Feline herpesvirus and calicivirus: Occurrence and pathology in cats with respiratory disease

Feline herpesvirus (FHV) and feline calicivirus (FCV) are the primary causative agents of respiratory disease in cats. Although fatal respiratory disease is frequently observed in routine pathological diagnostics, the role of viral involvement is rarely investigated. This study aimed to assess the involvement of FHV and FCV in necropsied cats with fatal respiratory disease using immunohistochemistry, and to characterize the affected tissues, along pathological and epidemiological findings of the positive cats. Sixty cats were assessed. Immunolabeling for FHV was observed in 12 cats, for FCV in seven cats, and for both viruses in three cats. These FHV and FCV-positive cats (n = 22) were included in the study. Co-infection with feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) was observed in 14 cats (six FeLV-positive, four FIV-positive, and four FeLV and FIV-positive). Gross lesions were mainly restricted to the respiratory and upper digestive tracts. Histological analysis revealed that the affected tissues included the nasal and oral cavities, esophagus, larynx, trachea, and lungs. All FCV-positive cats and most FHV-positive cats presented erosive, ulcerative, and fibrinonecrotic lesions in the upper respiratory/digestive tracts, concomitant with pneumonia (18/22). The remaining 4/22 cats, all infected with FHV, had lesions restricted to the upper respiratory tract. FHV-positive cats showed a bronchointerstitial pneumonia, whereas FCV-positive cats exhibited a fibrinosuppurative pneumonia. This study highlights pneumonia as an important cause of death in cats with FCV and FHV infection, particularly in those co-infected with retroviruses. It also emphasizes the importance of collecting samples from the respiratory and upper digestive tracts during necropsy and using immunohistochemistry to confirm the involvement of viral infectious agents—even in adult and aging cats. The findings of this study may contribute to further investigations into feline respiratory diseases.