What Are the Causes of Low White Blood Cell Count in Dogs?

A close-up of a veterinarian drawing blood.
A close-up of a veterinarian drawing blood. (Image: Jonathan Austin Daniels/iStock/Getty Images)

Your dog's blood is like a transportation system, ferrying oxygen, nutrients, waste and carbon dioxide throughout his body to be processed, used and removed. Blood is comprised of plasma -- the liquid portion -- as well as red and white blood cells -- which give blood its red color -- and platelets. When the vet orders a complete blood count, platelets and the different types of white and red blood cells are counted.

White Blood Cells as Defenders

Also called leukocytes, white blood cells are your pup's main defense against outside organisms, including fungi, viruses and bacteria. There are five types of white blood cells, each with its own defensive job. Eosinophils take on parasitic infections, while lymphocytes, neutrophils, monocytes and basophils battle infections or cellular invaders. A normal white blood cell count for a dog is between 6,000 to 17,000 per microliter (or ul). The quantity of each type of white blood cell informs the vet what's going on with your dog.

Types of White Blood Cells

The neutrophils in your dog's blood are formed in his bone marrow, usually numbering between 3,000 to 12,000/ul. High numbers of neutrophils indicate extreme stress or bacterial infection, while low numbers indicate viral infection. Eosinophils have a normal range of 0 to 1,900/ul; a high number of this type of white blood cell indicates parasitic infection or allergies, while a low number is considered normal since the range goes to zero. Formed in the lymphoid tissues, lymphocytes' normal range is 530 to 4,800/ul. Lower levels of these cells is referred to lymphopenia, and may indicate the early stage of infection or use of corticosteroids. Monocytes come from the spleen and bone marrow, with a normal range of 100 to 1,800/ul. Typically their numbers tend to stay steady unless cancerous leukemia impacts their volume. Basophils are the least common white blood cell, and often don't show up in the CBC.

Medical Causes of Low WBC

When your dog's body senses he needs help fighting off an invader, his bone marrow kicks in to help him out, producing more white blood cells. However, it can operate at a sustained level only for so long; a prolonged disease can exhaust the bone marrow's ability to keep up. Viral infections can cause the total WBC to fall. Coronavirus and infectious canine hepatitis are two conditions causing a moderate decline, while parvovirus will cause a much steeper decline in the WBC. Autoimmune disorders can cause your dog's body to destroy its own white blood cells and ehrlichia, caused by ticks, and some bone marrow tumors can cause a decline as well.

Medication Side Effects

Radiation therapy and other treatment protocols for cancer can destroy a dog's marrow cells, depleting his ability to produce white blood cells. Fever-reducing medication and certain antibiotics also can reduce the count, as can estrogen, sometimes used as a "mismating shot" for pregnancy termination.

Genetic Factors

Some dogs have a genetic predisposition leading to a lower white blood cell count. Cyclic hematopoiesis, a disease of gray collies, causes a decrease about every 11 to 14 days. Greyhounds tend to have a lower WBC, usually in the range between 3,500 to 6,500/ul. Giant schnauzers can have an inherited malabsorption of vitamin B12 leading to a lower white blood cell count.

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