Document Type



Virginia Institute of Marine Science

Publication Date



Journal of Shellfish Research





First Page


Last Page



The two haplosporidan parasites that cause diseases of oysters along the middle North Atlantic coast of North America differ in their habitats, in timing of oyster mortalities, and in their adaptations to the host. Haplosporidium nelsoni (MSX) kills oysters throughout the year over a wide range of salinities (about 15 to 30 ppt). It has a long infective period of nearly 6 months. This pathogen rarely completes sporulation in its life cycle in oysters. It is highly pathogenic and exhibits irregular activity suggesting that it is poorly adapted to the host species. In contrast, Haplosporidium costalis (SSO) has a short, well-defined mortality period of 4 to 6 weeks; it always sporulates in May-June and promptly kills the host oyster. The infective period is short and initial infections occur during the mortality period. Clinical infections appear after 8 to 10 months of incubation as hidden infections. This pathogen appears to be a native species and exhibits a regular life cycle in the oyster host.

Failure to achieve artificial infections with either pathogen has led most investigators to assume that some unknown host is the primary source of infective particles. Spasmodic attempts to achieve artificial infections are not convincing that direct transmission from oyster to oyster does not occur, given the scarcity of spores of H. nelsoni for experiments and the unavailability of H. costa/is spores to most laboratories.

In Virginia, H. nelsoni infections can be divided into early-summer and late-summer acquisitions that result in quite different mortality patterns of oysters. The late-summer infections of H. nelsoni exhibit patterns of incubation and mortality similar to those of H. costalis. Data are presented for many years of monitoring of late-summer MSX infections and subsequent June-July deaths the following year. The purpose of this report is to call attention to the value of life-cycle studies of H. costalis as a typical haplosporidan. The scarcity and dormancy of spores and the often long periods of obscure infections in H. nelsoni tend to make infection experiments difficult.


Haplosporidium nelsoni, MSX, oysters