Inodosporus fujiokai is a lethal microsporidian that forms cysts in the muscle of salmonids. Fish are infected by ingesting the invertebrate host, the common prawn Palaemon paucidens. However, the precise transmission routes of I. fujiokai remain uncertain. Three trials were carried out to infect amago salmon Oncorhynchus masou ishikawae and rainbow trout O. mykiss through oral intubation or immersion using I. fujiokai spores from infected prawns. Both methods effectively induced I. fujiokai infections and high mortalities in challenged fish. Oncorhynchus masou ishikawae appeared more vulnerable to I. fujiokai infection, with up to 100% mortality. The fish cohabited with spore-intubated fish also exhibited high infection prevalences, suggesting that spores excreted from fish which consumed infected prawns cause waterborne transmission to other fish. Spore immersion at 2.6 x 104 spores/mL and above led to 67% mortality and significant cyst formation in O. mykiss, while 2.6 x 102 spores/mL induced neither mortality nor cyst formation. Besides, fish fed with infected fish muscle tissue or cohabiting with infected fish did not show signs of illness or death, indicating that fish-to-fish transmission does not occur. This study provides significant insights into the transmission biology of I. fujiokai and establishes an experimental infection model for future research.
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 x 10(4) cfu/100 g body weight) and immersion (1 h, 1.0 x 10(6) 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.
