Immunoglobulins, antibodies and inhibitors of parainfluenza 3 virus in respiratory secretions of sheep.

Virus neutralising and haemagglutination inhibiting (HI) activities were monitored in the serum, nasal secretions and tracheo-bronchial secretions of lambs infected with Parainfluenza virus type 3 (PI3). HI activity was found in the secretions of both control and infected lambs, whereas neutralising antibody was found only in nasal secretions from infected lambs. Fractionation revealed that most of the HI activity in the secretions was due to a large molecular weight protein which also inhibited the haemagglutination (HA) of Parainfluenza virus type 1 (PI1) and type 2 (PI2) and Newcastle disease virus (NDV). This inhibitory activity was partially sensitive to receptor destroying enzyme (RDE). IgA antibodies specific for PI3 were also found in the respiratory secretions. However no increase in IgA levels was detected in the nasal secretions of the infected lambs. It is suggested that in certain reports non-specific inhibitors present in the nasal secretions of calves, may have been confused with PI3 specific IgA antibody.