Ecology of Simuliid Vectors of Onchocerciasis in Guatemala

Abstract

Onchocerca volvulus, the parasite causing human onchocerciasis, was first described from Africa by Leuckart in 1893. Although early workers speculated a good deal concerning the transmission of the parasite from one person to another, it was the discovery of the presence of the disease in Guatemala by Robles in 1915 (Calderon, 1917; Robles, 1919) that actually stimulated extensive investigations of onchocerciasis and its transmission. The disease is now known to exist in Africa, Guatemala, Mexico, and Venezuela. Robles (1919) after discovering the presence of onchocerciasis in Guatemala, was inclined to believe the vector was a day-biting insect since people who lived outside of the disease zone, and entered it only during the day to work on the coffee plantations, became infected. He then voiced his suspicion that two anthropophilic species of Simulium flies found in the endemic region were probably the vectors, but he did not carry out investigations to verify his belief. It remained for Blacklock (1926), working in Africa, to demonstrate that Simulium spp, might be the vectors. He was able to infect wild-caught Simulium damnosum on infected patients and to trace the subsequent development of the parasite in the gut, thorax, head and proboscis of the flies. Infective larvae were found in the proboscis as early as 7 days after the infective meal. Sharp (1927) and Bequaert (1929), working in Africa, Hoffmann (1930a, b, c, d, e; 1931), Vargas (1948), and Vargas and Ruiz Reyes (1949) in Mexico, and Strong (1931a, b, c; 1934) and De Leon (1940a, b) in Guatemala also studied the development of microfilariae in Simulium spp. Wanson, Henrard, and Peel (1946), working in the Belgian Congo, were the first to report the entire development of 0. volvulus in laboratory-reared S. damnosum. Like Blacklock, they found that the microfilariae reached the infective stage in only 6 to 7 days. In Guatemala, attempts to repeat the experiments of those investigators with local Simulium species have not met with equal success since it has not been possible to induce laboratory-reared flies to take blood meals. Gibson and Dalmat (1952) found a high percentage of wildcaught S. exiguum naturally infected with sausage forms of what they believe may be Onchocerca gutturosa of cattle. Gibson (1951a), using wild-caught