A survey of the health status of the Manila clam Ruditapes philippinarum in Ireland with specific reference to brown ring disease

摘要

The present work investigated the health status of the Manila clam in Ireland, with particular reference to brown ring disease (BRD) caused by V. tapetis which has been responsible for high mortalities of this bivalve throughout Europe. BRD was diagnosed in Ireland in the 1990s, causing heavy mortalities in Manila clam stocks in the north-west coast. In the current study, samples of clams from an Irish hatchery were obtained and screened from two sites, Drumcliff Bay, Co. Sligo and Dungloe Bay, Co. Donegal. Turbellarians and trematodes with some minor infections were the only parasites observed. Clams were examined for BRD, by analysis of shell valves for brown ring signs, and for V. tapetis by microbiological methods and by polymerase chain reaction (PCR). BRD was not diagnosed in cultivated clams from either site. It was, however, diagnosed in residual clams, which had survived the initial outbreak of BRD in the 1990s in Dungloe Bay. V. tapetis, however was detected in clams from both sites. Additionally, BRD was diagnosed, and V. tapetis isolated, in a once-off sample of clams from Mulroy Bay, Co. Donegal, where clams had been on-grown from imported seed. Minor heterogeneity at the 16S rDNA gene was observed between some sequenced products from Mulroy Bay and from Drumcliff Bay and Dungloe Bay indicating that in addition to V. tapetis, a V. tapetis-like strain may have been present in Mulroy Bay. The detection of V. tapetis in asymptomatic clams in Drumcliff Bay and Dungloe Bay may indicate that disease development may only occur when a number of factors combine to induce disease symptoms.