Serological, virological and histopathological study of an outbreak of sleeping disease in farmed rainbow trout Oncorhynchus mykiss

A prospective longitudinal survey for sleeping disease (SD) was carried out over a 20 wk period on a caged freshwater population of farmed rainbow trout Oncorhynchus mkyiss. Pancreas, heart and red and white skeletal muscle were examined histologically and the presence and severity of lesions recorded. Sera were tested for viraemia with Salmonid Alphavirus (SAV) and for virus neutralizing (VN) antibodies. Viraemia was detected for 4 wk, beginning at Week 6 and with a peak prevalence of 57.9% at Week 7. Clinical signs and mortalities appeared at Week 8. Total mortality in the study cage from Week 6 onward was 6.3 %, but other cages at the site had mortality levels of up to 47.2%. VN antibodies were first detected at Week 9, with seroprevalence increasing to 80% by the end of the study (Week 20). Geometric mean antibody titres peaked at 1/89.4 at Week 17. Histological lesions were first detected at Week 7 (pancreas only), before increasing in prevalence and severity to peak at Weeks 9 and 10. The majority of lesions were resolved by Week 15.