CHEMICAL GUIDANCE OF ONCHOCERCA-LIENALIS MICROFILARIAE TO THE THORAX OF SIMULIUM-VITTATUM

The behavioural responses of Onchocerca lienalis microfilariae (mf) to tissue factors of the surrogate black fly host, Simulium vittatum, we re studied using a novel in vitro bioassay. Mf accumulated towards tho racic tissues to a density 4 times higher than towards abdominal tissu es, despite the larger surface area and volume of abdominal tissues. M f migrated toward thoracic tissues regardless of whether or not contac t with thoracic tissues was possible. Therefore, mf directed themselve s toward the thorax rather than arresting their movement after they ma ke contact with thoracic tissues. Chemical cue(s) provided a principal guidance for mf to locate thoracic tissues. Mf lost their ability to differentiate thoracic and abdominal tissues following addition of tho racic attractant(s) to excised abdomens and reversed their differentia l response when excised thoraces were depleted of chemical cue(s). Mf did not respond to salivary gland product, but to other thoracic facto r(s). Intact thorax that was connected to 1-2 abdominal segments attra cted considerably less mf than excised thorax. However, intact thorax attracted more mf than excised abdomen. Ab dominal tissue size did not affect thorax attractiveness. Involvement of a large molecular weight protein(s) was suggested by the high attractiveness of the 100 kDa Ce ntricon concentrator retentate and complete precipitation of attractan t(s) by ethanol.