Equine blood testing

Professor Andy Durham explains what happens when a horse’s blood goes to the lab for testing

Equine blood testing and sampling plays a vital role in numerous equine medical disorders. Many different types of blood test can be used to help your vet confirm the nature and severity of a disease process as well as offering reassurance over absence of disease or exposure to an infectious organism.

However, all blood tests have their limitations to a greater or lesser extent and the procedure is never as simple as putting the blood sample in a machine that then spits out a result telling you exactly what is wrong with the horse! Interpretation of the result of a test should always be done with knowledge and awareness of the signs and symptoms (if any) demonstrated by the horse that has been sampled. Exactly the same test result could, and often does, mean very different things in 2 different horses. The following offers a brief overview of the main types of blood test in horses.

Haematology

This type of test examines the cells in the blood which are divided into red blood cells, white blood cells and platelets. Information on how many cells, their size and any apparent abnormalities in the cells is produced by the analyser. Additionally, the different subtypes of white blood cells are counted including neutrophils, lymphocytes, monocytes eosinophils and basophils; as differences in their relative numbers can infer important information about a disease process. This can be used to indicate problems such as anaemia or dehydration (red blood cells) or inflammation, allergy, tissue damage, stress and infection (white blood cells). Problems with platelets are rare but can be seen in horses with blood clotting problems.

Biochemistry

There are many chemical substances that circulate in the blood stream that can be used to indicate damage or disease in various organs. Different meanings can be attributed to the different blood markers. For example, there are several proteins (enzymes) that should be contained within cells in the liver. These include GGT and GLDH. When we see higher concentrations of these in the blood then that indicates that the liver cells have been damaged and have released their contents into the blood stream. Similarly, an enzyme called CK should be located within muscle cells and therefore high levels in the blood stream indicates damage to muscle cells. Unfortunately, these enzymes are not able to identify damage to all of the different tissues and organs in the body.

Other chemicals found in the blood can imply that certain organs are not functioning correctly. These are generally substances that accumulate to an abnormal extent because the specific organ involved is failing to get rid of that chemical. For example, the kidneys should remove creatinine and urea from the blood stream and therefore higher levels suggest that kidney function is impaired.

Similarly, the liver should remove bilirubin and bile acids and therefore higher levels indicate that the liver is not fully functional. Thus, whatever the values of the liver derived enzymes GGT and GLDH are in a case of liver disease, it is always far more worrying when bile acids are found to be abnormally high as that indicates the liver is so damaged that it can no longer perform its normal functions adequately.

Serology

Serology is the measurement of antibody levels in the blood stream. Antibodies are made by white blood cells when the body is exposed to an infectious organism such as a virus, bacterium or parasite. Serology testing can be confusing as it is one type of test where a positive result doesn’t necessarily mean that the horse has the disease, and a negative result does not rule out the disease! It is perfectly possible to have a positive result (a high antibody level) due to exposure to a disease several years previously from which the horse has now recovered. Additionally, it takes a couple of weeks or so to develop an antibody response and so a horse that has been exposed to an infectious disease within the last week may well still test negative. Careful interpretation is always needed! Nevertheless, when used carefully and appropriately, serologic testing can be used to indicate exposure (or not) to agents such as influenza, Strangles, Herpesviruses etc…

Negative serologic results are often required before mares are transported to stud. These may include testing for antibodies to Equine Infectious Anaemia (EIA), Equine Viral Arteritis (EVA) and strangles (Streptococcus equi subspecies equi). This offers some reassurance that the horse has not recently been exposed to these infections, but this reassurance is by no means absolute.

A common misconception exists relating to the use of strangles serology to indicate freedom from infection because strangles can exist in two separate forms:

either as an acute clinical disease with abscessation and nasal discharge;

or as a more long-term “silent carrier” status in the guttural pouches, which frequently is not associated with any external signs of infection.

Although strangles serology can be used to establish whether or not a horse has been recently exposed to strangles or has current acute clinical disease, the common usage of the strangles blood test to screen horses for evidence of silent carriage of disease pre-movement has been shown to be highly inaccurate. In a recent study including 9 silent guttural pouch strangles carriers, only 1 tested positive using the strangles blood test. Some serologic tests for parasites also now exist including those for small redworms (Cyathostomes) and tapeworms. Such tests require even greater caution and their link to actual parasitic infection can often be very weak.

Endocrinology

This refers to measurement of hormone levels in the blood. This has many possible applications but the understanding that the majority of cases of laminitis are related to hormonal conditions has greatly expanded the use of these tests. Ageing horses are rather prone to developing Equine Cushing’s disease (or PPID) which can be diagnosed by measuring the hormone ACTH in the blood. Unfortunately, this hormone is not very stable after collection and it is important that the sample remains chilled until it arrives at the testing laboratory. Sometimes the test accuracy can be improved by measuring ACTH following injection of another hormone called TRH into the horse 10 minutes before sampling in the so-called TRH stimulation test.

Equine Metabolic Syndrome is a further condition making horses and ponies prone to laminitis, often in association with being overweight and inactive. This is usually diagnosed and investigated via measurement of the hormones insulin and adiponectin which both indicate increased laminitis risk. Measurement of insulin will also heavily depend on what was eaten prior to testing and this should always be noted. Sometimes it can be useful to measure insulin following a set dose of sugar, and Karo Light Corn syrup often makes a convenient and palatable source of sugar for this test.

A few other hormones are often used in equine practice regarding reproductive queries. For example, pregnancy can be confirmed by measuring hormones such as oestrone sulphate from about 4 months of pregnancy, and other issues such as ovarian tumours or suspected rigs (cryptorchids) can be investigated using anti Mullerian hormone measurement. Overall, there are many applications for blood testing in horses that greatly help the management of clinical cases and disease suspects. However, it is important to realise that there are many influential factors on the test result that must always be carefully considered by the vet in charge of the case and that simple “yes” or “no” answers are often not forthcoming.