Again, molecular analysis will be required to verify this possibility and explain any differences in virulence. A feature that counters the previous argument is the appearance of highly vacuolated E-cells in the HP of shrimp challenged with 2HP, since these were not seen in shrimp that received the low dose challenges of 5HP. This phenomenon may be a unique feature of 2HP and may indicate that it possesses different virulence factors from AHPND bacteria. If so, the positive test result with AP2 would need to be explained. We could find no other reports of the vacuolated E-cell anomaly in shrimp or crabs, so it is difficult to speculate on its meaning and significance, except to state that it is associated with immersion challenge with 2HP.