[New data on the transmission of filariae]

This paper summarizes research of the past several years on two phases of filarial transmission: the ingestion of microfilariae by the vector and the regulation of the percentage of successful parasites by the vector. Experiments involved several different models for study: injection of inert particles into the bloodstream of a rodent and their subsequent ingestion by a vector; ingestion of gametocytes of different ages by vectors of a rodent malaria; and a monkey simultaneously infected with four different species of filariae. This has led to the idea that there exist two categories of microfilariae: those blood-borne microfilariae which cannot enter capillaries and those microfilariae which are said to be "dermic" but in fact live trapped in capillaries and are only taken up through the suction applied by vectors during their feeding. Thus, the lesions of onchocerciasis are caused by microfilariae accidentally leaving the capillaries and entering the surrounding tissues. Passage through the stomach wall of a vector by microfilariae is the essential stage in regulating the percentage of successful parasites: the phenomena of facilitation or limitation. Two examples of limitation are given. In the model--Onchocerca volvulus-Simulium sirbanum--the thickness of the peritrophic membrane which traps microfilariae is determined by the number of microfilariae ingested; the greater the number ingested the thicker is the membrane. In the model--Molinema dessetae-Aedes aegypti--the lytic reactions of individual digestive cells which are provoked by attacking microfilariae occur only in heavily infected mosquitoes. The "immunity" of the arthropod vector seems to take a number of forms which are distinctly different from those of vertebrates.