Delayed isotype switching in Dirofilaria immitis infection.

The anti-microfilarial immunoglobulin response in Dirofilaria immitis infection was investigated serially in a naturally infected dog. Spontaneous clearance of microfilariae was associated with IgM opsonizing antibodies which promoted in vitro killing of microfilariae by granulocytes. Over a 6- to 11-month period, there was a shift to a predominantly IgG response. The addition of fresh non-immune serum markedly enhanced killing mediated by both IgM and IgG. The findings document conversion from IgM to IgG isotype with chronic infection, and suggest that isotype switching in canine D. immitis infection is delayed relative to that seen in bacterial or viral infections.