ALTITUDINAL VARIATION IN SPECIES COMPOSITION AND INFECTION RATE OF ANOPHELES MOSQUITOES, AND THE RISK OF MALARIA AND ITS CONTROL ACTIVITIES IN THE HIGHLAND OF DIRASHE WOREDA, SOUTH ETHIOPIA

Abstract

condition of highlands creates a suitable environment for vector development and increase the chance of malaria transition locally, but little information is available on indigenous malaria transmission in most highlands. Hence, the current study aimed to investigate variation in species composition, infectivity rates of vectors, malaria infection and its control interventions along the altitudinal gradients in Dirashe Woreda, South Ethiopia. Longitudinal entomological study was carried out in three kebeles (Gato, Onota and Layignaw-Arguba) from August 2015 to April 2016. Thirty houses were randomly selected (ten from each kebele) for biweekly Anopheles mosquito collection. The circum-sporozoite proteins (CSPs) rate of Anopheles mosquitoes were detected using Enzyme-Linked Immuno-Sorbent Assay (ELISA). Health facility based malaria cases (August 2015-April 2016) were collected from laboratory registration book of four health posts and two health centers to determine malaria cases and Plasmodium species in each kebele. Malaria control intervention data were collected from 477 randomly selected households of the three kebeles. Nine species of Anopheles mosquitoes comprising An. arabiensis, An. demeilloni, An. cinereus, An. pharoensis, An. funestus group, An. pretoriensis, An. christyi, An. ardensis and An. tenebrosus from adult collections and six species such as An. arabiensis, An. demeilloni, An. cinereus, An. An. funestus group, An. pretoriensis, An. christyi from larval collections were documented in the study area. An. arabiensis was by far the most dominant in Gato whereas An. demeilloni was the dominant species in Layignaw-Arguba. No An. arabiensis were collected from Layignaw-Arguba and was rarely found in Onota. A total of 1102 Anopheles mosquitoes were tested for CSPs. Of which five Anopheleses were positive for P. falciparum CSP with the overall sporozoite rate of 0.45%. The P. falciparum sporozoite rate of An. arabiensis in Gato was 0.4% (3/754) and the estimated EIR was 8.6 infective bites per person per nine months. Two An. demeilloni from Layignaw-Arguba were positive for P. falciparum CSP and hence its sporozoite rate was 0.64% (2/311). No P. vivax sporozoite positive An. arabiensis and An. demeilloni were detected. The number of malaria cases was significantly different (p <001) with more malaria cases in Gato. P. falciparum (including the mixed infection) was the predominant Plasmodium species (88% in Gato and Onota (57.5%), whereas in Layignaw-Arguba P. vivax (59.4%) cases were dominant. Although malaria cases occurred in all age groups, increased malaria cases were observed in children age 5-14 years in Gato. The scenario was different in Onota and Layignaw- Arguba where more malaria cases were reported in age group >15 years. The study revealed that 92.7% of the households in the study area owned at least one LLIN and overall 77.6% of the households members slept under the net during the preceding night of the survey. The overall LLINs usage was significantly higher (86.4%) in the Gato compared to 71.3% in Onota and 64.6% in Layignaw-Arguba (χ 2 = 23.86, DF = 2, P < 0.001). In contrast, the utilization of bed net by mothers and under 5 years children were increasing from the lower altitude to high altitude. About 64.4% of the households in Gato were protected by IRS. However, the spray time was not considering the local information and performed when the indoor Anopheles density was very low and malaria cases were already built up and hence, less likely effective. In conclusion, the incrimination of Plasmodium positive Anopheles species in Layignaw-Arguba justifies the existence of local malaria transmission and the need of increasing bed nets distribution based on the family sizes for effective control of malaria. On the other hand, there is a need of further investigation and confirmation using more sensitive molecular techniques to consider An. demeilloni as a proven vector of malaria.