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Plasma is the straw-coloured liquid component of blood that is left after the red cells, white cells and platelets have been removed. It contains important proteins such as clotting factors, albumin and antibodies, and also electrolytes.

A plasma transfusion is given when a horse suffers a significant loss of blood proteins or antibodies. Depending on the cause, these horses may show only vague signs of illness, or more obvious problems such as weight loss, colic or diarrhoea.

When blood proteins drop below a certain level, fluid leaks out of the horse’s circulation. This results in swelling (oedema) on the underside of the abdomen and lower legs. A vet may use blood test results to calculate when a plasma transfusion is required, or base the decision on the severity of the horse’s clinical signs.

Beating bugs

Newborn foals are sometimes in need of antibody-rich plasma.

Antibodies are an essential part of the immune response to infectious challenges from viruses, bacteria and fungi, yet foals are born without antibodies in their circulation. A foal relies on transfer from his dam’s colostrum (first milk), during which antibodies pass through his intestine wall and into his bloodstream. This transfer process fails if the mare has poor-quality colostrum or if the foal does not drink enough milk in his first day of life.

A foal with very few or no antibodies in his blood is at high risk of serious infections for months, until his own immune system starts producing antibodies. He may show no signs that his antibody levels are dangerously low, so vets routinely recommend a blood test of all foals at 24 hourse old to measure concentrations of IgG (one of the antibodies) in order to detect the problem.

A foal found to be deficient will receive antibody-rich plasma, bought commercially or collected from a local donor.

Protein boost

Varied medical circumstances may mean that youngsters and adult horses lose protein and require plasma transfusions.

Blood proteins are often low due to leakage from diseased intestines or kidneys, although protein-losing kidney diseases are very rare in the horse compared to other species. A horse with kidney disease may drink and urinate more than usual and lose weight. High urine protein concentrations can be picked up on a simple urine dipstick test.

One cause of substantial protein loss and sudden, severe illness is colitis, where the intestinal wall becomes ulcerated and inflamed due to bacterial overgrowth in the gut. A horse with colitis may be very unwell, suffering from shock, septicaemia and protein loss.

The disease can be fatal, even if treated quickly and intensively. Plasma transfusion is a critical part of treatment, replacing lost blood proteins as well as providing much-needed antibodies to fight the septicaemia.

Parasitism can also lead to significant protein loss, especially in youngsters with heavy burdens of larvae and/or adult small redworm (Cyathostomins). Some of these cases benefit from plasma transfusions alongside more routine treatments such as steroid anti-inflammatory drugs and wormers.

Rarely, autoimmune diseases such as inflammatory bowel disease or intestinal tumours can damage the bowel enough for it to leak protein. A plasma transfusion can stabilise the horse before surgery, or support him while more specific treatment, such as steroids, takes effect.

Ref Horse & Hound; 20 April 2017