There are many reasons why your vet might recommend taking a blood test. Gil Riley MRCVS looks at what these tests reveal — and why they can form a useful piece of the health check puzzle

Blood tests can be very helpful in assessing the health status of a horse as an addition to a full clinical examination.

In some cases, blood tests can provide an exact diagnosis. It is much more usual, however, that the test acts as an aid to diagnosis, with results being considered alongside other findings. To understand why your vet might take a test, it’s helpful to understand the basics of blood.

What is blood?

A horse’s blood is made up of approximately 60% water and 39% red blood cells. The remaining 1% comprises white blood cells (WBC) and thousands of elements, electrolytes, enzymes and proteins.

Each individual red blood cell contains haemoglobin. This protein-iron complex gives blood its red colour and is the substance responsible for carrying oxygen around the body.

For every 100 red blood cells, there is roughly 1 white blood cell, the chief role of which is to fight infection. Other functions of the blood system are to carry clotting factors and repair cells to the site of damaged tissue; protein components of the immune system to where there is an infectious challenge, and nutrients from the gastrointestinal tract to the rest of the body.

In a healthy horse, these blood components are regulated and maintained in a way that varies only slightly between individuals — so the different components are largely consistent. This means we have a clear reference range for what is normal — but this changes when disease is present.

So because of this “predictability” of blood, a blood test can be a vital tool in identifying disease.

Types of blood test

These are generally divided into 4 main groups:

1. Haematology. This is the name for the examination of a horse’s red and white blood cells. The packed cell volume test, or PCV, reveals the percentage of blood volume comprised of red blood cells. A low number could indicate anaemia, often the result of other disease, including worm infestation. An increased PCV can indicate dehydration, among other possibilities.

Looking at the total number of white blood cells can help diagnose various types of inflammation. This number typically decreases with severe, acute inflammation and increases with slow-developing or chronic inflammation. Variations in the number of white blood cells present can help differentiate between say allergic-type inflammation or a viral illness.

The white blood cell count (WBC) is also useful in determining whether or not a condition is responding to treatment. The WBC number would increase with a mild to moderate bacterial infection, such as pneumonia, for example, but would decrease towards the normal range with treatment by an appropriate antibiotic. Specific white blood cells such as neutrophils are also examined for changes in cellular structure associated with illness.

With WBC counts, it is often beneficial to perform successive blood tests a few days to a fortnight apart, depending on the condition. A trend in any changes can then be identified.

2. Biochemistry. This is the study of the non-cellular component of blood. When a blood sample is collected and allowed to clot, all red and white cells, along with platelets (these help the blood to clot) and clotting proteins themselves, are removed.

The remaining “serum”, which is composed of water plus numerous products such as elements, electrolytes and enzymes, can offer many clues. Measurement of electrolytes can be helpful. Abnormal levels of electrolytes such as sodium, potassium or phosphorus can cause problems ranging from neurological disorders to kidney dysfunction or an abnormally low heart rate (bradycardia).

Serum proteins and inflammatory markers can also be usefully tested. Varying levels can indicate gastrointestinal disorders and some forms of cancer, as well as other problems including infection, such as lung abscesses in foals.

Analysing blood for specific antibodies is vital in identifying infectious diseases such as strangles. Although it is not a direct test for the presence of the infectious agent, it tells us indirectly that the body has mounted an immune response to the agent and has therefore been exposed to it. Sampling at a later date can then reveal if immunity is fading and therefore whether the challenge (disease) affected the horse recently or at an earlier date.

Serology, which includes measuring antibodies, is commonly used to identify viral illnesses including herpes. Biochemistry may also be used to measure enzymes, the proteins that can serve as important markers for identifying tissue damage. When the liver is damaged, for example, certain enzymes present in large quantities in the liver tissue will show high readings in the serum.

3. Endocrinology. Tests can be performed to measure hormone levels in the blood. There is now a reliable test for the hormone ACTH, which is excreted in excessive amounts by the pituitary gland when a horse is suffering from Cushing’s disease. Fortunately, an effective treatment is now available, so the ability to diagnose the condition quickly and easily with a specific blood test is a real breakthrough.

Endocrinology can also help in the diagnosis of equine metabolic syndrome, which is commonly seen in overweight ponies.

Rigs (males with either one or both testicles retained in the abdomen) can be identified with a recently developed and accurate blood test for the anti-mullerian hormone (AMH), which is excreted in large amounts by abdominally situated testicles. The same test is also useful for identifying mares suffering from a granulosal cell tumour, the most common tumour of the ovaries.

4. Genetic testing. Blood tests are commonly performed by breed societies to confirm parentage of a foal. Specific genetic markers can determine the foal’s DNA make-up and detect some inherited diseases too.

Regular screening

While blood tests are typically taken in response to a health problem, regular screening may be recommended by your vet in some cases.

An older horse is more prone to declining organ function and a regular (annual, at least) blood test can help determine whether any supportive treatment, care or dietary change is appropriate. Dosing long-term with an anti-inflammatory such as bute can affect gut lining and kidney function. A regular blood test will detect any damage at an early stage so the dose can be reduced or an alternative medication used.

Many competition horses have a routine haematology and biochemistry test to ensure they are in peak physical condition. As with human athletes, prime physical fitness involves levels of exercise that stress the body and render it more vulnerable to infection. Routines can then be adjusted if tests reveal a stressed or challenged immune system.

For a normal, healthy horse under a less rigorous workload, a routine blood test for general health is probably unnecessary. Providing the horse appears in good condition, such a test is unlikely to throw up any new information.

5 blood test facts

1. A test produces a snapshot of what’s happening in the blood when the sample is taken

2. Repeat testing can reveal trends or provide a means of assessing response to treatment

3. The results are just part of the picture when it comes to reaching an accurate diagnosis

4. Some conditions produce no measurable blood changes. These include gastric ulcers, lameness, tooth pain and most colics and skin conditions

5. Horses can be blood-tested to check for the presence of tapeworm, as well as other specific infections and diseases

Blood in numbers

54
the average amount of blood, in litres, in an adult horse

100
the approximate number of red blood cells for every one white

60
the percentage of blood made up of water

5
types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils

This veterinary clinic feature was first published in Horse & Hound magazine (3 January, 2014)