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Disease guidePacific oyster mortality syndrome (POMS)

What is it?

Pacific oyster mortality syndrome (POMS), or Infection with Ostreid herpesvirus-1 microvariant (OsHV-1 var) is a disease affecting pacific oyster.

Ostreid herpesvirus-1 (OsHV-1) is the only member of the genus Ostreavirus (family Malacoherpesviridae, order Herpesvirales). OsHV-1 var is a genotype of this virus.

Where and when might it occur?

The disease can affect all age groups of oysters, and there may be higher mortality in the younger life stages.

Higher mortality appears to be associated with higher water temperature and crowding. Infected adults may be a source of infection for larvae or spat. However, it is not certain if true vertical transmission occurs. Horizontal transmission has been demonstrated. Some adults may survive with subclinical OsHV-1 infections, and act as carriers of the disease.


The main sign of the disease is high mortalities of around 100 percent which occur within eight to 10 days of infection.

Gross pathological signs are:

  • cessation of feeding and swimming by larvae, which exhibit velar lesions
  • gaping in adults
  • pale digestive gland in spat and older oysters

Microscopic pathological signs are:

  • ulcerative and erosive lesions in the connective tissue of mantle, gills, labial palps and digestive tissue
  • nuclear hypertrophy, nuclear chromatin margination and pyknosis
  • inflammatory changes ranging from mild and localised, to severe and extensive

Source: Australian Government, Department of Agriculture, Fisheries and Forestry