Two sets of primers were used for the detection of HPV. The PCR protocol and primer set (H441F and H441R) described by Phromjai et al. (2002) was expected to yield a product of 441 bp, while the protocol and primer set 1120F and 1120R described by Pantoja & Lightner (2000) was expected to yield a product of 592 bp. For the detection of MBV, the nested PCR primers and protocol described by Belcher &
Young (1998) were employed. A nested PCR protocol (Hossain et al. 2001) was also used for WSSV detection. PCR was carried out in a 30 µl reaction mixture consisting of: 1 × PCR buffer; 5 pmol of each primer for HPV and WSSV, and 10 pmol of each primer for MBV; 50 µM each of dATP, dCTP, dGTP and dTTP; 0.9 U of Taq DNA polymerase; and 2 µl of DNA extract. Total DNA extracted from shrimp showing histopathological changes characteristic of WSSV, MBV and HPV were used as templates for positive controls, and DNA extracted from healthy freshwater fish Labeo rohita was used as the template for the negative control. Amplification was carried out in a thermocycler (M. J. Research). The amplified PCR products were electrophoresed in a 1.2% agarose gel containing 0.5 µg ml–1 ethidium bromide, and visualized with a UVtransilluminator (Gel doc system, Hero Lab).