MSX DISEASE (HAPLOSPORIDIUM NELSONI) IN CANADA
Follow-up report No. 1
Information received by OIE on 20 December 2002 from Dr Brian Evans, Executive Director, Canadian Food Inspection Agency, Ottawa:
End of previous report period: 21 October 2002 (see Disease Information, 15 , 212, dated 25 October 2002).
End of this report period: 18 December 2002.
A preliminary, histology-based survey to determine the distribution of MSX disease in Crassostrea virginica oysters throughout Atlantic Canada was completed on 18 November 2002. Oysters in the following areas were examined: Cape Breton (9 sample sites), Gulf Nova Scotia (2 sites), southwest Nova Scotia (1 site), eastern New Brunswick (6 sites), and Prince Edward Island (4 sites). Only the sample sites listed in "New outbreaks" below were found to be positive for Haplosporidium nelsoni during the survey.
Since the Emergency Report dated 21 October 2002, four additional positive sites have been identified in Cape Breton Island. Evidence of infection was found at two locations in Bras d'Or Lake, as well as at two sites on the north coast of the island. In addition, two of four oyster samples from Prince Edward Island showed signs of MSX.
A. Laboratories where diagnosis was made: see the Emergency Report.
B. Diagnostic tests used: histology and PCR(1).
C. Causal agent: at the newly identified sites, only evidence of subclinical plasmodial - early developmental or non-spore - infections was found.
Losses due to MSX have been associated solely with the heavy infections found in St. Patrick's Channel and Bras d'Or Lake, Cape Breton. At the other sites prevalence of infection is less than 7%. An expanded survey is being conducted.
Control measures during reporting period:
- Requests for transfers of bivalves between water bodies are reviewed on a case-by-case basis. Licences are only issued for oyster transfers where there is no evidence of MSX.
- A precautionary inclusion of sympatric bivalves in transfer restrictions has now been lifted for sites where there is no evidence of MSX disease.
- Harvesting and marketing from MSX affected areas are permitted under stringent conditions prohibiting relay, washing or wet storage away from the harvest site.
(1) PCR: polymerase chain reaction.