Blood tests can give us real insight into problems inside the body that might not be immediately obvious.
We can find out how well different internal systems are working, or whether a horse is fit, pregnant or fighting an infection. We can detect whether a foal has absorbed the antibodies from his mother’s milk. We can even determine the presence of certain genes that may cause illness, affect athletic performance or dictate the appearance of a horse’s offspring.
Tests can tell us about infections the horse has been exposed to in the past — or those that he could still be harbouring — and the medications he has recently received. In short, blood testing is crucial to determining the cause of disease, reducing the spread of infection and maintaining the integrity of equestrian competition.
How it works
Extracted blood is stored in special tubes before tests are performed. Some tubes allow the blood to clot, while others stop clotting and contain additives to preserve the blood cells or other components.
Certain tests can be run immediately at the yard; others are carried out at a veterinary practice or more specialised laboratories.
The blood continually mixes as it circulates through the body, but once sampled it can separate into different components:
- Cells: these can be examined under a microscope, or different cell types can be counted using a machine.
Red blood cells are most common in number and carry oxygen from the lungs to tissues such as the muscles. Higher numbers of red blood cells are found in fit and athletic horses than in less athletic breeds and many ponies. Red blood cell counts are also higher if a horse is dehydrated, because there is less liquid in the blood to dilute them.
White blood cells are used by the body to fight infection. There are several different types, and numbers change depending on the nature and stage of disease. White blood cells can decrease as they are used up, fighting infection, or increase once the body has had time to make more cells in response to the challenge.
- Platelets: small fragments of much larger cells. Their main role is to help with blood clotting, which is essential to stem blood loss after injury. A vet might assess clotting and check platelet numbers in horses that bleed excessively or unexpectedly.
- Plasma: the liquid the blood cells are carried in — a mixture of water, proteins and chemicals. If blood has been allowed to clot, this mixture is known as serum. Once the cells have separated away, plasma is usually a straw-yellow colour. It can become cloudy and fatty, particularly in seriously ill ponies and donkeys.
Other blood components include albumin, an important protein made in the liver which helps thicken plasma and keep blood within the blood vessels. Low levels can occur if the horse is not eating enough protein, if his liver cannot produce enough albumin, or if albumin is being lost through damaged tissues.
Certain proteins are produced in the early stages of disease as the horse tries to fight infection. Measuring these proteins can be helpful in signalling the presence of infection and the horse’s response to it. These “inflammation markers” cannot tell us what the disease is, however, or which parts of the body are affected.
Damaged cells often release their contents into the bloodstream. Blood tests can tell us which organ is affected and the severity of the damage. Measuring proteins (enzymes) called creatine kinase (CK) and serum aspartate aminotransferase (AST), for example, can help diagnose muscle diseases such as tying up, and determine when the horse can return to work after an episode. Liver cells also commonly release particular enzymes when they are damaged, indicating liver disease.
The body produces waste products which need to be removed from the blood — urea, which is removed by the kidneys, for example, or bile acids removed by the liver. High levels of these products tell us that these “waste disposal” organs are not working properly.
Antibodies are produced by the horse’s immune system to fight specific diseases. A single blood test cannot tell us when exposure took place, but can reveal if the horse has met the disease in the past — an example being the test for the bacterial infection strangles.
Blood carries hormones between different bodily organs. Hormone levels might be measured to test for pregnancy, or for hormonal conditions such as pituitary pars intermedia dysfunction (PPID, also known as Cushing’s) or insulin dysregulation.
We can also measure a foal’s antibody levels, to ensure that he has absorbed enough of these from the mare’s colostrum (first milk) to protect him from disease until he can produce antibodies of his own.
Risks and limitations
Blood tests are generally considered safe. Swelling occasionally occurs where the needle entered the vein, but this usually resolves quickly.
Behavioural training can be useful with horses who dislike needles — you’ll find advice at the British Equine Veterinary Association (BEVA) website, or by searching for “don’t break your vet” online.
There are plenty of things that blood tests cannot tell us. Many of the tests used to detect cancers in other species are not yet available for horses. Certain diseases, including most kidney conditions, cannot be detected until quite advanced.
Blood is usually sampled from the jugular vein, a large blood vessel that runs down the neck. Blood can be taken in almost any setting and the procedure is usually quick and simple. Some results can be altered by stress or recent exercise, so samples are often taken at the horse’s yard at a calm time.
Blood flows down the jugular vein and back to the heart. The vet or veterinary nurse will usually apply pressure towards the bottom of the neck to slow blood flow, so that the vein fills. A hairier horse may need to be clipped to reveal the vein.
A thin needle, about 2.5cm long and 1mm in diameter, is then pushed into the vein and blood is withdrawn using a syringe or a vacuum tube.
What’s the cost?
While some blood tests give a straightforward “yes” or “no” answer, others need to be interpreted more carefully or considered alongside a range of additional findings and test outcomes.
Results may point to several different causes, some mild and others more serious. Normal test results can be reassuring in that many of the horse’s systems are obviously working well, but frustrating when there’s clearly something wrong that tests have not detected.
Expect to pay from £10 to several hundred pounds, depending on the complexity of the tests required.
‘A real success story’
When his racing career was cut short due to health problems, thoroughbred gelding Breden (pictured, below) returned to Linda Jewell’s Kent racing yard to be retrained as a riding horse. During the horse’s rehab, Bell Equine vet David Sinclair instigated further treatment for a troublesome lung abscess.
“Breden was given a long antibiotic course,” explains David. “It can be difficult to see enough detail on X-rays and ultrasound scans to gauge the progress of a lung abscess, so regular blood testing revealed how Breden’s system was responding to the infection. The same tests were used to decide when to stop treatment and to monitor him for any recurrence as he was brought back into work.”
Against the odds, Breden was able to return to the racetrack. He has since won four times, at Newbury, Chelmsford and Kempton Park.
“It’s a real success story,” adds Linda. “We’re a small yard, so to have a horse as good as him means everything to us.”
Ref Horse & Hound; 17 January 2019