Mortality and cellular response in the skin and gills of plaice (Pleuronectes platessa L) to parasite and bacteria infection

During the culture of wild-caught plaice, Pleuronectes platessa L. gross skin lesions and patches of dark brown pigmentation were observed on the ocular side of the fish. Mortality increased approximately nine months a er capture, and a er 10 months, 21 of the 27 fish had died. The presence of Gyrodactylus unicopula (Glukova 1955) was demonstrated by histology and light microscopy studies of skin samples from affected fish. The parasite, probably assisted by secondary bacterial infection, caused extensive disruption of the skin and underlying tissue. No parasites or bacteria were seen in gill sections. However, epithelial hyperplasia was observed in the gills of affected fish, and suspected to be a result of the parasite infection. Histologically, malpigmentation was confirmed to be caused by melanocytes in the dermis and, to some extent, underlying tissues. These findings of high mortality and extensive ulceration of the skin suggest a direct cause-and-effect link to the specific parasite. However, the possibility of an interaction, or an additive effect of bacteria cannot be ruled out. This is the first report of mortality and skin responses of G. unicopula infection in plaice. Introduction Plaice, Pleuronectes platessa L, is a popular fish among consumers in many countries. However, reduced catches (FAO, 2007) and increased prices could make this a possible candidate for aquaculture. In wild fish stocks, parasites from a wide variety of groups are normally present in low numbers living in balance with their host. However, in aquaculture because of higher density of fish (Barton and Iwama, 1991) stressful conditions may increase susceptibility to diseases (Pickering and Po inger, 1989) and epizootics may occur if conditions are suitable (Johnson et al., 1996). The gills and skin of fish are highly susceptible to environmental influences and disruptions of these organs may have serious consequences for fish viability (Wendelaar Bonga and Lock, 1992; Noga, 2000). Different responses may occur in these organs including the occurrences of melanin-containing cells in inflammatory lesions (Roberts, 1975). The effects of parasites on wild and farmed stocks are receiving increased a ention due to economic losses and the impact on fish welfare (Scholz, 1999). The present histological study was conducted to investigate the aetiology of grossly visible skin hyperpigmentation and lesions in skin and increased mortality in a broodstock of plaice. * Corresponding author’s email: oddvar.o [email protected] Bull. Eur. Ass. Fish Pathol., 31(1) 2011, 17 Materials and methods