THE IMPACT OF AN IRRIGATION SCHEME ON MALARIA TRANSMISSION BY ANOPHELES (DIPTERA: CULICIDAE) IN A COASTAL SAVANNA AREA OF GHANA

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

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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.