Studies on Factors Influencing the Establishment and Development of Filaria in Mosquitoes

Abstract

Factors affecting the establishment and deveiliopment of Brugia and Wuchereria microfilariae in various strains of C. quinquefasciatus, C.p. molestus, An. stephensi and Ae . aaegypti SS were studied. The investigations concentrated mainly on the factors affecting exsheathment and migration of microfilariae from the midgut of refractory and susceptible mosquito species. It was shown t hat the rickettsia Wolbachia pipientis is not a requirement f or the successful infection of C. quinquefasciatus with W. bancrofti, and that the growth and development of W. bancroft I proceeded at the same rate and infecti ve larvae attained the same size in aposymbi otic and normal strains. Four geographical strains of C. quinquefasciatus and C.p. molestus were found to have low frequencies of the gene sb (fi ili a~ial susceptibility to B. pahangiL. These mosquitoes did ,not respondto selection for increased susceptibility or refractoriness to B. pahaagi, indicating that control of susceptibility was polygenic . Inoculation of C. quinquefasciatus (DAR) and An . stephensi with sheathed and exsheathed B. pahangi, increased the number of larvae that develop to the infective stage. Increasing t he heparin concentration in the bloodmeal decreased the migration rate of B. pahangi in C. quinquefasciatus mosquitoes . Exsheathment and migration of B. pahangi occurred normally in Ae. aegypti SS , a susceptible mosquito. It was shown that natural exsheathment of microfilariae in refractory mosquitoes does not appreciably increase migration. Neither did experimental exsheathment of microfilariae prior to infjecti on increase the migration rate. Feeding C-"· quinquefasciatus (DAR) on mixed infective bloodmeals of W. bancrofti and B. pahangi, increased the exsheathment and migration rate of B.pahangi i n .this mosqμito. Red blood cells played no role in inhibiting migration of B. pahangi In C. quinquefasciatus (DAR ) and it was shown that systemic factors in the abdominal haemocoel of this mosquito did not inhibit migration .However, the microfilariae which reached the thorax did not devel op further indicating a 'thoracic barrier'. The stimulus for migration was shown not to be located in the abdominal haemocoel. Cations decreased the rate of blood digestion and larval development of B. pahangi in Ae. aegypti SS but had no effect on exsheathment and mi gration. In vitro studi es on exsheathment of B. pahangi, showed·the presence of factors which inhibit endopeptidase induced exsheathment of B. pahangi in the abdomen homogenates, but there was no evidence that the factor was located in the midgut. In vitro exsheathment of B. pahangi occurred in the presence of midgut and thoracic tissues as well as in mosquito cells in ~uspension and in monoleyers~ Motility of B.~ pahangi microfilariae in the incubating medma was prolonged by the presence of midguts, particularly those of Ae. aegypti SS, and MK/VP12 + 20% FBS medium was shown to prolong motility of mi crofilariae, compared to other media.