Local immunity against Newcastle disease virus in the newly hatched chicken's respiratory tract.

Inoculation of 200 mean egg infectious doses (EID(50)) of lentogenic Newcastle disease virus strain B1 (NDV-B1) into the air sac of 4-day-old specific-pathogen-free chicks provided significant protection against challenge of the air sac with 100 chicken mean lethal doses (LD(50)) of velogenic NDV-H but no protection against reinfection when the challenge was by the eye. Conversely, inoculation of the eye with 200 EID(50) of NDV-B1 provided significant protection against challenge of the eye but not of the air sac with 100 chicken LD(50) of NDV-H. Birds that received both antiserum and intraocular immunization were subsequently protected against both eye and air-sac challenge. On the other hand, birds that received antiserum and air-sac immunization were protected only against air-sac challenge but not against ocular challenge. Low levels of passively administered antibody did not prevent infection of the eye or air sac but greatly reduced the mortality rate after inoculation of either the vaccine or the challenge viruses. Passively administered antibody also suppressed hemagglutination-inhibiting and virus-neutralizing antibody formation stimulated by air-sac infection but not antibody formation stimulated by ocular infection. These data are consistent with the hypothesis that local immunity is responsible for prevention of infection, since birds were immune to reinfection at one site and simultaneously susceptible at the other site of infection.