Introduction

Bronchiolitis obliterans is an airway disorder characterized by fixed obstruction of the bronchioles and, based on histologic findings, can be classified into the following two distinct types: constrictive bronchiolitis obliterans (CBO) and polypoid bronchiolitis obliterans (PBO).¹ Although both conditions typically occur secondary to airway epithelium injuries, PBO is characterized by fibroblast proliferation and plug formation within the bronchiolar lumen, whereas CBO is characterized by concentric intramural bronchiolar fibrosis with subsequent luminal obsruction.² Specific etiologies also differ as PBO is more likely to occur as a result of previous pulmonary infection.^3–5 Conversely, CBO is often the result of either inhalation exposure to chemical hazards or immune-mediated disease processes.^4,6 Both forms of bronchiolitis obliterans have been well described in humans, but only PBO has been described in veterinary species.³ This is the first report to provide a detailed description of the clinicopathologic features of CBO in a dog.

Case Report

A 2 yr old, neutered male rottweiler was evaluated at Michigan State University College of Veterinary Medicine for chronic cough of 3 mo duration, which had acutely worsened 3 wk prior to presentation. The dog had also been reported to intermittently vomit since the time of adoption at 1.5 yr of age, and the previous history was unknown. At the initial evaluation, the dog had mildly increased bronchovesicular sounds and a dry, nonproductive cough. The remainder of the physical examination was normal. Complete blood count and arterial blood gas analyses were performed. Hematologic evaluation revealed a mild lymphocytosis (4.4 × 10⁹/L; reference interval [RI]: 1.1–3.1). Arterial blood gas identified mild respiratory alkalosis (pH 7.42) characterized by decreased partial pressure of carbon dioxide (26.4 mmHg; RI: 36–40) with a compensatory metabolic acidosis characterized by a decreased bicarbonate concentration (17.1 mg/dL; RI: 20–24). Arterial pressure of oxygen was 101.1 mmHg (RI: 90–100), and the calculated alveolar-arterial gradient was 15.9 (RI < 15).

Diagnostic imaging was pursued to further investigate the dog’s respiratory disease. All imaging was interpreted by a board-certified radiologist (N.C.N). Thoracic radiographs identified an alveolar pattern in the left cranial lung lobe with a cranially displaced and tortuous bronchus. The right cranial lung lobe had a focal unstructured interstitial pattern with a scalloped ventral margin. A mild bronchial pattern was present throughout the parenchyma, most prominently around the caudal lobar bronchi (Supplementary Figure I). The radiographic findings were not typical for common canine pulmonary diseases; because of this, a computed tomographic (CT) examination was performed to further characterize the pulmonary lesions. Thoracic CT was performed with the patient under general anesthesia, and images obtained at a full inspiratory breath-hold at 20 cm H₂O. Precontrast images were obtained using a CT scanner^a, at a slice thickness of 2.5 mm. Following the precontrast phase, the patient received 80 mL of iopamidol^b contrast agent IV, and the CT series was repeated to obtain postcontrast images. On CT examination, both subsegments of the left cranial lung lobe had a diffuse alveolar pattern, were markedly decreased in volume, and had no aerated parenchymal tissue. The bronchial tree in both subsegments was irregular and tortuous with various degrees of dilation (Figure 1). The left caudal lung lobe was hyperinflated, and the cranioventral aspect of this lobe was more lucent than the remaining parenchyma. The right cranial lobe was mildly hyperinflated, and intraluminal soft tissue attenuating material was noted in the distal aspect of a large cranioventral bronchial branch; this bronchus tapered rapidly distal to this region. The right middle lung lobe appeared similar to the left cranial lobe but less severely affected. Collectively, the observed lesions were suggestive of severe airway pathology but were nonspecific in terms of etiology.

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Given the severity of diagnostic imaging abnormalities and the chronicity of clinical signs, an exploratory thoracotomy was performed to obtain pulmonary tissue for histologic evaluation. At surgery, the left cranial lung lobe was collapsed; most of the right middle lobe was similarly collapsed. The right cranial lobe was well inflated, and the right caudal and accessory lobes were grossly normal. Given the severity of changes, a left cranial lung lobectomy was performed; a right middle lung lobectomy was also performed as lesions appeared less severe and thus deemed potentially of value in determining the pathogenesis of lung disease. Samples of the resected lung were submitted to the Michigan State University Veterinary Diagnostic Laboratory for bacterial and fungal culture, and the remaining tissues were fixed in 10% neutral buffered formalin and prepared for histopathologic examination. Representative sections from both lung lobes were stained with hematoxylin and eosin, Masson’s trichrome, and Verhoeff-Van Gieson stains. Histopathology was interpreted by a board-certified pathologist (K.J.W).

Cultures for aerobic and anaerobic bacteria, Mycoplasma spp., and fungal organisms were negative for microbial growth. Approximately 60% of the examined alveolar parenchyma of the right middle lobe and 100% of the left cranial lobe was collapsed. Within the collapsed regions of both lobes, multifocal bronchial lumens were markedly reduced in diameter. The respiratory epithelium in these airways was often difficult to visualize, and there was replacement of the lamina propria with abundant, well-vascularized fibrous tissue, which surrounded and replaced submucosal glands (Figure 2A). The respiratory epithelium of nonoccluded bronchi was multifocally attenuated or hyperplastic, with the hyperplastic segments containing goblet cell hyperplasia. Small numbers of lymphocytes were present within and below the epithelium of lesser-affected airway segments.

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Numerous distal bronchioles were similarly affected, with marked concentric expansion of the lamina propria with fibrous tissue, resulting in reduction to near occlusion of the lumen of many bronchioles (Figure 2B, C). The extent and distribution of collagen accumulation was highlighted using Masson’s trichrome staining (Figure 2C), whereas Verhoeff-Van Gieson staining was used to identify elastic lamina present at the periphery to those airways difficult to identify from lumen occlusion by fibrosis (not shown). Fibrous tissue multifocally expanded adjacent alveolar septa; alveoli associated with the most severely affected bronchioles were effaced by fibrous tissue. Within the remaining aerated lung, alveolar septa were often fragmented, resulting in enlargement and coalescence of alveolar spaces (air trapping). Overall, the histologic changes were consistent with CBO with alveolar collapse secondary to fibrous airway obstruction.

Because CBO has been associated with immune-mediated processes in humans, immunohistochemistry was used in an attempt to identify colocalization of lymphocytes in affected and more normal airways using antibodies directed against the general leukocyte antigen CD18, CD3 (T cells), and CD20 (B cells). Small-to-moderate numbers of CD18-expressing cells were identified within and below the epithelium of intact bronchioles (not shown). These leukocytes were identified as primarily T cells through labeling with anti-CD3 antibody (Figure 2D). CD20 expression was rarely identified in these cells, suggesting that few B cells were present in the lymphocyte population. Although inflammatory infiltrates can be variable in humans, the finding in this dog is similar to some reports in humans, in which a T cell predominant population is observed.⁷ Very few lymphocytes were identified in the most severely remodeled airways (bronchi or bronchioles) (not shown).

Pending diagnostic test results, the dog was initially treated with a 2 wk course of antimicrobials. Based on the histologic findings, glucocorticoid therapy with prednisone was initiated at an immunosuppressive dose (2 mg/kg/day), which was gradually tapered over several months. At evaluation 1 mo after starting glucocorticoid therapy, the dog still had an intermittent cough, but less severe than upon initial evaluation. Arterial blood gas at this time revealed slight improvement with a pH of 7.41, partial pressure of oxygen of 104.6 mmHg, partial pressure of carbon dioxide of 30.1 mmHg, bicarbonate concentration of 18.9 mg/dL, and an alveolar-arterial gradient of 7.7. The dog was maintained on 0.5 mg/kg every other day dosing of prednisone and eventually transitioned over to an inhalant, fluticasone^c, at 220 μg twice daily. Over the next 2 yr, the dog remained stable with intermittent coughing. Periodically, when clinical signs would worsen, the dog would respond favorably to a combination of antimicrobials and increased glucocorticoid dosage. The chronic intermittent vomiting was eventually investigated and successfully treated with dietary therapy; however, the respiratory signs were unchanged. A right cranial lung lobe bulla was noted on a follow-up CT examination 1.5 yr after initial diagnosis; otherwise, repeat radiographic examinations revealed minimal changes during this time, and the dog was eventually lost to follow-up.

Discussion

Although the constrictive form of bronchiolitis obliterans has not been reported in domestic animals, it is well described in humans.^2,6–8 CBO describes concentric reduction of bronchiolar lumen diameter from progressive fibrosis in the bronchiolar wall, affecting terminal membranous and respiratory bronchioles.^1,2,6,7,9 Various inflammatory cell infiltration has been described accompanying the fibrosis in many cases in humans.² The distribution of lesions is frequently patchy and usually involves multiple lobes.¹⁰ Similar to the dog in our report, the most common clinical symptom in humans is a nonproductive cough. Dyspnea, most notable during exertion, is commonly reported with disease progression.^2,10 Results of pulmonary function tests are inconsistent and nonspecific. Consequently, some cases are initially mistaken for other lower-airway diseases.¹⁰ Decreased lung attenuation, air trapping, and bronchiolar destruction from fibrosis in advanced stages, as seen in the dog reported herein, are common CT abnormalities in human CBO.^1,9 Although characteristic features of CBO can be observed with CT, the results are not definitive, and lung histopathology is required for diagnosis.^2,9

The etiopathogenesis of CBO is not well understood; most cases are thought to result from bronchiolar epithelial injury, which triggers the progressive, concentric intramural fibrosis.¹¹ As such, CBO is usually assumed to represent a secondary reaction as opposed to a primary disease process in itself; however, CBO can be a component of immune-mediated disorders.^2,9 The most common inciting causes of CBO in humans are inhalation exposures to various toxins, drug reactions, and immune-mediated processes, although some cases are idiopathic. Less commonly, pulmonary infections can potentially trigger CBO.^2,6,7,9 Organ transplant recipients are frequently affected, although the mechanisms are undetermined; however, immune-mediated processes or potentially gastroesophageal reflux leading to aspiration could be contributing factors.^7,9,12 In one study, up to 80% of deployed military personnel with exertional dyspnea returning from Iraq or Afghanistan were affected by CBO.¹³ It was speculated that exposures to dust storms, sulfur fires, burn pits, human waste, and combat smoke contributed to CBO development. Other chemical hazards, such as 2,3-pentanedione used in the production of microwave popcorn, have been linked to CBO in a variety of manufacturing and production facilities in the United States.^2,10,11 The association between T cell infiltration in bronchioles and the apparent response to glucocorticoid therapy could be suggestive of an immune-mediated component to the development of CBO in this dog, although this is speculative at this time. Given the history of intermittent vomiting and the distribution of pulmonary lesions, recurrent bouts of aspiration pneumonia may have triggered the initial injury. The dog’s medical history prior to adoption was unknown. Therefore, it is unknown if previous housing environment, infection, or toxin exposure could have contributed to CBO development. Identification and study of additional cases of CBO in dogs will be important to further evaluate pathogenesis.

It is important to note that CBO is distinct from PBO.^2,3 PBO, which is far more common than CBO in people, has been commonly reported in cattle and sporadically reported in dogs.^3,14–16 Unlike CBO, most cases are the result of previous bronchopneumonia and represent aberrant healing within affected bronchioles.³ PBO involves fibroblast proliferation and plug formation within the airway lumen and more often involves a single lung lobe.^2,11 In contrast, CBO involves intramural concentric fibrosis and more often affects multiple lobes with a patchy distribution.² Beyond histologic differences, prognosis also differs between the two forms of bronchiolitis obliterans; resolution of PBO can occur with proper treatment, whereas CBO is typically a progressive and irreversible process.^6,7 The prognosis for CBO is guarded, and in severe cases in people, lung transplantation is necessary.^6,7,11 Supportive measures are often pursued, but no therapies have been shown to be effective.⁷ Treatment with corticosteroids or other immunosuppressive agents do not often result in improvement, but in some cases, symptomatic relief is observed which is presumably because of inflammation reduction.⁷ It is possible that a glucocorticoid-induced reduction of inflammation explains the slight improvement and stable disease observed in the dog in our report. Despite the overall guarded prognosis, a small subset of humans remain stable for prolonged time periods with similar treatments to the dog in our report.^7,13

Conclusion

In summary, this report describes the clinical features and pathology of a case of canine CBO. Similar to humans, clinical signs and diagnostic imaging findings are nonspecific and histopathology of affected tissue is necessary for diagnosis. The severe and characteristic histologic features of this disease, including concentric bronchiolar wall fibrosis and luminal collapse, are highlighted in the dog reported herein. Although additional studies are needed to further elucidate potential etiologies, treatments, and disease progression, clinicians and pathologists should be aware that CBO can affect dogs.