The development of Onchocerca volvulus in two temperate blackfly species, Simulium ornatum Meigen and Simulium lineatum Meigen

Acquired resistance to Onchocerca lienalis infections in Simulium ornatum Meigen and Simulium lineatum Meigen following passive transfer of haemolymph from previously infected simuliids (Diptera, Simuliidae)

This study describes experiments which demonstrate the presence of a parasite-induced or derived factor in the haemocoel of Onchocerca lienalis-infected simuliids. Haemolymph, when transferred from previously infected donor flies to previously untreated recipient flies, using fine glass needles, confers protection against a subsequent challenge to the recipients. In three trials using Simulium ornatum sl. or S. lineatum, significant levels of protection in 'immunized' compared to 'challenge control' groups were achieved (84% and 81% with S. ornatum sl. and 85% with S. lineatum). This is despite the fact that the worm burdens of the infected donor groups in the three trials were very different (means of 33.1 and 3.4 larvae/fly with S. ornatum sl. and 1.6 larvae/fly with S. lineatum). The use of sham-operated and bentonite-inoculated control groups demonstrate that this effect is not merely a non-specific reaction to trauma, but is probably parasitic in its derivation. The use of double infections separated by 4 days shows that the donor flies have themselves probably acquired resistance to O. lienalis. Frequency distributions of O. lienalis burdens in individual flies show that there is a shift from a normal to a skew distribution when comparing 'challenge control' flies with 'immunized'. This possible acquired resistance could have important implications in the transmission of forest and savanna human onchocerciasis by the Simulium damnosum complex.

Screening of some British simuliids for susceptibility to experimental Onchocerca lienalis infection

Ten species of Simulium from Britain were tested for susceptibility to Onchocerca lienalis infection by intrathoracic injection of microfilariae recovered from the skin of cattle at slaughter. All of the species examined supported development of a proportion of the microfilariae to third-stage larvae, but this was greatest at 53% in S. ornatum s.l., which is a known natural vector in Britain. Inocula of 20, 30 or 50 microfilariae resulted in similar levels of mortality in S. ornatum s.l. (42-53%), but a rise in the dose above 20 microfilariae per fly increased the proportion which developed to third-stage larvae. In a series of comparative experiments employing S. ornatum s.l. as a reference positive for blackflies of unknown susceptibility to infection, 6 other species (S. reptans, S. erythrocephalum, S. lineatum, S. equinum, S. nitidifrons and S. austeni) allowed a proportion of the microfilariae to reach third-stage larvae equivalent to 37-50% of those which developed in S. ornatum s.l. A small sample of S. angustipes supported development as well as its respective control group of S. ornatum s.l., but S. vernum and S. aureum were relatively refractory and were the only species in which third-stage larvae did not accumulate in the head. Fly survival rates varied between the species, but all were below those in S. ornatum s.l. It is concluded that some of these blackflies offer promise as surrogate vectors of O. lienalis in the laboratory and their possible role in natural transmission should receive further attention.

Workshop No. 15. Importation of tropical parasites to temperate regions

In 1979 the French population included 4200000 foreigners. While many came from European countries, mainly Spain, Portugal and Italy, a considerablc percentage came from the warmer belts of the earth: 20 % Algerians, 8 % Moroccans, 4 % Tunisians, 2 % black Africans. The total number has been on the decrease since 1978. However, metropolitan France has recently received, for political reasons, large groups from the Near East (Lebanese) and Southeast Asia (up to now 100000 Cambodians, Laotians and Vietnamese).