No abstract

A total of 101 fresh water fish Tilapia nilotica were bacteriologically investigated for pathogenic and potentially pathogenic organisms. True intestinal pathogens were obtained in 11.8% of the examined specimens including 7.9% with enteropathogenic Escherichia coli and 3.9% with Salmonella species. Serological typing of Salmonella (4 strains) revealed the detection of Salmonella typhimurium (1 strain), S. wangata (2 strains), and S. newport (1 strain). Other potentially pathogenic organisms were isolated in considerable frequencies, Proteus spp. (43.5%) atypable E. coli (13.8%), Micrococcus spp. (11.8%) and Providencia (9.9%). Vibrio parahaemolyticus and Staphylococci were not detected in the examined samples.

A total of 25 T. nilotica (S. niloticus) fish showing clinical altrations were subjected to clinical, P. M., complete parasitologic and bacteriologic examinations. Results of clinical and post-mortem exam inations revealed two forms of the disease: 1. The first was characterized by redness of the skin, presence of haemorrhagic patches of various size distributed all over the body and fins, exophthalmia and anal prolapse with abdominal distention, which was considered as the acute septicaemic form of the disease. 2. The second was characterized by roughness of the scales and their detachment from some parts of the body and formation of ulcers which were small at first, then became enlarged, have irregular shape, white centrally, surrounded by hyperemic zone and found on the lateral sides of the body. This was classified as chronic ulcerative form of the disease. Parasitologic examination of diseased fish revealed the presence of Ichthyophthirius multifilis protozoan which was considered as a stress factor. Bacteriologic examination of diseased fish revealed the presence of fourteen isolates of bacteria, morphologically and biochemically related to Aeromonas hydrophila. Biological experiments conducted on T. nilotica yearlings revealed that eight isolates of A. hydrophila were highly virulent, three isolates were moderately virulent and the other three isolates were avirulent to the tested fish. The infection with A. hydrophila to T. nilotica was transmitted experimentally by I/P and I/M routes and not transmitted by scarification of the epidermis. Crowdness was a major factor for increasing mortalities among inoculated tilapias. Besides, T. nilotica have been proved to be a good model for studying the biological properties of fish pathogenic bacteria.