False rumours of disease outbreaks caused by infectious myonecrosis virus (IMNV) in the whiteleg shrimp in Asia

Our continual testing by nested RT-PCR of shrimp samples suspected of IMNV infection from various Asian countries since 2006 has yielded negative results, except for samples from Indonesia. Our results are supported by the lack of official reports of IMNV outbreaks since January 2007 in the Quarterly Report on Aquatic Animal Diseases (QAAD) from the Network of Aquaculture Centers in Asia Pacific (NACA). In most cases, our shrimp samples for which tissue sections were possible showed signs of muscle cramp syndrome that also commonly causes muscle whitening in stressed whiteleg shrimp. Thus, we suspect that most of the false rumours in Asia about IMNV outside of Indonesia have resulted because of muscle cramp syndrome.Results from continual testing of suspected IMNV outbreaks in Asian countries other than Indonesia since 2006 and the lack of official country reports of IMNV outbreaks since January 2007, indicate that rumours of IMNV outbreaks in Asian countries outside of Indonesia are false. We suspect that confusion has arisen because muscle cramp syndrome causes similar signs of whitened tail muscles in whiteleg shrimp.Infectious myonecrosis virus (IMNV) is a double-stranded RNA (dsRNA) virus in the family Totiviridae near the genus Giardiavirus. Disease outbreaks in the whiteleg shrimp Penaeus (Litopenaeus) vannamei caused by this virus were first reported from Brazil in 2002 [1] and were characterized by gross signs of whitened abdominal muscles in the shrimp and by slow mortality persisting throughout culture (cumulative mortality reaching up to 70%). The causative virus was described in 2006 [2]. The viral particle is icosahedral and about 40 nm in diameter and the length of the whole genome is 7650 base pairs (GenBank AY570982). Although the black tiger shrimp (also called giant tiger shrimp) could be infected with IMNV in the laboratory, it did not die from the infection [3].At the end of June 2006, Centex Shrimp received shrimp samples from a suspected IMNV