Diagnosis of infectious bovine rhinotracheitis by direct immunofluorescence

Summary Clinical signs, virus excretion and immunofluorescence in nasal smears were studied in nine susceptible steers during a two week period, following intranasal expo-sure with IBR-virus. All animals responded with fever (ay. 3.9 days) and nasal discharge. IBR-virus was isolated from nasal swabs from 1 to 11 days after exposure (ay. 10 days), whereas fluorescence in nasal smears was observed from the second till the seventh day after infection (ay. 5.5 days). Fluorescence was most distinct 3 to 5 days after infection, which coincided with the period of fever and a serous nasal discharge. Smears from animals with a mucopurulent or slightly haemorrhagic nasal discharge were nearly always negative. For a reliable diagnosis on live animals by immunofluorescence, it is necessary to take nasal smears from several healthy looking animals with fever and a slight, preferably serous discharge. Air dried smears should be fixed in acetone within 24 hours. Seven yearlings were autopsied 3 to 11 days after intranasal exposure and subjected to a detailed investigation by the cryostat-immunofluorescence technique (IFT). The tonsils of all animals were positive, followed in declining frequency by the larynx, namharynx, nasal mucosa, and pharyngeal mucosa. Besides the organs already mentioned, fluorescence was often observed in the lungs and tracheal mucosa of animals that had suffered a fatal infection of IBR in the field. The tonsils should be regarded as the organ of choice. Fluorescent foci were localized in the epithelial lining of the tonsillar crypts and in the surface epithelium of the mucosae. The direct IFT on nasal smears of suspected animals and on cryostat sections of tissues collected at autopsy offers veterinary laboratories with no facilities for tissue culture a possibility of a rapid and reliable diagnosis of IBR infections.