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Dr McGregor's Equine Blood Analysis Series

The examination of readings from the White Cells of the blood give a large number of important indications of the standard health and fitness of the equine subject. It is important that the analysis is done by an experienced laboratory with a good overview of equine white cells.

Overall, the test gives two figures;

Firstly a total White Cell Count and then a percentage of the five types of White Cells that make up the total. These are the Neutrophils, Lymphocytes, Monocytes, Eosinophils, and Basophils. Within each group, there can be variations and these can be important so the quality of the laboratory recording is paramount to good results.

We will discuss each reading individually and then the ratios to make some generalised statements about a particular horse. Please remember not to place undue importance on any one figure; look at the overall picture before making judgement.

Total White Cell Count

Obviously, this is the total number of White Cells, usually expressed as the number of cells per nanolitre of blood, or per ml of blood. I usually use the number per ml of blood and will do so in this article.

The normal equine reading is between 6000 and 10 000, with the normal athletic reading being between 6000 and 8500. If the sample has clotted, even slightly, the count can be very low and this is an area to watch. Any clots mean that you must re-do the blood.

A number below 6000 means that large numbers of White Cells have left the bloodstream and entered the tissues of the body; or that there has been a destruction or under-production of White Cells. All of these things mean that there is excessive activity occurring and performance will be down. Common causes of low White Cell Counts in adult horses are viruses, acute inflammatory stimuli (eg. travel sickness), acute trauma such as bone fractures, reduced immune responses, some drugs, and prolonged chronic stresses (eg. growth, overwork, pain).

High White Cell Counts occur where the body has produced cells in response to bacterial infections, inflammatory processes of prolonged duration, cancers, allergies, parasites, excessive heat or cold, and where disease states have moved from being acute to chronic.

Cell Types and Functions

  • Neutrophils
    These are the shock troops of the body - responding quickly (in less than four hours) to a stimulus. They react to certain chemicals introduced into the body or released from the body's cells. Their aim is to swamp any invader and/or clean away and debris quickly. They usually total about 60% of the Total White Cell Count and, in absolute terms, number between 3500 and 6000.

    With certain stimuli such as severe infections, they may rise as high as 40 000. Neutrophil numbers in the blood drop during the early stages of viral infection and in acute stress situations such as transportation or severe inflammatory injury because they leave the bloodstream in large numbers. There is a type of cell called a Band Neutrophil - when these are seen in the bloodstream, this indicates that the horse is pulling up the young reserves from the bone marrow and the animal is under pressure.

  • Lymphocytes
    There are several types of these cells and since the work with human AIDS, the name T-Cell (one of the Lymphocytes) has become well-known.

    The Lymphocytes' function is to be the second line of defence. They carry immune system proteins and are involved in the production of antibodies. They are long-lived and are the major cells in the lymphatic system. The lymphatic system runs parallel to the bloodstream and is involved with fighting foreign proteins. Lymphocyte numbers are low in conditions of chronic stress and cortisone production (eg. growth, injury, travel, nervous disposition, teething and overwork). They are raised in cancers and some chronic infections, particularly viral ones such as Ross River Virus. Reactive lymphocytes also indicate a viral picture.

  • Eosinophils
    These are large red-staining cells which respond to allergic and irritation stimuli. They are raised in allergic reactions such as allergic bronchitis and irritated bowel syndromes. In Western Australia, they are often raised in sore throat conditions seen in horses during late Summer with the dust. Low readings really only occur at the very start of the problem when the cell goes into the tissue in large numbers and before new cells are made to replace it.

  • Monocytes
    Important cells which are the "clean-up" cells and are involved in disposing of the damaged tissue after the problem. They are a sign of repair and are often seen with chronic bleeders and horses with bowel or bowel blood vessel damage. Again, like the Eosinophils, the Monocyte numbers drop at the start of the stress response adn then often increase as it progresses.

  • Basophils
    Of limited value in our evaluation of equine blood pictures.


The ratio of Neutrophils to Lymphocytes has been used for years to assess stress states. If the percentage of Lymphocytes is greater than the Neutrophils, this is called a Reversal.

Normally, the Neutrophil/Lymphocyte ratio is 60:40. If it reverses (ie. 40:60), then it usually means that there is an acute stress (eg. virus, pain, tying up, travel). When the Neutrophil/Lymphocyte ratio goes from 60:40 to 70:30 or 80:20 then these are signs of chronic stress. Generally, if the ratio is greater than 2:1 (eg. 67:33) there is a chronic stress and the level is affecting performance. At 3:1 or 75:25, the horse is unwell and usually will not recover without time off or a spell. At 4:1, normally there is clinically a problem and at 5:1 we usually have colics and/or extreme pain. At 10:1, usually you do not have to do anything except get out of the way because the horse is about to fall on top of you!

Persistent chronic stress occurs when the Lymphocytes do not regenerate after a spell or ease-up and this will lead to an aborted preparation as the horse will not come up to anywhere near its ability.

I summary, I have mentioned the generalisations associated with White Series interpretations. Obviously we have not touched on all of the causes and effects but a very good summary of what causes White Cell changes is shown below...

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