The haplosporidian parasite Bonamia ostreae (OIE 2012) is the causative agent of bonamiasis, which is capable of causing severe mortalities of native European flat oysters Ostrea edulis. The impact of in - fection is evidenced by the drastic (93 per cent) drop in recorded production of O. edulis in France, from 20 000 t yr–1 in the early 1970s to 1400 t in 1982 (FAO data) following the occurrence of the disease, although infection with Marteilia refringens is also implicated in this decline (Alderman 1979).
Hudson & Hill (1991) documented the initial introduction, impact and spread of bonamiasis in England. They noted that it was first confirmed in England in wild stocks of Ostrea edulis in the River Fal, Cornwall, in 1982. In the same year it was also detected in Essex and in 1983 in the Helford River. By 1986, they showed that bonamiasis was found to have further spread to Poole (harbour and bay), the Solent and its adjacent south coast harbours.
Further spread of the disease at the time was successfully prevented by controls on movements of live shellfish made under the Molluscan Shellfish (Control of Deposit) Order 1974. This legislation recognised 3 separate bonamiasis-infected zones in the southwest, south and southeast of England (Hudson & Hill 1991). The reference to zones in the text is to a geographical unit of one or more contiguous hydrological areas defined by the diseases status. Each of these areas will comprise one or more sampling sites (farm or fishery location).
It is useful to consider the legislative background to the information on bonamiasis collected and presented in this paper as this influenced the monitoring. A change in the legislation controlling movements of live shellfish for re-laying became necessary with the coming of the Single European Market to the UK in 1993.
Movements of shellfish into the UK continued to be controlled because the UK was able to apply for Approved Zone status for listed molluscan diseases, i.e. Marteilia refringens at Member State level and Bonamia ostreae free zones.
Initially only bivalve molluscan shellfish originating from areas of equal or higher health status could be introduced into proposed Approved Zones, although the rules were later relaxed for species that were not susceptible to these diseases. An annual programme of sampling and testing from 1993 onwards, as described be low, of Ostrea edulis as the susceptible species to bonamiasis and marteiliosis was carried out in support of the application for and maintenance of Approved Zone status, which was not officially granted until 2002 (Commission Decision 2002/ 300/EC).
More recently, new aquatic animal health legislation has been agreed by all the EU Member States (Council Directive 2006/88/EC and Commission Regulation [EC] No 1251/2008) and transposed into UK national legislation.
Regular and routine sampling and testing for maintenance of disease-free status for marteiliosis was no longer required, provided that effective controls on movements were in place, and so a much reduced monitoring programme was initiated from 2008 onwards. Subsequent to the official confirmation (OIE 2011) of marteiliosis in a sample of mussels from the River Tamar in 2011 the monitoring has once again increased slightly. Sampling has also been (and continues to be) carried out in response to reported unusual mortality.
The 1993−2007 programme in support of Approved Zone status for marteiliosis entailed regular sampling from all of the areas within the bonamiasis-infected zones. As all samples are examined by histological methods to give a complete examination of any pathology, this together with results from previous and subsequent sampling efforts, enabled an assessment to be made of the status of bonamiasis in the 3 infected zones during at least the 15 yr period of this programme and in some cases longer.
An analysis of this information is presented in this paper, which includes data on outbreaks of bonamiasis in 7 new areas in England, Wales and Northern Ireland since 2005.
September 2014
