Infectious myonecrosis

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

• large numbers of sick animals and significant mortalities in juvenile and subadult pond-reared stocks of Penaeus vannamei

Clinical signs of disease in an infected animal

• acute form of disease produces gross signs and elevated mortalities, but disease progresses to a chronic phase with persistent low-level mortalities
• focal to extensive white necrotic areas in the striated muscle, especially of the distal abdominal segments and tail fan (which might become necrotic and reddened in some individuals)

Disease agent

The causative agent is infectious myonecrosis virus, an RNA virus.

Host range

Crustaceans known to be susceptible to infectious myonecrosis:

* naturally susceptible (other species have been shown to be experimentally susceptible)

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

• Infectious myonecrosis is a new viral disease identified recently in north-east Brazil in cultured P. vannamei. However, shrimps with similar gross signs have also been reported from other countries where P. vannamei is cultured.
• Affected life stages include juveniles and subadults. Significant mortalities occur in juvenile and subadult pond-reared populations.
• Extremes in salinity and temperature seem to be associated with disease outbreaks.
• Horizontal transmission has been demonstrated, but there is no information on vertical transmission.

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

White tail disease

Further images

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 Whom to contact if you suspect a disease.

Further reading

The currently accepted procedures for a conclusive diagnosis of infectious myonecrosis are summarised at www.oie.int/aac/eng/cards/en_diseasecard.htm