Studies on the Anopheline Mosquitoes of Metehara and \ Surrounding Areas in Relation to Malaria Transmission

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Addis Ababa University


The ecology and behaviour of Anopheles mosquitoes was studied in four selected sites in the Metehara area, Upper Awash Valley, Eastern Ethiopia. The sites represented two insecticide unsprayed villages (Metehara and Gelcha) and two sprayed villages (the Sugar Estate and Buse). Information on the prevalence of malaria cases was also gathered from the Metehara Sugar Estate Hospital and the East Shoa Malaria Control Sector (Nazareth). The results showed that a total of 24,799 microscopically diagnosed malaria cases out of 68,000 blood samples were registered between July 1999 and September 2000 in the two health service rendering organizations. Plasmodium falciparum and P. vivax were found to be responsible for the disease and occur nearly in equal proportions (52.7% P. falciparum and 47.3% P. vivax ). Larval collections from the different breeding habitats revealed the presence of four species of which Anopheles arabiensis was the predominant followed by. An. pharoensis. Similarly, a total of 3639 adult anophelines representing at least eight species were caught using different methods from July 1999 to September 2000. An. arabiensis was the predominant species, forming 95% of all collections followed by An. pharoensis (3.8%). The density of An. arabiensis resting indoors in the sprayed sites was much lower than the density in the unsprayed sites. Consequently, 18%, 21.3%, 66.1% and 69.4% of An. arabiensis exhibited exophily in Gelcha, Metehara, Buse and the Sugar Estate, respectively. The indoor to outdoor biting ratio of An. arabiensis varied between villages: 0.53 in the Sugar Estate, 1.97 in Buse, 0.75 in Gelcha and 2.12 in Metehara town, showing that the species was more endophagic in Buse and Metehara and more exophagic in Gelcha and the Sugar Estate, * : XI The highest man biting rate of 39.5, was recorded in Metehara town and the lowest 8.4 in Buse. An. pharoensis was mostly an outdoor feeder. Of 864 An. arabiensis and 63 An. pharoensis dissected from human bait collections, the average parous rate was 45.1% and 30.2%, respectively, showing An. arabiensis to be longerlived than An. pharoensis. The sporozoite rate of An. arabiensis was 0.77% in parous and 0.21% in nulliparous population, the overall being 0.46%. Similarly, An. pharoensis had sporozoite rate of 5.3% in parous and 2.3% in nulliparous population, the overall being 3.2%. The biting rhythm of An. arabienisis exhibited two to three peaks of activity before and after midnight . The highest biting density occurred after midnight indoors while variation was observed outdoors. The average daily entomological inoculation rate (EIR) of An. arabiensis was 0.05 while that of An. pharoensis was 0.01. The Human blood index (HBI) of An. arabiensis revealed variation between sites/villages being 1 in Metehara, 0.47 in Gelcha, 0.85 in Buse and 0.93 in the Sugar Estate, the overall being 0.65. The HBI varied also between dwelling conditions of mosquito sampling, being highest in human dwellings (0.78) and lowest in animal shelters (0.13) showing the opportunistic feeding behaviour of An. arabienisis. Clearly, An. arabiensis is the most important vector and An. pharoensis a secondaiy vector of malaria in the Metehara area. Insecticide susceptibility studies in Metehara showed that 30% arid 25% of An. \ arabiensis was resistant to DDT and pennethrin , respectively. The level of DDT and permethrin resistance in An. arabiensis does not seem to be epidemiologically dangerous, but requires frequent monitoring. An. arabiensis was found to be highly susceptible to propoxur (carbamate insecticide) .



Studies on the Anopheline, Mosquitoes of Metehara