Your horse may need a blood test any number of reasons. Vets take blood samples from sick horses, including those with diarrhoea, a high temperature, serious cough and nasal discharge, some cases of colic, or from those who is just generally unwell and lethargic. Alternatively, your horse might not be obviously sick, but you may feel that something is not quite right. Perhaps you want a blood test done in a healthy horse as a check-up to make sure all the levels are normal. But how much information can we get from blood tests and what exactly do they tell us?
Vets often start with a fixed panel of tests that covers a wide range of possible conditions, as as initial screening. If something abnormal shows up, then more specialised tests can be carried out to narrow the problem down.
There are also quite a few specialised blood tests to check for specific diseases or conditions, for example Cushing’s (PPID) and Equine Metabolic Syndrome/insulin resistance, while others can test for heart damage, antibodies to certain viruses, the blood itself for clotting, and of course, pregnancy. Selenium testing is another common blood test because while a lot of New Zealand pasture is low in selenium, too much selenium supplement is toxic. Testing to see what a horse’s levels are before recommending any supplement is strongly recommended, and so blood selenium levels are often added to a routine blood screen.
You may notice if the vet comes to take a blood test for a routine panel or screen, that they will use at least two different tubes; one has a purple or pink top and the other has either a red or green top. This is because the colour of the tube indicates to the vet if it contains an anti-coagulant (a chemical to prevent the blood from clotting) and which specific one it contains. Only the correct tube can be used for various tests.
The purple/pink one is used to look at the actual cells in the blood, which is called the haematology. The red or green one is used to look at different substances (chemicals, enzymes, proteins and electrolytes) circulating in the blood, and the term we use for this is biochemistry.
Haemotology
Blood is made up of fluid and different types of cells; there are red blood cells, of course, but there are also five different types of white blood cells, which are an important part of the immune system there to defend the body against disease, and some small cells called platelets. These are very important in helping the blood clot if your horse gets a wound.
What do the red blood cells tell us?
The red blood cells give the blood its colour, and are there to carry oxygen to the muscles and organs of the body. In a blood test, the amount of red blood cells as a percentage of the total blood volume is measured, and this is known as the PCV. The normal range is quite large, from around 32% up to 43%, and hot-blooded horses such as thoroughbreds tend to have a higher normal PCV than cold-blooded horses, like draught breeds.
If the blood has a higher percentage of red blood cells than the normal range, in a sick horse this can be due to dehydration. The PCV is something we use a lot in monitoring horses with colic and cases of diarrhoea, to assess how serious the problem is. A high PCV in a sick horse can be very serious indeed, and the horse needs urgent treatment.
In a healthy horse, the PCV can be increased due to excitement or stress; during a race, the PCV of thoroughbreds can be up to 65%. These extra red blood cells have been released from a store in the spleen so extra oxygen can be rapidly carried in the blood to the muscles, to allow galloping at a fast speed.
From this, we can see that the measurement of the PCV alone doesn’t tell us everything. A PCV of 65% in a sick horse wouldn’t be good news, but the vet needs to take into account what the horse was doing at the time the blood was taken ie. whether it was lying down in pain, or had just come off the racetrack after winning a race.
A low PCV, on the other hand, is anaemia. True anaemia, like we see in people, is uncommon in horses. Usually there is another cause, and once this is fixed then the anaemia improves.
There are three main causes of a low PCV in horses:
*bleeding. This can be from an external wound but also can be from internal bleeding, for example from ulcers in the stomach or intestines.
*long-term (chronic) disease. The disease may not even be that obvious, just something that grumbles on like viral respiratory disease or lung abscesses. In some cases, a low PCV on several blood tests alerts the vet that there may be a low-grade disease going on somewhere and more investigations can be performed.
*The red blood cells being destroyed. This is less common but can happen with certain toxins, and sometimes following treatment with certain drugs.
So, the red blood cell levels are important indicators both in the very sick horse, and of the general health of a horse. Some racehorse trainers like to monitor levels every two weeks to get a picture of what is normal for that horse, as there is such a large range. If a horse is normally around 42% then suddenly drops to 34%, then the horse may need to be checked over carefully. Of course, the testing needs to take place under the same conditions each time to avoid the effect of stress or excitement.
What do the white blood cells tell us?
As an important part of the immune system, the five different types of white blood cells have different roles in defending the body against disease. The different types are:
- Neutrophils. These defend against infection, and an increase or decrease is often linked with a bacterial infection (although increases can occur with other types of inflammation and also stress). When the numbers are very low, there is usually an acute and severe problem going on, as numbers are being used up quicker than they can be produced. Very high numbers usually mean the horse is fighting a bacterial infection.
- Lymphocytes. These cells make antibodies. The numbers decrease in stress (the opposite to neutrophils) and numbers also initially decrease with a viral infection, then increase. Sometimes the ratio of neutrophils to lymphocytes can give us an idea if there is a chronic (long-term) viral infection going on, even if both are in the normal ranges.
- Monocytes. These take longer to increase and so raised levels are often associated with longer-term (chronic) inflammation.
- Eosinophils. Increased numbers of these is often (but not always) associated with either allergy or parasites. Unfortunately for us, they don’t always show an increase in the blood with these problems
- Basophils. These are rarely increased in horses.
The interpretation of the white blood cell results is not always black and white. Different changes in the numbers of the different types of white cells occur with stress, bacterial infection, viral infection, parasites, allergy, overtraining and more. The white cell numbers help us work out what is going on, but we have to use other information, including the clinical examination of the horse or pony, and the results of other blood tests to get an overall diagnosis.
Biochemistry
In a routine biochemistry screen we look at levels of proteins, muscle enzymes, electrolytes, liver enzymes and the kidney function.
Protein levels
The proteins in the blood are divided into three main groups: albumin, globulin and “acute phase proteins”. If the amount of albumin in the blood is high then it can often indicate dehydration, whereas if it is low we are concerned about something going on causing a loss of protein. Protein can be lost from damaged intestines eg. with severe diarrhoea, a large worm burden or can be lost in the urine when there are damaged kidneys. Occasionally albumin will be low as the liver isn’t making enough,, but this is less common.
The protein called globulin increases slowly when there is inflammation whereas the ‘acute phase proteins’ increase more rapidly with inflammation. Two of these proteins that are commonly tested for are fibrinogen and serum amyloid A (SAA). SAA increases and decreases more rapidly even than fibrinogen so it is very useful to track the course of the inflammation and see if things are improving or not.
Muscle enzymes
There are two main muscle enzymes we look at that increase with damage to the muscles. Often a big increase is because the horse has suffered from a recent episode of tying-up, but there are other causes. Your veterinarian will work out the cause if your horse’s blood test reveals high levels of these enzymes.
Electrolytes
The electrolytes include sodium, potassium, chloride, magnesium and phosphate. Most significant changes in the levels of these electrolytes are because of either a disease like severe diarrhoea or kidney disease, or alternatively if the horse has lost a lot of electrolytes through sweating.
Liver enzymes
If liver enzymes are increased, it tell us that there is some ongoing damage (usually either infection or inflammation) happening to the liver. More tests can be then performed to look at the function of the liver to see if it is still able to work properly. The liver is able to regenerate if it is not too severely damaged once the cause of the damage is removed or treated.
Kidney
Substances called urea and creatinine increase when there is kidney disease; however, they also can increase with dehydration, which is a lot more common. If a horse has high levels and is dehydrated, then the vet will test these again when the horse is hydrated to make sure everything has returned to normal values.
Conclusion
Blood tests can give us a lot of information; however, they don’t always give us the total answer. Sometimes they just point the vet in the right direction. In other cases, blood tests are vitally important for the successful treatment of a very sick horse.
In each individual case the vet has to look at the clinical signs the horse is showing and any other findings, as well as the results of the blood test, to reach a diagnosis and make a plan for treatment or management.
