Abstract
Summary
1) Newcastle disease virus was modified by serial passage of the California 11914 strain through tissue cultures using minced chicken embryos suspended in Simm-Sanders medium. 2) The modified virus was administered intramuscularly to over 20,000 chickens of all ages without producing any nervous or respiratory symptoms, but produced an immunity as demonstrated serologically and by intramuscular challenge with a virulent strain of ND virus. One dose of the vaccine given to susceptible chicks at 5 days of age or over induced an immunity lasting at least 12 weeks. Two doses from 7 to 9 weeks apart have resulted in a high degree of immunity which was still evident 33 weeks later. 3) When administered to 2 flocks of chickens, one in acute stage of infectious bronchitis and the other with chronic respiratory disease, the virus apparently did not increase the severity of the respiratory disease condition. 4) Repeated trials showed that vaccinated chickens did not transmit the infection to unvaccinated susceptible birds within the same pen during 33 weeks. 5) The modified virus can be propagated on HeLa cells to produce a vaccine for poultry, thus avoiding the introduction of egg-borne infections to poultry flocks through vaccines produced in chicken embryonated eggs.
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