal. The hamlets were divided into pairs, which resembled each other as much as possible. Tossing a coin decided which hamlet continued as usual, and which would receive treated nets on top. All communities had agreed with this procedure beforehand. During the period of the study, about half the villages were sprayed under the routine national control program.

In the hamlets with extra mosquito nets the sand flies indoor were reduced by a quarter, but the number of infections was not significantly lower than in the control hamlets. Neither did the number of cases of leishmaniasis (luckily, not every infection leads to disease). To be precise: the disease risk went down with 1%, which could be just as well due to chance. Yet in the campaign-villages 90% of people slept under their nets for more than 80% of the nights, while in the control hamlets only 30% of people slept regularly under an (untreated) net.

In contrast, the number of malaria cases did significantly go down in the villages with the extra nets.

The most likely explanation is that sand flies bite more often outdoors than was previously assumed, where nets are not readily useable.

This study, the first on such a scale, shows that medicine shouldn't generalise its results too fast, and that it's always a good idea to do field research before launching grand scale campaigns. And that, for such a campaign against kala-azar to have effect, research into the behaviour of sand flies is urgently needed.