Importance of asymptomatic malaria and its infectivity to Anopheles mosquitoes in Mae Hong Son Province, Thailand.

This study aimed to record the prevalence of asymptomatic malaria and the development of malaria parasites in laboratory-bred Anopheles minimus mosquitoes fed on the blood of people with or without malaria symptoms. The intention was to identify where the main reservoir of infection was maintained. The study was conducted in three cantons in Muang district, Mae Hong Son Province, Northwest Thailand. Chapter 3 describes studies of epidemiology of malaria in this area, based on two cross-sectional surveys of populations in villages and people who visited a malaria clinic. A questionnaire was conducted in the local languages by trained interviewers, to identify people suffering from fever. The parasite prevalence was 0.7% of all the surveyed populations. The prevalence of slide positivity among males was significantly higher than females. All those with positive slides who had a body temperature >37.4°C reported fever. Some without measured body temperature >37.4°C also reported fever. Those people may have had fever a few hours previously. Contrary to expectation P. falciparum infections did not cause fever more than P. vivax infections. The prevalence of asymptomatic malaria was 47.1 % among those who were slide positive which corresponded to about 0.5% of the interviewed population. From the Clinic, 101 patients with malaria infection were interviewed. More than half of the infections had high parasitaemia. When people thought they had malaria, the malaria clinic and the hospital facilities were most commonly used, with the largest number going to the Clinic. A significantly higher malaria prevalence was found in the people who reported not having any form of mosquito protection. More than 99% stated that they owned mosquito nets, but only 44% of the nets had been treated with insecticide. 2 Chapter 4 reports a study of the infectiousness of malaria patients to An. minimus. Direct feedings were conducted on adults >15 years old and who gave their consent. All of these people had blood slides positive for P. falciparum or P. vivax and had been located from the village surveys (n = 28) or the Clinic (n = 92). After feeding, the mosquitoes were held for 7-9 days and then dissected and examined for oocysts. Feeding on about 40% of human subjects yielded some mosquitoes with oocysts. There was a significantly lower probability of infection with P. falciparum than P. vivax. Symptomatic or non-symptomatic people with parasites were equally able to infect mosquitoes. There was a significant association of probability of mosquito infection with presence of observable gametocytes, but there were some individuals without observable gametocytes who infected mosquitoes. PCR on 58 subjects with negative slides showed that 30% were positive for P. falciparum, but none yielded oocysts after mosquito feeding. PCR was also used to identify the parasite species in the mosquito's gut. The results revealed that none of the donors with visible P. falciparum infections carried cryptic P. vivax infections which could yield P. vivax positivity in mosquitoes fed on them. The occurrence of some cases with undetectable gametocytes who could infect mosquitoes suggests the question "Do gametocytes selectively enter the mosquito's proboscis during blood feeding"? However, a small study showed that the gametocyte density in the mosquito blood meals did not differ significant from that in the corresponding finger pricks, indicating that gametocytes do not selectively enter a mosquito's proboscis. The occurrence in some cases of high oocyst counts in mosquitoes fed on blood with no observable gametocytes remains unexplained. From the results, on infection of mosquitoes by different categories of people an attempt was made to estimate the number of people in the catchment area of the Clinic who were reservoirs of infection. On the basis of the number of patients visiting the Clinic per day, it was concluded that the main reservoir of infection for mosquitoes was not in patients feeling ill enough to be motivated to come to the Clinic. The present study suggests that directing anti-gametocyte drugs to all feverish 3 patients in the villages could have a major impact on the reservoir of infection. However, in Thailand there have not yet been reports on the infectivity of the gametocytes in mosquitoes after treatment of patients with gametocytocidal drugs (e. g. primaquine or artesunate). Such studies are strongly recommended