The fate and migration of microfilariae of a Guatemalan strain of Onchocerca volvulus in Simulium ochraceum and S. metallicum, and the role of the buccopharyngeal armature in the destruction of microfilariae.

The migration and fate of microfilariae of a Guatemalan strain of Onchocerca volvulus in Simulium ochraceum and S. metallicum was studied. The anatomical distribution of microfilariae at various times after ingestion was recorded from a study of histological sections of the flies. S. ochraceum showed higher microfilarial intakes than S. metallicum which is probably attributed to an inherent difference in their ability to ingest. The migration of microfilariae out of the mid-gut took place essentially through the expanded part (the stomach). Despite the high microfilarial intakes by S. ochraceum, only a few microfilariae were able to migrate successfully into the thorax (2.6%) whereas in S. metallicum showing a low microfilarial intake, a higher rate of migration was noted (74.5%). The main bulk of microfilariae ingested by S. ochraceum died and disintegrated in the stomach. This was due to the presence of a well-developed cibarial armature in the buccopharyngeal apparatus of S. ochraceum in contrast to S. metallicum where such an armature is lacking. Microfilariae were severely damaged by the cibarial teeth while en route to the stomach. They rapidly underwent profound changes in morphology and staining properties and were often surrounded by clusters of the phagocytic leucocytes in the blood mass. In some flies, microfilariae, apparently damaged, were still entangled in the cibarial teeth 5 minutes to 3 hours after taking a blood-meal.