Vector control in some countries of Southeast Asia: comparing the vectors and the strategies.

The use of information on malaria vector behaviour in vector control is discussed in relation to the area of Southeast Asia comprising Cambodia, Laos, Myanmar, Thailand and Vietnam. The major vectors in the region are Anopheles dirus, An. minimus, An. maculatus and An. sundaicus, of which An. dirus is the most important. Options for vector control and the biological features of mosquitoes, which would make them amenable to control by these measures, are listed. The methods with the greatest potential for controlling each of the four vector species are described. Experiences of vector control by residual spraying, insecticide-treated nets and larva control and of personal protection against the four vectors are outlined, and it is noted that choice of control strategy is often determined by epidemiological, economic and political considerations, whilst entomological observations may help to explain failures of control and to indicate alternative strategies. Future research needs include basic entomological field studies using the most appropriate indicators to detect changes related to rapidly changing environmental conditions, such as loss of forest and climate change. Further studies of the efficacy of insecticide-treated mosquito nets, with greater attention to study design, are needed before it can be assumed that they will work in Southeast Asia. At the same time, research to improve sustainable utilization of nets is important, bearing in mind that nets are not the only means to control malaria and should not drain resources from supervision and training, which improve access to diagnosis and treatment of malaria and other diseases. Research is needed to make decisions on whether vector control is appropriate in different environments, and, if so, how to carry it out in different health systems. Researchers need to play a greater role in making operational research (entomological, epidemiological, social, economic and health systems research) of good quality an integral component of implementation programmes.

The Anopheles punctulatus group of mosquitoes in the Solomon Islands and Vanuatu surveyed by allozyme electrophoresis.

Four species within the Anopheles punctulatus group of mosquitoes (Diptera: Culicidae) were identified by allozyme analysis of samples collected from thirty-three localities in Guadalcanal, Makira, Malaita, Temotu and Western Provinces in the Solomon Islands and six localities on Efate, Espiritu Santo, Maewo and Malekula Islands in Vanuatu. Three of these species are members of the An.farauti complex. A key is given to identify five species of the An.punctulatus group known to occur in the Solomon Islands using their isoenzyme characteristics. An.farauti No. 1 was widespread in coastal areas of the Solomon Islands and was the only species detected in Vanuatu, including Efate Island (where Faureville is the type locality of An.farauti Laveran sensu stricto). An.farauti No. 2 and An.punctulatus were common in the Solomon Islands in more inland areas. An.farauti No. 7, reported here for the first time, was found as larvae in freshwater at six localities on north Guadalcanal. Three other members of the An.punctulatus group which have been reported previously from the Solomon Islands: An.koliensis, An.renellensis and an electrophoretic variant of An.farauti sensu lato, were not found in our samples. Previously recognized vectors of malaria and bancroftian filariasis in the Solomon Islands are An.farauti No. 1 (i.e. An.farauti s.s.), An.koliensis and An.punctulatus s.s. Adult females of An.farauti No. 2 and An.farauti No. 7 were not attracted to human bait in areas where their larvae occurred, indicating that these two species are not anthropophilic and therefore unlikely to transmit human pathogens.

Malaria in Cambodia.

There are around half a million cases of malaria with 5-10,000 deaths per year in Cambodia. Incidence rates vary in different parts of the country. Malaria control is hampered by multiple drug resistance of Plasmodium falciparum, inaccessibility to the major vector, poor security in most malarious areas, and lack of resources. The control strategy emphasises improvement of clinical management and provision of prompt and accurate diagnosis in order to reduce morbidity and to prevent mortality. In addition health information and drug distribution systems are being improved. The use of pyrethroid-treated mosquito nets and health education are being promoted. Particular attention is given to returning refugees as they settle into the country.

Epidemiology of malaria in displaced Khmers on the Thai-Kampuchean border.

Malaria epidemiology from 1983 to 1985 in displaced Khmers living in camps on the Thai-Kampuchean border was studied for planning and evaluation of control measures. The annual parasite incidence per 1000 people fell from 359 in 1983 and 350 in 1984 to 116 in 1985. Incidence varied by camp according to the suitability of the habitat for the vectors. Camps with high incidence had a higher ratio of Plasmodium falciparum to P. vivax, a higher mortality rate, especially in young children and sometimes pregnant women, but a lower case fatality rate than camps with low incidence. Transmission occurred year round, but peaked in the rainy season in 1983 and 1984. In 1985 there was a steady decline in incidence, largely owing to evacuation of many camps away from forested areas. Mortality rates per 100,000 people were 62, 77 and 28 and case fatality rates 0.41, 0.31 and 0.42% in 1983, 1984 and 1985 respectively. The case fatality rate for cerebral malaria was 21 to 26%. Epidemiological information has been more useful for planning than for evaluating control measures.

Murine typhus among Khmers living at an evacuation site on the Thai-Kampuchean border.

An outbreak of febrile disease involving 170 Khmer adults at an evacuation site in Thailand occurred during the dry season of 1986, only 8 months after the camp was constructed. The illnesses were characterized by persistent fever, retro-orbital headache, myalgias, and clinical response to tetracycline within 2-3 days. The symptoms, effectiveness of tetracycline, and presence of a large rat population raised the suspicion of murine typhus. Fourteen (74%) of 19 patients had elevated or rising antibody titers against Rickettsia typhi, confirming the clinical diagnosis. Rats were caught, and they and their fleas were identified. In agreement with the known Thai host and vector, 80 (93%) of 86 rats were Rattus exulans, and all of 32 fleas were Xenopsylla cheopis. This first reported outbreak of murine typhus in Thailand is notable for its occurrence in a new human settlement only 8 months after construction.

Treatment of falciparum malaria with quinne and tetracycline or combined mefloquine/sulfadoxine/pyrimethamine on the Thai-Kampuchean border.

Three different regimens were compared for treatment of falciparum malaria in displaced Kampucheans living in encampments on the Thai-Kampuchean border in 1983: single dose 750 mg mefloquine, 1.5 g sulfadoxine, 75 mg pyrimethamine (MSP); 600 mg quinine 8-hourly for 3 days and 500 mg tetracycline 8-hourly for 7 days (Q3T7); or 600 mg quinine 8-hourly for 7 days and 500 mg tetracycline 8-hourly for 7 days (Q7T7). Radical cure rates were 98% (40/41) for MSP, 76% (32/42) for Q3T7 and 92% (33/36) for Q7T7. The criterion for treatment failure was reappearance of parasites by 35 days after commencement of treatment or no parasite clearance. Treatment failures comprised one case of reduction but no clearance of parasites (RII resistance) for MSP, 10 recrudescences (RI) for Q3T7 and 3 recrudescences (RI) for Q7T7. The radical cure rate for Q3T7 was significantly lower than that for MSP (P less than 0.01), whilst Q7T7 significantly from the other groups. Parasite clearance time was shorter (2.4 days) with MSP than with Q3T7 (3.5 days) and Q7T7 (3.3 days). There was little difference in side effects between the regimens, and tolerance was good. The MSP and Q7T7 regimens are both effective for treatment, but the single dose of MSP is much easier to manage than 7 days of quinine and tetracycline.

Occurrence of rickettsia-like symbionts among species of the Aedes scutellaris group (Diptera:Culicidae).

Rickettsia-like organisms were seen by electron microscopy in the oocytes and nurse cells of Aedes hebrideus and A. scutellaris scutellaris and in the oocytes of A. pseudoscutellaris, a Samoan stock of A. polynesiensis and in three Fijian stocks of A. polynesiensis. None were found in the ovaries of A. alcasidi, A. s. katherinensis, A. cooki nor in a second stock of Samoan A. polynesiensis. It is suggested that the distribution of rickettsia-like symbionts among species of the A. scutellaris group conflicts in some cases with the hypothesis that these organisms are responsible for cytoplasmic incompatibility in the group.