Practical and theoretical advances in the field of mosquito and malaria research

We report here advances being made in the field of mosquito and malaria research being made by Dr. Glyn Vale (SACEMA's Visiting Research Fellow based in Harare, Zimbabwe) and colleagues in the UK and elsewhere in Africa. We provide here the reference for the paper:

Torr SJ, Della Torre A, Calzetta M, Costantini, C and Vale, GA (2008). Towards a fuller understanding of mosquito behaviour: use of electrocuting grids to compare the odour orientated responses of Anopheles arabiensis and An. quadriannulatus in the field. Medical and Veterinary Entomology 22, 93-108.

In the words of Dr Vale the paper shows that:

"a fuller analysis of mosquito behaviour seems required, particularly to explore the prospects for refining the existing methods of bait control and surveys, and perhaps for developing new techniques for sampling and controlling mosquitoes and malaria".

In this regard the following remarks prepared, which provide a background to the above paper, seem to us of great interest.

Balance in Malaria research

We report here the results of some innovative work on mosquito behaviour, much of it carried out over the past three years at Rekomitjie Research Station in the Zambezi Valley of Zimbabwe. The work was initiated by Dr Glyn Vale who developed, forty years ago, the use of electrocuting grids for the study of the tsetse flies, Glossina spp. That development led to a revolution in the understanding of tsetse behaviour and to the way we think about the sampling of tsetse. It also led ultimately to the development of entirely new systems of efficient, cost-effective, more environmentally friendly methods tsetse control - based on the importance odour for tsetse in host location.

Because the importance of odour - and because odour is clearly important in host location by mosquitoes - Dr Vale has, over the years, tried in vain to interest the world of malaria research in the use of electric nets to study mosquitoes. Until recently nobody has picked up the ball and run with it; and as a consequence the important advances made in Zimbabwe in the late 1960s have never been made in the world of mosquito research - and that field is, as a consequence, 40 years behind tsetse research in Zimbabwe in simple terms of entomological understanding.

Dr. Vale and his colleagues are now taking the first strides in trying to drag mosquito research towards the 21st century - and their first paper is an eye-opener. What becomes apparent is that, as Vale has pointed out repeatedly over the years, the confusion in earlier trapping devices between the trapping device and the attractant used to bring mosquitoes to that device made it impossible to tease the components of the mosquito's behaviour. These observations, which have important synergies with the early advances that Vale made in tsetse behavioural research in the 1960s and 70s, lead to us to think that there may be important advantages to pursuing this type of research in the field of mosquito and malaria research.

We need to proceed carefully in this regard, particularly because it is clear that there are important distinctions between mosquitoes (and how they transmit malaria) and tsetse (and how they transmit trypanosomiasis).

For example, mosquitoes breed relatively quickly, are seasonal and diurnal, and only the females take blood. There are safe drugs for prevention and cure of malaria, but only curative drugs are available for human trypanosomiasis, and they are dangerous. However, there are many similarities too. Both diseases are caused by protozoans; each insect responds to host odours in a chain of responses covering activation, navigation to the host, alighting on it and feeding. Moreover, the insecticide-treated bednets used to kill mosquitoes are somewhat analogous to the ad hoc sticky screens that caught tsetse on the backs of men in 1910, and which were the forerunners of the more efficient bait methods used to control tsetse today.

The similarities ensure that scientists concerned with tsetse and trypanosomiasis are anxious about the apparent priorities in current research with malaria and mosquitoes. The great push towards developing a malaria vaccine is reminiscent of the efforts in the 1970s and 80s to produce a vaccine against trypanosomiasis. The longer that research progressed the disproportionately longer became the estimated time of delivery, and today the interest in a vaccine has largely evaporated. While the tsetse workers wish laboratory workers the heartiest success in vaccine development, they believe that studies of mosquito behaviour deserve enhanced encouragement.

Superficially, such beliefs might appear a little strange, perhaps even eccentric - given that mosquito behaviour has been worked over for many decades. However, in the eyes of tsetse researchers the current understanding of mosquito behaviour seems little better than for tsetse 50 years ago. Most of the data then were for catches in traps or hand-net catches beside live baits, i. e., for the apparent numbers of tsetse at or near the end of the chain of bait-orientated responses. Such catches cannot be used to analyse behaviour unless one knows also about the insects that did not reach the end; if behaviour is not analysed, the full scope for its practical exploitation cannot be assessed.

The greatest aids to the analysis of tsetse behaviour were electrocuting grids to catch the insects in various stages of response to baits. Such grids have been used occasionally by mosquito scientists, but mainly to confirm the reasonable expectation that the insects travel upwind when attracted to odour from a distance. The grids can achieve much more. This was exemplified, as a goodwill gesture between disciplines, by work performed as a brief side-line at Rekomitjie Tsetse Research Station, Zimbabwe, and reported in the attached article. Some points to emerge were that traps can give misleading indications for the numbers of mosquitoes attracted to them from a distance, and for the relative efficacy of different odours; the characterization of Anopheles arabiensis as anthropophilic or zoophilic depends on precisely how the insects are sampled.