By Mojca Kristan, Assistant Professor in Medical Entomology
As Plasmodium parasites are transmitted by Anopheles mosquitoes, vector biology plays a major role in the battle against malaria, promoting a better understanding of malaria life cycle, which in turn facilitates the discovery and use of more effectively targeted control strategies. LSHTM staff and their collaborators have been contributing to research on different aspects of vector biology and malaria transmission, as outlined in this section of the Report.
With a growing demand for rice in Africa and expansion of irrigated rice cultivation, there is a potential that vector densities will increase since rice fields are breeding sites for the main malaria vectors. New research suggests that due to epidemiological changes the relationship between rice growing, vector abundance and malaria should be reassessed. Moreover, there is a need to use novel rice growing techniques that could potentially reduce vector densities. In order to be able to measure the effect, mosquito abundance should be a parameter included in rice research to estimate the adult vector productivity of rice fields.
Different factors play a part in who or what the mosquitoes bite. Mosquito feeding choices partly depend on their dispersal but measuring mosquito dispersal in the field is extremely difficult. New molecular approach based on digestion of mosquito blood meals over time was developed by LSHTM researchers and can be used to estimate the dispersal rates of mosquitoes.
Malaria transmission is also affected by the willingness of mosquitoes to feed on humans. Although mosquitoes are frequently described as “anthropophilic” or “zoophilic”, recent meta-analysis shows that human blood index is more associated with where mosquitoes were captured rather than the species of mosquitoes. Furthermore, a recent study showed that attractiveness of humans to mosquitoes due to production of certain skin odour compounds is determined by human genes. Identifying genes linked to the production of these compounds could be used to develop novel malaria control methods. Another study showed that biting behaviour of mosquitoes, especially avoidance behaviour, together with insecticide resistance, can significantly impact malaria control efforts such as deployment and use of LLINs.
Presence of endosymbiotic bacteria such as Wolbachia can also affect transmission of vector-borne diseases. Until recently, Wolbachia was thought to be absent in Anopheles mosquitoes. A recent study described the presence of novel Wolbachia strains in five Anopheles species collected in different African countries. Further studies will be carried out to determine whether they inhibit Plasmodium transmission and could therefore be used for population replacement or suppression control strategies.
The ability to detect the presence of infectious mosquitoes is of vital importance for surveillance, control and elimination efforts. A study showed that sugar-feeding behaviour of mosquitoes can be exploited for surveillance purposes as mosquitoes expel sporozoites during sugar feeding. FTA cards can potentially be used as a simple and economical tool for collection of field samples for monitoring purposes.
Insecticide resistance remains a major threat to malaria control. A project in Uganda involves monitoring phenotypic and genotypic resistance across the country to determine the association between single nucleotide polymorphisms (SNPs) that confer resistance, mosquito survival post-insecticide-exposure, and sporozoite rates. Association between vector control interventions and the prevalence of SNPs that confer resistance is also being assessed.