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Introduction to Blood Disorders of Horses

By Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE, Emeritus Professor and Former Vice-Principal, Faculty of Veterinary Medicine, University of Glasgow ; Susan M. Cotter, DVM, DACVIM (Small Animal, Oncology) ; Wayne K. Jorgensen, BSc, PhD, Science Leader Applied Biotechnology Livestock, Agri-Science Queensland ; Susan L. Payne, PhD, Associate Professor, Department of Veterinary Pathobiology, Texas A&M University ; Nemi C. Jain, MVSc, PhD, Professor Emeritus of Clinical Pathology, Department of Veterinary Pathology, Microbiology, and Immunology, School of Veterinary Medicine. University of California


Blood cells form and develop mostly in the bone marrow, that is, the tissue located in the cavities of bones. Blood performs a variety of important functions as it circulates throughout the body. It delivers oxygen and vital nutrients (such as vitamins, minerals, fats, and sugars) to the body’s tissues. It carries carbon dioxide to the lungs to be exhaled and waste products to the kidneys and liver to be eliminated from the body. It transports hormones, which are chemical messengers, to various parts of the body, allowing those parts to communicate with each other. Blood also includes cells that fight infection and platelets that control bleeding.

There are 3 cellular elements of blood: red blood cells, white blood cells, and platelets. Basically, red blood cells supply the body with oxygen; white blood cells protect against infection; and, platelets start the formation of blood clots.

Blood is a complex mix of plasma (the liquid component), red blood cells, white blood cells, and platelets.

Blood disorders are quite diverse. They can occur as normal responses to abnormal situations; for example, a significant increase in the number of white blood cells in response to an infection or disease. They may also occur as primary abnormalities of the blood; for example, a deficiency of all cellular elements of the blood due to bone marrow failure. Furthermore, abnormalities may be quantitative (too many or too few cells) or qualitative (abnormalities in the way cells function). Like people, horses have different blood groups or types. Researchers have identified 8 blood types in horses: A, C, D, K, P, Q, U, and T. Horse owners and breeders will often have a record of their horse’s blood type. This should be included in the medical records for your horse.

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