40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



ceum up to 48 hours after the flies had taken an infective blood meal. 

 Gibson reported that, over a period of a year, he found natural infec- 

 tion in 0.38 percent of 1,839 ■^- ochraceiim, 1.04 percent of 1,734 S. 

 metallicum, and 0.62 percent of 162 5". callidum. These data on natural 

 infection rates, as well as the data on experimental infection rates, 

 are summarized in table 6. The data demonstrate : that 5. ochraceum, 

 metallicum, and callidum will support the development of the human 

 microfilariae and that most advanced development was observed in 

 6*. metallicum; that both metallicum and callidum appear capable of 

 surviving in captivity for longer periods than ochraceum; that all three 

 species support infections in nature. Recently Gibson (1951b) found 

 an advanced larva in a specimen of 6'. metallicum that was dissected 

 310 hours after feeding on an infected person. This larva was in a 

 preinfective stage, thus adding additional evidence to the probability 

 of this species serving as a vector of onchocerciasis. 



It has been our experience that not only do the Simulium flies ingest 

 the microfilariae and support their development, but they also seem to 

 attract the microfilariae to the region of their bite. In taking numerous 

 biopsies from an infected person, much fewer microfilariae are found 

 than in dissections of flies that have just fed on the same patient. 

 Biopsies taken in the immediate regions of bites, soon after the flies 

 are removed, will also contain more microfilariae than the biopsies 

 taken in other regions of the body, or in the same region prior to the 

 feeding of the fly. 



Gibson (1951b) also found the natural-infection rate in the three 

 principal anthropophilic species to be extremely low to zero from Oc- 

 tober through December, when the finca workers are most exposed to 

 the bites of the flies. This was not the experience of the present 

 author. It will be seen in table 5 that naturally infected flies were 

 caught during the dry months of November, January, February, 

 March, and April, No flies were collected during December, probably 

 owing to the small samples of the fly population that were dissected 

 during this month when the adult population, and, therefore, the dilu- 

 tion of infected flies, are at a peak. This table is further discussed 

 under "Natural Infection of Adults with Onchercerca volvulus" in the 

 section on ecology. 



The identity of the microfilariae in naturally infected flies was not 

 given by De Leon (1947) or Gibson (1951a). As stated by the latter, 

 skin biopsies of 884 local animals (mainly horses and cattle) showed 

 that 12 percent of the horses and cattle were infected with skin- 

 inhabiting microfilariae, superficially similar to those of human oncho- 

 cerciasis. There still remains the problem of developing adequate 



