Daily survival and human blood index of major malaria vectors associated with oil palm cultivation in Cameroon and their role in malaria transmission.

OBJECTIVE: To describe the daily survival rate, biting habits, feeding behaviour, sporozoite and entomological inoculation rates of Anopheles species and their relative contribution to Plasmodium falciparum transmission in large-scale oil palm-growing zones in Cameroon. METHODS: Entomological surveys by employing human-landing catches, both indoors and outdoors, two nights per month for 12 months from October 2004. RESULTS: A total of 2485 Anopheles were collected and four species recorded: An. funestus, An. hancocki, An. gambiae s.s. and An. nili. There was substantial indoors to outdoors variation and seasonality in the distribution of the Anopheles population. All four species showed similar nocturnal biting patterns and were sympatrically involved in malaria transmission simultaneously or replacing each other seasonally. Their constant presence throughout the year and longevity suggest that transmission can occur at any time if parasite reservoirs are present in the area. The circumsporozoite protein rates were 5.8%, 6.3%, 8.4% and 4.1%; mean anthropophilic rates were 97.1%, 94.5%, 73.9% and 77.6%, respectively, for An. funestus, An. gambiae s.l, An. nili and An. hancocki. The annual entomological inoculation rate was 1077 infectious bites per person. Of those An. gambiae specimens testing positive for P. falciparum sporozoites, 69.01% were M form and 30.99% were S form. CONCLUSION: Any vector control strategy intending to significantly reduce the malaria burden in the palm estate will have to take into account entomological heterogeneity in addition to ecological and socio-economic multiplicity.

Ecological transition from natural forest to tea plantations: effect on the dynamics of malaria vectors in the highlands of Cameroon.

From October 2002 to September 2003, an entomological survey was carried out in a rural forested fringed village in the highlands of Mount Cameroon region to determine the temporal dynamics of the anopheline population and the intensity of malaria transmission. A total of 2387 Anopheles spp. were collected, with A. funestus predominating (59.9%), followed by A. hancocki (24.4%) and A. gambiae s.l. (15.7%). Considerable differences were observed in the nocturnal biting cycles of parous mosquitoes, with peak activity in the latter part of the night. PCR revealed that all specimens of the A. funestus group were A. funestus s.s. and all specimens from the A. gambiae complex were A. gambiae s.s. of the S molecular form. Plasmodium falciparum sporozoite rates of 17.3% and 8.5% were recorded for A. funestus and A. hancocki, respectively, with an anthropophilic rate of 96.3%. A strong positive correlation (r=0.996) was found between the human-biting rate and the entomological inoculation rate (EIR). Malaria transmission was very high and perennial, with an estimated annual EIR of 460.1 infective bites per person per year. These results confirm that in high agricultural activity areas, A. funestus can be by far the major malaria vector responsible for malaria transmission.

Climate change and altitudinal structuring of malaria vectors in south-western Cameroon: their relation to malaria transmission.

An entomological survey was conducted in Cameroon between October 2004 and September 2005, in nine localities targeted for malaria vector control based on adult productivity and variability. Mosquitoes were collected by human-landing catches (HLCs) and pyrethrum spray catches. A total of 12 500 anophelines were collected and dissected: Anopheles gambiae s.l. (56.86%), An. funestus s.l. (32.57%), An. hancocki (9.38%), and An. nili (1.18%). Applying PCR revealed that specimens of the An. funestus group were An. funestus s.s. and An. gambiae complex were mostly An. melas and An. gambiae s.s. of the M and S molecular forms with the M forms being the most predominant. The natural distribution patterns of Anopheles species were largely determined by altitude with some species having unique environmental tolerance limits. A human blood index (HBI) of 99.05% was recorded. Mean probability of daily survival of the malaria vectors was 0.92, with annual mean life expectancy of 21.9 days and the expectation of infective life was long with a mean of 7.4 days. The high survival rates suggest a high vector potential for the species. This information enhances the development of a more focused and informed vector control intervention.