Sporadic bovine lymphosarcoma (SBL) is a rare type of cancer seen in cattle, characterized by the overgrowth of lymphoid tissue. It was first described in 1871 in Lithuania in a cow suffering from lymph node hypertrophy and splenomegaly. No specific causative agent has yet been discovered, as the disease appears to occur sporadically in cattle population, as the name suggests. This condition can manifest itself in three different forms: juvenile form (calf/multicentric lymphosarcoma), thymic form (thymic lymphosarcoma) and skin (cutaneous lymphoma). Although very similar in pathology to enzootic bovine leukosis, sporadic bovine lymphosarcoma is not related to infection from the retrovirus bovine leukemia virus (BLV). Bovine leukosis caused by bovine leukemia virus occurs frequently in adult cattle population, whereas sporadic bovine leukosis appears to affect only younger cattle. All forms of SBL are rare and are typically seen in cattle less than 2 years of age. Most commonly, B-cell lymphomas are seen as a result of bovine leukemia virus infection whereas tumor cells of sporadic lymphomas are all immature lineage of T-cells and less commonly B-cells. Previous findings have shown that the proto-oncogene C-Myb is expressed in most sporadic lymphomas but not in enzootic leukosis. These alterations to small sets of cellular genes appear to contribute significantly to the neoplastic transformation of cells, notably leukocytes and lymphocytes in this disease. All forms of sporadic bovine lymphosarcoma may have varying clinical presentation related to the affected organs.
Juvenile lymphosarcoma Juvenile or calf lymphosarcoma is typically seen in neonatal calves and cattle up to 18 months of age. Neonates are most commonly affected by the B-cell form of calf lymphosarcoma but the reasons and mechanisms around this remain unknown. Both the T-cell and B-cell form induce a diffuse lymphadenopathy causing palpable enlargement of peripheral lymph nodes in affected calves. Out of all of the dairy breeds, it appears that the milking shorthorn has the highest incidence of juvenile/calf lymphosarcoma, making researchers question the genetic implication of this disease.
Clinical signs of juvenile lymphosarcoma Juvenile lymphosarcoma can be characterized by a progressive weight loss and generalized peripheral lymphadenopathy, in addition to a diffuse lymphoid hyperplasia. Clinical signs will vary significantly depending on what organs have been affected. Upon examination, increased heart and respiratory rate along with bronchial sounds are often noticed. Additionally, fevers, tachycardia, dyspnea, bloat, diarrhea and hind limb ataxia (or posterior paresis) were also reported. Posterior ataxia was found to be caused by the formation of tumors near the spinal cord, causing a compression in the lumbar area. Animals with juvenile lymphosarcoma usually perish within 4 to 6 weeks following the onset of clinical sign.
Diagnosis of juvenile lymphosarcoma The hematological parameters observed in cows affected with SBL are variable. Most commonly, calves will have a high incidence of circulating immature lymphocytes and roughly 50% will be leukemic. Complete blood counts (CBC) can show both leukocytosis (>17,000/mL) and lymphocytosis (>10,800/mL), in addition to normocytic and normochromic anemia (<28% PCV). Blood serum biochemical testing often shows increased activity of gamma-glutamyltransferase (GGT > 23 U/L) secondary to neoplastic infiltration of hepatic tissue. Also hypophosphatemia (<5.9 mg/dL) and hypomagnesemia (<1.9 mg/dL) can occur as a result of anorexia. Bone marrow biopsy may reveal neoplastic lymphocytes and a macrocytic anemia. In most cases, blood biochemistry will appear normal, yet some cases may show decreased globulins and increased albumin, causing an imbalanced A/G ratio. The presence of neoplastic cells within the medullary cavity of the bone has eroding consequences on the cortical bone and fat. This condition can lead to marrow embolism, peripheral blood cutopenias, pathologic fracture, and necrosis of medually bone and bone marrow elements. Similar to enzootic bovine leukosis, enlarged lymph nodes and infiltration of neoplastic lymphocytes within the spleen, liver, bone marrow and sometimes within the body of the lumbar vertebrae and the spinal cord will be observed at post mortem. In calves between the age of 4 and 7 months, development of visceral tumours, bone marrow lesions and peripheral leukemia have also been reported.
Thymic lymphosarcoma The thymic form of sporadic lymphosarcoma is an aggressively invasive malignant tumour which appears in cattle between the age of 6 and 24 months of age, although it has been reported in 30-month-old cattle. Secondary to the findings of Ogawa (et al. 1986) on the involvement of the proto-oncogene C-myb, it was suggested by Oliver-Espinosa et al. (1994) that the thymic form may be hereditary, secondary to the transfer of these mutated genes. The lymphosarcoma appears, or begins its rapid growth in the thymus, where it may then affect to a lesser extent lymph nodes and other organs such as the liver, spleen, kidney and lungs. This form may involve the cervical and/or intrathoracic thymus.
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