Intensity and efficiency of transmission and the development of microfilaraemia and disease: their relationship in lymphatic filariasis

Abstract

Currently available methods for measuring the intensity of transmission in the lymphatic filariases are all to some extent unsatisfactory. Clinical methods for defining the presence of filarial disease are also less than perfect, and many infected persons remain free of symptoms and signs for very long periods. Incidence rates of microfilaraemia calculated from prevalence rate data are combined with 'entomological inoculation rates' obtained by direct observations of vector landing rates, infective larval rates and infective larval densities in field studies to determine efficiencies of transmission, and to relate transmission parameters to observed microfilarial and disease prevalence rates. Published studies from various endemic areas of the world which provide sufficient data have been analysed, using reversible catalytic models. In general, it seems that much less intense levels of transmission are needed in sub-Saharan Africa to produce given rates of microfilaraemia and disease than in Asia and Oceania, in the case of Wuchereria bancrofti infections; similarly, the genus Anopheles appears to produce infection and disease much more efficiently than the genera Culex and Aedes when transmitting W. bancrofti. The only example of Brugia malayi transmitted by Mansonia spp. analysed provided the highest level of transmission efficiency found. Tolerable levels of transmission analogous to those used in onchocerciasis cannot at present be defined for the lymphatic filariases; it seems that the intensity of transmission required to produce new cases of disease is probably below that required to produce new cases or episodes of readily detectable microfilaraemia.