It is curious that isolate 2HP did not cause AHPND pathology, even though it gave a positive result with the AP2 method designed for detection of AHPND bacteria. Since this isolate did cause mortality using the immersion challenge at 108 cfu/ml followed by 106 cfu/ml (Tran et al., 2013) and since the moribund shrimp showed flatten HP tubule epithelia similar to that of moribund shrimp subjected to immersion challenge by isolate 5HP at 103 cfu/ml, it is possible that 2HP is related to AHPND isolates but has lower virulence than the isolates that caused characteristic AHPND histopathology.