Histopathology of cultured shrimp showing gross signs of yellow head syndrome and white spot syndrome during 1994 Indian epizootics.

Two epizootics of cultured shrimp occurred in India in 1994. The first case of mass mortalities of tiger shrimp Penaeus monodon in July 1994 was very similar to that caused by yellowhead virus (YHV) infection with regard to gross clinical signs, host species and size of shrimp affected, but it was histologically atypical. Very interestingly, intranuclear inclusions typical of a white spot syndrome virus (WSSV) infection were present in shrimp showing gross signs of yellow head syndrome, suggesting a dual infection. The second case of mass mortalities of P. monodon and P. indicus of all age groups and sizes in November 1994 was typical of a WSSV infection, clinically and histopathologically. Densely stained, round intracytoplasmic inclusions typical for YHV infection found in the lymphoid organ and haematopoietic tissue of WSSV-infected shrimp indicated a possible dual infection.