This case is considered atypical because the clinical signs are exemplary of both the systemic and localized forms of the disease. Although diseases are commonly described and differentiated as either multisystemic or localized, as demonstrated here, disease expression can be a continuum between 2 distinct phenotypes.

A 10-month-old, Limousin-cross feedlot heifer was presented for chronic bloat (day 1). The heifer, purchased from a local sales barn 1 wk earlier, was first noticed to be bloated 2 d before presentation. The owner had administered dioctyl sodium sulphosuccinate (Bloat-Aid; Citadel Animal Health, Cambridge, Ontario), 300 mL, PO. The bloat was refractory to this treatment, and the owner sought veterinary assistance.

The heifer and her pen mates were in good body condition and of similar size. However, she had a distended left paralumbar fossa and dried blood on the right nostril. Closer examination revealed several masses causing asymmetry of the mandible. A firm mass, approximately 5 cm in diameter, was found in the rostral extremity of the body of the left mandible, adjacent to the mandibular symphysis. The left mandibular canine and 3rd incisor teeth were loose. A 2nd firm mass, 7 cm in diameter, had eroded the ventral margin of the ramus of the left mandible. The mucous membranes of the oral cavity were pink, and a smooth, erythematous, and pedunculated mass, approximately 4 cm in diameter, was visualized in the right oropharynx. A moderate degree of dyspnea was evident, but the heifer was alert and did not cough. The respiratory rate was elevated (42 breaths/min; reference range, 10 to 30 breaths/min), and mild crackles were heard on auscultation of the craniodorsal thorax. Lung sounds in both cranioventral lung fields were decreased, and there was bilateral dullness on percussion. A high-pitched metallic sound (‘ping’) could be heard upon simultaneous percussion and auscultation of the rumen, consistent with a large rumen gas cap. The rectal temperature and pulse rate were within normal limits, and cardiac auscultation revealed no significant findings. Rectal examination demonstrated normal feces and transrectal palpation identified masses, varying in size, throughout the uterus. The prefemoral and prescapular lymph nodes were not enlarged.

Neoplasia was the prime differential diagnosis. It was postulated that a large mass in the cranial thorax was impinging upon the esophagus, trachea, and lungs, causing a free gas bloat and dyspnea, and that the other masses were metastases.

Lymphosarcoma and fibrosarcoma both occur in cattle. Mandibular masses might also be caused by lumpy jaw (Actinomyces bovis osteomyelitis), foreign body granuloma, or trauma. Differential diagnoses for dyspnea and a dull cranioventral lung field include interstitial and bronchopneumonia, traumatic reticulopericarditis, and congestive heart failure. Because of the poor prognosis for this heifer, euthanasia was recommended. As the bloat was not life threatening, the owner opted to delay until he could inquire about compensation from the sales barn where she had been purchased.

By day 3, the bloat had progressed, jugular distension was evident, and there were muffled heart sounds upon auscultation of the thorax. The heifer was euthanized with an overdose of sodium pentobarbital (Euthansol; Schering-Plough Animal Health, Pointe-Claire, Quebec) and a field necropsy was performed.

Subcutaneous edema was present along the ventral thorax and brisket. There was a large (20 to 25 cm in diameter), white-tan mass with multiple yellow foci of apparent necrosis in the cranioventral thorax, with multiple tough adhesions to the costal pleura and pericardium. This mass was assumed to be the primary site of neoplasia. The lung and thymus could not be distinguished grossly. Only the dorsocaudal lung lobes appeared normal. The trachea, esophagus, and heart were displaced caudally by the large mediastinal mass. There were multiple small nodules (approximately 1 cm in diameter) on the visceral pleura of the right middle lung lobe. Mediastinal lymph nodes appeared enlarged, and white parenchyma bulged above the cut surface when nodes were bisected. A white, raised nodule, 5 mm in diameter, was evident on the endocardial surface of the apex of the right ventricle. A chain of 3 distinct, grey nodules, each approximately 1 cm in diameter, was attached to the caudal vena cava.

Samples from the abdominal, thoracic, and mandibular masses were collected into 10% buffered formalin and submitted for microscopic examination. The mandible could be cut easily with a knife. The mass in the oropharynx was firm, white on cut section, and involved the palatine bone. Colic lymph nodes were enlarged and appeared white on cross section. A large, friable mass of coagulated blood was found surrounding the colic lymph nodes, but the carcass did not appear pale. Three focal, raised, white nodules, ranging in size from 3 to 8 mm in diameter, were found on the surface of the liver. No other cellular infiltration was evident upon gross examination of the liver. Many nodules ranging in size from 1 to 2 mm to 1 to 2 cm could be palpated in the wall of the uterus. The uterine wall appeared diffusely white on cross section. Although the spleen and kidney appeared normal on gross examination, samples were collected for microscopic examination. Numerous monomorphic lymphocytes were observed in impression smears of a mediastinal lymph node and the oropharyngeal mass stained with eosin and methylene blue (Hema Quick Trend Scientific, Kalamazoo, Michigan, USA).

The histopathological report described a monomorphic population of lymphocytes with small amounts of cytoplasm, round nuclei with coarsely granular chromatin, and numerous mitotic figures in samples examined from the lymph nodes (mediastinal, colic, lumbar aortic), mandibular masses, lung, uterus, heart, and liver. The large mass in the cranioventral thorax was considered to be of thymic origin, with a diffuse infiltration of neoplastic lymphocytes. A focal and diffuse infiltration of neoplastic lymphocytes obliterated the uterine submucosa and penetrated the muscularis. A single subcapsular infiltrate of neoplastic lymphocytes was found in the sample of liver submitted for microscopic examination. Similarly, a focal, subendocardial, nodular lymphocyte infiltration was identified at the apex of the right ventricle. The neoplastic lymphocytes in the mandibular masses were contained within a fine, fibrous stroma, adjacent to spicules of woven bone, with increased osteoclastic activity in bone nearest the neoplastic cells. The final histopathologic diagnosis was lymphosarcoma.

Lymphosarcoma in cattle is classified into 2 major epidemiological and etiological subgroupings: enzootic bovine leukosis (EBL) and sporadic bovine leukosis (SBL) (1,2). Enzootic bovine leukosis affects primarily adult cattle 3 to 7 y of age and is virally induced (3,4). The etiological agent, bovine leukemia virus (BLV), is an oncovirus from the family Retroviridae; it is transmitted horizontally by infected B lymphocytes (1,3,5). The BLV integrates into the host genome through a double-stranded DNA copy of its RNA (4). Clinical expression of the disease is influenced by the age of the animal, herd seroprevalence, opportunities for transmission, concurrent disease or stress, and status of the dam. In contrast, genetic susceptibility is the most important factor in the epidemiology and pathogenesis of EBL (2). Host genetics influence the probability of infection, the chances of overcoming infection versus persistent infection, and, finally, disease expression (lymphocytosis or multicentric lymphosarcoma) (2). The agar gel immunodiffusion (AGID) test, a serological herd screening test, was not used in this heifer because of the test's low sensitivity in the individual animal (1,4). An enzyme-linked immunosorbent assay (ELISA) for BLV or polymerase chain reaction (PCR) test for BVL provirus would have been necessary to definitively rule out EBL (1,4). Due to the costs of these tests and the age of the heifer, EBL was discounted in this case.

The precise etiology of SBL is unknown, but 3 different clinical presentations have been described (1,3,6). These include a cutaneous form, a calf or juvenile form, and a thymic or adolescent form (1,3,6). In the cutaneous form, lesions involve the dermis, not the subcutis, and start on the perineum, hind legs, and dorsum (2,5). Initially, the skin thickens, and lesions progress to alopecia and white, scabby, powdery plaques (5), which may ulcerate and appear painful (2). Rarely, cutaneous SBL progresses to multicentric lymphoma (3,5). The cutaneous form most commonly affects cattle that are 1.5 to 2 y of age (3,4,7). This heifer showed no cutaneous lesions.

The juvenile form of SBL is a multisystemic neoplastic disease that results in infiltration of many organs and massive enlargement of lymphoid structures (2), usually in calves 2 wk to 6 mo of age. Alternatively, it may be delayed until adolescence or the 2nd year of life (3,4); it has occasionally been documented in newborns (2). Weight loss, reduced appetite, and depression usually precede fever, anemia, and symmetrical enlargement of all palpable lymph nodes (3,4). However, clinical signs vary with the organs infiltrated. Bone marrow necrosis is the consequence of massive lymphocytic infiltration and always results in microcytic, hypochromic anemia, frequently accompanied by thrombocytopenia and leukemia (4,8). In this case, epistaxis from the right nostril and the coagulated blood around the colic lymph nodes suggested the possibility of thrombocytopenia., The heifer did not appear pale either ante- or postmortem, but a complete blood cell count and bone marrow analysis were not performed. The clinical course of SBL is extremely rapid, with animals dying within 2 to 6 wk of the onset of clinical signs (8). At postmortem, all lymph nodes are enlarged, and the spleen, lymph nodes, thymus, and other organs are infiltrated (1). Renal and spinal canal involvement is quite common, with gross abomasal, uterine, and cardiac infiltration being less common (1,4,8).

A multicentric distribution of infiltration was apparent in this heifer. However, only the thymus, uterus, mandible, and a few lymph nodes were enlarged, and there was minor cardiac and hepatic involvement. There was no gross evidence of renal, splenic or abomasal involvement. Clinically, this heifer appeared to have the thymic form of SBL, but the pattern of infiltration was more compatible with the juvenile form. The thymic form occurs more commonly in adolescent beef cattle rather than in dairy breeds, with some suggestion of a familial predisposition (6). The usual age of onset is 6 mo to 2 y, with clinical signs being caused almost exclusively by enlargement of the thymus (3,4,7). Interference with ventilation, distended jugular veins, regional edema, and bloat are commonly observed complications, as demonstrated in this case (7,9,10). Fluid may accumulate in the thorax causing muffled heart sounds, suggesting a diagnosis of traumatic reticuloperitonitis or primary cardiac disease (7,9,10). Depression, reduced appetite, and weight loss often occur initially, but the clinical course is only a matter of weeks once thymic enlargement reaches a critical point (7,9). Lymph node enlargement and bone marrow infiltration may occur, but anemia and lymphocytosis are rare (7,10).

Sporadic bovine leukosis is described as occurring in distinct syndromes. However, there tends to be a continuum in which animals show clinical signs associated with several forms of the disease. The disease in this heifer had some characteristics of both the thymic and juvenile forms of SBL.

Approximately 0.5 to 1.2 individuals per 100 000 cattle are estimated to be affected by SBL (4). Although SBL is rarely diagnosed, its clinical expression is variable and it should be considered a differential diagnosis in animals with localized or multisystemic disease.