First confirmed outbreak of mud crab reovirus (MCRV) causing mass mortality in orange mud crab Scylla olivacea in India.

Mud crab reovirus (MCRV) has emerged as a major challenge in mud crab farming, often leading to sudden mortality and severe economic losses. This study documents an outbreak of MCRV in a Scylla olivacea fattening facility at Bideipur, Odisha, India. Crabs in poor health condition were collected for analysis. The affected animals were sluggish, showed a grey and unhealthy hepatopancreas with autotomy. Histological examination revealed basophilic inclusion bodies together with necrotic changes in the hepatopancreas, gills, and muscle. Transmission electron microscopy confirmed the presence of non-enveloped, icosahedral viral particles arranged in crystalline arrays predominantly within the cytoplasm, together with nuclear alterations including chromatin condensation and nucleolar displacement. Reverse transcription (RT)-PCR produced a distinct amplification at ~433 bp, verifying MCRV infection. Experimental challenges using oral exposure, waterborne transmission, and cohabitation all resulted in complete mortality, demonstrating both the virulence of the virus and its ease of transmission. This study provides the first evidence of MCRV infecting S. olivacea in India. This outbreak underlines the need for consistent health screening, the use of virus-free seed, and strict biosecurity protocols if mud crab aquaculture is to remain sustainable.