Diagnostic Challenges: Equine Thoracic Neoplasia E

96 EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2013

Review Article Diagnostic challenges: Equine thoracic neoplasia E. G. Davis and B. R. Rush* Clinical Sciences, Kansas State University, Manhattan, Kansas, USA. *Corresponding author email: [email protected]

Keywords: horse; diagnostic; thoracic; neoplasia; pulmonary

Summary cell carcinoma, fibrosarcoma, metastatic melanoma, The diagnosis of thoracic neoplasia in the horse can be difficult mastocytoma and undifferentiated sarcoma. The clinical due to the nonspecific nature of the clinical signs and their features of these tumours are generally nonspecific overlap with other pulmonary diseases. Haematological and and often relate more to the primary site of tumour formation. serum biochemical evaluation, thoracic ultrasonography, radiography, endoscopic examination, and, where appropriate, thoracocentesis and pleural fluid cytology may Introduction all be helpful in reaching a diagnosis. Granular cell tumours Equine patients suffering from pulmonary neoplasia can are the most frequently reported primary pulmonary tumours of present a diagnostic challenge. Case details, evidence of horses. They occur as single or multiple masses adjacent to respiratory distress, depressed demeanour, pyrexia, ventral bronchi and bronchioles, and the mass typically extends into oedema and poor body condition collectively aid the the airway, resulting in partial or complete occlusion of the clinician in establishing the presence of advanced pulmonary lumen. Thymic tumours are classified as benign or metastatic, disease. However, allergic and infectious pulmonary diseases based on evidence of tissue invasiveness, even though they can present with overlapping clinical features. The majority uniformly appear benign histologically. These tumours are of horses suffering from infectious pulmonary disease derived from epithelial reticular cells of the thymus and are have a history of chronic, progressive respiratory disease rare in horses. Other primary thoracic neoplasms originate characterised by pyrexia, cough, nasal discharge, travel from various pulmonary tissues and are primarily reported as history and/or respiratory disease among cohorts. Allergic single case reports: pulmonary and bronchial carcinoma and airway disease is commonly accompanied by appropriate adenocarcinoma, bronchogenic squamous cell carcinoma, environmental factors that trigger the clinical manifestation of bronchial myxoma, pulmonary chondrosarcoma, pulmonary disease. Features of inflammation, including neutrophilia, leiomyosarcoma and pleuropulmonary blastoma. Clinical monocytosis, hyperfibrinogenaemia and elevated globulins, signs of these primary pulmonary neoplasms are dependent often provide insight involving the magnitude and chronicity of on the tumour type and location, but commonly include the pulmonary disorder, yet these findings are not specific to chronic cough, weight loss, anorexia, fever and respiratory establish an aetiological diagnosis. difficulty; ventral oedema, pleural effusion and epistaxis are Specific evaluation of pulmonary disease involves also frequently observed. Mesothelioma is a rare primary haematological and serum biochemical evaluations, thoracic pleural tumour arising from the mesothelium of the pleura, ultrasound, radiography and, potentially, endoscopic pericardium and peritoneum. The clinical presentation examination. When pleural effusion is present, thoracocentesis in horses includes weight loss, respiratory difficulty and and cytological examination may reveal diagnostic evidence large volume pleural effusion. The tumour appears that aids in making the distinction of septic vs. neoplastic ultrasonographically as multiple small nodules on a thick disease. However, cytological evaluation of pleural effusion serosal surface and pleural biopsy is diagnostic. Lymphoma is from a patient with nonexfoliative thoracic neoplasia may the most common haematopoietic neoplasm in horses, which not provide definitive diagnostic information, which is the can present with 4 main manifestations of lesions: mediastinal, case with many primary metastatic thoracic neoplasms. multicentric, alimentary and cutaneous. Common clinical Conversely, a neoplastic effusion may be colonised by a features include chronic weight loss, lethargy, anorexia, secondary bacterial pathogen, further masking the diagnosis subcutaneous oedema, lymphadenopathy, colic, bleeding of primary neoplasia. tendency and diarrhoea. Coughing and laboured respiratory Thoracic ultrasonography is a common diagnostic effort are often apparent in individuals suffering from procedure used to examine equine patients with respiratory mediastinal masses. In such instances, pleural effusion may disease. A valuable component of this examination is the result in severe pulmonary atelectasis and pulmonary function opportunity for the clinician to identify the presence of pleural is significantly compromised. Haemangiosarcoma is the fluid and establish whether pulmonary consolidation or second most common metastatic thoracic neoplasm in atelectasis is present. Large volume anechoic effusion is horses. Disseminated haemangiosarcoma is aggressive and frequently seen in horses suffering from thoracic lymphoma and rapidly progressive. The clinical presentation often includes mesothelioma (Reef 1991). Differentiation from septic exudate tachypnoea, pale or icteric mucous membranes, respiratory must be established in suspect individuals since distress, epistaxis, and subcutaneous, cutaneous or pleuropneumonia may present with similar clinical features intramuscular masses. Other tumour types that metastasise to that include depression, respiratory distress, pyrexia and the thoracic cavity include adenocarcinoma, squamous anorexia.

Haemothorax appears as echoic swirling fluid observed during imaging of the thoracic cavity (Reef 1991). The character of the pleural fluid will provide the clinician with an indication of the disease process; cytological analysis will be an important accompanying diagnostic modality to determine the aetiology of the disease process. Trauma or haemangiosarcoma are important differential considerations when haemorrhagic pleural effusion is present. Pleuropneumonia resulting from polymicrobial challenge will result in abundant hyperechoic echoes resulting from free gas within the pleural fluid, sometimes referred to as a composite effusion. Radiographic imaging of the thorax provides little additional information in horses with a large volume of pleural effusion. However, after evacuation of pleural fluid, thoracic radiography may reveal abnormalities of the pulmonary parenchyma or mediastinum. Not all thoracic neoplasms are associated with pleural effusion. In cases of suspected thoracic neoplasia, due to chronic, unresponsive respiratory disease or evidence of hypertrophic osteopathy, thoracic radiography may be valuable to identify pulmonary and mediastinal masses. Thoracoscopy is a valuable adjunct diagnostic procedure in horses with thoracic masses and nondiagnostic effusion. As Fig 1: Endoscopic examination identifying a granular cell tumour described in the accompanying article (Lee et al. 2013) and obstructing the right mainstem bronchus. in previous reports (Peroni et al. 2001), thoracoscopy is particularly safe, useful, efficient and cost-effective for the exploration of the pleural space including the mediastinum (Ford et al. 1987; Rossier et al. 1990; Vachon and Fischer 1998). the literature (Pusterla et al. 2003). Bilateral disease occurs in Diagnostic information obtained from this procedure <20% of the case reports. There is no breed predilection and includes thorough visual inspection, as well as fluid and tissue more females have been reported than males. Despite their sample collection. Thoracoscopy has been described for size and propensity to occlude large airways, granular cell the ante mortem diagnosis of a variety of tumour types tumours may result in minimal clinical signs and represent an that include squamous cell carcinoma, haemangiosarcoma incidental necropsy finding in some horses. and cholangiocellular carcinoma (Ford et al. 1987; Mueller In clinically-affected horses, the presenting complaints et al. 1993; Pollock and Russell 2006). Although lymphoma has are chronic cough, exercise intolerance, tachypnoea and been reported to be the most common thoracic neoplasm in weight loss. Horses with advanced disease demonstrate horses (Mair and Lane 1989; Mair and Brown 1993), other an increased effort of respiration during expiration. Depending lesions may be present within the pleural cavity and on tumour size, auscultation may reveal decreased breath pulmonary parenchyma in equine patients. Thoracoscopy sounds over one hemithorax. Based on age (mean age 13 provides the clinician with a valuable additional diagnostic years) and clinical signs, horses with granular cell tumour are modality that may aid in establishing an accurate, efficient often assigned a presumptive diagnosis of reactive airway diagnosis. disease (heaves) (Pusterla et al. 2003). Failure to respond to therapy leads the clinician to more aggressive diagnostic testing. Focal pneumonia may develop in the pulmonary Primary thoracic tumours parenchyma distal to the mass due to poor clearance of Primary lung tumours are less common than metastatic respiratory secretions and inhaled particulate material. Horses pulmonary neoplasms and constitute <10% of all pulmonary with focal pneumonia present with fever, depression, tumours (Sweeney and Gillette 1989). Granular cell tumour is abnormal lung sounds and leucocytosis, in addition to the most frequently reported primary pulmonary tumour of coughing. There are no reports of pleural effusion, epistaxis or horses (Mair et al. 2004). Despite being the most common, only mediastinal lymhadenapathy in horses with granular cell approximately 30 cases have been reported in the literature tumour. Hypertrophic osteopathy has been reported as a (Misdorp and van Gelder 1968; Parker et al. 1979; Nickels et al. paraneoplastic complication of granular cell tumour in some 1980; Turk and Breeze 1981; Scarratt et al. 1993; Sutton and horses (Alexander et al. 1965; Sutton and Coleman 1995; Mair Coleman 1995; Goodchild et al. 1997; Pusterla et al. 2003). et al. 1996; Heinola et al. 2001). These tumours have been described as myoblastomas and Routine blood work provides little evidence to advance appear to originate from Schwann cells (Bouchard et al. 1995). the diagnosis of granular cell tumour. Thoracic radiography They occur as single or multiple masses adjacent to bronchi identifies a single, large mass or multiple pulmonary masses and bronchioles and are locally invasive with no reports of near or caudal to the hilus (Mair et al. 2004). Evaluation of metastasis. The mass typically extends into a large calibre magnification in left and right radiographic projections can airway, resulting in partial or complete occlusion of the lumen identify the hemithorax affected by the tumour. Rarely, the (Fig 1). Some authors suggested a propensity for the right tumour cannot be visualised via thoracic radiography. In some hemithorax, which is not supported by a cumulative review of cases, the mass may be obscured by focal pneumonic lung

Fig 2a a) ( ). Focal pneumonia in an atypical pulmonary location (i.e. central or dorsal lung fields) should trigger a suspicion of tumour or inhaled foreign body. Using the tracheobronchial map of the lung reported by Smith et al. (1994), the approximate location of the tumour in the pulmonary tree can be determined for endoscopic examination. There is no report of visualisation of the tumour mass via ultrasonographic examination; however, decreased movement of the pleural surface over the affected hemithorax has been reported in horses with large granular cell masses (Pusterla et al. 2003). The surface of granular cell tumours is smooth pink to white in colour and occludes or nearly occludes a large calibre airway (Video S1). A main stem bronchus is the most commonly affected airway (Kelley et al. 1995). Biopsy confirmation is difficult and may not be necessary, since the endoscopic appearance of the tumour is distinctive. The external surface of the mass consists of normal b) respiratory epithelium; therefore, a biopsy sample obtained via endoscopy may be nondiagnostic due to insufficient size and depth (Facemire et al. 2000; Pusterla et al. 2003). To obtain a larger tissue sample, with a greater chance of achieving a diagnosis, a biopsy instrument (such as uterine biopsy forceps) can be passed through a tracheotomy incision at the level of the thoracic inlet (Facemire et al. 2000). On histopathological examination, neoplastic cells are benign and appear rounded to polyhedral with hyperchromatic nuclei, numerous eosinophilic cytoplasmic granules and indistinct cytoplasmic margins (Kelley et al. 1995; Pusterla et al. 2003). Histochemical and immunohistochemical staining results of these tumours are well-described and suggest that they are composed primarily of neural crest cells, probably myelinating Schwann cells (Bouchard et al. 1995). Morphological features of the equine pulmonary granular cell tumours are similar to those of human endobronchial granular cell tumours (Bouchard et al. 1995; c) Kelley et al. 1995). In other species (dogs, cats and man), granular cell tumours are reported to occur at many other sites besides the thoracic cavity, including the oral cavity and central nervous system. Granular cell tumours are presumably slow growing and may be an incidental finding at post mortem examinations. Conservative treatment may produce an acceptable clinical outcome for years in horses with stable clinical signs (Pusterla et al. 2003). The presenting complaint for the horse shown in Figure 2a reflected focal pneumonia, including cough, fever and depression. The granular cell tumour obscured a third generation bronchus (Fig 2c) and compressed only a small portion of pulmonary parenchyma. The mare was treated with a 2 week course of broad-spectrum antibiotics and managed for several years with minimal clinical signs (Fig 2b). Ohnesorge et al. (2002) removed the intraluminal portion of a granular cell tumour mass via transendoscopic electrosurgery. The remaining tumour surface was irradiated using a neodymium:yttrium-aluminium-garnet (Nd-YAG) laser to Fig 2: a) Thoracic radiograph demonstrating focal pneumonia coagulate and kill residual tumour cells. In most cases, the in an atypical (perihilar) location obscuring the presence of tumour observed in the airway represents only a small a granular cell tumour in an 18-year-old mare. b) Thoracic proportion of the total tumour mass. Horses with large tumour radiograph obtained one year later. The granular cell tumour is masses require more aggressive therapy. Facemire et al. readily apparent in this image without accompanying focal (2000) removed the entire right lung that was affected by pneumonia. c) Endoscopic examination reveals near complete multiple large masses. In the reports by Ohnesorge et al. obstruction of a third generation bronchus, with minimal (2002) and Facemire et al. (2000), there was no tumour compression of pulmonary parenchyma. recurrence after 2 years.

Necropsy examination reveals a single large mass or, more case reports: pulmonary and bronchial carcinoma and commonly, a large mass with multiple small nodular masses adenocarcinoma (Dill et al. 1986; Uphoff and Lyncoln 1987; that compress the surrounding parenchyma and invade the van Rensburg et al. 1989; Anderson et al. 1992; Mair and Brown lumen of a large airway. There are no reports of metastasis to 1993); bronchogenic squamous cell carcinoma (Schultze et al. other organs, primary neoplasia originating from other sites, 1988); bronchial myxoma (Murphy et al. 1978); pulmonary metastasis to regional lymph nodes or invasion of chondrosarcoma (Clem et al. 1986); pulmonary nonpulmonary tissue (Mair et al. 2004). leiomyosarcoma (Rossdale et al. 2004); and pleuropulmonary Thymic tumours are classified as benign or metastatic, blastoma (Pérez-Écija et al. 2009; Woolford et al. 2010). based on evidence of tissue invasiveness, even though they Clinical signs of primary pulmonary neoplasms are uniformly appear benign histologically. These tumours are dependent on the tumour type and location. Chronic cough, derived from epithelial reticular cells of the thymus. Tumour weight loss, anorexia, fever and respiratory difficulty are classification has historically included those with a lymphocytic common clinical findings in horses with pulmonary neoplasia component, epithelial or mixed composition. More recently, regardless of the tissue of origin. Ventral oedema, pleural the World Health Organization (WHO) has provided effusion and epistaxis are not unusual in horses with pulmonary classification recommendations to include: A, AB, B1, B2 and neoplasia. Most case reports involve aged horses (>12 years), B3 (Dadmanesh et al. 2001). The equine case report that although pleuropulmonary blastoma has been reported utilised this classification scheme describes a type A tumour in a neonate and a young mature horse (Pérez-Écija et al. characterised by proliferation of spindle cells without nuclear 2009). As in human patients, this tumour is characterised atypia that contain no more than a few non-neoplastic by mixed epithelial and mesenchymal elements and lymphocytes and inconspicuous nuclei (Shahriar and Moore aggressive malignancy. The clinical presentation of pulmonary 2010). Thymic tumours are rare in horses. In an original report of leiomyosarcoma is similar to granular cell tumour, with the 2 cases, they were considered incidental post mortem findings exception of the presence of epistaxis (Rossdale et al. 2004). without evidence of metastasis (Migaki 1969). Mesothelioma is a rare primary pleural tumour arising from Malignant thymoma has rarely been reported to affect the mesothelium of the pleura, pericardium and peritoneum. In horses (Whiteley et al. 1986; Furuoka et al. 1987; Shahriar man, this malignancy is associated with asbestos exposure. The and Moore 2010). A mediastinal mass, associated clinical presentation in horses includes weight loss, respiratory lymphadenopathy and marked pericardial involvement was difficulty and large volume pleural effusion. Differentiation of reported in one case (Furuoka et al. 1987). In addition neoplastic mesothelial cells from reactive mesothelial cells is to cranial thoracic disease, the affected Percheron mare difficult on cytological examination of pleural effusion. The had evidence of pulmonary lymph node involvement, tumour appears ultrasonographically as multiple small nodules extensive parenchymal pulmonary disease, abdominal and on a thick serosal surface, and pleural biopsy is diagnostic. retroperitoneal lesions. A separate report described a mixed There is no treatment and the prognosis is grave (Straub et al. breed mare affected by a squamous cell thymoma, which 1974; Kramer et al. 1976; Carnine et al. 1977; Wallace et al. demonstrated a markedly more aggressive nature (Whiteley 1987; Colbourne et al. 1992; Mair et al. 1992; Fry et al. 2003). et al. 1986). This latter report described the lesions to extend Most cases of primary thoracic neoplasia have an from the intermandibular space to the thoracic inlet. Post extended history of cough and nonspecific signs of weight loss mortem examination revealed the mass to involve the and anorexia (Mair et al. 2004). The most common first-opinion mediastinum and pericardial sac. Pulmonary nodules were diagnosis in horses with pulmonary neoplasia is heaves, diffusely present and were confirmed on histopathology. followed by low-grade pneumonia and pleuropneumonia. In A recent report describes an 18-year-old Tennessee some cases, the definitive diagnosis is not identified for months Walking Horse which died suddenly while on a trail ride to years. With the exception of granular cell tumour, there are (Shahriar and Moore 2010). Post mortem examination limited options for therapeutic intervention and the prognosis is revealed the pleural cavity and pericardial sac to contain a grave at the time of diagnosis (Sweeney and Gillette 1989). combined volume of 300–500 ml of serosanguineous fluid. A large mass was found to occupy the cranial thoracic cavity and was adherent to the thoracic wall. The architecture of the Metastatic thoracic neoplasia right atrium was significantly distorted, which also involved the Equine lymphoma is the most common haematopoietic tricuspid valve. Histopathological evaluation of the mass neoplasm in horses, which can present with a variety of clinical revealed few characteristics of neoplasia, consistent with signs that have been previously described (Neufeld 1973a,b; previous reports of thymic tumours in horses. This tumour was Cotchin 1977; van den Hoven and Franken 1983; Platt 1988; classified in accordance with the WHO classification system as Mair and Brown 1993; East and Savage 1998; Taintor and a type A thymoma with proliferation of spindle-shaped cells Schleis 2011). Classification involves 4 main manifestations of with oval to elongated nuclei, lack of nuclear atypia and a lesions: mediastinal, multicentric, alimentary and cutaneous. lack of neoplastic lymphocytes with inconspicuous nuclei. A When present in the thoracic cavity, this manifestation of predominant feature of this neoplasm and the findings in this lymphoma is not considered a true primary neoplasm, since it reported case is the lack of neoplastic nature identified on originates from an extrathoracic site (Mair and Brown 1993). histopathology, despite the aggressive behaviour of the Lymphoma is most commonly a disease of mature horses tumour in the host. In the reported case, cardiac invasion was without a tendency toward breed or gender (Platt 1988). attributed to lymphatic drainage or direct local invasion, Clinical signs of lymphoma typically relate to the primary although haematogenous spread was also considered a organ system involvement. Common clinical featuresof the possibility (Shahriar and Moore 2010). disease include chronic weight loss, lethargy, anorexia, Other primary thoracic neoplasms originate from various subcutaneous oedema, lymphadenopathy, colic, bleeding pulmonary tissues and are primarily reported as single tendency and diarrhoea (Rebhun and Bertone 1984; Reef et al.

component of the physical examination to determine whether abdominal lymphadenopathy is present. A recent report of lymphoma described an 18-year-old mare with a presenting complaint of severe pruritus, alopecia, pyrexia, mammary gland enlargement and lymphadenopathy (Mendes et al. 2011). This mare was determined to be suffering from primary T cell lymphoma that primarily affected the mammary gland. Haematological dyscrasias that may be present in lymphoma cases include hyperglobulinaemia, hypercalcaemia and anaemia. Rarely, leukaemia may be identified, which typically represents bone marrow involvement. The mare reported by Mendes et al. (2011) suffered from pyrexia and anaemia, which resulted from paraneoplastic complications. Pruritus has been previously described to occur as a paraneoplastic condition in horses (Finley et al. 1998) and has been identified in approximately 25% of human patients suffering from Hodgkin’s lymphoma. A recent report described a mare with significant Fig 3: Submandibular lymphadenopathy in a 13-year-old mare submandibular lymphadenopathy and respiratory distress suffering from lymphoma. that was later diagnosed with lymphomatoid granulomatosis (Keen et al. 2004). Although the mare had clinical findings 1984; Adams et al. 1988). Although the disease has an insidious to suggest a more typical case of lymphoma, this mare progression, it is not uncommon for cases to present with a had evidence of marked lymphocytosis on haematological relatively acute change or deterioration in condition. It is likely examination. Morphological evaluation of these cells revealed that the time of presentation relates to a stage of disease where that approximately 98% of the cells were classified as atypical the clinical manifestation has become pronounced. T cells. Additional clinical findings included anaemia Ventral oedema is a common clinical finding in association and thrombocytopenia. On post mortem examination, with lymphoma, resulting from lymphatic obstruction. unencapsulated nodules were scattered throughout all the Coughing and laboured respiratory effort are often apparent lung lobes, without obvious involvement of other organs. in individuals suffering from mediastinal masses. In such However, microscopic examination of the skin revealed instances, pleural effusion may result in severe pulmonary perivascular and mural infiltrates of neoplastic mononuclear atelectasis and pulmonary function is significantly cells closely associated with affinity for vascular elements. compromised. Neoplastic cells were characterised to have oval or cleaved Haematological and serum biochemical testing are nuclei with characteristics of neoplasia represented by nonspecific with regard to provision of diagnostic evidence for multiple nucleoli and evidence of mitosis among cells. the presence of thoracic lymphoma. Thoracic radiography Lymphomatoid granulomatosis is a rare form of lymphoma may identify a pleural fluid line and, in some instances, reveals that has been previously identified in man (Liebow et al. 1972), the presence of a mediastinal mass. Thoracic ultrasonography dogs (Lucke et al. 1979; Postorino et al. 1989; Berry et al. 1990; provides specific evidence regarding the presence, depth Leblanc et al. 1990; Fitzgerald et al. 1991) and cats (Valentine and character of pleural fluid. Thoracocentesis is an important et al. 2000). The findings in this mare were supportiv of diagnostic procedure that will provide the clinician with this diagnosis for the first time in a horse. Interestingly, her evidence of the nature of fluid. Cytological examination lesions involved primarily the submandibular lymph nodes, provides evidence of lymphoma, when the tumour is pulmonary tissue and skin. The leukaemic nature of disease in exfoliative. In some instances, concurrent sepsis may be this horse was believed to represent a consequence of the identified; therefore, careful examination of lymphocyte angioinvasive progression of disease. This differential morphology should be performed. When overlapping consideration should be considered for clinical disease that conditions exist, such as sepsis with mediastinal lymphoma, a presents in a similar fashion with clinical and histopathological lack of response to appropriate medical therapy should alert findings to support this diagnosis. the clinician to the potential for complicating factors rather Classification of tumour type has become a routine than a primary pleuropneumonia. Thoracoscopy may be component of the clinical evaluation of lymphoma. particularly helpful in these cases. When enlarged peripheral Specifically, diagnostic modalities utilised for tumour lymph nodes are present, biopsy is strongly recommended to classification include immunophenotypic examination with aid in diagnostic confirmation of lymphoma (Fig 3). Fine flow cytometry and immunohistochemical staining on tissue needle aspirates do not provide information regarding samples. Immunophenotypic evaluation of pleural fluid can (disrupted) nodal architecture and are difficult to cytologically be performed to determine lymphocyte surface marker differentiate from reactive lymph nodes. expression, which can aid with the characteristic nature of Peripheral lymphadenopathy is an uncommon clinical neoplasia. Flow cytometry of fluid effusions is analysed for feature of disease (Meyer et al. 2006), yet when surface expression of major histocompatibility II (MHC II), CD4, lymphadenopathy develops, the prescapular or CD5, CD8a or CD8/a/b (Roberts 2008). When tissue samples submandibular nodes are most commonly enlarged. Post are available, immunohistochemistry is utilised to establish cell mortem examination typically reveals conclusive evidence of surface expression, which provides criteria for tumour lymphoid pathology (Fig 3). When examining a patient with classification (Kelley and Mahaffey 1998; Meyer et al. 2006). suspected lymphoma, rectal palpation is an important Cellular staining provides diagnostic evidence for the

distinction between T and B cell lineage neoplasms. Standard staining protocols involve identification of CD3 for T cell lesions and CD20, CD21 and/or CD79a for B lymphocyte surface marker. Several veterinary laboratories offer diagnostic services that can aid in the specific identification of tumour type. Examination of DNA ploidy may aid in characterisation of neoplastic cell populations in some clinical investigations (Ross 1996; Davis et al. 2002). Classification of equine lymphoma has been hindered by documentation of relatively few cases. An original report examined 31 horses with diagnostic confirmation of lymphoma. Among these cases, 24 (77%) horses had lymphoma derived from B lymphocytes with an infiltration of non-neoplastic T lymphocytes, therefore termed T cell rich B cell lymphoma (TCRBCL) (Kelley and Mahaffey 1998). This investigation concluded that not all equine lymphomas can be classified; however, among equine tumours that can be classified, there is an apparent tendency towards TCRBCL. In contrast, a more recent investigation that utilised Fig 4: Subcutaneous mass on the lateral thorax of a patient with immunophenotyping, classified 37 cases of equine lymphoma haemangiosarcoma and large volume haemorrhagic pleural fluid. (Meyer et al. 2006). Among all tumours, 34 (91%) of the neoplasms involved multiple lymphoid tissues in addition to tachypnoea, pale or icteric mucous membranes, respiratory abdominal or thoracic organs. Twenty-six (70%) of the cases distress, epistaxis, and subcutaneous, cutaneous or were identified to be of T cell origin, 7 of B cell origin and 4 intramuscular masses (Fig 4) (Jean et al. 1994). Anaemia, were not able to be classified. Immunophenotyping was thrombocytopenia and neutrophilia are the most common performed on effusions and were found to be consistent with abnormalities on routine blood work (Southwood et al. 2000). the immunohistochemical findings in 6 tumours (Meyer et al. A large volume of haemorrhagic pleural effusion is present 2006). The investigation of this population concluded in approximately 20% of the cases (Fig 5), accompanied that most horses had large T cell tumours, a concurrent by ventral oedema and marked respiratory distress. inflammatory response was common and many horses had Thoracocentesis provides relief for horses with large volume mediastinal masses. This population of affected horses was pleural effusion. Pleural fluid is typically serosanguinous uncharacteristically young, aged <5 years. Anaemia was the and may be characterised as haemothorax. Cytological most common cytopenia and was frequently associated with examination may or may not reveal neoplastic cells. Trauma is agglutination and hyperglobulinaemia. Thrombocytopenia the most common first opinion diagnosis in horses with and neutropenia were identified in association with haemothorax due to haemangiosarcoma (Southwood et al. myelopthesis (Meyer et al. 2006). Although equine lymphoma 2000). is relatively uncommon, an effort to establish a diagnosis early Ante mortem diagnosis is uncommon (Southwood et al. in the course of disease would be expected to enhance the 2000), but has been achieved via cytological evaluation chance for a favourable case outcome should chemotherapy of pleural fluid (Fig 6) and pleuroscopic guided biopsy (Rossier be a therapeutic option. et al. 1990). Biopsy of a haemangiosarcomatous mass can Post mortem examination typically reveals large volume result in further haemorrhage. Plans to manage haemostasis pleural effusion and associated ventral pulmonary atelectasis. should be considered prior to completing a biopsy in cases of Cranial mediastinal masses are commonly present and are suspected haemangiosarcoma. At post mortem examination, often composed of coalescing enlarged lymph nodes. Such neoplastic tissue is widely distributed to many tissues including masses may occlude the thoracic inlet contributing to the heart, spleen, kidney, skeletal muscle and central nervous obstruction of blood flow and lymphatic drainage. Regional system. The spleen is the most common organ of origin, and local lymph nodes are commonly enlarged. In some although many other tissues have been reported to be the site instances the pulmonary parenchyma may be infiltrated with of primary tumour formation. Occasionally, the thoracic cavity neoplastic lesions as well as other organs that may be involved is considered the primary tumour site with disseminated including: liver, kidney, spleen and, potentially, gastrointestinal metastasis to distant sites. tract (Mair and Brown 1993; Scarratt and Crisman 1998). Disseminated haemangiosarcoma should be Haemangiosarcoma appears to be the second most differentiated from focal haemangiosarcoma of the distal limb common metastatic thoracic neoplasm in horses. In a series in horses aged <3 years (Johnson et al. 2005). These tumours do of 35 cases, pulmonary parenchyma and pleura were not demonstrate the same aggressive biological activity and involved in 77% of the horses (Southwood et al. 2000). The age are unlikely to metastasise at the same rate. Surgical resection distribution of affected horses is clustered around middle-aged can be curative and in some cases, the tumour may resolve (mean age 12 years, range 3–27 years). Pulmonary spontaneously. haemangiosarcoma is not unusual in horses aged 6–7 year Other tumour types that metastasise to the thoracic cavity (Valentine and Ross 1986; Johnson et al. 1988; Rossier et al. include adenocarcinoma (Prater et al. 1989; East et al. 1998), 1990). Disseminated haemangiosarcoma is aggressive and squamous cell carcinoma (Ford et al. 1987), fibrosarcoma rapidly progressive. Most horses present in good body (Jorgensen et al. 1997), metastatic melanoma (Murray et al. condition with an abbreviated history of anorexia and 1997; MacGillivray et al. 2002), mastocytoma (Tan et al. 2007) depression. The clinical presentation often includes or undifferentiated sarcoma (Sweeney and Gillette 1989; Mair

Fig 5: Large volume of haemorrhagic pleural effusion (approximately 60 l) collected from a patient with haemangiosarcoma.

comprehensive understanding of this condition and specific manifestations in equine cases (Hollis 2011; Axiak and Johnson 2012). Fever is a common paraneoplastic disorder observed in association with equine lymphoma resulting from cytokine production by the tumour. Interleukin (IL)1, IL6 and tumour necrosis factor are pyrogens that are believed to increase prostaglandin E production by hypothalamic endothelial cells. When pyrexia is identified in combination with anaemia, the differential list must include potential infectious aetiologies such as equine infectious anaemia and piroplasmosis. Immune mediated disorders should also be considered as a primary disease or manifestation of paraneoplastic disease that may also involve intermittent or persistent fevers. Pruritus has been clearly demonstrated to develop as a paraneoplastic condition associated with equine lymphoma (Finley et al. 1998). Altered T cell function is believed to result in modified cytokine synthesis and secretion. Additional aetiologies for pruritus include nerve entrapment or compression, tumour growth and hepatic involvement leading to bile duct obstruction (Hollis 2011). Anaemia may result from immune-mediated mechanisms or reduction of bone marrow production, such as with myelophthisis. Anaemia of chronic disease is well-recognised and occurs secondary to a variety of inflammatory conditions. μ 20 m Changes in bone marrow function occur secondary to altered cytokine synthesis. Cytokines that have been implicated in this mechanism include transforming growth factor B, IL1, IL6 and interferon gamma, which upregulate hepcidin synthesis. Fig 6: Cytological evaluation of sarcoma-type cells identified Hepcidin will antagonise gastrointestinal iron uptake and from pleural effusion from a patient with haemangiosarcoma. Ante utilisation. In addition, erythropoietin antagonism results from mortem diagnosis based on identification of neoplastic cells in tumour necrosis factor affecting bone marrow production and pleural effusion is uncommon. function (Hollis 2011). Hypercalcaemia (calcium >140 mg/l) may result from a paraneoplastic syndrome, although the clinician should and Brown 1993). The clinical features of these tumours are consider other differential possibilities as well such as: chronic generally nonspecific and often relate more to the primary site renal failure, iatrogenic hypervitaminosis D, consumption of wild of tumour formation. Some metastatic tumours will produce a or day-blooming jasmine (Estrum diurnum) particularly in the neoplastic effusion and/or damage to intrathoracic structures south eastern USA, rapid administration of calcium containing (Jorgensen et al. 1997; Murray et al. 1997). Cytological solutions, laboratory error and hyperparathyroidism. evaluation may or may not identify neoplastic cells. The mechanisms of hypercalcaemia occurring in patients Thoracoscopy is an important diagnostic tool to obtain a tissue suffering from neoplastic disease may include lytic bone sample to confirm neoplasia and identify the tissue type metastases, malignant hyperparathyroidism, ectopic tumour (Peroni et al. 2001; Vachon and Fischer 1998). production of parathyroid hormone-like hormone (PTHLH, PTHrP), tumour produced prostaglandins (PGE1 and PGE2) and Paraneoplastic syndromes tumour produced osteoclast activating factor (Blackman Paraneoplastic conditions develop in association with et al. 1978; Weir et al. 1988; Dascanio et al. 1992). The cDNA neoplasia yet are often unrelated to the primary site of tumour sequence for equine PTHLP has been reported (Accession development. Recent reviews will provide the reader with a NP_001157453). Although this protein has been clearly defined

that described HO in horses include a total of 42 cases (Sweeney et al. 1989; Mair et al. 1996, 2004). These reports are divided among individual case reports that date back to 1944 and a more recent retrospective that describes the clinical features of 24 cases (Mair et al. 1996). In this report 71% of the horses were ultimately diagnosed with pulmonary disease. Among those with pulmonary lesions, 40% were neoplastic in origin. In cases with a treatable pulmonary condition, resolution of HO occurred following clearance of the primary pulmonary disease. Limb swellings were present on all 4 limbs and typically occurred in a bilaterally symmetric fashion. In some instances the limbs were cool and comfortable while in others they were warm and sensitive to palpation. Head lesions were present in 2 of the cases that involved the mandible and maxilla (Mair et al. 1996). Stiffness and pain were commonly reported in affected individuals (Fig 7). When intrathoracic disease was present, clinical signs also included cough and dyspnoea. The radiographic findings associated with HO included periosteal new bone formation that was in a palisade fashion. Similar to findings in human and canine cases, in those individuals with a treatable condition, clinical signs resolved upon resolution of the primary disorder. Interestingly, in this report, 3 cases were not identified to have a primary disorder, yet clinical signs resolved following Fig 7: Radiograph of the distal limb of a mare with hypertrophic symptomatic therapy with phenylbutazone. In man, osteopathy. Note the periosteal proliferation on the first phalanx. indomethacin is a nonsteroidal anti-inflammatory drug that has been reported to provide clinical benefit when used as the primary therapy for the management of HO (Leung et al. to result in hypercalcaemia associated with canine tumours 1985). such as with lymphoma and anal sac adenocarcinoma (Weir et al. 1986), further investigations are required to more clearly define the relationship of PTHLH in the horse. Reported cases of Less common differentials hypercalcaemia in equine cases suffering from neoplasia are Differentiating a thoracic tumour from a number of infectious somewhat limited to include disseminated lymphoma and conditions of the thorax can be surprisingly difficult. lymphoid leukaemia (Jaeschke and Rudolph 1991, 1992; Finley The accompanying article (Lee et al. 2013) demonstrates a et al. 1998). common dilemma for clinicians in differentiating neoplastic Hypertrophic osteopathy (aka Marie’s disease, HO) effusion from pleuropneumonia or a mediastinal abscess. has been reported in domestic species and is most commonly Middle-aged and aged horses with solid pulmonary tumours associated with primary and metastatic pulmonary tumours are often mistaken as being affected with heaves. Hydatid (Brodey et al. 1958; Brodey 1971; Fawthrop and Russell 1993). cysts and fungal pneumonia are less common conditions that The pathogenesis of HO remains incompletely understood. The may mimic signs of thoracic neoplasia. primary syndrome involves periosteal proliferation on the Equine cases of hydatid cyst may present with a cortices of long bones (Fig 7). The condition is classically large volume of pleural effusion. In Europe, hydatidosis described to occur secondary to an intrathoracic mass, yet (Echinococcus equinus) is generally well tolerated in horses, may also develop secondary to extrathoracic lesions. and cysts in the liver and lung may be an incidental finding at Although HO is more common in man and dogs, it also occurs post mortem examination (Blutke et al. 2010). In the USA, in horses, where it has been reported in association with a Echinococcus spp. and an unidentifiable aberrant, acephalic variety of pulmonary conditions including infection, neoplasia metacestode have been identified in the liver and lungs of and trauma (Mair et al. 1996; Mair and Tucker 2004). Thoracic horses. Occasionally the cyst will rupture, resulting in clinical neoplasia is an uncommon disease in horses with HO being respiratory difficulty due to large volume pleural effusion. In the rarely reported in affected individuals. This finding is in USA, this clinical syndrome has been associated with the contrast to the findings in man where up to 10% of patients acephalic metacestode. with thoracic neoplasia develop HO, often characterised Affected horses may have intermittent fever, depression, by clubbing of the fingers, dermal changes, limb swelling rapid shallow respiration, pectoral oedema and nonspecific and arthropathy (Fawthrop and Russell 1993). In canine laboratory findings indicative of inflammation (Blutke et al. patients suffering from HO, the most common predisposing 2010). A large volume of pleural effusion is a consistent finding factor is consistently pulmonary neoplasia (Brodey 1971). in horses with pulmonary and pleural metacestode infection. The presence of HO in an equine patient should alert the The effusion has low to moderate cellularity (5–80 x109 cells/l), clinician to be aware of the potential for intrathoracic disease 20–80% neutrophils, markedly increased protein concentration to exist. (50–80 g/l) and may be difficult to differentiate from neoplastic Although HO is a relatively uncommon condition in horses, effusion (Rush and Mair 2004). Bacterial and fungal culture of it has been described to occur in association with pulmonary the pleural fluid is negative. Ultrasound examination may and extrapulmonary disease. Previous retrospective reports reveal a large fluid-filled cyst within the pulmonary

Fig 8: Ultrasound examination that revealed a large fluid-filled cyst within the pulmonary parenchyma on the surface of the diaphragm in a patient with a metacestode cyst.

parenchyma, on the surface of the diaphragm (Fig 8), and/or within the hepatic parenchyma. Metacestodes may be attached to a thickened pleural surface, or hypoechoic cysts (1 ¥ 4 mm) may be seen floating within the pleural or peritoneal fluid. The authors treated one horse with an unidentified Fig 9: Gross findings of a fungal mediastinal granuloma, which aberrant, acephalic metacestode using albendazole appeared radiographically similar to mediastinal lymphoma. (10 mg/kg bwt per os s.i.d. ¥ 30 days), thoracic drainage, and examination (Fig 9) (Rush Moore et al. 1993). The site is difficult surgical debridement of the pleura and cyst (10 ¥ 10 ¥ 17 cm) to biopsy percutaneously and is best identified via on the surface of the diaphragm. Disruption of a cyst by thoracoscopy. centesis or surgery may result in an anaphylactic reaction or seeding of daughter metacestodes within the thoracic cavity. Conclusion Surgical intervention was performed in this case after 2 weeks Diagnosis of thoracic neoplasia may be delayed because the of antiparasitic therapy. The horse was asymptomatic 6 weeks clinical signs of disease are non-specific. Horses with solid after treatment, and remained athletic for years. intrapulmonary tumours typically present with chronic cough The radiographic appearance of fungal pneumonia can and weight loss, and are often treated for heaves for a period of be similar to disseminated pulmonary neoplasia. The most months before performing additional diagnostic tests. common isolate from horses with fungal pneumonia Neoplastic conditions that produce pleural effusions may is Aspergillus sp., and the majority of cases occur in horses as a mimic the clinical signs of infectious pleuropneumonia, and complication of severe gastrointestinal disease. Prolonged secondary bacterial infections further complicate the diagnosis neutropenia is a primary risk factor. Clinical signs of fungal of thoracic neoplasia. Pleural effusion and intrathoracic masses pneumonia include tachypnoea, fever, nasal discharge, are readily visualized using thoracic radiography and epistaxis, nasal plaques or erosions, abnormal lung sounds, ultrasonography; however, the diagnosis of neoplasia may not and pleural friction rubs. Most horses have a history of be apparent based on imaging alone. Pleural cytology does progression of respiratory disease despite aggressive not always reveal neoplastic cells. Endoscopic examination of antimicrobial therapy. A miliary or reticulonodular interstitial the lower airway and thorascopic biopsy are valuable tools to pattern is common with multifocal, coalescing nodules definitively diagnose thoracic neoplasia. Early diagnosis of visualised in peripheral lung fields in both fungal pneumonia thoracic neoplasia may improve the outcome for horses with and metastatic neoplasia. Low volume pleural effusion can be granular cell tumour, and timely diagnosis of neoplasia in horses detected by thoracic ultrasound, and fibrin plaques may with a grave prognosis will minimize patient suffering and occur on visceral and parietal pleural surfaces. If pleural reduce client costs. effusion is identified, pleural fluid samples should be submitted for cytological evaluation and culture. Observation of fungal spores on cytological evaluation and/or culture of fungal Authors’ declaration of interests elements from transtracheal aspirate samples are not strong No conflicts of interest have been declared. indicators of a diagnosis of fungal pneumonia in horses. Culture and histological evaluation of lung biopsy samples References provide definitive evidence of fungal pneumonia and Adams, R., Calderwood-Mays, M.B. and Peyton, L.C. (1988) Malignant differentiation from neoplasia. Fungal mediastinal granuloma lymphoma in three horses with ulcerative pharyngitis. J. Am. vet. appears similar to mediastinal lymphoma on radiographic med. Ass. 193, 674-676.