Feline infectious peritonitis (FIP) is one of the most important infectious diseases in cats and is a major cause of death. It has a worldwide distribution and mainly affects young cats from shelters and catteries [1]. It is caused by FIP virus (FIPV), which is a mutant virulent biotype of feline coronavirus (FCoV) characterized by a switch from primary viral replication in enterocytes to replication and activation in circulating monocytes, thereby transforming a local infection into a systemic one [[2], [3], [4]].

FIP pathology is characterized by generalized pyogranulomatous inflammation associated with vasculitis, mainly due to inflammation of veins [5]. Differentiation between the ‘wet’ and ‘dry’ forms has been used to distinguish cases with effusion from those with only pyogranulomas, but both forms represent the same disease entity [1]. Clinical signs differ according to the affected tissues but commonly include abdominal distension [1]. Respiratory signs are less frequently observed, with dyspnoea being more commonly related to pleural effusion [6].

Pathological findings in cats with FIP are well established but there is little information about FIP-associated respiratory disease. It is known that FIPV can cause pyogranulomatous pneumonia [[6], [7], [8]] but the significance of FIP for the respiratory system is poorly understood. The lungs are not considered a primary target of FIP and its involvement is often considered to be a consequence of systemic disease, generally with localized lesions in the parenchyma underlying the pleura [9,10]; however, some cats seem to develop respiratory signs such as sneezing and nasal discharge at an early stage of FIP [[11], [12], [13], [14]]. Furthermore, there is a recent report of a case of pyogranulomatous rhinitis associated with local immunohistochemical detection of FIPV, suggesting that respiratory infection is part of the FIP-associated disease spectrum [14].

This study aimed to investigate the pathological findings in the respiratory tract of cats with FIP infection and the occurrence and distribution of FIPV antigen in lung tissue, and to define lung injury patterns in these cats.