Histopathological Study of Yellowtail Seriola quinqueradiata Experimentally Infected with Lactococcus formosensis

Abstract

The histopathological features of yellowtail Seriola quinqueradiata experimentally infected with Lactococcus garvieae (strain Lg-Aq-33) and L. formosensis (Lg-Aq-55 and Lg-Aq-103) were compared to elucidate the histopathology of lactococcosis caused by L. formosensis. Experimental infections were performed in duplicate for each bacterial strain using injection (1.0 × 10⁴ cfu/100 g body weight) and immersion (1 h, 1.0 × 10⁶ cfu/mL) methods. The cumulative mortality trend after experimental infection in each experimental group did not differ between the two experiments. All infected fish showed lesions typical of lactococcosis, such as inflammation of the inner operculum, ulceration of the caudal peduncle, epicarditis, and encephalitis. Histopathological observations confirmed previously reported lesions of lactococcosis, and revealed new lesions of the disease and those that are characteristic of L. formosensis infections. New histopathological features of lactococcosis in yellowtail included bacterial invasion and granulomas in the parenchyma of the heart, as well as bacterial invasion in the spleen and kidney. Lactococcosis caused by L. formosensis had more frequent granulomatous inflammation and verrucos endocarditis than L. garvieae infection.