Aeromonas Causes Severe Skin Lesions in Rainbow Trout (Oncorhynchus mykiss): Clinical Pathology, Haematology, and Biochemistry

Aeromonas infection caused mass death of rainbow trout, Oncorhynchus mykiss (Walbaum) weighing 394 69 g, at a water temperature of 4 o C in April. In a bioassay, the disease was induced by an Aeromonas strain whose biochemical characteristics most closely resembled Aeromonas sobria and Aeromonas caviae. The development of the skin lesions started as depigmented spots surrounded by a hyperaemic zone with the formation of ulcers, or the changes on the skin resembled furunculosis, taking the form of very large prominent bulges filled with clear exudate which, when broken, revealed haemorrhagically altered muscle. Some fish showed exophthalmus; inflammation around pectoral fins; hyperaemia of the wall of the swim-bladder and petechial haemorrhages on the liver were found inside the abdominal cavity. Severe anaemia was characterized by a reduced erythrocyte count and lower haematocrit and haemoglobin levels. Clinical chemistry analyses in the diseased fish indicated reduced levels of total protein, cholesterol, triacylglycerol and total calcium and an increase in the urea level. Among the five enzymes and isoenzymes analyzed, catalytic concentration reaching multiples of the normal level was found in alanine aminotransferase, lactate dehydrogenase, -hydroxybutyryl dehydrogenase and -glutamyl transferase. Electrophoretic analysis indicated a reduced level of albumin in the diseased fish. These results point out the importance of mesophilic motile Aeromonas as causal agents of severe skin affections in salmonids. The findings encourage efforts to extend the knowledge of clinical haematology for the identification of health disorders and specific responses typical of the individual diseases.