The filaria Litomosoides galizai in mites; microfilarial distribution in the host and regulation of the transmission

The mites, Bdellonyssus bacoti, are engorged on rodents having 800 to 60,000 microfilarie/10 mm3 blood. Quantitation of L. galizai larval development shows that an additional blood meal improves development and that high microfilaremiae do not result in a proportional increase in the number of infective larvae. The first important stage of transmission regulation occurs during ingestion of microfilariae: the numbers of ingested microfilariae are lower than expected in cases of high microfilaremia. This phenomenon cannot be ascribed to the mite vector that engorges a constant blood meal whatever the level of microfilaremia. Contrarily, one finds that microfilarial density in the small peripheral blood vessels (blood drawn from incision of the dorsal skin) increases less than in large blood vessels (retro-orbital sinus). A similar observation was reported by Dickerson et al. (1989) working with Wuchereria bancrofti. We assume that in both cases, the high microfilaremiae cause the small blood vessles accessible to the vector to become saturated with parasites. Although regulation during engorging is not the sole factor to monitor the infection in B. bacoti (another one operates during larval development of L. galizai), demonstrating its existence seems to us fundamental: it points out the concept that sub-ingestion, as well as over-ingestion, shows the inequalities of microfilarial densities in the host which seem to be dependent on mechanical factors such as the diameter of blood vessles and the size of microfilariae.