Anaemia in horses

WHAT IS IT?

Anaemia is when the number of circulating red blood cells in the bloodstream is lower than normal. The normal ‘packed cell volume’ (PCV) in horses is 32-52%. Packed cell volume measures the percentage of fluid in the blood vessels taken up by red blood cells. The range for packed cell volume is so wide because red cell count is influenced by the age and breed of the horse. For example, ‘hot blooded’ horses like Thoroughbreds have a higher PCV than ‘cold blooded’ horses like Clydesdales. An individual horse will have a narrow range of normal values specific to its age and breed.

Red blood cells are essential as they carry oxygen to all the cells of the body. Therefore, clinical signs of anaemia are related to the fact that cells and organs in

the body are not receiving enough oxygen to function as they should.

Horses that suffer from chronic anaemia will not show clinical signs until a lower blood red cell count is reached than horses with acute anaemia. This is because the horse can adapt to a slow loss of red blood cells (and therefore a slow loss of oxygen) over time, whereas they cannot cope with a rapid drop in the circulating blood cells from a significant loss of blood in a short space of time.

CAUSES OF ANAEMIA

Broadly speaking, there are three causes of anaemia: blood loss, blood cell destruction, and inadequate blood cell production.

BLOOD LOSS (HAEMORRHAGE) Blood loss can occur externally, e.g. with a nosebleed, or internally, e.g. with a tumour or a large worm burden. External blood loss is usually very obvious, but internal blood loss can be difficult to recognise, particularly if it is small volumes over a long time, such as gastrointestinal losses. Blood loss from the last part of the gut can be seen as fresh red blood in the manure, but blood from the stomach, small intestine and most parts of the large colon is only detectable with laboratory tests. This often means that owners won’t know that a horse is losing blood until it is quite late.

DESTRUCTION OF BLOOD CELLS (HAEMOLYSIS) Autoimmune haemolysis is one of the most common causes of haemolysis. This involves particular proteins (antigens) on the red blood cells being recognised as foreign by the body’s immune system and so being destroyed. This can occur in foals, where the foal inherits red blood cell antigens from its sire, against which the dam has antibodies. The foal receives these antibodies when it suckles the dam’s colostrum (first milk containing antibodies). These antibodies attack the foal’s red blood cells, destroying the cells. The foal becomes increasingly weak, pale and jaundiced and will ultimately die if the condition is not recognised quickly. Sometimes particular drugs or infections can initiate secondary autoimmune haemolysis in the adult horse by having proteins very similar to those on blood cells, which trigger the immune system to attack the infection, drug and the horse’s own red blood cells.

Some parasites and infectious disease, such as Babesia, can cause destruction of red blood cells. Toxins from some plants, e.g. onions, red maple and bacteria (e.g. Clostridia, Streptococci), can also cause rupture of red blood cells. Small blood vessel disease and genetic diseases are other causes of haemolysis.

INADEQUATE RED BLOOD CELL PRODUCTION Inadequate red blood cell production secondary to another chronic disease process is the most common cause of anaemia in the horse. This frequently occurs with infection, inflammation, tumours, endocrine disorders, and liver and kidney disease. Loss of old blood cells is not matched by new production and release of red blood cells from the bone marrow. Red blood cells last approximately 150 days in the horse, and so this type of anaemia develops slowly as old cells are not replaced with sufficient new cells. This type of anaemia is usually only picked up when a blood sample is taken as it is often not associated with any obvious clinical signs.

BLOOD LOSS (HAEMORRHAGE) BLOOD CELL DESTRUCTION (HAEMOLYSIS) INADEQUATE PRODUCTION (DYSERYTHROPOIESIS)

Severe wound

Small strongyles (worms)

Severe gastric ulceration

Gut tumours

Exercise-induced pulmonary haemorrhage Autoimmune disease

Babesia (blood parasite)

Toxins e.g. onion

Equine Infectious Anaemia Chronic disease

Iron deficiency

Bone marrow failure

IRON SUPPLEMENTATION

Anaemic horses are generally not iron deficient unless it is very long-standing, e.g. chronic gastrointestinal bleeding. Iron deficiency anaemia can be identified under the microscope as pale and small red blood cells, but iron deficiency is rare in horses. This means iron supplementation is usually unnecessary and will not boost red blood cell numbers, oxygencarrying capacity, or performance in non-anaemic horses.

HOW IS ANAEMIA DIAGNOSED?

Anaemia is diagnosed by taking a blood sample to determine the number of red blood cells present. Different ages and breeds of horses have different normal red cell counts, and therefore age and breed-specific values must be considered. Exercise and stress will temporarily raise the red cell count due to the release of stored red blood cells from the contractile spleen, and so blood samples should be taken when the horse is at rest and relaxed if possible.

Information about the specific type of anaemia can be given by looking at the size, colour and shape of the red blood cells. For example, small, pale red blood cells occur in iron deficiency anaemia. Then, consideration of the concentration of other markers in the blood, such as proteins and bilirubin (a breakdown product of blood), can help identify if anaemia is from blood loss or destruction of blood cells. A laboratory test called a Coombs test can determine if an autoimmune reaction is occurring.

Blood cell precursors and immature blood cells reside in the bone marrow, so further detail can be given by analysing a bone marrow sample. These samples are usually taken from the sternum in a standing sedated horse but can also be taken from a rib or the hip.

Finally, evidence of blood loss or other disease processes should be sought to help to elucidate the cause further.

SIGNS OF ANAEMIA

Signs of anaemia in horses include: • Pale gums • Weakness • Poor performance • Fever • Increased respiratory rate • Depression • Increased heart rate • Lethargy • Loss of appetite • Signs of blood loss, e.g. nosebleed • Jaundice (yellow-tinged gums and sclera) • Red spots (petechiae) on gums and mucosa

HOW IS ANAEMIA TREATED?

Treatment depends on the specific cause and the severity of the anaemia. The circulating blood volume in a horse is about 8% of his body weight, e.g. 40 litres for a 500kg horse. A sudden loss of 20% blood volume (8 litres in our 500kg horse) will result in marked signs of anaemia and may require a blood transfusion, particularly if the loss continues. The decision as to whether to give a blood transfusion is usually made by considering the severity of clinic signs, PCV and cause of anaemia (if known). Blood from another horse will only last a short period (a few days) but can provide temporary lifesaving benefits.

The blood for transfusion should ideally be from a matched donor, i.e. the donor and recipient horses are the same blood group so that the donor does not have antibodies against red cell proteins in the anaemic recipient horse. There are eight major equine blood groups: A, C, D, K, P, Q, U, and T, each of which is further subdivided into subtypes because of slight variations in the proteins on the surface of the red blood cell and so it is virtually impossible to find an exact match. Horses with blood groups A, C and Q, should not be used as donors as these are associated with the more severe transfusion reactions if not compatible. In an emergency where blood typing is not possible, blood should be used from an unrelated gelding.

The reason for any blood loss or destruction should be identified and treated accordingly. Likewise, any primary disease process causing anaemia of chronic disease should be addressed. Autoimmune disease may need to be treated with steroids. Chronic blood loss may cause secondary iron deficiency, in which case iron supplementation will be beneficial, although this is rare in the horse.