Relationship of Serum and Nasal Secretion-Neutralizing Antibodies in Protection of Calves Against Parainfluenza-3 Virus

SUMMARY Twenty-one calves were exposed to parainfluenza-3 ( pi -3) virus by aerosol. Some of the calves had serum antibody and nasal secretion ( ns ) antibody from previous exposures to aerosol and by intramuscular inoculation. After exposure to viral aerosols, the calves underwent a gradient of clinical responses, ranging from signs of illness, with shedding of virus, to no apparent effect. Increases in ns antibody and serum antibody occurred in all calves, regardless of their degree of clinical reaction after challenge and their preexposure titer levels. The degree of protection from illness was reflected more by the ns antibody level than by the serum antibody. Also, serum antibody was associated with reducing the severity of illness, once it occurred.