Interaction between live avian pneumovirus and Newcastle disease virus vaccines in specific pathogen free chickens.

One-day-old specific pathogen free White Leghorn chicks were vaccinated with live avian pneumovirus (APV) vaccine, live Newcastle disease virus (NDV) vaccine or both. At intervals up to 28 days after vaccination, distribution of the virus in the tissues was studied, together with humoral and mucosal antibody responses in lachrymal fluid and tracheal washes. APV vaccine was detected for almost twice as long in the dual vaccinates as in the single vaccinates. Higher numbers of isolations of ND virus vaccine were obtained from the dual rather than the single vaccinates at 7 days post-vaccination but the reverse occurred at 14 days. APV serum antibodies were significantly lower in the dual rather than the single vaccinates. However, there were similar levels of local APV-specific IgA in the lachrymal fluids of both single and dual APV vaccinates. NDV serum antibody titres in the dual vaccinates were significantly higher than in the singly NDV-vaccinated chickens. It appears that simultaneous vaccination of chicks with live APV and NDV vaccines causes temporary suppression of APV vaccine proliferation and reduces humoral antibody responses to it, although the antibody response to NDV is enhanced.