Ten 7-day-old gnotobiotic Beagle puppies were inoculated intraperitoneally with virulent canine distemper virus (R-252-CDV). The dogs were killed and perfused with paraformaldehyde/glutaraldehyde from eight to 36 days after inoculation. The developing teeth of the mandibles were examined by light microscopy, and the teeth from three dogs were examined by electron microscopy. Necrosis of individual cells in the stratum intermedium of the developing tooth was the first change, detectable at day 9 post-inoculation. At day 16 post-inoculation, there was disorganization of the ameloblasts. In the stratum intermedium, multinucleate giant cells and large eosinophilic cytoplasmic viral inclusions were prominent. Ultrastructurally, these inclusions consisted of clusters of tubular aggregates typical of canine distemper virus nucleocapsids. At 28 to 36 days post-inoculation, the changes were seen in the reduced enamel epithelium. Multinucleate cells were seen, but no inclusions. Some necrotic cells were seen. In these teeth, ameloblastic cells of the root were morphologically normal. Our results suggest that distemper virus affects developing teeth by direct infection of the enamel organ.
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