Cellular Hemolymph Response of Simulium vittatum (Diptera: Simuliidae) to Intrathoracic Injection of Onchocerca lienalis (Filarioidea: Onchocercidae) Microfilariae

Abstract

Hemolymph cellular changes in Simulium vittatum Zetterstedt in response to intrathoracic injection with Onchocerca lienalis Stiles were characterized by increased numbers of tissue fragments that contained cells embedded within a noncellular matrix. Cell numbers within the matrix fragments increased both for sham and microfilariae-injected flies, indicating a blastogenic response to injection alone. Morphological characteristics of the fixed, Giemsastained cells within these tissue fragments were most similar to those previously described for prohemocytes. The total population of freely circulating hemocytes collected 24 h after injection also responded to injection alone. Differential cell counts showed a complex pattern of changes that was influenced strongly by increased numbers of prohemocytes at 24 h in microfilariae-injected flies. Fat body fragments collected in hemocoel perfusates at 24 h were fewer when flies were maintained at 27°C than at 21°C. More fat body fragments were collected from microfilariae-injected flies than from control and sham-injected flies held at 27°C. Injection of 1.25-5 μg of lipopolysaccharide per fly did not elicit similar hemolymph-associated matrix tissue and fat body changes, indicating that S. vittatum response to microfilaria infection and bacteria infection are likely to involve different mechanisms.