Pseudotuberculosis caused by Photobacterium damselae subsp. piscicida is a serious problem in fish culture industry. The initial stage of host-pathogen interaction is primarily important to understand infectious disease. However, the mode of transmission and route of infection of the agent are not fully studied. In this study, changes in the distribution of the bacterium on and in yellowtail Seriola quinqueradiata body during infection process was investigated using immersion challenge. The result revealed that external regions such as the lower jaw, pectoral fin, skin and gills permitted bacterial adherence soon after immersion. Viable bacterial counts increased in the gills soon after infection, then they increased in the blood, spleen and kidney. Moreover, fish were exposed to bacteria at the gills or posterior part of body surface to elucidate the primary infection route. The fish group challenged at gills showed higher mortality than the other group. These results reveal that the gills are the primary regions for P. damselae subsp. piscicida to invade and proliferate in yellowtail.
Infection dynamics of Microsporidium seriolae causing the beko disease of yellowtail Seriola quinqueradiata and amberjack S. dumerili were investigated at fish farms and an experimental facility. A 4-year survey (2006-2009) showed a sharp increase in prevalence of infection with M. seriolae in June or July soon after stocking net cages with yellowtail and amberjack wild fingerlings. Most cysts blackened and finally disappeared in November in the following year. However, in 2006 when the levels of infection were relatively high, some visible cysts still remained even in autumn of the following year. Infective periods and effects of fish size on M. seriolae infection were investigated via natural exposure of hatchery-reared yellowtail juveniles by transferring to an open sea cage in an endemic area. Invasion of the parasite into fish occurred in June and July but not after the mid-August. A comparison of M. seriolae infection among four size classes of yellowtail showed that the smallest fish (1 g in mean weight) had the lowest prevalence of infection.
We investigated the histopathological features of the gill lesions of diseased ayu Plecoglossus altivelis that had been diagnosed with ‘Boke’. The features were classified into three types: one characterized by atypical gill epithelial cells, one characterized by numerous long Gram-negative rods, and one characterized by both atypical gill epithelial cells and long rods. TEM revealed the presence of poxvirus-like virus in the atypical gill epithelial cells. Our results indicated that ‘Boke’ was composed of different kinds of diseases. We propose ‘atypical cellular gill disease (ACGD)’ as the new name of novel gill disease in ayu.
The utility of chromogenic media for determination of α-glucosidase (α-glu) activity in 116 Nocardia seriolae isolates was investigated. In all isolates, the results obtained using chromogenic media were identical to those of the API ZYM test. All the α-glu-positive isolates (n = 27) were erythromycin (Em)-sensitive, and more than half of them (n = 18) were oxytetracycline (OTC)-resistant. On the other hand, all the α-glu-negative isolates (n = 89) were OTC-sensitive, and most of them (n = 81) were Em-resistant. These results supported the relationship between the α-glu activity and drug susceptibility profiles in N. seriolae. Therefore, chromogenic media can be used as a simple and reproducible one-step test to determine the α-glu activity and presumptive drug susceptibility profiles in N. seriolae.