Distribution of rabies antigen in infected brain material: determining the reliability of different regions of the brain for the rabies fluorescent antibody test.

To assist in making recommendations for sampling of brains for the fluorescent antibody test (FAT), a study was conducted to determine the regions of the brain where rabies antigen is found most reliably. Each identifiable part of 252 rabies-positive brains of various species was re-tested using routine FA tests. It was found that there was frequent variation in the quantity of antigen between regions of the brain. The thalamus, pons and medulla were the most reliable parts of the brain as they were positive in all specimens tested. The cerebellum, hippocampus and different parts of the cerebrum were negative in, respectively, 4.5, 4.9 and 3.9-11.1% of positive brains. It is recommended that specimens for rabies diagnosis must include the brain stem. The structure of choice would be the thalamus as it was positive in all specimens and had the most frequent prevalence (97.8%) of abundant antigen. These findings contradict many old studies that state that the hippocampus should be the structure of choice for rabies diagnosis. The current data demonstrate that the reason for the old recommendations is that the hippocampus has the highest frequency of large inclusion bodies, as the reliability of the histological tests used previously depended on inclusion body size.