White spot syndrome virus infection of cultured shrimp in China

In 1993 and 1994, an epizootic of disease (white spot) occurred among cultu red shrimp in China, resulting in mass mortality. During the periods of out break epizootiological surveys were undertaken. Based on these surveys it i s known that the disease occurred among populations of cultured Penaeus chi nensis, P. japonicus, and P. monodon, infecting shrimp from 2.4 cm to adult . The disease presumably spread among farms as a result of transport of con taminated shrimp seedlings and seawater. Water temperatures exceeding 25 de grees C caused the disease to spread more rapidly. Challenge experiments sh owed the causative agent was highly virulent. Both naturally and experiment ally infected shrimp exhibited white spots on the carapace. Moribund shrimp contained turbid hemolymph, hypertrophied lymphoid organ, and a necrotic h epatopancreas. By electron microscopy, viral particles were observed in gil ls, stomach, lymphoid organ, and hypodermal tissue of infected shrimp. The virions were slightly ovoid with an envelope and averaged 350 X 150 nm and nucleocapsids measured 325 X 120 nm in size. By use of negative stains, com plete virions were 375 X 157 nm and unenveloped nucleocapsids averaged 395 X 83 nm.