Onchocerca lienalis in Simulium vittatum: Navigation of microfilariae and the fly defense response.

Abstract

A rapid clearance of a third of the number of Onchocerca lienalis microfilariae injected into Simulium vittatum, a laboratory host, occurred within 2 hr post inoculation (pi) regardless of dose. A second injection, 2.5 hr after the first, resulted in a lower proportion of microfilariae eliminated, suggesting that availability of active factors were reduced after the first inoculation. Microfilariae did not differ in their susceptibility to clearance as microfilariae that were recovered 2 hr pi and reinoculated into other flies were eliminated faster than unexposed controls. In S. jenningsi, a natural vector, rapid microfilariae clearance occurred similarly and many immotile microfilariae were observed (> 85% at 12 hr pi). Rapid clearance and immobilization of microfilariae represent newly-described immune responses to macroparasites of black flies. Migration of O. lienalis microfilariae into the thorax of S. vittatum proceeded in 2 phases: 0-2 and 6-12 hr pi. Migration success 12-24 hr pi was only 36%, indicating that a large proportion of microfilariae failed to reach the thorax. Migration success was density independent. Microfilariae that arrived into the thorax within 2 hr pi had similar migration potential to microfilariae that remained in the abdomen at this time. The number of microfilariae in the thorax at each time point, but not the proportion of microfilariae in the thorax, was negatively correlated with microfilariae loss, suggesting that microfilariae loss affected migration success. The behavioral responses of O. lienalis microfilariae to tissue factors of S. vittatum, were studied using a novel in vitro bioassay. Microfilariae accumulated towards thoracic tissues at densities 4 times higher than towards abdominal tissues. Microfilariae directed themselves toward the thorax rather than arresting their movement after they make contact with thoracic tissues as contact with thoracic tissues was not necessary. Chemical cue(s) provided principal guidance for microfilariae to locate thoracic tissues. Microfilariae lost their ability to differentiate between thoracic and abdominal tissues and reversed their differential response following experimental manipulation in chemical cue(s) distribution. Exposure of thoracic tissue increased its attractiveness but intact thorax attracted more microfilariae than excised abdomen. Abdominal tissue did not affect thorax attractiveness. Involvement of a large molecular weight protein(s) was suggested.