Re-examination of archived materials reveals infection of zebra mussels Dreissena polymorpha in UK waters with the parasite Haplosporidium raabei

Recent increased interest in the role of invasive bivalve species in the translocation of non-native parasite and symbiont species prompted the re-examination of archived samples of zebra mussels Dreissena polymorpha collected in 2012 and 2013 from Cardiff Bay, Wales and Grafham Water, England. Histopathological survey of parasites and pests identified the presence of haplosporidian-like cells. Comparison of these cells with the published description and digitally scanned tissue sections of the type of material of Haplosporidium raabei indicated the presence of H. raabei in established populations of zebra mussels in the UK. Confirmatory FFPE DNA extraction and targeted PCR confirmed the molecular identity of the parasite. This is the first record of the lethal, but presumed rare, zebra mussel parasite in UK waters, and demonstrates the value of archive samples for invasive parasite records. The translocation and establishment of this freshwater mussel pathogen should be further researched, given the current ecologically threatened status of native freshwater mussels.