Trials were carried out to study the humoral immune response of blackflies to filariae following infection using the intrathoracic injection technique. An induced 66 kDa protein was abundant in the haemolymph of the European species Simulium ornatum following infection with bovine Onchocerca lienalis. This protein was apparently at higher concentrations in the haemolymph of sham-inoculated flies, i.e. flies that received sterile medium without the parasites. A molecule of the same size was also observed in the haemolymph of infected S. damnosum s.l. following infection with human O. volvulus or bovine O. ochengi. However, the level of this protein was lower in blackflies injected with microfilariae of bovine O. dukei. Unlike O. volvulus and O. ochengi this species is not transmitted by S. damnosum s.l. under natural conditions. No such reaction was observed if the African blackflies had received a sham inoculation. Feeding experiments with wild-caught nulliparous S. damnosum sl. on Onchocerca-infected cattle supported the results of the injection trials. The 66 kDa protein could only be found in the haemolymph of specimens infected via a blood meal. This 66 kDa molecule was identified as phenoloxidase. It appeared in the haemolymph due to the activation of the prophenoloxidase system following the filarial infection and we hypothesize that it may be sequestered by the parasites, as part of a non-self recognition system.

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The vectors of filariasis, mosquitoes and blackflies, are capable of mounting a defence response to the infection. This selective review describes the molecules that are involved in these immune systems. Several antibacterial peptides are known to be induced and secreted into the haemolymph by the fat body and the circulating haemocytes. In addition, haemagglutinating lectins with carbohydrate specificities to the surface of the developing filarial larvae appear. Activation of a range of proteases occurs rapidly as does activation of the prophenoloxidase pathway. The possible roles of these and other molecules is discussed, together with mention of a working hypothesis as to how these molecules may be regulated.

SummaryA range of protease inhibitors and carbohydrates were administered to the haemolymph of the vector Simulium damnosum s.l. to test for their effects on the success of an Onchocerca ochengi infection in vivo. We found that serine protease inhibitors led to a significant increase of parasite survival. Two sugars, d(+)-galactose and methyl--d-mannopyranoside, had the same effect. These effects are possibly due to the successful in vivo blocking of the two respective types of inducible immune molecules, the serine proteases and the carbohydrate binding lectins, both of which have been identified in simuliids.keywords onchocerciasis, insect immunity, proteases, lectins, S. damnosum, O. ochengi correspondence Dr

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