Insecticide-treated nets and treatment service: a trial using public and private sector channels in rural United Republic of Tanzania.

The Rotary Net Initiative, implemented in Kilombero District, southern United Republic of Tanzania, allowed us to explore different sales channels for the distribution of insecticide-treated nets (ITNs) and the insecticide treatment service in a rural area of very high malaria transmission. Several types of ITNs were promoted and sold through different channels in the public and private sector, i.e. hospital pharmacy, mother and child health (MCH) clinic, net committee, village health workers and retail shops. The ITNs were sold for US$ 5.0-9.2, with profit margins of 9-16%. Net treatment cost US$ 0.33, with commission fees of 75%. Net transport and treatment were partially subsidized. Some outlets established their own fund by ITN sales. Sales of nets and treatments were seasonal, and certain net types were preferred. Demand for insecticide treatment was generally low. Changes in net coverage were assessed in two villages. A range of outlet features were compared qualitatively. Our experience supports suggestions that ITN technology should be delivered through MCH care services and demonstrates that specific promotion and innovation are necessary to achieve substantial net treatment levels. A large-scale ITN project in the same area and other ITN studies should lead to better understanding of ITN implementation at the population level.

Variability in the relationship between weight and wing length of Anopheles gambiae (Diptera: Culicidae).

The relationship between wing length and body weight of female Anopheles gambiae Giles reared at 3 densities and at 3 temperatures was examined. Although overall, weight was proportional to wing length raised to the 4th power, the relationship within treatments was linear. The slope of the regression line varied significantly among treatments. Therefore, wing length of mosquitoes caught from field populations may not be an adequate measure of body weight if the conditions under which the mosquitoes have developed are not known.

Effects of adult body size on fecundity and the pre-gravid rate of Anopheles gambiae females in Tanzania.

The influence of adult body size on the pre-gravid state and fecundity was studied in Anopheles gambiae Giles females hand-caught inside houses and virgin females collected as pupae in Tanzania. Blood-fed mosquitoes were kept for 2-3 days before dissection and examination for insemination and ovarian condition. Those females which did not develop eggs were classified as pre-gravid. The number of mature eggs in those mosquitoes which became gravid was counted. Virgin females were fed and kept for egg maturation in the laboratory. Wing-length of females was measured as an index of mosquito size. The overall pre-gravid rate in the resting An.gambiae population was found to be 21% and, of these, 66% had been inseminated. In the virgin females the pre-gravid rate was 92.6%. The mean wing-length of wild females which became gravid was significantly larger than those which remained pre-gravid. There was a positive correlation between fecundity and wing-length. Smaller females tended to require two or three bloodmeals to facilitate completion of the first gonotrophic cycle. The critical size permitting oviposition from the first blood-meal was a wing-length of 3 mm.

Relationship between body size of adult Anopheles gambiae s.l. and infection with the malaria parasite Plasmodium falciparum.

The influence of adult female body size of Anopheles gambiae s.l. on development of midgut and salivary gland infections by the parasite Plasmodium falciparum was investigated in a field study carried out in Tanzania. The proportion of mosquitoes infected during a blood meal was independent of size. However, the number of oocysts harboured by infected mosquitoes increased with size of the mosquito. The proportion of mosquitoes with sporozoites, and thus potentially infective to humans, was highest in intermediate-sized mosquitoes, whereas the largest and smallest mosquitoes were less likely to have sporozoites. This pattern is interpreted as a combination of high survival rate of large, uninfected mosquitoes and of low survival rate of mosquitoes infected with many oocysts.

Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. Part 3. Effects on the prevalence of malaria parasitaemia and fever.

Children aged 1-10 in five villages were contacted fortnightly. Their axillary temperatures, reports of fevers and blood slides were taken. Following the introduction of permethrin impregnated nets into two estate villages the slide positivity for falciparum malaria declined markedly. In traditional villages the introduction of impregnated nets had less convincing effects than in the estate villages and DDT spraying had no perceptible effect on malaria. Over all villages there was a clear relationship between axillary temperature greater than 37.4 degrees C, reports of fever and high parasitaemia. We defined malaria fever in this way, and found in some cases significant reductions in occurrence of such fever following some time after introduction of permethrin impregnated nets. No such effects were found with lambdacyhalothrin nets or with DDT spraying.

Human malaria infectiousness measured by age-specific sporozoite rates in Anopheles gambiae in Tanzania.

In an area of holoendemic malaria in Northern Tanzania, Anopheles gambiae s.l. females were age-graded by Polovodova's method and dissected for sporozoites. Age-specific sporozoite rates implied that mosquitoes acquired new infections at all ages. The extrinsic period lasted just over 3 gonotrophic cycles (9-11 days). Very high sporozoite rates in the oldest females implied the absence or rarity of genetic refractoriness to infection. A method is described for estimating the proportion of bloodmeals which result in mosquito infection. This method makes relatively few assumptions about mosquito behaviour, and could be useful for evaluating transmission-blocking interventions. Overall, it is estimated that about 21% of meals are infectious. This is much higher than previous estimates derived either from experimental mosquito feeding studies or from similar age-grading data collected from the same area in 1962. Various alternative explanations are considered, and it is concluded that there has been a 2.5-fold increase in human infectiousness in the last 25 years. This is partly attributable to suppression of human infectiousness by widespread chloroquine usage during the 1960s, followed by removal of this effect by drug resistance. It is argued that chloroquine would be expected to select for increased infectivity in the parasite, and this may also have contributed to the observed increase.

Septic tank mosquitoes: competition between species in central Nigeria.

Exit traps, placed over the air vents of septic tanks, were used to examine species diversity and relative abundance of mosquitoes breeding in ammonia-rich waters of septic tanks. Of the six species found, Culex decens Theobald and Culex cinereus Theobald appeared to be competing successfully with Culex quinquefasciatus Say during the wet season but not during the long dry season. The seasonal timing of their displacement by Cx quinquefasciatus was variable and did not correlate well with climatic factors. The three other species present, generally during the wet season and early dry season, were Culex tigripes G. & C., Culex horridus Edwards and Aedes aegypt (L.). Experimental bucket ovitraps were used to assess preference towards covered (dark) septic tank water in comparison with sunlit septic tank water, covered and sunlit compost water. These were colonized by Cx quinquefasciatus, Cx decens, Ae. aegypti and Ae. vittatus Bigot. The covered septic tank water was more abundantly colonized by Cx quinquefasciatus and marginally so by the two Aedes species. Cx decens appeared to colonize the exposed compost water more readily in the dry season, but changed to the covered septic tank water in the wet season. The discussion centres around competition between these mosquitos species and concludes that it would be useful to know what environmental factors, or what aspects of competition, lead to severe natural reductions in the abundance of the major pest species Cx quinquefasciatus.