Equine Thoracic Neoplasia

Fabio Del Piero • Pamela A. Wilkins

Surveys of equine neoplasms indicate a low prevalence of thoracic neoplasia in the horse. In an abattoir survey in London of 1308 horses, only 2 horses had pulmonary tumors: one granular cell tumor and one bronchiolar adenoma.¹ Two other necropsy surveys of 155 and 687 equines reported no thoracic neoplasms.^2,3 A report on chronic pulmonary disease in the horse states that the practical importance of lung tumors in this species is negligible.⁴ University of Pennsylvania researchers examined 5629 horses between 1968 and 1987.

The prevalence of primary lung neoplasms of any type is reported to be less than 1% of all reported neoplasms in domestic animals.⁶ The lungs are susceptible to tumor emboli because of the filter action of the capillary bed associated with small capillary diameter and, perhaps, specific adhesion factors. It can sometimes be difficult or even impossible to distinguish metastatic disease from primary pulmonary neoplasia; thus an important part of the diagnosis is exclusion of possible primary sites elsewhere in the body. Primary pulmonary tumors reported in the horse include granular cell tumor, bronchial myxoma, adenoma, adenocarcinoma, anaplastic bronchogenic carcinoma, and pulmonary carcinoma (Fig. 31.39). Other primary thoracic tumors reported in the horse include pulmonary chondrosarcoma, pleural mesothelioma, thymoma, and lymphosarcoma. The majority of these neoplasms generally occurs in the mature or aged horse. The exception is lymphosarcoma, which may also be observed in young animals.

Antemortem diagnosis of thoracic neoplasia depends first on recognition of thoracic disease. Most of the reported cases of thoracic neoplasia involve metastatic disease.

Granular cell tumors are the most frequently reported primary pulmonary neoplasm of the horse. They are usually confined to the lungs and are characterized by single to multiple well-defined, whitish, dense nodules associated with a major bronchus, often protruding into the bronchial lumen or more distally within the parenchyma. Most granular cell tumors have been diagnosed only at postmortem examination, often as incidental findings. These homogeneous growths of granular eosinophilic cells express S-100 and occasionally neuron-specific enolase.⁷ The ultrastructural morphology of this tumor may suggest a neural origin as well. Primary pulmonary chondrosarcoma has been reported in the horse.⁸ Cytologic examination of pleural fluid and antemortem needle aspirate of the pulmonary mass revealed neoplastic chondrocytes. Another report describes metastatic chondrosarcoma in the lungs of a horse with a primary tumor in a rib.⁹ A primary bronchial myxoma, characterized by loose spindloid mesenchymal cells, was reported in a 25-year-old Arabian mare with a history of intermittent coughing, hyperpnea, and respiratory disease of 2 years' duration.¹⁰

Primary pleural tumors are also rare. The type reported in the horse is pleural mesothelioma. The tumor is frequently associated with a large volume of pleural effusion. Cytologic examination of the pleural effusion revealing numerous pleo­morphic mesothelial cells may aid in the antemortem diagnosis.

Thymomas are neoplasms of thymic epithelial cells, regard­less of the presence or absence of lymphocytes.

Gastric squamous cell carcinomas commonly metastasize to the thoracic cavity. Cytology of pleural fluid with identifica­tion of neoplastic epithelial squamous cells has allowed for antemortem diagnosis of the carcinoma in several cases. Metastatic squamous cell carcinoma of the thorax was found in 14% and 5% of the cases in the University of Pennsylvania⁵ and University of Bristol¹⁶ surveys, respectively. Neoplastic epithelial cells may also be identified in biopsy specimens via indirect immunohistochemistry using antibodies recognizing cytokeratins of various molecular weights.

FIG. 31.39 A, Metastatic undifferentiated sarcoma. B, Granulosa cell tumor. C, Metastatic hemangiosarcoma. D, Metastatic squamous cell carcinoma.

Hemangiosarcoma with pulmonary involvement was found in 9% of the horses in the University of Pennsylvania survey.⁵ Hemothorax, anemia, and dyspnea were commonly seen with this type of neoplasm. Tentative antemortem diagnosis of the hemangiosarcoma can be made by transcutaneous direct thoracoscopy and observation of hemorrhages over the visceral and parietal pleural surfaces as well as tissue biopsy of these sites. Identification of neoplastic endothelial cells demarcating irregular, sometimes blood-filled cavities allows diagnosis from the biopsy specimen.

Malignant melanoma in a 20-year-old gelding had widespread infiltration of many organs, including the lungs and pleura.¹⁶ This pattern of distribution is not uncommon in malignant, less pigmented melanomas. Amelanotic melanomas are rare in the horse, and microscopic identification of melanin simplifies the diagnosis of these tumors. Equine melanocytes express S-100 and PNL2 proteins, and their detection can be used to diagnose melanocytic neoplasms.

Other pulmonary metastatic tumors reported are mammary carcinoma,¹⁷ seminoma,¹⁸ and malignant pheochromocytoma.¹⁹ Pulmonary hamartomas are occasionally observed in the newborn foal. Although they are nonneoplastic growths, they appear as tumorlike masses that can compress the surrounding parenchyma and cause systemic passive congestion and hydrops of the amnion in the fetus. Their histologic characterization is that of bronchiolar-like structures lined by cuboidal epithelium with lack of alveolar development or by normal alveoli, bronchi, and blood vessels but with an alveolus/artery ratio greater than normal. Generally they are not compatible with a long period of extrauterine life.

An unusual progressive idiopathic multifocal granulomatous pneumonia of adult horses may resemble neoplasia behaviorally and morphologically during clinical examination.^20,21 Pulmonary biopsy may allow the identification of equine multinodular pulmonary fibrosis (EMPF), which has been associated with EHV-5 infection²¹ (see Figs. 31.29 and 31.30).

Clinical signs of thoracic neoplasia are often inapparent or nonspecific—for example, depression, inappetence, weight loss, and pyrexia. More specific signs include cough, epistaxis, and dyspnea. Cytologic examination of tracheobronchial aspirate, BAL fluid, or pleural fluid and histologic examination of a thoracic mass tissue biopsy with ancillary histochemistry and indirect immunohistochemistry may allow for antemortem diagnosis of the neoplasm. Occasionally, electron microscopy may provide additional information. In equine patients with respiratory signs, all causes of infectious or allergic lung disease should be eliminated before neoplasia is considered in the differential diagnosis.