Experimental transmission of white spot syndrome virus (WSSV) from crabs to shrimp Penaeus monodon

White spot syndrome virus (WSSV) of the black tiger prawn Penaeus monodon is a recently discovered baculo-like virus disease which is currently the cause of very serious and widespread losses in the shrimp industry in Thailand and elsewhere in Asia. Three suspected crab carriers of this virus commonly found in shrimp-rearing areas were investigated. These were Sesarma sp., Scylla serrata and Uca pugilator. All these crabs could be infected with WSSV by injection and they sustained heavy viral infections for up to 45 d (confirmed by normal histology, specific in situ DNA hybridization and PCR amplification) without visible signs of disease or mortality. All of them also transferred the disease to P. monodon via water while physically separated in aquarium cohabitation tests. Transfer of the virus to the shrimp was monitored using in situ DNA hybridization and PCR assay at 12 h intervals after cohabitation began. With U. pugilator, WSSV could be detected in the shrimp cohabitants after 24 h using PCR amplification and after 60 h using in situ hybridization. With S. serrata, the shrimp were positive for WSSV after 36 h using PCR and after 60 h using DNA in situ hybridization. With Sesarma sp. they were positive after 48 h using PCR and 72 h using in situ hybridization. These laboratory studies demonstrated that crab carriers of WSSV may pose a real threat to cultivated shrimp. However, the studies were carried out in containers with a small volume and with relatively clean sea water as compared to shrimp cultivation ponds. Pond-based studies are now needed to determine whether factors such as pond volume, pond water quality and shrimp and crab behavior can influence the rate and success of transfer.