A 9-year-old, 3.5 kg, spayed female domestic shorthair cat was evaluated for a 1.5-month history of persistent labored breathing. Three months prior to the presentation, the cat developed paroxysmal tachypnea that gradually worsened and was treated with theophylline, bromhexine and cetirizine by the primary veterinarian. An improvement was observed in the tachypnea, but signs of increased breath effort persisted. The owner reported that the cat had a 1-year history of infrequent cough, occurring approximately twice a month. The cat had not been vaccinated for uncountable years and was housed entirely indoors. The deworming prophylaxis was not administered on a regular basis. According to the owner, there was no known exposure to any indoor pollutant or toxic substance. At presentation, auscultation of the thorax revealed decreased lung sounds and subtle crackles. Hematology and serum biochemistry were unremarkable. Serological tests for Toxoplasma gondii IgG antibody, feline heartworm antibody, cryptococcus antigen, and FIV/FeLV antibody were negative. Thoracic radiography identified significant lung hyperinflation and diffuse bronchointerstitial pattern with focally increased opacity in the right middle lung lobe and caudodorsal lung fields. Pulmonary function test was noninvasively performed using barometric whole-body plethysmography (BWBP), showing increased minute volume (906 mL/kg; in normal control cats, 299.8 ± 52.1 mL/kg) and late-expiratory flow limitation (pTBFVL PEF/EF25 ratio = 2.79; in normal control cats, 1.64 ± 0.18) under natural breathing []. Terbutaline (0.01 mg/kg) was administered subcutaneously as a bronchodilation test, as well as a treatment trial. After approximately 30 min, BWBP recording was repeated and revealed reduced minute volume (537 mL/kg; in normal control cats, 299.8 ± 52.1 mL/kg) and PEF/EF25 ratio (2.13; in normal control cats, 1.64 ± 0.18), which were still abnormally high, suggesting an incomplete response to the short-acting bronchodilator. The differential diagnosis at this stage was considered to be etiologies associated with lung hyperinflation and obstructive lower airway diseases, including feline lower airway disease (FLAD) with a phenotype of irreversible bronchoconstriction, emphysema, or lung bullae. Further diagnostics, such as lung HRCT, were suggested, and the owner preferred scanning under sedation alone to reduce the risk associated with anesthesia. Terbutaline (0.14 mg/kg PO q8h), prednisolone (0.6 mg/kg PO q12h), and enrofloxacin (2.5 mg/kg PO q12h) were prescribed as a trial for possible FLAD with concurrent lower respiratory tract infection. Lung HRCT was performed 2 days later revealing no evidence of emphysema or lung bullae. However, dorsally distributed and distinct marginated subpleural ground-glass opacities (GGO) were observed in the bilateral caudal lung lobes and a small part of left cranial lobe. There seemed to have more severe bronchiectasis in the bronchi toward those regions. Other findings included tree-in-bud opacities and mildly thickened bronchial walls []. CT images did not support the tentative diagnosis of typical FLAD, and the suspicion of an infectious etiology was raised. Prednisolone was temporarily withdrawn in concern of the presence of an active infection, and clindamycin (12 mg/kg PO q12h) was prescribed additionally. After a 10-day course of antibiotics and bronchodilator treatment, the cat experienced an improvement in activity levels at home but still showed labored breathing, which seemed to become worse gradually. The owner agreed to the BAL procedure at this time point, and the sampling was scheduled in 6 days with immediate withdrawal of antibiotics. Nonbronchoscopic BAL was performed by aseptically passing a sterile 8-Fr polyvinylchloride tube through the endotracheal tube to be wedged in the distal airway. Dorsal recumbency was used in this cat to obtain samples from the dorsal region of the lung, which had multiple subpleural GGO on CT images. Two boluses of 7.5-mL warmed sterile saline (followed by 2-mL air) were instilled, and the recovered fluid (8.2 mL) was turbid and had substantial amounts of mucus. Only one lung was sampled to avoid compromising the patient’s condition further. BAL fluid was processed immediately after collection, and cytology showed hypercellularity (1037 cells/μL; reference interval [RI] 200–400/μL) with 88.5% neutrophils (RI < 7%), 7.7% macrophages (RI 65%–80%), 3.4% lymphocytes (RI < 10%), and 0.4% eosinophils (RI < 17%) [–]. Routine microbiological examination, including aerobic and anaerobic bacterial culture, Mycoplasma PCR, and fungal culture were all negative. Additional workup was attempted to investigate other common viral pathogens by PCR, including feline coronavirus, calicivirus, and herpesvirus; the first two were negative, and the latter was positive in the BAL sample. Considering the presence of herpesvirus in the BAL fluid, as well as negative results of other pathogens, the lack of adequate vaccination in this cat, and other clinical findings, herpesvirus-induced bronchiolitis was diagnosed accordingly. Owing to no known standard treatment for viral bronchiolitis in small animal medicine, the therapeutic strategy was focused on supportive care, bronchodilation, mucolytics, and anti-inflammatory treatment. In the subsequent 6 weeks, multiple attempts of different bronchodilator (terbutaline, PO or SC; sustained-released theophylline, PO; ipratropium, nebulized) were used separately or in combination; none of these could effectively alleviate the labored breathing of the cat. Inhaled form of corticosteroids was considered, but the owner failed to achieve cooperation from the cat. Oral prednisolone was prescribed at a relatively low anti-inflammatory dose of 1.1 mg/kg/day, with careful observation for possible reactivation of the virus. Previously prescribed enrofloxacin was continued with the aim of preventing secondary bacterial infection, and it was later replaced with doxycycline for its potential immunomodulating effect. Lysine was supplemented at a dose of 1000 mg/day, and no other antiviral agent was considered due to the absence of strong evidence for treatment efficacy. The activity, appetite, and breathing effort of the cat waxed and waned in the 5 weeks after BAL, but the overall clinical status deteriorated rapidly over the last few days. The cat died 8 weeks after the initial presentation. Necropsy showed a hyperinflated lung with geographic, dark-red, well-demarcated, and slightly firm foci on bilateral caudal lobes and the left cranial lobe. There were numerous small air bubbles in the pulmonary parenchyma of every lobe which is suggestive of overinflation of alveoli. Excessive yellow-green mucopurulent exudate filled the bilateral bronchi. Histopathology identified that most lesions were centered on the bronchioles, with extension into the interstitium and subpleural distribution. Some bronchi were also affected, and both the bronchial and the bronchiolar lumina were filled with abundant mucus, amorphous cell debris, neutrophils and macrophages, with epithelial squamous metaplasia. Submucosal gland hyperplasia with moderate to marked lymphoplasmacytic inflammation was also present. There was prominent bronchiolar epithelial hyperplasia with severely hypertrophic smooth muscles surrounding the bronchi and bronchioles, and the adjacent alveolar septa revealed moderate interstitial fibrosis. The small bronchioles exhibited a varying degree of submucosal concentric fibrosis compressing and reducing the diameter of the lumen, with lymphoplasmacytic infiltrates. These histologic changes were consistent with constrictive bronchiolitis obliterans [,, ]. Within the remaining aerated lung, the alveolar spaces were enlarged and coalescent. However, typical pathognomonic lesions associated with feline herpesvirus infection, such as tissue necrosis, syncytial cells, and viral inclusion bodies, were not found in the histopathological examination. A piece of lung tissue from the dorsal region of the caudal lung lobe was collected and submitted for feline herpesvirus PCR, but the virus was no longer detected. Considering the time from symptom onset or BAL to necropsy, it was speculated that the pathological changes could be different from that of cats with acute herpesvirus pneumonia. Therefore, constrictive bronchiolitis obliterans induced by a preceding feline herpesvirus infection remains highly suspected in this case.