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A horse may require a blood transfusion when his life is at risk due to the volume of blood he has lost. This could be due to external haemorrhage (bleeding from blood vessels into the outside world), internal haemorrhage (bleeding into a body cavity), or because the horse’s own immune system has destroyed its red cells due to autoimmune disease.

A vet makes a decision on the need for a transfusion based on the horse’s vital signs. Certain parameters will be checked for evidence of haemorrhagic shock, including his heart rhythm and rate, respiratory rate and effort, and pulse strength.

The temperature of the horse’s extremities may drop, leaving his ears and lower legs feeling cooler than normal. His mucous membranes (the insides of his mouth and nose and under his eyelids), typically pale pink, may turn white.

External haemorrhage may be caused by lacerations to superficial blood vessels by sharp objects (wire, glass, metal spikes or shards of wood) or, rarely, gunshots. Loss of blood from surgery is another hazard.

As a general rule, wounds with infrequent drips of blood are unlikely to result in life-threatening haemorrhagic shock, whereas those with a constant stream could lead to loss of life. The amount of blood loss will be greater and the bleeding is less likely to stop (clot) on its own.

It is important to remember that lacerations and puncture wounds can also damage underlying tissue and joints.

It is always best to consult your vet as a matter of urgency when a wound is detected with any significant bleeding or near a joint.

Significant internal haemorrhage can result from blunt trauma to the chest or abdomen. This trauma tends to occur during high-speed collisions with solid objects such as cross country fences, paddock fencing or vehicles, or when the horse hits the ground with force (perhaps in a rotational fall, or if he rears up and falls backwards or sideways).

Other medical causes of internal haemorrhage include rupture of uterine vessels in heavily pregnant or recently foaled mares, spontaneous rupture of pulmonary (lung) vessels or the aorta while exercising, and abdominal tumours.

Occasionally, mares may haemorrhage significantly from an ovary during ovulation.

Finding donors

There are no horse blood banks in the UK. This is mainly because stored donor red blood cells don’t last very long in the circulation of the recipient horse, compared to fresh blood cells. The optimal scenario is to collect fresh blood from a donor and transfuse immediately into the recipient.

Veterinary hospitals have different approaches to sourcing donors. Some keep old, retired horses in fields by the hospital —others hold a list of local donors.

Vets will sometimes collect blood from another horse out on the yard, with the owner’s permission, or will send a suitable donor with a sick horse when he travels to hospital if they suspect that he will need a transfusion.

There are eight blood groups in horses — far more than in humans. There’s no such thing as a “universal” donor blood group, but most blood types can be transfused from one horse to another without causing a significant adverse reaction.

The most reactive groups tend to be found in thoroughbreds, so these are not suitable donors. Cross-match testing involves mixing tiny volumes of the donor’s and recipient’s blood, but this does not reliably predict if a horse will have a bad reaction after transfusion. Vets tend to select a donor horse on the basis of its breed and history, rather than the cross-matching results.

Size matters when it comes to choosing a donor. It is more time-efficient to use large horses (weighing more than 500kg) than smaller ones, as a larger volume of blood can be collected in one go.

We don’t use breeding horses as donors because they may carry sexually transmitted diseases without showing outward signs (silent carriers), which could also be passed on in the blood to a recipient horse. For similar reasons, horses that have been imported from Europe are not suitable donors because they may be silent carriers of infectious diseases (such as equine infectious anaemia or piroplasmosis) that could be transmitted via blood to another horse.

Ideally, every donor horse is tested for such infectious diseases before blood collection, but this is not always achievable in emergency situations, so these groups of horses (imports and breeding stock) are avoided to minimise this risk.

Horses with a past history of receiving blood or plasma are not suitable as donors, because the immune system develops antibodies against the donor blood cells and proteins.

Giving blood

Up to 20% of a horse’s blood volume can be collected safely at one time. A 550kg horse, for example, could safely donate 8.8 litres of blood.

Blood is collected through a drip line attached to a catheter in the jugular vein, into collection bags containing anti-coagulant. Local anaesthetic is used for insertion of the jugular catheter and then the horse must stand fairly still for 20-30 minutes while collection takes place. Usually, a full haynet will keep the donor horse’s attention, but sometimes mild sedation is used.

Donor horses are rested for two weeks after collection, by which time their bone marrow will have replaced the donated blood cells.

At Liphook Equine Hospital, donors are given a fleece rug embroidered with “LEH Blood Donor” as a gesture of gratitude to horse and owner. These rugs also raise awareness, hopefully sparking interest in other owners so that they might feel able to offer their horse for blood donation.

Blood is given to the recipient horse through a jugular vein catheter and specialised filtered drip-tubing. A small volume is transfused over 15 minutes. If the horse’s temperature and heart rate remain stable, the rest of the blood transfusion is given at a faster rate.

The volume of blood required by a sick horse depends on his size, the severity of blood loss and whether the cause of blood loss has been controlled.

The vet will sometimes measure blood lactate levels to estimate the degree of shock and to help decide how much blood is required.

Between nine and 12 litres can be collected from an average-sized 550-700kg donor horse, so this amount is usually given before the vet assesses the patient’s response. If the sick horse’s vital signs have not improved sufficiently, further transfusions are recommended.

Ref Horse & Hound; 13 April 2017