Response of calves to exposure with swine influenza virus.

In calves inoculated with live swine influenza virus (SIV) A/sw/IL/75 (H1N1) intranasally, SIV was isolated for 7 days, and respiratory tract disease was observed. Antibody was detected in serum of inoculated calves from postinoculation day 9, and virus-neutralization antibody was demonstrated on postinoculation days 14 and 21. The primary response was low, but readily differentiated from the secondary response after calves were challenge exposed with homologous SIV. Pneumonic lesions were observed at necropsy, and histopathologic changes in airways and lungs were consistently found. Fluorescent staining revealed viral activity in epithelial cells of airways. The virus was transferred to healthy calves housed with inoculated calves.