Bloodwork Basics – Complete Blood Counts in Horse Health

Veterinarians use complete blood counts (CBCs) routinely to monitor horse health or understand the cause of a horse’s symptoms. Complete blood counts quantify blood cells and measure characteristics such as the size and hemoglobin content of red blood cells. These characteristics can indicate different conditions or health concerns, and the information CBCs provide alongside other symptoms and tests can help veterinarians diagnose, treat, and monitor horses' health.

Why run a CBC?

Complete blood count reports typically provide 19 measurements, giving veterinarians information about a horse's health status. Veterinarians often use CBCs to determine why a horse is sick and monitor their health over time. Complete blood count results show which blood cells are above or below normal levels, how old they are, and whether inflammation or illness may be present. It is important for a CBC to be interpreted by a veterinarian alongside a horse's medical history, symptoms, and other test results. A CBC does not definitively test for disease but provides veterinarians with more information about how a horse is responding to illness or injury.

What do different types of blood cells do?

Whole blood is made up of red blood cells, white blood cells, platelets, and plasma. Plasma is the liquid portion of the blood and carries proteins, nutrients, and other substances throughout the body.

Red blood cells are responsible for carrying oxygen from the lungs to different tissues in the body. Hemoglobin is the protein in red blood cells that binds to and carries oxygen. The content of hemoglobin in red blood cells plays a key role in their ability to transport oxygen efficiently throughout the body.

White blood cells are responsible for defending the body from infection and initiating the repair of damage. Some of these cell types are responsible for surrounding and removing harmful foreign organisms; some are responsible for removing damaged tissue, and others alert the rest of the body to damage and attract more white blood cells.

Platelets are carried in plasma and are cell fragments responsible for blood clotting. They also play a role in the inflammatory response and can be higher in response to inflammation. Platelet counts can be low due to conditions such as anaplasmosis.

What information does a CBC provide?

The table below lists the common measurements that a CBC provides, as well as a brief description of each measurement.

Why might CBC values shift outside of normal values?

One of the primary reasons veterinarians run CBCs is to determine what kind of illness or infection a horse has. Abnormal white blood cell counts and ratios often provide insight into this question. When a horse has inflammation or a bacterial infection, neutrophil and monocyte counts usually increase. Neutrophils fight infection while monocytes remove damaged cells and tissue. This often occurs at the cost of lymphocyte production, which may decrease as the body prioritizes neutrophils and monocytes. On the other hand, a viral infection often leads to increased lymphocyte counts, and the percentage of white blood cells made up of lymphocytes rises to equal or even exceed the percentage of neutrophils.

Exercise or stress can also affect CBC results due to contraction of the spleen, which releases more blood cells. This response can elevate red blood cell parameters because the spleen stores a large reserve of red blood cells. Dehydration can also increase red blood cell values because there is less liquid portion of blood (plasma) relative to red blood cells.

Horses may have low red blood cell counts, known as "anemia," due to blood loss, chronic inflammation, or nutritional deficiencies. Blood loss can contribute to low red blood cell counts even when it is not outwardly visible, such as from severe gastric ulcers. Chronic inflammation can also lead to anemia because proteins released in response to inflammation prevent the production of red blood cells. Additionally, shortages of nutrients required for hemoglobin and red blood cell production can negatively impact the body's production, limiting the number of cells circulating. Shortages of iron, copper, or B vitamins could be responsible for some cases of anemia.

Platelet values are most often lower than reference values due to clumping. Platelets are responsible for blood clotting, so they often clump together when blood samples are collected. A blood film viewed under a microscope can be used to see if low platelet counts are due to clumping or a true shortage of platelets.

It is important to consider a CBC as a snapshot of the blood at a single moment in time rather than a fixed measure of overall health. New blood cells are constantly being produced and released while others leave the bloodstream to address infection and damage. Repeated blood tests over time can provide better context for results and interpretation. Horses also have individual variation, so it's important to know what is normal for each horse. Repeated CBCs over time can help create an individualized expectation for what a "healthy" CBC looks like for each horse. Make sure to talk with your veterinarian if you have any concerns about your horse's CBC.

Take-Home Messages:

1. Complete blood counts provide veterinarians with information that can help them assess the nature of an immune response.
2. Complete blood count parameters may shift outside of normal ranges due to infection, inflammation, anemia, or nutritional deficiencies.
3. Cell populations are ever-changing, so repeated CBCs can provide important context.

The content of this publication, including text, graphics, and images, is educational only and not intended to be a substitute for veterinary medical advice, diagnosis, or treatment. Always seek the advice of a licensed doctor of veterinary medicine or other licensed or certified veterinary medical professional with any questions you may have regarding a veterinary medical condition or symptom.