In studies on the vectors of lymphatic filariasis in Africa, species of biting mosquitoes collected include the genera Aedes, Culex Anopheles and Mansonia. Species of these genera are known to transmit lymphatic filariasis in one place or another worldwide. However, in Africa only Culex and Anopheles are reported as vectors. Culex species are important vectors in urban areas of East Africa [19, 20], but not in West Africa. A recent study carried out on Culex species in Ghana revealed that they are refractory . In West Africa, including Ghana, Anopheles species have been reported as the only vectors of W. bancrofti[1–3], although there has been an earlier report of L3 of W. bancrofti in M. uniformis from Boké, Guinea . Even though Mansonia species are known vectors of filariasis in some endemic countries in Asia they had not been considered as vectors in Africa. A recent study however, revealed that infective M. uniformis was possible in laboratory experiments but this was not the case in the wild . Therefore, this is the first report of infection of all stages of W. bancrofti in Mansonia species in Ghana and the only one in West Africa since 1958.
The global strategy for the elimination of LF is based on the vector –parasite relationships of facilitation and limitation. Anopheles vectors of W. bancrofti have been reported to exhibit facilitation and hence elimination is feasible through MDA alone where they are the vectors. Thus West Africa is among the areas where LF elimination through MDA is expected to progress without any hitch. However, in some sites in Ghana and Burkina Faso, the prevalence of infection is not what is expected after 5 to 8 years of annual MDA. For example in the KEEA district a study conducted after 6 annual MDA showed a prevalence of 2.1% (23/1107) in some neighbouring communities to the study sites (D. Boakye unpublished report). There have been various explanations for this apparent residual infection and non-compliance by infected people to take the drugs have been postulated as was found in Haiti . Others have hypothesized that on-going transmission could account for the prevalence observed and that not all Anopheles species may exhibit facilitation . A recent study by Amuzu et al. at the Gomoa District sites showed that An. melas may be exhibiting limitation and could be responsible for transmission at low level parasitaemia . It has also been suggested that even if vectors pick up parasites after MDA, the number of parasites ingested could be too low to sustain effective transmission. In this study, we found an individual infective Mansonia species that harboured up to 11 L3 larvae. This indicates that an infective bite could transmit many parasites that could set up an active infection. Hence, transmission could be sustained even if at a low level. Relatively, therefore, it does appear that Mansonia species could be more important compared to An. gambiae species especially at low level microfilaraemia.
Previous studies in some parts of West Africa and in Accra Ghana as summarized by Laurence, shows that Mansonia africana was more widely distributed than Mansonia uniformis and that their distribution does not completely coincide . We found this to be the case since both study sites have similar ecology and housing structure but Mansonia uniformis was localized in only one study area. Even though sampling at both sites was carried out in the rainy season, the study was limited by the fact that it was not carried out at the same time. The sampling methods used at the study sites were also different and these could have been a contributing factor for the differences in the distribution of the two species.
There is currently an ethical debate on the use of the human landing collection (HLC) for sampling vector species. However, HLC provides accurate estimates of transmission indices due to anthropophilic vectors, which are difficult to obtain from other collection methods. For example, mosquitoes obtained from other methods such as the indoor residual spraying collections require analysis of bloodmeals to indicate the level of anthropophily (human biting index). In the current study, the mosquitoes processed from the Gomoa district were part of sampling to accurately determine the impact of MDA on transmission of W. bancrofti hence the use of HLC.