Abstract:
Ability of mosquitoes to ingest microfilariae (mf), promote their maturation to the infective stage, and their survival rate to parasite maturation for transmission to humans seems to differ according to geographic mosquito strains. The proportion of ingested mf that develops successfully into L3 may decrease (limitation) or increase (facilitation) with higher mf uptake. Transmission intensity depends on a number of factors such as the level of infection in the human population, vectorial capacity, vectorial competence, and climatic factors. Results obtained from a preliminary study after three years of mass drug administration (MDA) showed a decrease in annual transmission potential (ATP) of Anopheles funestus but no change in An. gambiae s.s. This study was conducted to determine the vector competence of these two Anopheles species in the transmission of Wuchereria bancrofti at low mf levels. Mass screening for mf was done using 100μl finger-prick blood and consented positive individuals who volunteered and slept under mosquito nets that had one side opened. Wild mosquitoes that fed on them were collected hourly from 21:00 to 06:00 hours GMT. Approximately half of the mosquitoes were killed immediately and dissected to count the number of mf ingested. The remaining mosquitoes were maintained for 13 days to observe parasite maturation after which they were dissected. Alongside the mosquito collection, 100μl finger-prick blood was collected hourly to observe mf level in peripheral blood. The overall prevalence of mf in the study community (N = 1083) was 1.6%. The levels of mf varied from 0 to 59 mf7 100μl blood, with a geometric mean intensity of 1.1 mf7 ml. Some variation in intensity with age-group was observed, however neither the intensities in age group (P = 0.40) nor the intensities in the male and female subjects (P = 0.91) were significant. Out of the 564 mosquitoes collected, 62.1% were Anopheles species, 32.3% Mansonia species, 5% Aedes species and 0.7% Culex species. Anopheles funestus and An. gambiae formed 8 8 .6% and 9.1% of Anopheles caught respectively. Both mf level in peripheral blood and biting rates of the Anopheles mosquitoes peaked between 00:00 and 03:00 hours. Six mosquitoes each of Anopheles (1.7%) and Mansonia (3.3%) were found infected but none was infective after day 13 of maintenance. Molecular studies showed all Anopheles gambiae s.l. to be An. gambiae s.s. out of which 70% were M form. All infected Anopheles gambiae were M forms. A total of 8 6% of the An. funestus were identified as An. funestus s.s. with 6% being An. leesoni. Although these Anopheles species were not competent in promoting the maturation of the parasites when mf is low, a repeat of this study targeting larger mosquito numbers is required to ascertain the role played especially by M forms of An. gambiae in the transmission of lymphatic filariasis when parasite levels in the community are low. Considering the fact that the study was conducted in the natural setting, this finding will help as to whether the combination therapy with ivermectin and albendazole is enough to eliminate the disease or vector management has to be integrated for the success of the GPELF in areas like Ghana where Anopheles gambiae and An. funestus are the main vectors.
