The occurrence of virus in leukocytes of vaccinated chickens following challenge with virulent Newcastle disease virus.

Chickens that had been vaccinated with a lentogenic strain 2 to 9 weeks before challenge with a virulent virus remained clinically normal following challenge and circulated virus within leukocytes for as long as ten days. The virulent virus was detected by inoculating embryonating eggs with leukocytes separated from blood shortly after drawing. Freezing the leukocytes before inoculating the eggs reduced isolation 64.8%. Virus was demonstrated most readily in chickens challenged when antibody was decreasing, about 9 weeks after vaccination. The observation that virus in blood may be contained within a cellular component and be readily demonstrable only if the cells are separated and washed is significant both to an understanding of the immunological responsiveness of vaccinated chickens exposed to field challenge and to an understanding of epizootiological events.