Résumé
Six isolates of Yersinia ruckeri [Y. ruckeri], the aetiological agent of Enteric Red Mouth [ERM] disease were recovered from healthy and septicaemic Oreochromis niloticus [O.niloticus] fish collected from River Nile at Giza Governorate. The isolate No. 2 was the most pathogenic strain of the six isolates, causing high mortality percent among the experimentally infected fish under different stress conditions, namely overcrowdness, external parasitism and transportation with mortality rates of 83%, 70% and 60% in O. niloticus respectively. The mortality rate was 100% in intraperitoneally inoculated O. niloticus and Clarias lazera [C. lazera], while it was 40% in subcutaneously inoculated C. lazera. The main recorded clinical signs in experimentally infected fishes were general septicemia and redness of the eyes. The in-vitro antibiotic sensitivity test for Y. ruckeri isolates revealed their high sensitivity to oxytetracycline