Iridovirosis

Signs of disease

Important: animals with disease may show one or more of the signs below, but disease may still be present in the absence of any signs.

Disease signs at the farm level

• high mortality

Gross signs of disease in an infected animal

• occasional yellow or green pustules on mantle or adductor muscle
• yellow patches on gills and mantle that spread as the disease progresses
• patches turn brown at the centre as tissue dies, leaving a hole in the gill structure

Disease agent

Gill necrosis virus is an icosahedral DNA virus.

Host range

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

• A number of iridoviruses causing disease in oysters have been identified, but not all are associated with gill necrosis virus disease, and some affect oysters at different life stages.
• Transmission is direct from the water column through the surface of the gills.
• Little is known about the ecology of the organism responsible for this condition, but iridoviruses are generally considered to be distributed in oceans worldwide.
• Outbreaks usually occur in spring and sometimes in summer.
• A protistan, Thankatostrea polymorpha in the Sarcomastigophora phylum, has also been associated with this disease.
• Surviving oysters do not repair perforated gill structures and are potential carriers of the virus.

Differential diagnosis

The differential diagnostic table and the list of similar diseases appearing at the bottom of each disease page refer only to the diseases covered by this field guide. Gross signs observed might well be representative of a wider range of diseases not included here. Therefore, these diagnostic aids should not be read as a guide to a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

Similar diseases

Infection with Perkinsus marinus, P. olseni, Mikrocytos mackini, Bonamia sp.,B. ostreae, B. exitiosa

Sample collection

Because of uncertainty in differentiating diseases using only gross signs, and because some aquatic animal disease agents might pose a risk to humans, you should not try to collect samples unless you have been trained. Instead, you should phone your state or territory hotline number and report your observations. If samples have to be collected, the agency taking the call will advise you on what you need to do. Local or district fisheries/veterinary authorities could advise you on sampling.

Emergency disease hotline

For your state or territory emergency disease hotline number, see who to contact if you suspect a disease.