The red blood cells are responsible for carrying oxygen to the various cells of the body so they are able to function and move. Each cell in the horse's body needs oxygen to do its job and without all cells working properly the horse will experience any number of health related problems.
Anemia, or lack of red blood cells in the body results in poor blood aeration or ability to carry oxygen to the body. Anemia is actually relatively common in horses of all ages and breeds, although Arabians and some Thoroughbred lines in particular are more prone to anemia than some of the coldbloods or heavier horses.
There are actually three main reasons why horses may develop anemia or lack of red blood cells. The first and most obvious is blood loss due to a health condition or injury. Often this can be seen by dark colored bloody looking urine or swelling in the abdominal area in the case of internal bleeding. Nosebleeds may also be a sign of anemia and internal bleeding. In the case of injury the blood loss may not be evident on the horse, but the cut or injury may be significant so that the owner would reasonable expect there has be large amounts of bleeding. In severe cases of blood loss transfusions may be required, but typically stabling and careful temperature control, high quality feeds and lots of water is all that is needed to allow the healthy horse's body to generate red blood cells.
The second and third causes of red blood cell problems in horses are caused by more difficult to determine factors. These can either be genetic or disease conditions that cause the red blood cells to break down in the body or prevent the formation of new red blood cells.
Typically the first symptoms of problems with blood aeration and anemia in horses is a higher heart rate, often up to 50 beats a minute from the normal 30 as the body tries to get more oxygen to the various cells and tissues. The mucous membranes of the nose and gums as well as the tissue around the eyes may appear white or even grayish in color rather than the normal healthy pink color. The horses will often not want to eat or drink and will have trouble completing even mild amounts of exercise. Often the horse will appear despondent or depressed and seem to stand with their head hanging, very disinterested in what is going on around them.
When red blood cells are being destroyed in the body by various types of viruses, blood diseases and poisons the urine will be very red and dark, the horse will typically have a fever, problems in breathing, as well as yellowish gums, eyelids and mucous membranes.
The vet can determine anemia through a blood test that is called the PCV or packed cell volume test that measures the number of red blood cells present. Normal horses have about 40% of the blood as red blood cells whereas anemic horses are under 30%. After the anemia has been diagnosed the vet can prescribe feed supplements, limited exercise and can treat the primary cause of the anemia. Horses that are left untreated will not usually survive if their PCV number drops below 15%.