Infection with Mikrocytos mackini

What is it?

Infection with Mikrocytos mackini is an intracellular protozoan parasite that causes lethal infection of American, European, Olympia and Pacific oysters. M. mackini is the only species described in the genus and is unrelated to Bonamia spp.

Where and when might it occur?

Severe infections appear to be restricted to oysters over two years old.

The disease is associated with low temperature and high salinity. Most mortalities occur during April and May (spring in the Northern Hemisphere). There is a three to four month pre-patent period when temperatures are less than 10˚C.

The Pacific oyster appears to be more resistant to the disease than other species challenged experimentally under laboratory and field conditions.


The disease is often identified by mortalities.

Gross pathological signs are:

  • focal yellowgreen lesions up to 5 mm in diameter within the body wall or on surfaces of the gonad, labial palps, gills or mantle
  • brown scars on the shell adjacent to lesions on the mantle surface
  • gaping oysters (due to impaired adductor muscle contraction)

Microscopic pathological signs are:

  • focal intracellular infection, mainly of vesicular connective tissue cells, resulting in haemocyte infiltration and tissue necrosis
  • intracellular microcell protozoa, 23 nm in diameter, in vesicular connective tissue cells immediately adjacent to lesions

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