Small-plaque variant of canine herpesvirus with reduced pathogenicity for newborn pups

The plaque characteristics of 13 field isolates of canine herpesvirus were found to be similar. After 312 passages of the F-205 strain in dog kidney cell cultures incubated at 35 degrees C, the virus still was virulent for newborn pups; it also had the macroplaque morphology of the wild-type virus. However, after 20 additional passages at 30 degrees C, a microplaque (mP) variant had emerged as the predominant type. Cloned mP virus retained the small-plaque characteristic after 66 transfers at 30 degrees C. The natural history of the mP strain and some of its biological properties are reported, the most significant being its lack of pathogenicity for newborn pups.