Evidence of respiratory infection by bovine coronavirus in a neonatal calf

BACKGROUND: Bovine coronavirus (BCoV) is a major pneumoenteric pathogen that causes diarrhea in calves and respiratory disease in cattle of all ages. While BCoV-associated diarrhea is well-documented, reports of respiratory infection in neonatal calves remain limited, particularly in the Republic of Korea (ROK). Here, we describe a case of BCoV-induced pulmonary infection in a calf that presented without clinical respiratory signs. CASE PRESENTATION: A 7-day-old Holstein calf developed white, watery diarrhea and dehydration that subsequently progressed to astasia, showing no respiratory signs. Following euthanasia due to poor prognosis, necropsy revealed no gross lesions. However, real-time RT-PCR detected BCoV only in the lungs, whereas all other tissue samples, including the intestines, tested negative. Histopathologic examination revealed mild bronchiolitis and interstitial pneumonia, and immunohistochemistry confirmed the presence of viral antigen in bronchiolar epithelial cells and alveolar macrophages. CONCLUSIONS: This case represents a naturally occurring BCoV pulmonary infection in a neonatal calf in the ROK. These findings indicate that BCoV can establish primary respiratory infection at a very early age without typical respiratory signs. Overall, this observation underscores the potential role of BCoV as an important contributor to respiratory disease in calves.