| Summary: | In the late 1950s, oyster populations in the Chesapeake and Delaware Bays suffered mortality from the parasite, Haplosporidium nelsoni that is now present from Florida to Maine. In 2001 and 2002, H. nelsoni caused oyster mortality in the Bras d’Or Lake, Cape Breton, N.S. Temperature and food availability are two factors thought to influence the development of H. nelsoni based on field observations from the U.S. and mathematical models developed for the parasite. This research investigated the role of environmental parameters that influence the progression of H. nelsoni. Field studies showed that (i) H. nelsoni prevalence differed among oysters within tens of metres, (ii) high prevalence does not always result in high mortality, (iii) detailed field studies could provide more valuable information than laboratory experiments, and (iv) current models for infection and disease progression could be tested and modified at a small scale in Cape Breton. To examine the role of temperature in the progression of the parasite, oysters were maintained at 5oC, 10oC, 15oC, 20oC, and 24oC. Surprisingly, the parasite prevalence was reduced under controlled conditions. Food availability treatments showed there was a reduction in the parasite burden of treated oysters compared to the field samples, but there was no clear pattern with respect to parasite progression or reduction and the amount of food available. These studies suggest that temperature and food availability may have been influencing some factor(s) outside of the host-parasite direct interaction such as the abundance of intermediate and/or reservoir host(s). Alternatively, it is possible that laboratory conditions had an effect on the results. Eighteen years after the initial oyster mortality in the Bras d’Or Lake, oysters from several locations within the Bras d’Or Lake having different histories of exposure to H. nelsoni were placed in an area known to have high H. nelsoni activity. Following 16 months exposure, high mortalities were observed from all originating oyster populations, providing evidence of a lack of resistance to mortality in any of the originating locations. The lack of mortality in some originating locations was likely due to low level or absent H. nelsoni in those areas. A potential model that includes the parasite, intermediate/reservoir hosts, and oyster mortality is proposed to explain some of these observations across Cape Breton.
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