ONCHOCERCIASIS 243 
observations with reference to Simuliwm damnosum as the transmitter of On- 
chocerca volvulus are apparently the first to confirm the very important ones 
of Blacklock,! already published several months before our observations were 
made. Brumpt had suggested that several insects might transmit the parasite 
to man, and Rodhain, Van den Branden, and Blanchard and Laigret performed 
experiments with certain mosquitoes, flies, bedbugs, and ticks without, however, 
demonstrating insect transmission of the parasite (see page 850). Robles,? in 
Guatemala, also had previously suggested, even before Blacklock’s findings, that 
coffee flies, Simuliwm dinellii, might transmit Onchocerca caecutiens, but he made 
no investigation of any kind on the subject. 
Blacklock * found that larvae ingested by Simuliwm damnosum leave the gut 
within twenty-four hours and within forty-eight hours have reached the thorax, 
by which time they have become stumpy forms with the characteristic caudal 
appendage. After several moults they pass to the head and proboscis, about seven 
or more days after infection. He also observed that heavy infection appears to 
be fatal to the insect. By allowing wild flies to feed on heavily infected areas 
of the skin in human eases the gut infection of the flies was raised to eighty per 
cent in one experiment and the thorax infection to nearly eighty-two per cent in 
another experiment. The developing forms of O. volvulus were found in the 
thorax after the infecting feed up to the seventh, eighth, and tenth days, after 
which period no insect survived. Blacklock also attempted to infect two mon- 
keys, which were inoculated intra- and subcutaneously, the first in the flank and 
the second in the head, with advanced forms of larvae contained in the heads 
of the flies. Neither of the monkeys showed any important reaction, the incisions 
healing by first intention, and they had shown no evidence of infection up to 
the time the report was published. Perhaps the species of monkey employed 
(not stated) might not be susceptible to Onchocerca infection. 
From Blacklock’s observations, confirmed by our own, it would appear that 
Simulium has at least been shown to harbor a parasite pathogenic for man. 
An actual demonstration, by biting experiments, that this fly transmits the 
filariae to man, would for obvious reasons be very difficult. However, there is 
no reason to doubt that the fly when feeding on human cases of infection ingests 
the embryonic form of the filaria, and it has been demonstrated that some trans- 
formation and development of the filaria takes place in the fly. 
From all the evidence available, it seems highly probable that Simuliwm 
damnosum is the natural agent of transmission of the infection. 
It was formerly repeatedly claimed that Szmuliwm samboni was the trans- 
mitting agent in pellagra, but there was never any experimental proof of this 
and the idea has since been proved to be erroneous.' It has also been suggested 
that Simulium may sometimes transmit cutaneous leishmaniasis and anthrax. 
Simulium amazonicum is very common in the Amazon basin along the Rio 
Braneo. We dissected large numbers of these flies there in 1924 but never found 
1 Blacklock: Ann. Trop. Med. and Parasit. (1926), XX, 1. 
2 Robles: Bull. Soc. Path. Exot. (1919), XII, 448. 
§ Blacklock: Ann. Trop. Med. and Parasit. (1926), XX, 203; Brit. Med. Jour. (1927), p. 219. 
4 Sambon: Jour. Trop. Med. and Hyg. (1910), XIII, pp. 271, 287, 290, 305, 319. 
