Cellular hemolymph response of Simulium vittatum (Diptera: Simuliidae) to intrathoracic injection of Onchocerca lienalis (Filarioidea: Onchocercidae) Microfilariae

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, Giemsa-stained 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 degrees C than at 21 degrees C. More fat body fragments were collected from microfilariae-injected flies than from control and sham-injected flies held at 27 degrees C. Injection of 1.25-5 micrograms 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