Similarity between the histopathology of White Spot Syndrome Virus and Yellow Head Syndrome Virus and its relevance to diagnosis of YHV disease in the Americas

Abstract

The similarity of lesions, especially of the lymphoid organ (LO), caused by severe white spot syndrome virus (WSSV) infection and those once thought to be diagnostic for yellow head virus (YHV) infection have been the cause of misdiagnosis of YHV disease at several pathology laboratories in the Americas. In order to illustrate this similarity, and to demonstrate the risk of misdiagnosis, specimens of shrimp Penaeus setiferus and P. vannamei, experimentally infected with WSSV, were examined by conventional H&E histology and by in situ hybridization (ISH) with nonradioactively labeled gene probes specific for detection of each virus. H&E analysis of shrimp with severe WSSV infection revealed the presence of severe necrosis of the lymphoid organ and of the fibrous connective tissue that were very similar to that caused by YHV infection. Five representative shrimps, displaying severe lymphoid organ necrosis, were selected and subjected to in situ hybridization with the gene probe specific for detection of YHV, and the results were negative. When the same specimens were subjected to in situ hybridization with the WSSV-specific gene probes, a strong positive reaction to the probes was observed both systemically and within the lymphoid organ. Clinical samples of 10 shrimps originating from several Latin American countries and diagnosed with severe WSSV infection by H&E histology and displaying severe lymphoid organ necrosis were also examined by in situ hybridization. No reaction to the YHV probe was observed in any of these specimens. Hence, we have concluded that severe infection by WSSV may result in systemic necrosis, and necrosis of the lymphoid organ, in particular, which is very similar to that caused by YHV. Since the presence of YHV has not been confirmed in the Americas, it is recommended that in cases when YHV infection may be suspected after conventional H&E analysis, additional evidence should be obtained by other means such as in situ hybridization with a YHV-specific probe and/or by RT-PCR.