Backgrounder - SSO Disease of American (Eastern) Oysters

What is SSO Disease?

This disease is caused by a microscopic parasite – too small to see with the naked eye – with the scientific name Haplosporidium costale. SSO stands for “Seaside Organism” since it was first found in oysters on the ocean side of Virginia, unlike MSX (a closely related oyster disease) that is found in estuarine waters.

Does it affect Human Health?

No. SSO is purely a health problem for the American oysters. In the United States, oysters are routinely marketed from populations that carry SSO with no human health concerns.

What does it look like?

Under a microscope, two stages of SSO can be detected in the oyster tissues:
(a) the ‘plasmodium’, which is a multinucleate stage that spreads throughout the tissues, is very similar to the plasmodium of MSX – but slightly smaller; and
(b) the spore stage, which develops in the connective tissues, unlike the spores of MSX that develop in the walls of the digestive ducts of the oyster.

Oyster soft tissues removed from shell (bottom left) and “steak” cut for microscope examination: spores stain pink in connective tissue (bottom right), plasmodial stage in connective tissue (top right)

Where does it occur normally?

SSO has been found solely in American oysters between Long Island Sound, New York to Cape Charles, Virginia, in waters over 25ppt salinity.

What does it do to the Oyster?

When the parasite enters the oysters’ tissues it multiplies and spreads. The plasmodial stage first appears in early summer (May to June). By late June/July, spores develop throughout the connective tissues, and it is at this stage of infection that mortalities are observed. An oyster mortality rate of up to 40% has been reported in the USA.

Some oysters may survive infection, but the SSO parasite is difficult to detect until the following spring. This differs from MSX in two ways: i) MSX spores develop gradually throughout the summer and are located in the digestive tubules; and ii) MSX causes two waves of mortality – one in late summer and another in late spring.

How does it spread?

SSO cannot be spread directly from oyster to oyster, but is picked up from some unknown stage in the water. It is believed that SSO uses an unknown, intermediate host to spread infection. Oysters exposed to salinities of less than 25 ppt do not appear to be susceptible to SSO, as infections appear limited to higher salinities.

Does it affect any other shellfish?

There is no evidence that SSO can infect other bivalve molluscs, such as clams, mussels and scallops. However, SSO could be accidentally carried on or in other shellfish, thus, caution is required when moving any shellfish from affected areas.

Who should I call if I find dying oysters?

If you find dying oysters, do not move them. Report them to one of the following contacts:

• DFO Halifax – René Lavoie (902) 426-2147
• DFO Moncton – Maurice Mallet (506) 851-3176 or
  Mary Stephenson (506) 851-6983
• DFO Ottawa – Sharon McGladdery (613) 991-6855 or