Effects of feed additives and feed contaminants on the susceptibility of chickens to viruses.

Several feed additives and contaminants are recognized which may alter avian immune responsiveness or disease resistance. Exposure of poultry to such chemical substances may in some instances explain the failure of vaccines to confer expected levels of disease resistance and may explain certain flock health problems associated with infectious agents generally presumed to be commensals. Limited studies with BHT and ethoxyquin did not associate decreased resistance to NDV infection with dietary intake of either substance. Conversely, dietary intake of BHT protected chickens against NDV infection, and the protection may have resulted at least in part from direct antiviral activity of BHT. Exogenous histamine administered in the diet increased mortality associated with avian reovirus infection, but the clinical significance of this observation is unknown.