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Akoya Oyster Disease

What is it?

Akoya oyster disease, also known as mass mortality of Japanese pearl oyster or red adductor disease of pearl oyster, is a disease affecting the Akoya oyster.

The causative agent of Akoya oyster disease has not been determined. Based on some infection experiments, a viral aetiology is suspected.

Where and When Might it Occur?

Akoya oyster disease affects oysters more than one year old, including juvenile, adult and seeded oysters.

The disease results in high cumulative mortality (reported up to 80 per cent), particularly during summer and autumn when water temperature is higher (above 20C).

Susceptibility may vary with strains and subspecies of the Akoya oyster.

The disease is thought to be transmitted horizontally.

Diagnosis

The disease is characterised by mass mortalities.

Gross pathological signs are:

  • stunted growth
  • slow closing of valves
  • atrophied and shrunken soft tissue
  • watery appearance, due to lack of nutrient storage
  • redbrown adductor muscle (as opposed to creamy white in healthy oysters)
  • decreased quality of pearls, including dark spots, gross pearl deformities, dark colouration and reduced lustre.

Microscopic pathological signs are:

  • degenerated loose connective tissue in the mantle and an increased number of host cells
  • necrosis, atrophy, swelling and vacuolisation of muscle fibres, with increased connective tissue supporting framework.

Control/Treatment

The only effective control methods appear to be avoidance by restricting movement around high-risk areas and moving oysters to cooler water during the summer.

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