Bovine respiratory disease complex (BRDC) is a multi-etiologic disease that significantly impacts the cattle industry. Nasopharynx-associated lymphoid tissue (NALT) is the nasal mucosal immune system that protects against various pathogens
however, reports on its pathology are limited. We compared the pathologic impact of BRDC-related viruses on the NALT and lungs of 10 pneumonic and 3 negative control Japanese black (JB) calves. Three bovine viral diarrhea virus (BVDV) persistently infected Holstein calves were examined as positive control
all 3 calves had BVDV in situ hybridization (ISH)-positive signals in NALT follicular macrophages and lymphocytes, suggesting viral replication in these cells. NALT and lungs of pneumonic JB calves had weak-positive BVDV results in PCR and negative results in ISH, indicating late-stage transient BVDV infection. The finding of BVDV in unvaccinated pneumonic JB calves confirmed the involvement of a BVDV field strain. PCR detection of bovine coronavirus (BCoV) and bovine respiratory syncytial virus (BRSV) in NALT along with ISH BCoV-positive signals in NALT epithelial cells confirms infection by those viruses. Pneumonic JB calves had nasopharyngitis and pneumonia, with the same bacteria present in nasopharyngeal swabs and lungs, indicating that, in these animals, the bacteria proliferating in the nasopharynx probably migrated to the lungs via inhalation. BVDV transient infection in the NALT may induce local immunosuppression
BCoV and BRSV infections are thought to damage epithelial cells, facilitating bacterial infection of the NALT and lungs. Our results confirm that bovine NALT is a location for viral replication and may be associated with BRDC development in cattle.