Newcastle disease. Environmental and host influences on immunity induced by non-parenteral vaccination.

Many millions of doses of Newcastle disease vaccine have been given to chickens by two non-parenteral routes; in aerosols and in drinking water. These vaccines, that reach either or both the respiratory and digestive tracts, have afforded protection against disabling disease for millions of chickens but not without irregularity in the adequacy and duration of the response induced. Among the factors that affect the response are the nature of the delivery system (i.e. aerosol particle size), the dose received, the time involved in delivery, the environmental circumstances (i.e. ambient temperature) and the precondition of the host from the withholding of feed and water and from handling and crowding. The site of initial replication is critical, presumably because by inducing nonspecific or specific blockage of other replication sites, it shapes the immune response. Most events in the development of a vaccine-induced infection are dependent upon both thresholds and inherent periodicities. Individual host responsiveness is an interplay between the virus, environmental factors and intrinsic systems of the host.