On histological examination there was a focal distribution of virus-infected hepatopancreatic tubule cell nuclei. Up to 36 cells in one transverse section of a
tubule could show nuclear pathology while adjacent tubules were normal. Infected cells were usually R-cells, though nuclear pathology was seen in other differentiating cells. Infected nuclei were irregular to round in shape, greatly hypertrophied, and up to five times normal size (Fig. 6). The strongly or weakly staining basophilic inclusions entirely occupied the nucleus and tended to be amorphous, though a finely
granular structure could be seen occasionally. Scattered strands of nucleolar material were present in some of the inclusions. The earliest nuclear changes
detected were a slight hypertrophy of the nucleus, a pale nucleolus or marginal nucleolar fragments, a thin marginal band of strongly basophilic chromatin, and
infrequent fine eosinophilic granules scattered through a nonstaining nucleoplasm (Fig. 6). An intermediate stage was represented by hypertrophied nuclei
containing a single, pale purple-red to reddish amorphous inclusion. These nuclei had a distinct margin of basophilic chromatin and a central nucleolus or
circle of nucleolar remnants. The marginal chromatin stained strongly with the carbol fuschin counterstain in Gram’s stain. Inclusions in the advanced infections
had a diffuse red reticulum throughout with the same stain. Inclusions did not stain with PAS, Giemsa, or alcian blue and were Feulgen-negative. Virus related
nuclear pathology was only seen in the hepatopantreas. All affected nuclei examined under electron microscopy appeared to be at an advanced stage of infection.
A loose granular virogenic stroma filled most of the nucleus. The chromatin that remained was margin