Infectious Myonecrosis (IMNV) in Shrimp: A Disease Guide

Infectious myonecrosis virus causes whitish muscle necrosis and elevated mortality in farmed Penaeus vannamei. There is no treatment; control relies on SPF stock and biosecurity.

Overview

Infectious myonecrosis (IMN) is a viral disease of farmed penaeid shrimp caused by infectious myonecrosis virus (IMNV), a non-enveloped, icosahedral, double-stranded RNA virus of the family Totiviridae with a non-segmented genome of about 7,561 base pairs and a virion roughly 40 nm in diameter. The disease was first recognized in northeast Brazil in 2002-2003 and was later introduced to Indonesia around 2006. IMN is a notifiable disease of the World Organisation for Animal Health (WOAH).

Affected species

The principal host is the Pacific white shrimp, Penaeus vannamei (Litopenaeus vannamei). Other penaeids, including Penaeus stylirostris and Penaeus monodon, are also susceptible. The disease has had significant economic impact in the shrimp-farming Americas and Southeast Asia, and outbreaks are often triggered by periods of acute stress.

Clinical signs

Whitish, opaque necrosis of the skeletal muscle, most marked in the distal abdominal segments and tail fan
Sometimes reddening of affected areas
Lethargy and reduced feeding
Elevated mortality, especially following acute stress such as temperature or salinity changes
On histology, coagulative muscle necrosis and lymphoid organ spheroids

Transmission

IMNV spreads horizontally, including through cannibalism of dead or moribund shrimp and likely through water. Vertical transmission from infected broodstock to progeny is also suspected. The international spread of the virus has been linked to the movement of live infected shrimp between regions.

Diagnosis

Diagnosis is confirmed by RT-PCR, supported by histopathology, which shows coagulative necrosis of skeletal muscle progressing to liquefactive necrosis with haemocytic infiltration, along with lymphoid organ spheroids. Because the white-muscle appearance can resemble non-infectious muscle necrosis from stress, laboratory confirmation is important.

Treatment, control and prevention

There is no treatment for IMN, and no widely available licensed vaccine. Control in endemic areas centers on excluding the virus from the farm and on reducing the acute stress that precipitates clinical disease.

Stock specific-pathogen-free (SPF) and PCR-screened postlarvae from certified hatcheries
PCR-test broodstock, eggs, and nauplii and discard positives
Disinfect eggs and larvae and maintain hatchery and pond biosecurity
Minimize acute stressors such as rapid temperature and salinity changes
Remove and dispose of dead shrimp to limit cannibalism-driven spread