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Ultrastructural studies conducted to elucidate and compare the microanatomy of the South American (simuliid-transmitted) and the Caribbean (culicoid-transmitted) microfilariae of Mansonella ozzardi revealed that these microfilariae are essentially identical. The only difference detected was the larger central body of the Colombian microfilariae. Although the microanatomy of M. ozzardi microfilaria is essentially the same as that of other microfilariae, some of the subtle differences noted between M. ozzardi and other microfilariae include: amphids of unequal lengths, a large nucleus located approximately 30 microns from the anterior end, and an esophagus which appears to be less developed than that of Loa loa and Brugia spp. The microfilaria has a small cephalic hook, intracellular bacteriae in the hypodermal cells, and crystalloid inclusions which are most prominent in the R-1 cell and are also present in the hypodermal and esophageal cells. These crystalloids appear to be unique to M. ozzardi microfilariae.

Comparative morphological studies were conducted at the light microscope level to determine whether any difference could be detected in thin smears, thick smears, or Knott samples, between Colombian (simuliid-transmitted) and Haitian (culicoid-transmitted) Mansonella ozzardi microfilaria. Results indicated that the microfilariae from these two geographical areas are essentially identical. The small differences observed in the dimensions of microfilariae in thick and in thin smears can be attributed to different preparative procedures. The consistent morphological differences, noted in PAS-hematoxylin-stained microfilariae, from Knott smears, was that the central body was thinner and stained less intensely with the Schiff reagent in the Haitian than in the Colombian microfilariae. Although these observations suggest a physiological difference between these two forms, additional studies are needed to exclude the possibility that these differences are not indications of the age or a state of physiological activity of the microfilariae.