ONCHOCERCA LIENALIS - RAPID CLEARANCE OF MICROFILARIAE WITHIN THE BLACK FLY, SIMULIUM-VITTATUM

A rapid decrease of about a third of the number of Onchocerca lienalis microfilariae (mf) parenterally inoculated into Simulium vittatum bla ck flies occurred within 5 hr postinoculation (pi). The change of mf c ounts over time was modeled by a segmented linear regression. During 2 hr pi the slope was -3.5 mf/hr (P less than or equal to 0.001) and be tween 2 and 24 hr pi the slope was -0.1 mf/hr. Although significantly different from the former slope (P < 0.001), the latter was not signif icantly different from zero (P > 0.2). The decrease could not be attri buted to excretion of mf. Microfilariae (especially those heat-killed prior to inoculation) in intermediate stages of destruction were obser ved in flies dissected 5 hr pi but not immediately after injection. No short- or long-term (24 hr pi) effects of the injection procedure alo ne on mf survival were evident. A constant proportion of mf was elimin ated regardless of dose within a range of 5 to 100 mf/fly during 24 hr pi. However, a second injection of 50 mf/fly 2.5 hr following an inje ction of the same dose resulted in a significantly lower proportion of mf eliminated. These results suggest that the availability of an acti ve factor(s) in the fly was reduced 2.5 hr after the first inoculation . The change in the availability of this factor(s) may partly explain the change in clearance rate occurring 2 hr pi. Soluble factor(s), rat her than a sequence of cellular responses, seems to be involved in the rapid clearance because it occurred in freshly killed flies at a simi lar rate to that observed in live flies. The hypothesis that mf differ in their innate susceptibility to rapid clearance was rejected as mf that were recovered 2 hr pi and reinoculated into other flies were eli minated faster than unexposed controls. It is concluded that the rapid clearance of mf represents an as yet undescribed immune response to m acroparasites of the fly host. (C) 1994 Academic Press, Inc.