Possible limitations of diagnostic methods recommended for the detection of the protistan, Bonamia ostreae in the European flat oyster, Ostrea edulis

Screening of heart smears is one of the OIE recommended methods for diagnosis of the protistan Bonamia ostreae in the native oyster, Ostrea edulis. Within a recent study, 130 oysters were brought to the laboratory from Lake Grevelingen, a Bonamia-endemic area as part of a planned experiment. A sub-sample of 30 oysters was screened using heart smears, to determine baseline prevalence of infection at time zero, and no infection was observed. The remaining oysters were maintained in the laboratory and screened over a six-month period, but three weeks after the trial began two oysters out of 100 had Class 4 infections, and further infection was observed over the remaining months. These results indicate that oysters found at the beginning of the trial to be free of Bonamia, using a standard method i.e. heart smears, were in fact infected. The oysters may have been experiencing a latent infection; latent infections cannot be detected by presently recommended screening techniques. Alternatively the sample size used may have been too small. The results raise concerns about limitations of the diagnostic methodology for Bonamia ostreae for research and regulatory purposes.