Leukemia, i.e., the neoplasia of one or more cell lines of the bone marrow, although less common than in other species, it is also reported in horses. Leukemia can be classified according to the affected cells (myeloproliferative or lymphoproliferative disorders), evolution of clinical signs (acute or chronic) and the presence or lack of abnormal cells in peripheral blood (leukemic, subleukemic and aleukemic leukemia). The main myeloproliferative disorders in horses are malignant histiocytosis and myeloid leukemia, the latter being classified as monocytic and myelomonocytic, granulocytic, primary erythrocytosis or polycythemia vera and megakaryocytic leukemia. The most common lymphoproliferative disorders in horses are lymphoid leukemia, plasma cell or multiple myeloma and lymphoma. Lymphoma is the most common hematopoietic neoplasia in horses and usually involves lymphoid organs, without leukemia, although bone marrow may be affected after metastasis. Lymphoma could be classified according to the organs involved and four main clinical categories have been established: generalized-multicentric, alimentary-gastrointestinal, mediastinal-thymic-thoracic and cutaneous. The clinical signs, hematological and clinical pathological findings, results of bone marrow aspirates, involvement of other organs, prognosis and treatment, if applicable, are presented for each type of neoplasia. This paper aims to provide a guide for equine practitioners when approaching to clinical cases with suspicion of hematopoietic neoplasia.

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SummaryReasons for performing study: Endurance races are the competition with the biggest metabolic demands for the sport horse. During races, some horses show homeostasis alterations, having repercussions in diverse biochemical parameters and negative consequences on performance and health. Objectives: To evaluate the utility of biochemical analysis in the early diagnosis of metabolic stress and to determine cut-off values of biochemical parameters to assist in the prevention of metabolic alterations in endurance horses. Methods: This study involved 36 CEI races and 283 horses (41 eliminated because of metabolic disturbances). Blood samples were taken before competition, after the vet-gates and after finishing the race or veterinary disqualification. Packed cell volume (PCV), activities of CK, AST and LDH, and concentrations of total plasma proteins (PP), urea, creatinine (Cr), uric acid (UA) and plasma lactate were determined. Successful horses were compared with horses eliminated due to metabolic conditions in the values obtained in the phase prior to being removed from the competition. Factors associated with metabolic elimination were further analysed using multiple logistic regression analysis. Dichotomisation for each variable was made using the receiver-operating characteristic curve to enter into the model. Results: PCV>52%, PP>82 g/l, standardised Cr>30 mg/l 100 km, UA>72 mg/l, standardised CK>12.6 ui/l km and standardised AST>6.2 ui/l km were associated with the development of metabolic alterations. Of the horses with an imbalance between PCV and PP, 30% had metabolic elimination in the following phases. Muscle enzymes and Cr were directly related to the distance covered. Conclusions: Selected biochemical markers are evident in some endurance horses before their elimination. However, most horses developed metabolic disturbances without any important alterations in the variables determined in this study. Potential relevance: Analysis of selected plasma biochemical parameters could be useful in the prevention and early diagnosis of metabolic stress in endurance horses.

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UA could be useful in the assessment of metabolic response during endurance exercise.