Insecticide-treated bed-nets for malaria mosquito control.

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.

Comparison of different insecticides and fabrics for anti-mosquito bednets and curtains.

Various formulations of six insecticides (a carbamate and five pyrethroids), were impregnated into bednets and curtains made from cotton, polyester, polyethylene or polypropylene fabric. For bioassays of insecticidal efficacy, female Anopheles gambiae mosquitoes were made to walk on the fabrics for 3 min and mortality was scored after 24 h. The main concentrations tested were: bendiocarb 400 mg/m2, cyfluthrin 30-50 mg/m2, deltamethrin 15-25 mg/m2, etofenprox 200 mg/m2, lambda-cyhalothrin 5-15 mg/m2 and permethrin 200-500 mg/m2. Field trials in Tanzania used experimental huts (fitted with verandah traps) entered by wild free-flying Anopheles gambiae, An.funestus and Culex quinquefasciatus mosquitoes. Results of testing the impregnated fabrics in experimental huts showed better personal protection provided by bednets than by curtains. Permethrin cis:trans isomer ratios 25:75 and 40:60 were equally effective, and the permethrin rate of 200 mg/m2 performed as well as 500 mg/m2. Bioassay data emphasized the prolonged insecticidal efficacy of lambda-cyhalothrin deposits, except on polyethylene netting. Most of the impregnated nets (including the 'Olyset' net with permethrin incorporated during manufacture of the polyethylene fibre) and an untreated intact net performed well in preventing both Anopheles and Culex mosquitoes from feeding on people using them overnight in the experimental huts. Anopheles showed high mortality rates in response to pyrethroid-treated nets, but only bendiocarb treated curtains killed many Culex. Holed nets treated with either cyfluthrin (5 EW formulation applied at the rate of 50 mg a.i./m2) or lambda-cyhalothrin (2.5 CS formulation at 10 mg a.i./m2) performed well after 15 months of domestic use. Treatment with deltamethrin SC or lambda-cyhalothrin CS at the very low rate of 3 mg/m2 gave good results, including after washing and re-treatment.

Anopheles arabiensis and An. gambiae chromosomal inversion polymorphism, feeding and resting behaviour in relation to insecticide house-spraying in Tanzania.

Differential responses of the mosquitoes Anopheles arabiensis and An. gambiae sensu stricto to house-spraying with DDT or lambda-cyhalothrin were evaluated in relation to chromosomal inversion polymorphism, feeding and resting behaviour of these malaria vectors in Tanzania. Blood-fed mosquitoes from pit traps outdoors, exit traps on windows and indoor-resting catches were identified cytogenetically and the chromosomal inversion frequencies compared between samples and species. Their outdoor-resting behaviour was assessed by a mark-release-recapture experiment and by determining the proportion of freshly blood-fed individuals in exit traps. The source of bloodmeals was analysed by an ELISA method. Endophagic females of An. arabiensis were more likely than those of An.gambiae to exit from a house on the night of blood-feeding. Only in one out of three villages was there evidence that chromosomally distinct individuals within a species had different preferences for resting sites. There were indications, but not conclusive evidence, that mosquitoes caught indoors or outdoors had a tendency to return to the same type of resting site. In villages sprayed with either insecticide, the mean age of the vector populations was greatly reduced, compared with those in the unsprayed villages. An.arabiensis females exited from DDT sprayed houses after blood-feeding, whereas with lambda-cyhalothrin those exiting were mostly unfed and there was a decline in the human blood index. The excitorepellency of DDT was perceived as a disadvantage, whereas lambda-cyhalothrin apparently had more impact on malaria transmission by An.arabiensis.

Age determination in the blackfly Simulium woodi, a vector of onchocerciasis in Tanzania.

Age-dependent changes in the ovarioles of Simulium woodi (Diptera: Simuliidae) and the age structure of a wild population of this species in the Amani hills of north-eastern Tanzania were studied using the ovarian oil injection technique. Contrary to previous theories, egg sacs degenerate completely and do not form dilatations. The physiological age of the females could be determined only by gonotrophic dilatations formed from degenerating follicles during gonotrophic cycles. In individual females, the proportions of ovarioles with degenerating follicles were 3-54% (mean 22%) in the first cycle and 8-61% (mean 36%) in the second gonotrophic cycle. Agonotrophic degenerating follicles occurred in 2% of ovarioles in 12% of females. Additional information with regard to the degree of parity is given by the level of granulation in the basal body of each ovariole, a group of six to eight cells in the calyx wall enclosed by the end of the ovariolar sheath. Their granulation progressively increased in intensity following each subsequent ovulation.

Mortality of under-fives in a rural area of holoendemic malaria transmission.

From January 1992 to December 1993, a total of 361 births and 243 deaths were recorded by village reporters in five villages in Muheza District, north eastern Tanzania. Among those aged less than one year 48 deaths were recorded, giving an infant mortality rate of 133 per 1000 live births (95% CI 97.9-168.0). There were 42 deaths among the censused population of 845 children aged 1-4 years (24.8/1000/year, 95% CI: 17.4-32.3). Verbal autopsy questionnaires were administered to parents or close relatives of 83 of the dead infants and children. From analysis of these, 30 of the deaths were tentatively attributed to malaria. The results are discussed in relation to other studies in East and West Africa and to the prospects for reducing mortality by use of insecticide impregnated bednets.

PIP: The rural areas of Muheza District in northeastern Tanzania are holoendemic for malaria. In Tanzania, malaria is the largest single cause of hospital attendance, the second largest cause for hospital admission, and one of the leading causes of hospital deaths. Hospital data, however, only reveal part of the picture of childhood mortality because most such deaths in rural areas occur at home. The authors investigated the causes of infant and child mortality in five villages near Muheza to collect baseline data in preparation for aa large-scale study planned to explore the impact of impregnated bednets upon infant and child mortality. Records of births, deaths, ages at death, and supposed causes of death were collected by village reporters, while verbal autopsies were obtained from parents or close relatives of dead infants and children. The study 361 births and 243 deaths were recorded by the village reporters from January 1992 to December 1993. There were 48 deaths to infants under one year old for an infant mortality rate of 133/1000 live births. 42 deaths were recorded among the censused population of 845 children aged 1-4 years. Verbal autopsy questionnaires were administered to parents or close relatives of 83 of the dead infants and children. On analysis, 30 deaths were tentatively attributed to malaria. These results are discussed in relation to other studies in East and West Africa, and to the prospects for reducing mortality through the use of insecticide-impregnated bednets.

Identification of the E3900 family, a second family of rye B chromosome specific repeated sequences.

A second family of highly repeated sequences has been identified on the B chromosome of rye (Secale cereale). The E3900 family was detected as a variant band in EcoRI digests of +B DNA. A clone of the basic repeat of the family was obtained, and the organization of the family was investigated by genomic hybridization. The E3900 family has no apparent homology to the A chromosome sequences of rye or other members of the Gramineae. The family has been localized by in situ hybridization to the end of the long arm of the rye B chromosome. The previously characterized E1100 sequence shows in situ hybridization to the same location as the E3900 family. These results are discussed in light of current theories of the origin of B chromosomes.

Various pyrethroids on bednets and curtains.

Verandah trap huts in a Tanzanian village were used to assess the effectiveness of impregnated bednets and curtains in preventing hut entry and feeding by, and in killing of, Anopheles gambiae and An. funestus. Permethrin, deltamethrin, lambdacyhalothrin and pyrethrum were used for impregnation of damaged or undamaged bednets, sisal eaves curtains or bed curtains made of polypropylene fibre. The performance of the three synthetic pyrethroids did not differ statistically significantly, except that on a damaged net permethrin was better at preventing feeding. Sisal eaves curtains deterred mosquitoes from hut entry but did not kill those that had entered. In assessing damaged nets and curtains it must be recognised that anything less than the best vector control may have no appreciable impact on holoendemic malaria.

Various pyrethroids on bednets and curtains

Verandah trap huts in Tanzania village were used to assess the effectiveness if impregnated bednets and curtains in preventing hut entry and feeding by, and in killing of, Anopheles gambiae and An. funestus. Permethrin, deltamethrin, lambdacyhalothrin and pyrethrum were used for impregnation of damaged or undamaged bednets, sisal eaves curtains or bed curtains made of polypropylene fibre. The performance of the synthetic pyrethroids did not differ statistically significantly, except that on a damage net permethrin was better at preventing feeding. sisal eaves curtains deterred mosquitoes from hut entry but did not kill those that had entered. In assessing damaged nets and curtains it must be recognised that anything less than the best vector control may have no appreciable impact on holoendemic malaria

Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. Part 1. Operational methods and acceptability.

In five Tanzanian villages, nets impregnated with permethrin or lambdacyhalothrin were given out. The people received them enthusiastically and brought their nets for re-impregnation at six monthly intervals. Bioassays showed that the insecticidal power of permethrin impregnated nets remained adequate for six months unless the nets were washed. Nets with 30 mg lambdacyhalothrin/m2 retained high insecticidal power despite washing, but this dose caused temporary cold-like symptoms in those sleeping under freshly treated dry nets. Methods by which durable bednets might be made affordable by Tanzanian villagers are discussed.

Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. Part 2. Effects on the malaria vector population.

The malaria vector population consisted mainly of Anopheles gambiae s.s. with a small contribution from An. funestus and An. rivulorum. The mosquitoes coming to bite in bedrooms were monitored with light traps set beside untreated bednets. When impregnated bednets were provided for all the other beds in a village the Anopheles populations declined but the Culex quinquefasciatus populations were unaffected. The survival of An. gambiae (as measured by the mean number of ovarian dilatations) and the sporozoite rate declined following introduction of the nets and the estimated sporozoite inoculation rates into people not under their nets declined by more than 90%. The net introductions caused sharp declines in the number of mosquitoes resting indoors, but the evidence was inconclusive regarding diversion to outdoor resting, animal biting, earlier biting or outdoor biting. DDT spraying greatly reduced the Anopheles populations.

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.

The life expectancy of phlebotomine sandflies: first field estimates from southern France.

1. A field study of Phlebotomus ariasi Tonnoir, the vector of Leishmania infantum Nicolle in southern France, addressed the following questions: Is it possible to estimate reliably the life expectancy of this sandfly; can spatial or temporal variation in the life expectancy be detected, and is such variation significant for disease transmission? 2. Life expectancy was estimated by examining follicular relics in the ovaries of more than ten thousand females caught in light traps at seven sites in the Cévennes and the Garrigues, throughout their active period in 1985 and 1986. Whilst the distinction between nulliparous and parous flies was easily made, assessments of the number of times a parous fly had laid eggs were unreliable. Best estimates of life expectancy were therefore calculated from the parous rate. 3. Large samples collected from one site in the Cévennes in both years gave very similar estimates of life expectancy. 4. There was also no significant difference between estimates obtained from the Cévennes and the Garrigues, despite their distinct vegetation and climates. Therefore, large regional differences in sandfly population size and the prevalence of canine leishmaniasis cannot be explained by a difference in adult survival rate. 5. With no systematic annual or regional variation, a useful mean life expectancy can be calculated from the data collected at all sites in both years. It is 1.54 (SE 0.04) ovarian cycles. However, this estimate is sensitive to the assumption that survival rate is a discrete rather than a continuous variable. 6. Local variation in the parous rate may be associated with the proximity of traps to P.ariasi emergence sites.

Two populations of Phlebotomus ariasi in the Cévennes focus of leishmaniasis in the south of France revealed by analysis of cuticular hydrocarbons.

Two distinct populations of Phlebotomus ariasi Tonnoir have been identified in the Cévennes focus of leishmaniasis in the south of France using gas-liquid chromatography (GLC) of cuticular hydrocarbons extracted from individual dried female flies. Results were obtained after analysis of flies collected from CDC light traps from a domestic and a sylvatic site separated by a distance of 900 m. Flies were provided for GLC analysis as six blind samples. Using cluster and discriminant analysis techniques, five of the samples were shown to form two distinct groups, while a sixth was identified as a mixture. These findings were subsequently confirmed to correspond exactly with the way the samples had been presented. Samples grouped together on the basis of the flies' cuticular hydrocarbon profiles had been taken from the same site, while the ungrouped sample had been deliberately mixed. Using a jack-knifed estimator, it is shown that specimens can be correctly allocated to the population to which they belong with a 92% success rate. These results confirm the value of the technique for the identification of populations of medical vectors. The implication of the findings for the epidemiology of leishmaniasis is discussed with special reference to the need to determine if both populations are vectors and to study differences in behaviour. In addition to adults, profiles of larval sandflies have been obtained for the first time.

Preliminary field observations on the flight speed of a phlebotomine sandfly.

From observations with a portable wind tunnel used in the field in southern France, it is estimated that the maximum flight speed of Phlebotomus ariasi is in the range 0.65-0.70 m/sec (2.3 to 2.5 km/h).

Ecology of leishmaniasis in the south of France. 20. Dispersal of Phlebotomus ariasi Tonnoir, 1921 as a factor in the spread of visceral leishmaniasis in the Cévennes.

The dispersal of Phlebotomus ariasi was studied in mark-release-recapture experiments in the summer of 1980 in a valley on the north-eastern slopes of the Oiselette range in the Cévennes mountains, in the commune of Roquedur, Gard, 50 km north of Montpellier, France. More than 5,000 specimens of P. ariasi were marked with fluorescent powders and released in 9 batches at 3 different places. Seven batches were engorged females and two were unengorged females and males. From 1-29 days after release, 497 marked sandflies (approximately 9%) were recaptured by active searches with UV lamps or in 58 CDC light traps set up in groups of 4 or 5 at 12 recapture stations. Females released engorged generally remained within 250 m of the release point for the first eight days while the bloodmeal was being digested after which there was a tendency to disperse to distances greater than 350 m presumably in a search for oviposition sites or another bloodmeal. The furthest distance to which a female released engorged was shown to move was 925 m; it was caught 12 days after release. Some of the females released unfed quickly moved away from release points, sometimes to distances of 1,000 m or more. One of these was caught 68.5 hrs after release at a station 2,200 m from the release point. Male sandflies tended to stay near the point of release and were not recaptured at distances greater than 600 m. There was no evidence that the movement of the sandflies was assisted by wind. Observations on the dispersal of female sandflies confirm that leishmaniasis can be more widely spread than generally assumed by the movements of the vector.

Observations on the biting of Anopheles triannulatus bachmanni from the Mato Grosso, Brazil.

Foram coletados em Aripuana, Mato Grosso, Anopheles triannulatus bachmanni com isca humana. Em capturas aos pares menos de 5% dos insetos foram coletados picando dentro de casas, a maioria dos insetos foram coletados nas margens da mata ao por do sol. A determinacao fisiologica da idade indicou uma distribuicao de idade uniforme de femeas, mas o comportamento do voo dos insetos fa-los-ia de pouca importancia como vetor da malaria