Infectious Myonecrosis Virus

Name

Infectious Myonecrosis, familiarly known as Myo.

Clinical signs

Symptoms of Myo in shrimp include a pale body, cramped muscles, and a color change of shrimp muscles to white. In severe stages, shrimp turn red from the lower segments to the tail, experience muscle tissue necrosis, and develop lesions at the tail end. Clinical signs typically appear when stressors are present in the pond water, such as poor water quality, high stocking density, or harvesting where factors that cause stress in shrimp are not controlled, such as an overly high temperature or prolonged duration.

Method of diagnosis

Myo disease can be diagnosed by polymerase chain reaction (PCR) test.

Source: Taukhid, Yani Lestari

Nama

Infectious Myonecrosis Virus (IMNV)

Type of pathogen

Virus

Synonyms

No data.

Characteristics

IMNV is a small virion from the Totiviridae family with a 40 nm double-stranded RNA molecule that replicates in the cytoplasm of target cells.

Toxicity

Myo is classified as a chronic disease, or a disease which takes a long time to cause mortality. Mortality typically occurs 9-13 days after infection. Post-larval (PL), juvenile, and adult shrimp at 60-80 days of culture (DoC) are susceptible to this virus, with a mortality potential of 50-70% of the pond population. Low salinity (<30 ppm) accelerates virus replication, while at 35 ppm, replication occurs more slowly.

Predispositional factors

IMNV can cause disease in densely populated aquatic environments. Environmental stressors such as sudden changes in water quality (temperature and salinity) can make shrimp more susceptible to IMNV. Additionally, accumulated feed residues at the pond bottom that turn into ammonia can stress shrimp, making them more prone to disease.

Transmission

IMNV is transmitted horizontally due to cannibalism and through water, as well as vertically from broodstock to fry.

Epidemiology

In Indonesia, Myo was first reported in vannamei shrimp from ponds in Situbondo, East Java, in 2006, with a prevalence of 11.11% and clinical symptoms similar to Myo outbreaks in Brazil in 2002. Therefore, it is believed that Myo occurred due to vannamei shrimp imports from Brazil to Asia.

Host or vector

Myo can be transmitted from broodstock to fry.

Infectious dose.

No data.

Incubation period

This disease can emerge at DoC 40-60.

Susceptibility to drugs

No data.

Susceptibility to disinfectants/probiotics

No data.

Physical inactivation

Field observations show that inactivating IMNV with routine pond disinfection, such as sun drying or chlorination, is more challenging than with other viruses like IHHNV, YHV1, WSSV, and TSV.

Early warning

Shrimp that are infected with Myo will have a paler body and white clumps on the abdomen, then they will turn red in the lower part of the body to the tail.

Prevention

Myo can be prevented by applying strict biosecurity measures. Some steps that can be taken include using shrimp fry from broodstock proven to be disease-free or SPF (Specific Pathogen Free). Additionally, reduce stocking density and ensure adequate aeration. Partial harvests can also be done to maintain pond capacity.

Before starting a new cycle, dry the pond for 2 weeks. If chlorine is applied, rinse the chlorine out of the pond by flowing water and draining it. Additionally, regularly change the water and apply plankton and probiotics to break the disease transmission chain.

Other steps to prevent Myo include installing nets or plastic at the pond bottom to prevent aquatic organisms like crabs from entering and using bird nets. Biosecurity should also be implemented across an entire farming area that shares the same water source and fry.

Treatment

No vaccination is effective against IMNV. During the early infection stage when mortality is still low, stabilize the water quality, particularly the temperature, salinity, and pH. Increase aeration and administer additional feed which contains vitamin C, molasses (25% of FR/day) and probiotics, and reduce the amount of feed or stop feeding temporarily.

Eradication

Farm clustering can be implemented to facilitate management agreements among farmers in the same area. This can help coordinate the treatment of water to be discharged if a Myo outbreak occurs, minimizing the spread of disease to other ponds.

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