ABSTRACT. A 12-month-old Holstein heifer with anorexia, lameness, and enlargement of peripheral lymph nodes was suspected of having bovine leukosis. Although lymphocytosis was not observed, cytology of fine needle aspirate from a superficial cervical node, and increased serum lactate dehydrogenase and thymidine kinase activities, strongly suggested lymphosarcoma. Increased numbers of mononuclear cells as well as mitotic cells were observed in synovial fluid collected from swollen joints. Pathological examination confirmed B-cell calf form bovine leukosis and joint swelling related to neoplastic cell infiltration. Both interleukin-2 receptor and thymidine kinase 1 genes were highly expressed in cells from superficial cervical lymph node aspirate. KEY WORDS: calf form bovine leukosis, gene expression, joint infiltration.doi: 10.1292/jvms.12-0090; J. Vet. Med. Sci. 74(9): 1225-1228, 2012 Bovine leukosis is one of the most common neoplasms detected in dairy cattle and is generally divided into 2 types: enzootic bovine leukosis caused by bovine leukemia virus (BLV) and sporadic bovine leukosis, which has not known cause and does not appear to have any association with BLV [1]. The sporadic form of bovine leukosis can be further subdivided into calf, juvenile/thymic, and cutaneous forms [3,15]. Thymic and calf forms of leukosis can affect calves and heifers from birth up to 18 months of age [15]. Younger cattle are generally affected by the B-cell calf form [15,17]. The most obvious clinical sign of the calf form is diffuse lymphadenopathy that results in obvious and palpable enlargement of peripheral lymph nodes [3]. Tumor cells often infiltrate many organs, including the thymus, lymph nodes, liver, spleen, lung, bone marrow, mammary gland, salivary gland, gall bladder, adrenal gland, and pancreas [11]. Ataxia is another common clinical sign of the calf form, because tumor cells often invade the central nervous system [11,14]. However, tumor cell infiltration into the musculoskeletal system, such as muscles and joints, is not common in the calf form. In this report, we describe a clinical case of calf form bovine leukosis with lameness and joint swelling related to neoplastic cell infiltration in a Holstein heifer. Additionally, we evaluated enzymatic activity and gene expression related to leukosis as biomarker candidates for the onset of bovine leukosis.A 12-month-old Holstein heifer presented with anorexia and lameness. At initial examination, the heifer had a body temperature of 38.8°C and heart rate of 108 beats/min (bpm). Diffuse lymphadenopathy was noted. Despite treatment with antibiotics and dexamethasone, the general condition of the heifer did not improve. The heifer was taken to the Veterinary Teaching Hospital at Obihiro University of Agriculture and Veterinary Medicine on day 9. Upon admission, emaciation, depression, lameness, abdominal posture − standing under, and swollen carpal and tarsal joints were noted (Fig. 1). Rectal temperature was 39.9°C, heart rate was 120 bpm, and respiratory rate was 56 b...