Mark-release-recapture experiments were carried out in Sabah, East Malaysia on the malaria and filariasis vector, Anopheles balabacensis. Samples of wild females were marked with different colours of fluorescent pigments, released in man-baited huts fitted with exit traps. Simultaneous collections and releases were also made in night-biting catches on a water buffalo and on four men. All subsequent recaptures were made in the same situation in which the mosquitoes were marked. The same individual mosquitoes were caught biting men and buffalo on different occasions and the numbers caught showed a strong preference for man over buffalo. The length of the oviposition cycle in the field was found to be 3.0 days. After blood-feeding on man in a hut, An. balabacensis were found to exit on the night or early morning. The same individual mosquitoes were found resting in the hut or exit trap on different occasions. The results indicate that there is strong evidence for the existence of genetic variability in the tendency of An. balabacensis to rest in houses and to bite man and buffalo. The obvious existence of this phenomenon is considered discouraging for the prospects of interruption of malaria transmitted by An. balabacensis in nature.
Susceptibilities of two colonies of the taxon An. dirus (one from Perlis and from Thailand) and one colony of An. balabacensis from Sabah to DDT, dieldrin, malathion, fenitrothion and propoxur were determined. DDT and fenitrothion tolerance was found in An. dirus species B and An. balabacensis. No resistant strain was isolated as the two colonies were not either homozygous or nearly so for resistance. Field testing of the susceptibility of the adults of An. balabacensis to DDT was carried out between 1957 to 1976. The results indicated a progressive rise in the LC50 levels greater than 1% in almost all instance. The variation in the number of sprays applied in some districts have resulted in varying sensitivities. Association between the changes in levels of DDT susceptibility and exophilic habit of An. balabacensis has been observed but needs further confirmation. The significance of these findings and the difficulties in distinguishing tolerant from truly resistant individuals are discussed in relation to accurate species identification.
Anopheles (Cellia) litoralis King and Anopheles (Cellia) sundaicus Rodenwaldt, vectors of malaria, were collected from the same brackis and sea-water habitats in six localities in Sabah. They share the same breeding habitats with predominance of one species over the other. The two species although distinct have small morphological differences and are taxonomically separated by certain wing characters. Hybridization between the two species was successful. Reciprocal crosses produced viable progeny which appeared to develop normally to adults. Hybridized females laid fewer viable eggs in comparison with the parents. The F1 hybrids resembled the litoralis parent in most characters. Backcrosses of both litoralis and sundaicus parents with the F1 hybrids yielded no eggs. F1 male hybrids were thus assumed to be sterile. The results obtained from cross matings between the two species suggested something more than subspecific status.
Observational and survey methodologies were used to probe human behavioral factors influencing the use of insecticide-impregnated bednets to control malaria in rural Sabah, Malaysia. One aim was to investigate why a field trial of such nets in an interior area yielded disappointing results. A second aim was to gather baseline data prior to a field trial proposed for a coastal area. Interior villagers reported a significantly higher net usage rate than that observed directly, suggesting that subject self-reports need to be validated in some way. The poor results of the interior field trial appeared related to reluctance to regularly use nets, which were not in wide use previously. Prospects for reducing malaria transmission through bednets appeared better for the coastal area since nearly half of observed villagers were sleeping in them. However, significantly more coastal than interior villagers were observed watching television at night, an activity that may increase malaria risk by keeping villagers awake and out of bednets.
Insecticide-impregnated bednets appear to be a potentially cost-effective intervention against endemic malaria in the tropics, but this has yet to be confirmed by field trials. There are two aspects to consider in assessing such trials: (1) the extent to which subjects use nets regularly and properly, and (2) the effectiveness of nets which are truly used regularly and properly in reducing malaria transmission. The second aspect is currently of primary concern, to determine if human-vector relationships for a particular at-risk population are such that bednets can be effective. But to give bednets a "fair" test in this regard requires regular and proper use in the first place. The study described here suggests they did not get a "fair" test in one field trial in Sabah, East Malaysia. The study also strongly suggests that direct observations, rather than post hoc questioning of subjects, may be essential to accurately gauge bednet usage rates. Accurate usage rates are required to determine what proportion of a population needs to use nets to reduce malaria transmission, and to evaluate the effectiveness of promotional programs over time. Direct observations can also yield valuable data on night-time activities that increase malaria risk, such as television viewing that keeps people awake and out of bednets.
An exophilic population of the vector mosquito Anopheles balabacensis Baisas was investigated in two mark-recapture studies (16.ix-13.x.1986 and 6-26.i.1987) at an inland, foothill village in Sabah, Malaysia. Wild female mosquitoes were intercepted as they came to feed on man or buffalo, given a bloodmeal, marked with fluorescent dust and released. The recapture rate was about 12%. A new method of analysis is proposed which uses cross-correlation and a time series model. The estimated survival per oviposition cycle was 0.48-0.54 and the oviposition cycle interval 2-3 days.
Two separate observations from recent electrophoretic studies of the systematics and population genetics of laboratory-reared populations which had a long history of colonization in various laboratories, were found to be inconsistent with the present study which used wild-caught populations from East Malaysia. Reanalysis of the two data sets generally indicated a low amount of genetic variation in laboratory colonies. The latter is characterized by higher frequency of monomorphic loci, low average heterozygosity values and, in one extreme case, no variability at two loci. However, natural populations of An. balabacensis and An. leucosphyrus showed more protein variability by the use of polyacrylamide gel electrophoresis. Since laboratory-maintained mosquitoes are genetically and phenotypically different from those in the field, results of laboratory studies on the systematics and population genetics of Anopheles species complexes may be biased.
The district of Kudat has one of the highest and most persistent malaria transmission levels in Sabah, Malaysia, with annual parasite incidence of 102 per 1,000 inhabitants per year. Due to this situation and the failure of DDT spraying to control malaria, a community participation health program (Sukarelawan Penjagaan Kesihatan Primer or SPKP) was developed as an adjunct to current anti-malarial measures during 1987-1991. SPKP is made up of unpaid community workers known as village health volunteers (VHVs). VHVs are selected by a village development and security committees training and supervision a member of the Vector-Borne Diseases Control Program (VBDCP). The beneficiaries of SPKP consisted primarily of Runggus people and other remote, and mobile populations who visit the home of a VHV for diagnosis and treatment. This group of febrile patients and their children who attend a participating school submit finger prick blood and personal details to the VHV. and receive a presumptive treatment for malaria. Thick and thin blood smears are examined by a VBDCP microscopist who then prepare and forward a radical or curative treatment to the VHV so that it can be administered to the microscopically-positive patient free of charge. Between June 1987 to June 1991, VHVs from 32 kampungs (villages) and 22 schools collected 56,245 slides representing 24.7% of total slide collection compared to 74.9% collected by passive case detection (PCD) posts in health centers and district hospital. The average volunteer treated 11.8 (range 10.4-13.4) and 31.4 (range 26-49) patients per month in kampungs and schools respectively. In contrast, non-SPKP posts in a district hospital, health centers and flying doctor service treated an average of 616.3 patients per month (range 134.8-1032.8). The slide positivity rate of blood smears taken by VHVs was 8.43% compared with 7.37% for non-SPKP posts. Average slide collection and slide positivity rates varied considerably from one community to another, despite their close geographic proximity. The monthly number of VHV-diagnosed patients from the school and kampungs communities and the monthly number of true malaria patients in the two groups were significantly correlated. Sustainability of SPKP was linked to an ongoing process of social change which involved co-operative networking between the government health sector and the community. This in turn provided a stimulus for malaria abatement efforts. When Runggus people themselves control and maintain ownership of community-based malaria programs, the function of SPKP as a malaria surveillance system and an antimalarial drug distribution network is vastly improved.
During the intermonsoon period from mid-September to mid-October 1986, wild-caught Anopheles balabacensis Baisas females were marked and released in a host-choice experiment. Association between capture and recapture of marked mosquitoes from human and bovid hosts and blood meal host identification of recaptured females were determined on a daily basis. Although the mark-recapture and blood meal data indicated behavioral heterogeneity between buffalo and human biters, restriction endonuclease fragment length polymorphism analysis revealed no differences in repeat sequence profiles. Doubly-marked recaptures strongly indicated a "learning" component involved in a separate host preference experiment. In a "habitat loyalty" experiment conducted in January 1987, females of An. balabacensis preferentially returned to the resting sites (indoor surfaces and exit traps) where they were first caught. Of nine isozyme loci found to be polymorphic, the genotypic frequencies of Esterase-3 and Isocitrate dehydrogenase-3 were different in "faithfully" endophilic and exophilic subpopulations. Genetic heterozygosity, as determined by polyacrylamide gel electrophoresis, was greater in exophilic than endophilic population components. These results confirm that genetic and learning components can significantly influence house resting and host seeking behavior and may contribute to local epidemiological patterns of malaria transmission observed in Sabah, Malaysia.
Seven villages in Banggi Island, Sabah, Malaysia, were surveyed four times to evaluate the roles of local mosquitoes as vectors of malaria and Bancroftian filariasis. 11 species of Anopheles were found biting man. 53.9% of the anophelines caught were An. flavirostris, 27.1% An. balabacensis, 6% An. donaldi and 4.2% An. subpictus. Infective malaria sporozoites, probably of human origin, were found in two of 336 An. flavirostris and 12 of 308 An. balabacensis. Sporozoites, probably of a non-human Plasmodium, were found in An. umbrosus. Nine of 1001 An. flavirostris and four of 365 An. balabacensis harboured L2 or L3 filarial larvae identified as those of Wuchereria bancrofti. This is the first record of An. flavirostris as a natural vector of malaria and W. bancrofti in Sabah.
A small-scale trial was carried out in the Upper Kinabatangan district of Sabah, Malaysia, to determine the effect of using permethrin-impregnated bednets on malaria transmission. A total of 306 nylon bednets with cotton borders, impregnated at a dose estimated to have been 0.062 g permethrin/m2 of nylon netting, were distributed to 139 households in five villages. At the time of distributing bednets, mass drug administration with Fansidar plus primaquine was also administered to the human population to clear all parasitaemias due to Plasmodium falciparum Welch. In another village, for comparison, mass drug administration was the only intervention. After intervention measures in December 1984 and January 1985, the parasite rates in children declined in all villages during the first month, significantly more in the villages with impregnated bednets than in the control, thus proving that the nets had an impact on malaria. However, after about 2 months, parasite rates started to increase again. After 4-6 months, parasite rates in the villages with bednets approached the rate in the control village without nets. The increase in parasite rates was paralleled by a significant deterioration in the quality, physical condition and the degree of non-utilization of bednets. Entomological evaluation proved the efficacy of permethrin-impregnated nets for controlling Anopheles balabacensis Baisas and other anophelines. Bioassays (1 h exposure) of permethrin-impregnated bednets gave 100% mortality initially and 44-61% mortality after 85-106 days. Mosquito collections in treated bednets were significantly reduced for at least 217 days. The project failed to achieve prolonged suppression of malaria transmission for a combination of entomological, sociological and practical reasons which are discussed in relation to the objectives and implementation of future bednet studies.
Holoendemic malaria transmission in two small isolated forest communities and a coastal village was studied by (1) all night human bait collections of Anopheles species from inside and outside houses and (2) buffalo-biting and CDC light-trapping catches during March and November 1984. During the same period thick and thin blood films were collected from the human population, and spleen rates were determined in children from two to nine years of age. Using both the immunoradiometric assay (IRMA) and the dissection technique, more sporozoite-positive infections were detected in An. balabacensis and An. flavirostris in November than in March. IRMA confirmed the presence of Plasmodium falciparum sporozoites. An average of 76.2% of the An. balabacensis population lived long enough to have reached a point where infectivity with P. falciparum was possible in November. Although fewer than five adult females bit humans per night at any time, a resident could theoretically have received more than 160 infective bites in one year. A high frequency of feeding on humans, coupled with increased anopheline life expectancy, contributed to high estimates of falciparum malaria vectorial capacity (number of infections distributed per case per day); for An. balabacensis (1.44-7.44 in March and 9.97-19.7 in November) and for An. flavirostris (0.19-5.14 in March and 6.27-15.8 in November). These high values may explain the increased malaria parasite rates obtained from at least two forest communities. Correlation between actual and calculated rates of gametocytaemia was poorest in Kapitangan due to inadequate sampling of the human population. In Banggi island, malaria is stable and holoendemic, and the population enjoys a high degree of immunity.