This study shows that competition elicits a classic physiological stress response in horses and that different training programmes induce different responses. It suggests that horses involved in competition can provide a good model to study the exercise-induced stress response.

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.

Metabolic responses to exercise differ between Andalusian horses and other breeds, although changes in plasma muscle enzymes have not been reported and most useful information is obtained from animals subjected to different training programmes. The objectives of this study were to 1) describe the changes in plasma enzymes during exercise in different horse breeds in relation to other biochemical parameters (Experiment A) and 2) assess the effect of training duration on these measures (Experiment B). Twenty stallions, 9 Andalusian (AN), 7 Arabian (A) and 4 Anglo-Arabian (AA), age 5-10 years, were studied. They performed 3 exercise tests (ET), consisting of a warm-up of 800 m at 0.7 km/h and 4 workloads at 15, 20, 25 and 30 km/h, at respective distances of 1250, 1670, 2080 and 2500 m, with 5 min active recovery between each workload (Experiment A). Three ETs were performed at the beginning and after 2 and 6 months of training (Experiment B). Venous blood samples were collected during the ETs and plasma glucose (GLU), free fatty acids (FFA), lactate (LA), creatine kinase (CK), lactate dehydrogenase (LDH), alpha-hydroxybutyrate dehydrogenase (HBHD), aspartate aminotransferase (AST), Na+, K+ and Cl- were measured. AN horses responded to exercise with greater increases in GLU, HBHD, LDH, CK and AST compared to the other breeds. An unexpected result in Experiment A was the lack of interbreed differences in plasma peak LA concentrations, since it is commonly accepted that AA and A horses have greater athletic potential. Although the glycolytic response to exercise was reduced after 2 months of training in the AA and A horses, and after 6 months of training in the AN horses, at the end of Experiment B, AN horses produced more lactate than the other 2 breeds. Most of the adaptations linked to training were found in the AN breed. The more striking changes in plasma enzyme activities corresponded to CK in AN horses after 2 months of training. The attenuation of CK response to exercise was related to lower extrafibrilar GLU utilisation with LA formation and greater fat metabolism. The results show that plasma muscle enzyme concentrations for the diagnosis of equine myopathies must be interpreted in relation to breed and training.