Evaluation of a nondestructive diagnostic test for Kudoa thyrsites in farmed Atlantic salmon ( Salmo salar)

Postharvest myoliquefaction (soft flesh) associated with Kudoa thyrsites infections is a concern to the Atlantic salmon ( Salmo salar) industry in the Pacific Northwest. Infection with this parasite is not macroscopically visible in salmon, and thus infected fish are not readily detectable on the processing line. Identification of infected fish relies on either histological or whole-mount evaluation of muscle tissue. A nondestructive, inexpensive diagnostic test for detection of K. thyrsites infection in the musculature of Atlantic salmon was evaluated in this study. The results indicated that the presence or absence of K. thyrsites in the hyohyoideus ventralis muscle of the operculum, as detected in wet-mount preparations, is a good indicator of the presence or absence of the parasite in the body musculature (fillets) of Atlantic salmon. The sensitivity and specificity of the diagnostic test were 79.0% and 94.6%, respectively. The sensitivity of the test was increased to 93% when the test was used to detect only heavily infected fillets. Therefore the presence or absence of K. thyrsites spores in the hyohyoideus ventralis muscle is a good indicator of heavily infected fish fillets, but is slightly less accurate at detecting fillets with lighter infections of K. thyrsites.