DAO 30:99-105 (1997)  -  doi:10.3354/dao030099

Fish-to-fish transmission of the marine myxosporean Myxidium leei was experimentally demonstrated in sea bream Sparus aurata L. A group of specific-pathogen-free (SPF) fish of ~11 g each were placed in a wire-mesh cage immersed in a tank holding infected fish. A second group was placed in a tank receiving water discharged from another tank holding diseased fish. After 9 wk, the fish were sacrificed and 12 of the 38 (31.6%) test fish from the mesh cage were found to harbor trophozoites, sporoblasts and spores in the posterior gut epithelium, as was readily diagnosed by standard paraffin histology. Of the fish exposed to water discharge, 10 out of 30 (33.3%) showed similar infection. None of the fish examined displayed any proliferative stages of the parasites in the blood, spleen, kidney, liver or gill samples. All of 100 control fish examined remained uninfected. A third group of SPF fish was fed once daily for 7 d on pieces of freshly dissected M. leei-infected gut, after which the fish were maintained on a commercial pellet diet for a further 4 wk. Control fish in this experiment were fed only commercial pellets for 5 wk. The fish were sacrificed after 5 wk, and 4 out of 30 test fish (13%) were found to be infected. All control fish remained uninfected. Examination of the water sampled from all tanks in which infected fish were held revealed presence of exfoliated gut tissue and mucus casts containing trophozoites, sporoblasts and spores of M. leei. Examination of existing potential intermediate hosts yielded definitively negative results for actinosporeans. It is suggested that M. leei is transmitted between fish by ingestion of excretions from infected fish. The results reveal that sharing facilities with diseased fish as well as exposure to contaminated water is a route for parasite transmission. In general contrast to the freshwater myxosporeans studied to date, the present study of a marine species provides evidence that direct transmission can take place without need for actinosporean development in an alternate (oligochaete) host. It is suggested that this may be a model for the development of other marine myxosporeans as well.

Direct transmission · Myxidium leei · Sea bream