Abstract:
The impact of irrigation schemes on malaria risk has been a perennial subject of debate in that transmission in irrigated areas can appear higher, less than or similar in neighboring non-irrigated villages. To study its impact on malaria transmission in the coastal savanna area of Ghana, adult Anopheles mosquito populations were studied during the dry season of 2002 - 2003 in two villages, Okyereko which is situated within a rice-growing irrigation scheme and Bewadze located 9.5km away. Indoor and outdoor human landing catches of adult mosquitoes were carried out simultaneously over 2 days each month at the villages followed by pyrethum spray catches. The mosquitoes were morphologically identified and sibling species of Anopheles gambiae s.l. identified using polymerase chain reaction based (PCR) method. Molecular forms of Anopheles gambiae s.s were further identified using Hhal restriction of the PCR products. Anopheles mosquitoes were dissected for parity and Plasmodium falciparum sporozoite infections and blood meal source were determined by ELISA. A total of 8893 mosquitoes; 8233 from Okyereko and 660 from Bewadze were caught using both HLC and PSC during the study period. Anopheles gambiae s.l. was the dominant species at Okyereko comprising 73.9% of the total collection followed by Culex species, An. pharoensis, An. funestus and Mansonia species, Aedes species and An. coustani respectively. At Bewadze, An. funestus was the dominant species accounting for 33.5% of the total collection followed by An. Gambiae s.l., Culex species, Mansonia species, An. phaoensis and Aedes species repectively. A total of 798 Anopheles gambiae s.l. from both sites (602 from Okyereko and 196 from Bewadze) were all identified as Anopheles gambiae s.s by PCR. The M and S molecular forms constituted 91.7% (66/72) and 8.3% (6/72) at Okyereko and 66.7% (52/78) and 33.3% (26/78) at Bewadze respectively. The mean biting rate of anopheline mosquitoes was approximately 17 times higher at Okyereko than at Bewadze (126.3 vs 7.5 bites/man/night). Parous rates however were similar being, 65% (874/1345 dissected) at Okyereko and 63.8% (157/ 246 dissected) at Bewadze. At Okyereko, the sporozoite rates of An. gambiae s.s. and An. funestus were 0.2% (4/2,411) and 0.5% (1/213) respectively. However, it was higher at Bewadze, where it was 2.1% (4/188) and 5% (9/179) respectively for the two species. The overall infection rate (due to both An. gambiae and An. funestus) was about 17.5 times lower at Okyereko compared to Bewadze (0.2% vs 3.5%). None of the An. pharoensis and the An. coustani examined was sporozoitepositive. The estimated yearly EIR for An. gambiae s.s. and An. funestus were 66.7 and 12.9 at Okyereko, and 28 and 69.7 infective bites/man/night respectively at Bewadze. The yearly EIR (due to both species) was 1.2 times lower at Okyereko than at Bewadze (81.9 vs 95.6 infective bites/man/night). The HBI of An. gambiae s.s. and An. Funestus was 85.3% and 82.8% at Okyereko, and 92% and 62.2% respectively at Bewadze. The results obtained indicate that despite the higher number of Anopheles vector species at the irrigated area, the malaria challenge was lower there than in the neighbouring nonirrigated community.
