Competition between nonplaquing and plaquing strains of Newcastle disease virus as affected by temperature.

If lentogenic (nonplaquing) strains of Newcastle disease virus were inoculated into a monolayer of chicken embryo fibroblasts before velogenic (plaquing) strains, the plaquing of the latter were inhibited partially or completely. An interval of 22 hours between the two inoculations gave greater inhibition than did 1 hour. In general, lentogenic strains grew better at 37 C (inhibited plaquing more effectively) than at 42 C. Enzootic velogenic strains that produce a neurotropic form of the disease in chickens were inhibited more than were exotic velogenic strains that produce a viscerotropic form. The lentogenic strains differed markedly in ability to interfere with velogenic strains.