There are many reasons why your horse might have a blood sample taken. Many vet practices now encourage a ‘well horse’ scheme, which may include annual blood tests to screen your horse for his overall health. This is especially useful for elderly equines or those on long-term medications such as bute, where early signs of organ disease may be detected on a blood test a long time before the clinical signs develop. When detected early, many diseases or problems with organ function can be successfully treated or managed to lengthen or improve the quality of your horse’s life.
If your horse appears under the weather or isn’t performing well, a blood test is likely to be your vet’s first port of call, and blood tests are commonly used to diagnose hormonal diseases such as PPID (Cushing’s disease) or to look for signs of possible tapeworm infection using the tapeworm ELISA. They are also used to screen horses for exposure to or possible carrier status of strangles, and when horses are travelling abroad to ensure they aren’t infected with certain diseases when arriving in or leaving the UK.
How is the blood collected?
Blood is usually obtained from the jugular vein in your horse’s neck and it can be taken from either side. It’s collected in special containers, often specially designed tubes called Vacutainers, which can be plain or contain different substances to preserve the blood for a variety of different tests. Blood can be collected from other blood vessels, such as the cephalic vein on the inside of the front leg, but the jugular vein is usually used because it’s readily accessible and most horses don’t seem to mind the procedure.
Different types of test
Blood contains many components and can be tested as whole blood, or after being split into either serum (the liquid part of the blood that contains no cells) or plasma (which is serum plus proteins that are involved with clotting).
Whole blood testing
Whole blood is examined to look at its red blood cells, white blood cells and platelet content…
- red blood cells are the oxygen-carrying cells. Horses normally have a relatively high packed cell volume (PCV) because they are designed to be athletes and, therefore, need a lot of oxygen-carrying cells. Thoroughbreds will often have a PCV of 35–45%, with warmbloods and draft horses having a slightly lower PCV of 30–40%.
- white blood cells are the cells of the blood that fight infection. There are several different types of white blood cell – neutrophils, lymphocytes, monocytes, eosinophils and basophils. The cells vary in what they do, where they are made and how long they last in the bloodstream. An increase in the total white blood cell count is known as a leucocytosis, and can be seen in times of stress or excitement, during exercise, and when infection and inflammation occur. A decrease in the total white blood cell count is known as leucopaenia and is usually a sign of an infection, as the white blood cells are diverted out of the bloodstream towards the area where they need to fight the problem. Rarely, leucopaenia may be a result of a lack of production of white blood cells. The most numerous white blood cell is usually the neutrophil, making up around 50–70% of the white cell count in healthy horses. The next most numerous is the lymphocyte, which makes up 30–45% of the total white cells. Eosinophils, monocytes and basophils occur in small numbers.
- platelets are part of the blood clotting mechanism. They will often clump together during normal blood analysis, making the count inaccurate when it’s done with a machine. If a problem with clotting is suspected, your vet will collect blood into a special tube designed to minimise platelet clumping, then a blood smear will be looked at under a microscope to confirm the machine’s platelet count.
- clotting times may need to be analysed in some circumstances. This may be performed before a liver biopsy (because the liver is very important for producing proteins that control bleeding) or because your horse is suspected to have a blood clotting problem. This involves specialised tests performed on blood collected in special tubes.
Serum and plasma testing
Serum or plasma is tested to investigate other potential problems…
- muscle enzymes (CK and AST) are evaluated to look for signs of muscle damage
- liver enzymes (commonly GGT and AST, but also occasionally other enzymes such as SDH) are evaluated to investigate the liver. The bile acids are often analysed in cases where there is an increase in GGT or AST, as this is a measure of liver function. Bilirubin can also be used to investigate liver function, but is much less specific and changes can be seen with other conditions, most commonly when your horse is simply not eating as much as usual
- urea and creatinine are a measure of kidney function
- protein level in the blood is monitored both as the total protein and as the albumin and globulin levels, which are the major proteins that make up the total protein. An increase in the total protein may be due to an increase in albumin, globulin or both
- fibrinogen and SAA are evaluated to look for signs of infection or inflammation, and to monitor the response to treatment. The SAA increases very quickly with infection and, once the infection starts to resolve, will decrease very quickly, so this can be a good indicator that your horse is on the appropriate treatment. Therefore, SAA may be monitored frequently – for example, every 48–72 hours – in very sick horses, where it’s used as an early indicator of things getting better or worse
- electrolytes are often included as part of a routine blood profile and increases or decreases can be caused by many different conditions, so changes in electrolyte concentration should be interpreted carefully
Specialist blood tests
As well as checking your horse’s general health, blood tests can be used to look for evidence of exposure to certain pathogens, infection with certain diseases and to look at hormone levels…
- PPID is extremely common in older horses and can cause a lot of clinical problems, such as increased risk of laminitis, infections, hoof abscesses and dental problems. Thankfully, it’s now very easy to test for and treat the disease, as a simple blood test can be done to look for a hormone known as ACTH. When the ACTH levels are very high, the diagnosis of PPID can be confidently made. There can be borderline or confusing results, so sometimes other tests are added to clarify these results, but in most cases the diagnosis is easily made. The test can then be used to monitor ACTH levels once the horse is being appropriately treated, to make sure that the hormone levels return to and remain within the normal range.
- equine metabolic syndrome (EMS) is a disease that leads to problems regulating glucose and insulin levels, leading to abnormal fat deposition, a predisposition to gaining weight and an increased risk of laminitis. Blood glucose and insulin testing can help with the diagnosis and management of EMS.
- tapeworm ELISA is used to monitor tapeworm burdens because, unlike other types of worm, tapeworms can be hard to diagnose on a faecal worm egg count, as they do not reliably shed eggs. This blood test has been developed to look for antibodies to tapeworms to help diagnose whether a horse has been recently exposed to a high number of tapeworms. A very high result would be an indication to use a specific wormer targeting tapeworms. However, the antibody levels take a long time to decrease, even after successful treatment, so repeated positive results have to be interpreted with caution.
- strangles blood testing has been developed to help look for the presence of antibodies to the bacteria that causes strangles. This is a very useful test, especially because some horses can carry the bacteria without showing any clinical signs. The antibody levels will not increase until two weeks after infection with the disease, so it can’t be used to rule out a horse who has very recently been exposed, but in most cases a negative result means the horse poses no risk of spreading strangles to other horses. If the antibody levels come back as very low – a negative result – the horse is generally considered to be safe to move to a new yard, as he isn’t a carrier and isn’t likely to be incubating the disease if he doesn’t come from a yard suspected of having strangles. The blood test can also be used as part of the management of a strangles outbreak.
- other diseases. Horses are usually required to have negative tests for various diseases prior to being used for breeding or being transported abroad. Blood tests can be used to look for antibodies to diseases such as equine infectious anaemia, equine viral arteritis, and piroplasmosis (a tick-borne disease). Negative test results will allow the horse to be bred or transported with no risk to other horses.
Like humans and dogs, horses have several different blood groups. It’s possible to give blood transfusions to a horse, but ideally cross matching would be performed before a blood transfusion is given. This is especially important if giving blood to a broodmare or to a horse who has already had a blood transfusion, because they are more likely to be sensitised and react to a blood group that is different to their own.
Thankfully, blood transfusions are rarely needed in horses, but this means that there are very few laboratories that can provide a cross matching service. Often, if your horse is unlucky enough to need a transfusion, blood will be collected from a horse who is likely to be a universal donor (usually a draft type gelding) rather than waiting for a cross matching result, which can take many hours to achieve.
Interpreting the results
Blood testing is very complicated and there are lots of reasons why your vet may choose to take a blood sample from your horse. It’s also important to remember that there are many things that can cause a blood test result to be abnormal, so if you have any concerns about your horse’s test results, the best person to talk you through it is your vet.