No abstract

No abstract

Eleven freshwater salmonid hatcheries in southern Australia were surveyed for bacterial pathogens and diseases between 1981 and 1985. Twenty-five populations of fish were examined in the study, representing a total of 2755 fish, from which kidney, liver, spleen, and in some cases peritoneum, blood and faeces were cultured. Bacteria of pathogenic significance isolated included Aeromonas hydrophila, Streptococcus sp., Lactobacillus piscicola. Yersinia ruckeri. Mycobacterium fortuitum, M. chelonei and a filamentous aeid-fast organism of uncertain taxonomic position. Eactobacillus piscicola and Streptococcus sp. were associated with clinical and subelinical peritonitis. Mycobacterium ehelonei was isolated from visceral granulomas in an externally normal fish. Aeromonas salmonicida, Renibacterium salmoninarum and Edwardsiella tarda were not isolated, indicating that the diseases furunculosis, bacterial kidney disease and edwardsiellosis are exotic to Australian saimonids. Similarly, while Y. ruckeri was isolated, enteric redmouth disease had not been recorded and is eonsidered an exotic disease.

The histopathology of Myxidium giardi infections in wild and cultured elvers, fingerlings and adult eels is deseribed. The lesions in the target organs of the kidney and gills were minimal. Marked granulomatous changes were evident in the spleen, peritoneal fat and, to a lesser extent, the liver and gas gland. The granulomatous reaction consisted of a mononuclear infiltration into the spore mass and proliferative fibrosis of the surrounding connective tissue. Histologicai evidence of an immunological response is described. The types of M. giardi cysts are described-a primary interstitial cyst of the kidney and gill, a secondary cyst of the gills and a glomerular spore mass. Primary interstitial eysts in the kidney were observed to rupture and formed spore masses in the interstitial tissue. Evidence of vascular spread and the presence of spore emboli is presented. It is suggested that primary multiplication occurs in the primary interstitial cyst of the kidney and gills, and secondary multiplication in the secondary gill cyst and glomerular spore mass from which mature spores are released to the environment.

The clinical signs and histopathology of Saprolegnia sp. [related to S. diclina Humphrey (syn. S. parasitica Coker)] are described in cultured Anguilla anguilla L. elvers under an intensive production system. The main lesions observed were loss of epithelium leading to ulceration, oedema and myofibrillar degenerative changes of the muscle mass. The oedema resulted in swelling of the inter-myotomal connective tissue, loss of nuclei and minor host reaction. Traumatic lesions with subsequent bacterial infections had a mononuclear inflammatory response. It is suggested that the rapid extension of the lesion is due to the loss of integrity of the integument and the widespread oedema, which altered the viability of tissues and assisted the Saprolegnia infection. The condition was fatal for infected elvers.