2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 25 



are still unknown. In Mexico and Guatemala the nodules are found 

 preponderantly in the region of the head and shoulders of infected 

 individuals, while in Africa the nodules are more prevalent around the 

 waist. Ocular involvement, including blindness, is an important mani- 

 festation of the disease. 



Although early workers speculated a good deal concerning the trans- 

 mission of the parasite from one person to another, it was the dis- 

 covery of the presence of the disease in Guatemala by Robles in 191 5 

 (Calderon, 1917; Robles, 1919) that actually stimulated extensive 

 investigations of onchocerciasis and its transmission. The disease is 

 now known to be endemic in large areas of central Africa (Puyuelo 

 and Holstein, 1950), in Guatemala, Central America (Strong et al., 

 1934), Mexico (Puig Solanes et al., 1948), and in Venezuela, South 

 America (Potenza et al., 1948). A single case, diagnosed as probably 

 being onchocerciasis, was reported by Hartz (1950) from Surinam, 

 Dutch Guiana. The recent discovery of the presence of the disease 

 in Venezuela (Potenza, Cordero, and Anduze, 1948) suggests the 

 possibility of still wider distribution of onchocerciasis than is known 

 at the present time. Although accurate statistics concerning the inci- 

 dence of the disease are not available, it is believed that almost a mil- 

 lion people in these various areas are infected. The estimate given for 

 Mexico is about 35,000, and for Guatemala about 25,000. The number 

 of cases in Venezuela still has not been determined, although it is be- 

 lieved to be low in comparison to the figures for Guatemala or Mexico. 

 The degree of infection in the endemic zones of Guatemala varies 

 greatly from one locality to another. On some fincas (plantations) as 

 few as 5 percent of the population is infected, while on others, not 

 very distant, nearly 100 percent is infected. From the statistics made 

 available by the Department of Public Health of Guatemala, it ap- 

 pears that a general average of about 35 percent of the population in 

 the disease zones is infected. Ocular involvement is manifest in more 

 than half of the persons infected, and blindness (pi. i, fig. i), the most 

 serious sequela, occurs in approximately 5 percent. 



Robles (1919; see also Calderon, 1917), after discovering the pres- 

 ence of onchocerciasis in Guatemala and after making preliminary 

 epidemiological studies, was inclined to believe the vector ^ was a day- 

 biting insect, probably two anthropophilic species of SimuHum flies he 

 found in the endemic region. It remained for Blacklock (i926a,b), 

 working in Africa, to be the first to infect Simulium adults with the 

 microfilariae of Onchocerca volvulus and to trace subsequent develop- 



2 Throughout this paper the word "vector" is used interchangeably with "inter- 

 mediate host." 



