Investigations on pathogens and parasites in larval blackflies in and near the Guatemalan areas of endemic onchocerciasis were made from September 1978 to March 1979. Two mermithids (Isomermis sp. and Mesomermis sp.), eight microsporidans (Thelohania bracteata, T. fibrata, two Thelohania spp., Pleistophora multispora, P. debaisieuxi and two Pleistophola spp.), a fungus (Coelomycidium sp.) and two viruses (iridescent virus and cytoplasmic polyhedrosis virus) were found. Mermithids, microsporidans, and fungi infected, respectively 2.4, 1.2, and 0.2% of 23,346 blackfly larvae. Nineteen of 23 simuliid species examined harbored one or more groups of parasites. Mermithids infected one species of Gigantodax, one of Cnephia, and eight of simulium, Microsporidans parasitized one species of Gigantodax and 13 of Simulium, while Coelmycidium sp. infected one of Gigantodax and four of Simulium. Iridescent virus infection was recognized in three species of Simulium, and cytoplasmic polyhedrosis virus only in S. aureum. Mermithid, microsporidan, and fungal infection levels for the three onchocerciasis vectors varied considerably. S. ochraceum did not harbor mermithids, whereas S. metallicum and S. callidum frequently did so, with incidences of 8.9% in 27 of 51 habitats and 4.9% in 11 of 32 habitats, respectively. Microsporidan infections (always at low incidence) were found in all three vectors. Microsporidiosis was rare in S. ochraceum but common in S. metallicum and S. callidum. Fungal infection was found in 0.04% of S. ochraceum but not in the other two species.
Present address: Medical College of Oita, Hasama, Oita, Japan.