The significance of low density microfilaraemia in the transmission of lymphatic filarial parasites.

Low density microfilaraemia (mf) is a density of circulating mf which is often undetected by standard survey techniques; it occurs naturally, after anti-filarial drug administration and after vector control. Its occurrence in human populations is closely related to the observed mf frequency distributions in them, and it is an important cause of underestimation of mf prevalence rates in epidemiological surveys. In the present paper it is defined quantitatively as a count of less than 4 mf 20 microliters-1 of capillary blood or less than 30 mf ml-1 of venous blood. Detection of low intensity transmission of parasites is difficult; detection by clinical, entomological or immunological methods may be more sensitive than the usually employed parasitological techniques, due to the extreme inefficiency of the transmission process. Mosquito vectors of filariasis ingest and develop low density mf readily; since they exhibit limitation or proportionality, Aedes, Culex and Mansonia spp. vectors do this more efficiently than Anopheles spp. which exhibit facilitation. Field studies indicate that low level microfilaraemia can initiate a resumption of transmission after very efficient control programmes where Aedes spp. are vectors, whereas eradication has been achieved in areas of Anopheles transmission by levels of vector control which fall far short of eradicating malaria. The situation in the extensive endemic areas where Culex spp. are vectors is less clear, and should be a research priority.