AN INVESTIGATION OF CILIATE XENOMAS
IN CRASSOSTREA VIRGINICA
Emily Scarpa, Susan Ford, Bruce Smith, David Bushek
Since the late 1990s, unusually high prevalences of xenomas have been noted during routine histological examination of
oysters from Great Bay, New Hampshire. Xenomas are formed when intracellular parasites accumulate within host cells, causing
them to hypertrophy. Although in fish xenomas are commonly caused by microsporidians, in oysters xenomas are caused by
ciliates, genus Sphenophrya, and are rare. Because they are macroscopically visible on gills, the marketability of
infected oysters has been questioned.
In this study, samples were collected every fall from 1997 through 2005 and processed using normal histological
procedures. In 2005, counts were also made of macroscopically visible xenomas.
Prevalence varied according to site within Great Bay and also by year. In histological sections, it has increased
notably since 1997, when only 1% of oysters were affected. The xenomas were located in gill water tubes and were often large
enough to occupy the entire cross sectional area. They cause localized epithelial erosion and most likely impede water flow.
Nevertheless, the histological appearance of the remaining tissues was not obviously affected and there was no clear
correlation between oyster size and infection. There was an inverse relationship between the prevalence of
Haplosporidium nelsoni (MSX) infections and that of xenomas, although the reason is presently unclear.
Continuing research will utilize electron microscopy to further study the causative agent of this condition. We
will also explore the conditions that allow these xenomas to become so common.
