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Infectious Myonecrosis

What is it?

Infectious myonecrosis (IMN) is a viral disease of penaeid shrimp caused by infection with infectious myonecrosis
virus (IMNV).

The principal host species in which IMNV is known to cause significant disease outbreaks
and mortalities is Penaeus vannamei.

Where and When Might it Occur?

Outbreaks of IMN with sudden high mortalities may follow stressful events such as capture by cast-net, feeding, sudden changes in salinity or temperature, etc.

This usually appears in early juvenile, juvenile, or adult P. vannamei, in regions where IMNV is enzootic, or in P. vannamei introduced from infected regions or countries.

So far, the disease has been reported in north-eastern Brazil (where the expected carriers are) and South-East Asia, including the island of Java.

Due to a non-enveloped dsRNA virus structure, it is likely that IMNV will remain infectious in the gut and faeces of seabirds that have fed on dead or moribund shrimp at farms with ongoing IMN epizootics, and therefore the disease may be spread within and among farms by faeces or regurgitated shrimp carcasses.

IMNV has also been demonstrated to be transmitted from shrimp to shrimp by cannibalism. Transmission via water and vertical transmission from broodstock to progeny probably occurs.

With vertical transmission, it is not known if the mode is transovarium or by contamination of the spawn eggs.

Diagnosis

Shrimp in the acute phase of IMN disease will present focal to extensive white necrotic areas in striated (skeletal) muscles, especially in the distal abdominal segments and tail fan, which can become necrotic and reddened in some individual shrimp.

Severely affected shrimp become moribund and mortalities can be instantaneously high and continue for several days.

Mortalities from IMN range from 40 to 70 per cent.

Control/Treatment

No effective vaccines for IMNV are available.

Disinfection of eggs and larvae is a good management practice recommended to reduce the potential of IMNV contamination of spawned eggs and larvae produced from them.

Source: OIE