Demonstration of rabies viral antigen in paraffin tissue sections: Comparison of the immunofluorescence technique with the unlabeled antibody enzyme method

The immunofluorescence technique and the peroxidase-antiperoxidase method were used to demonstrate rabies antigen in a retrospective study on formalin-fixed, paraffin-embedded brain tissues from 34 naturally infected wild and domestic animals. Rabies was confirmed with immunofluorescent staining on fresh brain tissue at the time of necropsy of the animals. There was a perfect correlation (serial sections from a given brain area were always positive by both methods), but the peroxidase-antiperoxidase technique was preferred, since no trypsin digestion was required. Twenty six of the 34 animals were immunohistochemically positive and had encephalitis, and in 21 of these 26, the hematoxylin and eosin-stained sections contained detectable intracytoplasmic inclusion bodies in at least 1 brain area. Of the remaining 8 animals (with no inflammatory lesions), 7 were positive for rabies antigen and 2 had no inclusion bodies. Rabies antigen was apparent in 62% of the brain areas in which inclusion bodies were not found in the corresponding hematoxylin and eosin stained sections. Thus, together with the inclusion body positive areas, which were all immunohistochemically positive, it was possible to diagnose rabies in a total 84% of the areas examined. Both techniques greatly facilitate the diagnosis of rabies and may be a reliable help to the diagnostic pathologist when only formalin-fixed tissues are available. However, the methods should not be considered substitutes for the immunofluorescence technique and the mouse inoculation test with fresh brain tissue.