Frozen commodity shrimp: Potential avenue for introduction of white spot syndrome virus and yellow head virus

Since 1992, white spot syndrome virus (WSSV) and yellow head virus (YHV) have caused mortalities in cultured shrimp throughout Asia. By 1995, WSSV was detected in Texas and South Carolina, and the virus has also been recently reported in Central and South America (Nicaragua, Honduras, Guatemala, Panama, Colombia, Peru, and Ecuador). The importation of live infected shrimp is the principal mechanism by which exotic viruses may be introduced to new geographic regions. However, another probable mechanism is via the importation of infected commodity shrimp from regions where the pathogens are enzootic. Ten different lots of imported frozen tails of Penaeus monodon were screened for WSSV and YHV by polymerase chain reaction (PCR) and reverse transcriptase (RT) PCR analysis. In 8 of 10 samples tested, WSSV was detected, and YHV was found in 3 out of the 10 samples. Six of the 10 sample sets of frozen shrimp gave strong positive tests for WSSV or YHV by PCR, and these were selected for bioassay with specific pathogen-free P. vannamei, which were used as the indicator shrimp for infectious virus. The indicator P. vannamei were exposed to WSSV- or YHV-positive tissues either per os or by injection. Infectious WSSV resulted in 100% mortality in the indicator P. vannamei in four of nine bioassays. One of the three YHV bioassays produced mortalities in the indicator shrimp, and the other two in vitro assays demonstrated the presence of the virus in the tissues of the indicator shrimp. Infections were confirmed by histology, PCR or RT-PCR, or by in situ hybridization. The results of the study indicated that WSSV and YHV were present in the different lots of imported frozen P. monodon that were tested, and that both viruses were infectious to P. vannamei either by injection or by per os exposure.