NO. I BLACK FLIES OF GUATEMALA — DALMAT 375 



TRANSMISSION OF HUMAN ONCHOCERCIASIS 

 IN GUATEMALA 



Since Robles' hypothesis (1919) concerning the transmission of 

 human onchocerciasis in Guatemala by Simulium flies, and Blacklock's 

 experimental proof that Simulium damnosum transmits the disease in 

 Africa (1926a and b), it has been generally accepted that these Diptera 

 are the vectors. The investigations of Strong (1931a, b, and c), Hoff- 

 mann (1930a, b, c, d, e ; 1931a), De Leon (1940a, b), Vargas (1948), 

 and Gibson (1951a), all working in Guatemala or Mexico, have cor- 

 roborated the evidence of Robles and Blacklock that Simulium species 

 do transmit onchocerciasis. Upon epidemiological grounds, 6^. ochra- 

 ceum, S. metallicum, and ^. callidum have been considered the probable 

 vectors. Based principally on the anthropophilic nature of Simulium 

 ochraceum and the apparent coincidence of its geographic distribution 

 with the endemic regions of onchocerciasis, this species has been ad- 

 judged the principal vector. 



It is true that Simulium species do abound in all regions with en- 

 demic onchocerciasis and that the transmitting species would neces- 

 sarily have to be one that attacks humans. However, in Guatemala 

 and Mexico there are several species that bite man, and it is therefore 

 advisable to weigh all available evidence to determine which of the 

 species actually transmit the disease. 



Simulium ochraceum, metallicum, callidum, haematopotum, exig- 

 uum, and veracruzanum are the species that readily attack man and, 

 therefore, can be considered possible vectors of onchocerciasis. In 

 considering the distribution of these species (see "Distribution of the 

 Guatemalan Simuliidae"), only 5". ochraceum, metallicum, and calli- 

 dum inhabit the highly endemic sections of the disease zones along the 

 Pacific slopes of the volcanic cordillera. 6". exiguum and haematopo- 

 tum are more restricted to the lower limits of the zones, and vera- 

 crusanum to the upper limits. Since callidum is only found sporadi- 

 cally throughout its range, and then in only small numbers, it can 

 probably be disregarded as an important transmitting agent. Although 

 both S. ochraceum and metallicum are usually found throughout the 

 onchocerciasis zones, metallicum is also widespread in all parts of the 

 country outside of the disease zones, from the coastal region to well 

 above 6,000 feet. Where S. ochraceum exists outside of the disease 

 zones, it is found in only small numbers. From these distributional 

 data alone, it would seem that 5. ochraceum is probably the most im- 

 portant vector of onchocerciasis. 



Now let us consider the frequency with which the different Simu- 

 lium species attack man, their biting preferences, and their natural 



