Simple one-and multi-generation models are used to show the essential features of the dynamics of selection for resistance to one insecticide or two unrelated insecticides in insect populations, part of which are unexposed to either compound. The use of a mixture appears advantageous provided that resistance is not fully dominant, but, if it is, linkage disequilibrium builds up rapidly and nullifies the advantage of the mixture.

Insecticide-treated bednets (ITNs) are commonly used as a means of personal protection from malaria transmission by anopheline mosquitoes (Diptera: Culicidae). Long-lasting insecticidal nets (LLINs) have special treatments intended to remain effective after many washes. The present trials assessed the efficacy and wash-resistance of several production batches of PermaNet (polyester net coated with polymer resin containing pyrethroid insecticide deltamethrin 55 mg ai/m2) against malaria vectors in Pakistan, Iran and Tanzania compared to ITNs conventionally treated with alphacypermethrin 15 or 20 mg ai/m2, or deltamethrin 25 or 50 mg ai/m2. Insecticidal efficacy of the nets before and after repeated washing (using W.H.O. recommended and traditional local washing procedures) was monitored through contact bioassays with Anopheles and by experimental hut and outdoor platform tests. Local washing regimes gradually reduced the insecticidal efficacy of conventionally treated nets, but they were not exhausted, even after 21 washes. Using a more rigorous laboratory washing method, insecticide was more readily stripped from conventionally treated nets. PermaNet retained high efficacy after 21 washes, giving more than 97% mortality of Anopheles in contact bioassays with 3-min exposure. Using the more sensitive bioassay criterion of 'median time to knockdown', PermaNet showed no loss of insecticidal activity against Anopheles after washing repeatedly in 2 out of 6 trials; whereas in a further three trials knockdown activity of PermaNet and conventional ITNs declined at comparable rates. Higher mortality levels of Anopheles in contact bioassays did not always translate to superiority in experimental hut or enclosed platform trials. In only one of four comparative field trials did PermaNet out-perform conventional ITNs after washing: this was in the trial of PermaNet 2.0--the product with improved quality assurance. Because PermaNet and conventionally treated nets were both quite tolerant of local washing procedures, it is important in field trials to compare LLINs with conventional ITNs washed an equivalent number of times. Our comparison of PermaNet 2.0 against conventionally treated deltamethrin nets (CTDN) in Pakistan demonstrated superior performance of the LLIN after 20 washes in phase I and phase II bioassays, and this was corroborated by chemical assays of residual deltamethrin. Although PermaNet 2.0 has received WHOPES interim recommendation for malaria control purposes, its performance should be monitored in everyday use throughout its lifespan in various cultural settings to assess its durability and long-term effectiveness for malaria prevention and control. As many millions of conventionally treated nets are already in routine use, and these will require regular re-treatment, programme strategies should be careful to preserve the effectiveness of ITNS before and after establishing the reliability of LLINs in long-term use.

A dual port olfactometer was used to study the response of Anopheles gambiaeGiles sensu stricto to odours of human and animal origin. Human odour consisted of human skin emanations collected on a nylon stocking, which was worn for 24 h. This was tested alone or together with 4.5% carbon dioxide, the concentration in human and cattle breath. Cattle odours consisted of cow skin emanations and/or carbon dioxide. Cow skin emanations were collected by tying a nylon stocking (‘cow sock’) around the hind leg of a cow for 12 h. Anopheles gambiae s.s. was consistently highly attracted by human odour, which is consistent with the high degree of anthropophily in this mosquito. Anopheles gambiae s.s. was not attracted by human or cattle equivalent volumes of carbon dioxide and this gas did not enhance the effect of human skin residues. Furthermore, A. gambiae s.s. showed a high degree of aversion to cow odour. When human odour and cow odour were tested together in the same port, mosquitoes were still highly attracted, indicating that whilst cattle odour may deter A. gambiae s.s., these mosquitoes can detect human odour in the presence of cattle odour. It was concluded that carbon dioxide plays a minor role in the host seeking behaviour of A. gambiae s.s., whilst host specific cues such as human skin residues play a major role and very effectively demonstrated anthropophilic behaviour in the laboratory.

In an organism with a life-cycle modelled on that of Plasmodium two separate resistance genes are assumed, each protecting against one of two unrelated drugs. The model was used to compare the rates of build-up of resistance in a population where the two drugs are used either as a mixture or in sequence. The model suggests that the use of a mixture would be advantageous if: both resistances are initially rare; there is recombination between the genes; a large proportion of the parasite population is unexposed to the drugs.

In the town of Makunduchi, Zanzibar, wet pit latrines provided breeding places for Culex quinquefasciatus and there were few other mosquito breeding places available. The adult mosquito population in bedrooms was monitored for a year and found to consist almost entirely of Cx quinquefasciatus of which 2.45% carried infective larvae of Wuchereria bancrofti. It was estimated that each person received about 24,993 bites per year, of which 612 were potentially infective. After a year of baseline data collection, polystyrene beads were expanded in boiling water and applied to form floating layers on all infested pits. Subsequently the adult mosquito population declined remarkably so that the estimated number of bites per person per year was only about 439. To retain control, surveillance for newly wet pits continued to be necessary. Initially the microfilarial rate in the human population was 49.5%, with many cases of filarial disease. The microfilarial rate was reduced to 10.3% by mass treatment with diethylcarbamazine, with no sign of resurgence a year after treatment. The infective rate among mosquitoes was reduced to 0.41% and the combined effect of the vector and microfilarial control was to reduce the number of infective bites per person per year by 99.7%.

Eighteen strains of the complex of Culex pipiens L. from Africa, Asia and Europe were bioassayed for resistance to chlorpyrifos and electrophoresed and stained for esterases. Susceptible strains showed only low activity esterase bands. The resistant strains of C. quinquefasciatus Say from hot countries (Liberia, Nigeria, Sri Lanka, Tanzania, Thailand) all showed the same two high intensity esterase bands (Rm 0"60 + O -82). Different patterns of high esterase were found in resistant C. pipiens strains from cooler localities in Nairobi, Kenya (Rm 100), and Montpellier, France (Rm 0 -50). Selection experiments on strains originally polymorphic for resistance and esterase pattern showed, without exception, that high esterase remained associated with resistance, and it is concluded that the association is almost certainly causal and not merely due to genetic linkage. The high intensity esterase bands were probably due to alleles of the loci Est-l, Est-2 and Est-3, separated by crossover distances of approximately 2 -4 and 55 units, respectively. Strains monomorphic for what appeared to be the same high esterase pattern varied markedly in resistance level.Enzyme assays showed a direct relationship between levels of enzyme activity and resistance. Bioassays with fenthion and chlorpyrifos revealed differences in the relative resistance of C quinquefasciatus from Colombo (Sri Lanka) and Dar-es-Salaam (Tanzania). Despite these differential degrees of cross-effectiveness, it is concluded that high intensity esterases are reliable indicators of organophosphate resistance in mosquitoes of the C. pipiens complex, although the possibility of other resistance mechanisms means that the lack of abnormally active esterases does not necessarily indicate the absence of resistance.

Pyrethroid-treated bed-nets act against late-night biting mosquitoes, like traps baited by the body odor of the occupant. The personal protective effect of treated nets is considerable, even if they are torn. However, some biting of the occupants does occur, as shown by matching microsatellite alleles in mosquito blood meals to those of net occupants. When whole communities were provided with treated nets, ovarian age grading showed that mosquito survival was reduced, and so was the number of sporozoite-positive mosquitoes in malarious communities. Thus, a high percentage of coverage of all members of malaria-endemic communities is considered to be the most effective way of providing protection for highly malaria-vulnerable children and pregnant women. Teams distributing nets or retreating them free of charge show high productivity, and we consider this the most cost-effective way to proceed. There is evidence for reduced anti-malaria antibody levels in children in communities where treated nets have long been used. However, overall benefits in reduced anemia and mortality are sustained. A high frequency of the kdr resistance gene has not prevented pyrethroid-treated nets from functioning, but it is important to develop alternative fabric treatments in case stronger forms of resistance emerge.

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