Objective—To test the hypothesis that feedlot cattle
with acute interstitial pneumonia (AIP) have bacterial
infection of the lung or liver and concurrent bovine
respiratory syncytial virus (BRSV) infection significantly
more often than pen mates without AIP.
Animals—39 feedlot cattle with signs consistent
with AIP and no history of treatment with antimicrobials
and 32 healthy control cattle from the same
Procedure—Lung and liver specimens were
obtained postmortem for bacterial or mycoplasmal
culture and histologic examination; lung tissue was
assessed for BRSV infection immunohistochemically.
Results—Among affected cattle, 26 had AIP confirmed
histologically. Lung tissue from 11 cattle with
AIP yielded microbial respiratory tract pathogens on
culture; tissues from control animals yielded no
microbial growth. In 4 cattle with AIP and 2 control
animals, liver abscesses were detected; bacteria
were isolated from abscessed tissue in 3 and 1 of
those animals, respectively. Immunohistochemically,
9 cattle with AIP and no control animals were BRSV-positive.
Histologically, 9 AIP-affected cattle had only
acute alveolar damage with exudation, and the other
17 had acute exudation with type II pneumocyte
hyperplasia. No lesions of AIP were detected in control
animals. Only 4 AIP-affected cattle had bacterial
infection of the lung with concurrent BRSV infection.
Conclusions and Clinical Relevance—Results indicated
that microbial respiratory tract pathogens are
more common in cattle with AIP than in healthy pen
mates. Control of bacterial pneumonia late in the
feeding period may reduce the incidence of AIP at
feedlots where AIP is a problem. (Am J Vet Res 2004;65:1525–1532)