Larval biology of six filariae of the sub-family Onchocercinae in a vertebrate host.

The development of six filariae of the sub-family Onchocercinae-Litomosoides sigmodontis, Acanthocheilonema viteae, Molinema dessetae, Monanema martini, Brugia malayi, B. pahangi-was compared in rodents, following a single inoculation of a low or high dose of infective larvae. Analysis was done with 105 rodents dissected and 53 rodents fixed for histopathology. The percentage of larvae which developed corresponded to the proportion of those which were able to penetrate into the sub-cutaneous lymphatic vessels; this percentage was determined during the first day (phase 1) and was characteristic of the filaria-host pair, and independent of the number of larvae inoculated. It could remain stable for a long time, more than eight months with M. martini (phase 2); the phenomena of regulation appeared later (phase 3). The larvae migrated through the lymphatic system, which represents a medium less protected and thus less aggressive than the blood system. The coelomic cavities, almost devoid of inflammatory cells, represented an ultimate shelter, as well as the joint-cavities (colonized by some Dirofilariinae). Localizations in the cardio-pulmonary blood system were accidental and occurred when, during the migrations, some larvae penetrated into the thoracic channel and arrived in the superior vena cava, then the right ventricle and the pulmonary arteries (the biology of Dirofilaria immitis resulted in a secondary adaptation); such accidents may occur with adult filariae, especially, after drug treatment. One may expect similar events in human filariasis. These "occult" filariae, more frequent than it is usually thought, influence the immunological status and the pathology.