FISH-TO-FISH TRANSMISSION OF A MARINE MYXOSPOREAN

Fish-to-fish transmission of the marine myxosporean Myxidium leei was experimentally demonstrated in sea bream Sparus aurata L. A group of s pecific-pathogen-free (SPF) fish of similar to 11 g each were placed i n a wire-mesh cage immersed in a tank holding infected fish. A second group was placed in a tank receiving water discharged from another tan k holding diseased fish. After 9 wk, the fish were sacrificed and 12 o f the 38 (31.6%) test fish from the mesh cage were found to harbor tro phozoites, sporoblasts and spores in the posterior gut epithelium, as was readily diagnosed by standard paraffin histology. Of the fish expo sed to water discharge, 10 out of 30 (33.3%) showed similar infection. None of the fish examined displayed any proliferative stages of the p arasites in the blood, spleen, kidney, liver or gill samples. All of 1 00 control fish examined remained uninfected. A third group of SPF fis h was fed once daily for 7 d on pieces of freshly dissected M. leei-in fected gut, after which the fish were maintained on a commercial pelle t diet for a further 4 wk. Control fish in this experiment were fed on ly commercial pellets for 5 wk. The fish were sacrificed after 5 wk, a nd 4 out of 30 test fish (13%) were found to be infected. All control fish remained uninfected. Examination of the water sampled from all ta nks in which infected fish were held revealed presence of exfoliated g ut tissue and mucus casts containing trophozoites, sporoblasts and spo res of M. leei. Examination of existing potential intermediate hosts y ielded definitively negative results for actinosporeans. It is suggest ed that M. leei is transmitted between fish by ingestion of excretions from infected fish. The results reveal that sharing facilities with d iseased fish as well as exposure to contaminated water is a route for parasite transmission. In general contrast to the freshwater myxospore ans studied to date, the present study of a marine species provides ev idence that direct transmission can take place without need for actino sporean development in an alternate (oligochaete) host. It is suggeste d that this may be a model for the development of other marine myxospo reans as well.