Humoral Immune Response of Ayu, Plecoglossus altivelis to Glugea plecoglossi (Protozoa: Microspora)

The humoral immune response of ayu (Plecoglossus altivelis) to the microsporidian parasite Glugea plecoglossi was investigated. The antibody levels of sera from ayu, to which G. plecoglossi spores were administered orally, were measured by indirct ELISA. While the cysts appeared from 27 D.P.I. (days post-infection), the antibodies against the intact spores were first detected at 10 D.P.I. and no more significant increases in the antibody levels were observed. The uptake of spores through the gut epithelium was histologically observed. Additionally, sera from ayu in a cultured stock which showed 50% prevalence of infection were obtained and the antibody levels were measured by the ELISA. The antibody level was variable but no important difference was observed between fish with or without cysts. The intensity of infection (number of cysts) was not related to the antibody level both in the artificially and naturally-infected fish. These results suggest that the antibody production against the intact spores of G. plecoglossi plays no protective role against G. plecoglossi infection.