Does good-quality housing protect against malaria

News article 1 Apr 2014
12

The LSTM Seminar Series continued on Friday with a presentation delivered by Professor Steve Lindsay, from the School of Biological and Biomedical Sciences at the University of Durham. His seminar entitled “Does good-quality housing protect against malaria?” was introduced by LSTM’s Dr David Weetman.

As an historical precedent for the value of mosquito-proofing houses, Prof. Lindsay highlighted the pioneering studies of Angelo Celli in Rome at the turn of the 20th century. By screening houses to prevent mosquito entry of some railway workers, but not others, Celli,  starting from a pilot study which was subsequently expanded, demonstrated a dramatic reduction in malaria, providing proof of mosquito bites in the home as the primary source of malaria. Such findings still ring true in sub-Saharan Africa with an estimated 80% of infectious bites occurring in houses. In Africa, many homes have open eaves, which is of particular significance because Anopheles gambiae, the main malaria vector, display the unusual behaviour of upward flight (which would orient them toward eaves) when they come to a flat surface. But is house entry still a key factor in malaria transmission?

Clearly, contemporary evidence was required and Prof. Lindsay described a two-year trial carried out in The Gambia. His team, with field work led by Matt Kirby, carried out a three-armed trial involving 462 homes, comprising of (a) houses which were not screened, (b) houses having a ceiling net and (c) the other group of houses being fully screened (doors, windows, and eaves). The study recorded the number of mosquitoes (and also flies) that were found in the homes at regular intervals and, as a symptomatic malaria index, anaemia was recorded in children at the end of transmission seasons. When the data were analysed it was found that those houses with screened ceilings had up to 50% fewer mosquitoes collected, and that houses that were fully screened had around 62% fewer. In terms of clinical results, the inhabitants of those houses subject to either form of screening suffered about half as many cases of anaemia as those from the control houses. The study, subsequently published in The Lancet concluded that screening prevents house-entry by mosquitoes and thus can dramatically reduce infectious bites received in the home. Perhaps a telling observation is that at the end of the trial all those who took part were offered a choice of each of the three trial options: the majority of householders opted for full screening.

At the time the trial was conducted, population coverage of insecticide treated bednets (ITNs) and long-lasting insecticidal nets (LLINs) in the trial villages was relatively low, but in The Gambia and elsewhere in sub-Saharan Africa, coverage has increased dramatically. This widespread availability is linked as a major causal factor in the 25% fall in malaria during the last decade. However, Prof. Lindsay highlighted the growing spectre of insectide resistance, a key topic of research for LSTM, and in particular the threat to ITN/ LLIN efficacy which depends on only one class of insecticide (the pyrethroids), that are safe enough for close human contact. The Gambia trial involved untreated screening materials, but could benefit further from the use of insecticide treatment, and unlike bednets could utilise a full spectrum of available insecticides, which might help to combat resistance.

Prof. Lindsay looked at trends in African housing and, in particular the widespread shift away from thatched to metal roofing, which reduces, but by no means excludes chances for mosquito entry. He concluded by looking at how to exploit future opportunities that housing changes could present for vector control. Whilst acknowledging that treated nets and also indoor residual spraying should remain the cornerstones of control, over the coming years housing reforms could also play a major part With many African economies upwardly mobile, widespread housing improvements are both a current and future reality. With appropriate anti-mosquito designs or modifications, better housing offers substantial hope to continue the current downward trends in malaria rates.