[Retention of the Newcastle disease virus in immunized poultry]

The period of virus harbouring was followed up in a total of 50 experimentally infected (Newcastle disease) birds. Two methodes were used to isolate the challenging virus: the ordinary virologic procedures and the method of tissue cultures. Studied was also the effect of the humoral immunity on the persistence of the virus. Investigations by the tissue culture method revealed a persisting latent infection in the course of 70 day, which in certain cases could assume an acute course and become apparent. The Newcastle disease virus was isolated from birds having either low or very high levels of serum agglutinins. The currently employed virologic methods failed to demonstrate the virus after the 16th day of infection.