378 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 25 



cerciasis ? Why is the natural-infection rate in metallicum higher than 

 in ochraceumf 



Let us discuss the first question, relative to the nonconformity of the 

 distribution of ochraceum with that of the disease. As has already been 

 mentioned, ochraceum is present primarily in regions of highest en- 

 demicity of onchocerciasis, although relatively small numbers are 

 also in evidence outside of the known disease zones. The endemic 

 areas can be extended by either of two means : migration of infected 

 flies from a disease region to regions free of the disease where they can 

 infect other individuals ; or entrance of infected individuals into non- 

 endemic regions, giving ochraceum the opportunity of ingesting micro- 

 filariae. In the studies of flight range and longevity of infected flies 

 (see section on ecology), it was seen that infection with O. volvulus 

 does reduce the flight capacity and life span of the flies. Thus the 

 probability of migration of infected flies from infected to noninfected 

 regions is greatly reduced. In the case of ochraceum living outside of 

 the infected region, it is possible that certain environmental factors 

 might reduce either their anthropophilic tendencies or their ability to 

 support development of the Onchocerca larvae. Peterson (1953) 

 found that four species of black flies in Utah would bite humans only 

 within certain limits of its altitude range, above and below which it was 

 not anthropophilic. Since our records of S. ochraceum outside of the 

 infected regions were based primarily on collections of pupae, it can- 

 not be ascertained to what extent emerging adults would have favored 

 human hosts. The number of adults collected outside of the zone was 

 too small to be of significance. Taking into consideration the low 

 natural-infection rate in ochraceum, it can be seen that in a highly 

 dispersed agricultural population there would have to be an abundance 

 of ochraceum in order to adequately establish an endemic disease 

 region. 



To answer the second question, we need only to refer to the heavy 

 mortality in the flies of this species caused by infection with O. volvu- 

 lus. This was mentioned in the discussion of flight range and longevity 

 of infected flies. Knowing that 5. ochraceum undoubtedly ingests tre- 

 mendous numbers of microfilariae, owing to its extended and non- 

 interrupted blood meals, it becomes evident that a very large propor- 

 tion of the infected flies must die because of that infection. 



The last question has already been alluded to in "Animal Associa- 

 tions" in the section on epidemiology and in the earlier part of this 

 section. Since metallicum bites both human and other animal hosts, 

 and since it actually prefers hosts other than man, it is quite probable 

 that its high natural infection is, at least in part, of equine or bovine 



