The problem of cell blockade or interference has been studied using Newcastle disease of chickens as a model. Embryos may be protected against the uniformly lethal effect of the virus by previous inoculation with ultraviolet-irradiated virus. It was necessary to use 0.5 to 1 mg. of partially purified washed virus in order to demonstrate this effect. Blockade by inactive virus in the embryo was not complete, since it could be overcome by inoculating increasing amounts of active virus or by injecting the active virus into the allantoic sac instead of placing it on the membrane. The lethal effects of small doses of Newcastle virus could also be blocked by previous infection of the embryo with either swine influenza virus or human influenza A. Again this blockade may be overcome by using larger doses of active Newcastle virus.
Simultaneous injection of chickens with viruses of equine encephalomyelitis and a virulent strain of Newcastle disease virus merely delayed the incubation period of the Newcastle virus a day or so. Simultaneous inoculation of chickens with virulent and avirulent Newcastle strains caused complete blocking of the virulent strain. This blocking or interfering effect of the avirulent strain could be demonstrated 1 or 2 days after the inoculation of the virulent strain but was not effective after symptoms of the virulent disease had set in.