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Development of Onchocerca volvulus Larvae in Simulium pintoi in the Amazonas Region of Venezuela^*

Hiroyuki Takaoka, Hiroshi Suzuki, Shinichi Noda, Isao Tada^||, Marií G. Basáñez^** AND Luis Yarzábal^**

Division of Medical Zoology, Medical College of Oita, Oita, Japan
Department of Virology, Institute for Tropical Medicine, Nagasaki University, Nagasaki, Japan
Department of Medical Zoology, Faculty of Medicine, Kagoshima University, Kagoshima, Japan
^|| Department of Parasitic Diseases, School of Medicine, Kumamoto University, Kumamoto, Japan
^** Section of Parasitic Immunology, National Institute of Dermatology, Caracas, Venezuela

The intake and development of Onchocerca volvulus in Simulium pintoi from the Parima mountain region of the Federal Territory of Amazonas in Venezuela, were studied experimentally. When wild females fed on the lower half of the legs and lower third of the back of an onchocerciasis patient harboring 23 and 264 microfilariae per skin snip, respectively, at each site, an average (median) of 14 (range, 1–77) and 245 (range, 58–495) microfilariae was ingested. However, within 24 hours of microfilarial ingestion a mortality of 47% (16/34 flies) was observed in the group of flies which fed on the back, as compared with 2% (2/101 flies) in the other group which fed on the legs. At a temperature varying between 16°C and 24°C, the development of O. volvulus larvae in S. pintoi was synchronous and orderly; no abnormal nor deformed larvae were observed. Third-stage larvae were first seen in the head of flies dying between 8 and 9 days after microfilarial ingestion, and 98 of 100 larvae recovered from days 10–16 were in the third stage. The proportions of females harboring third-stage larvae among flies which lived through day 8 in the two groups which fed on the legs and back, respectively, were 55% (21/38 flies) and 63% (5/8 flies). Although only two of five positive flies in the latter group contained third-stage larvae in the head (1 and 12, respectively), 71% (15/21 positive flies) of the former group had an average of 2.7 third-stage larvae in the head (range, 1–10). In conclusion, it is suggested that S. pintoi is an efficient vector of O. volvulus due to its high susceptibility, in spite of the high mortality caused by an excessive intake of microfilariae.

Accepted for publication November 17, 1983.

^* This study was supported by the grant "Oversea Scientific Research" from the Ministry of Education, Culture and Science, Japan (Nos. 57041041 and 58043041) and by grants from the National Council for Scientific and Technological Research (No. S1-1128) and PROICET Amazonas of Venezuela.