Leukocytes, or white blood cells, include neutrophils, lymphocytes, monocytes, eosinophils, and basophils (see Blood Disorders of Dogs: White Blood Cells of Dogs). These cells vary with regard to where they are produced, how long they circulate in the bloodstream, and the factors that stimulate them into going in or out of the intricate network of blood vessels that branch out through the tissues of the body. The normal numbers of each type of white blood cell also vary between species. Leukocytosis is an increase in the total number of circulating white blood cells; leukopenia is a decrease. In addition to an overall increase or decrease in white blood cells, increases or decreases in each type of white blood cell can lead to—and help diagnose—disorders.

Leukograms are blood tests that count the number of different white blood cells circulating in the bloodstream. By counting the cells and examining their form your veterinarian gains valuable information that can help diagnose a wide variety of disorders.

Disorders Related to Increased or Decreased White Blood Cells

Neutrophilia is an increase in the number of neutrophils in the bloodstream and is caused by inflammation. Structural changes in neutrophils may occur during severe inflammation and are referred to as toxic changes. Neutropenia is a decrease in the number of neutrophils in the bloodstream. It may occur due to the white blood cells sticking to the walls of damaged blood vessels, destruction of neutrophils, or reduced formation in the bone marrow. Neutropenia may occur in all species during overwhelming bacterial infections. Adverse reactions to drugs may result in neutropenia or even pan-cytopenia (a reduction in red and white blood cells and platelets).

Eosinophilia is an increase in the number of eosinophils, which are involved in allergic reactions and in controlling parasites. Increases are caused by substances that promote allergic reactions (for example, histamine) and by certain antibodies. Eosinophils increase during infections with parasites such as heartworms or fleas. Eosinophilia also may occur with inflammation of the intestines, kidneys, lungs, or skin. A decrease in eosinophils is known as eosinopenia. It is a common reaction to stress or treatment with corticosteroids.

Basophils are rare in domestic animals. Basophils produce histamine and, like eosinophils, are involved in allergic reactions and combating parasites. Basophilia (an increase in basophils) is uncommon, but does occur in some dogs with heartworm disease.

Lymphocytosis is an increase in the number of lymphocytes in the bloodstream. It can be caused by certain hormones, stimulation of the immune system by infections, chronic diseases like arthritis, and leukemia, which is a cancer of the immune system. Lymphopenia is a decrease in the number of lymphocytes. It is most commonly caused by corticosteroids (either those naturally occurring in the body or given as treatment for a disease). Lymphopenia may also be caused by other conditions, such as decreased production of lymphocytes, some viral infections, and hereditary diseases.

Monocytosis is an increase in monocytes and may be associated with longterm inflammation.

Leukemia and Lymphoma

Leukemia is a malignant cancer that is characterized by an increase in abnormal white blood cells in the bloodstream. Lymphoma is a related cancer of certain white blood cells that begins in a lymph node or other lymphoid tissue (see Blood Disorders of Dogs: Canine Malignant Lymphoma). Leukemia should be considered a potential cause when there is an increase in the number of white blood cells in the bloodstream.

Gray Collie Syndrome

This syndrome, also called cyclic hematopoiesis, is an inherited deficiency of the immune system that affects gray (but not merle) Collies. Signs of the disease include an extensive decrease of neutrophils that occurs in 12‑day cycles, overwhelming reoccurring bacterial infections, bleeding, and a pale coat and nose color. The start of this disease is thought to be a defect in the maturation of the cells that form red and white blood cells and platelets in the bone marrow. Blood cell growth factors and other hormones also have a cyclic pattern.

Affected puppies often die at birth or during their first week. Most dogs with the disease die by 6 months of age. Surviving dogs may be stunted and weak and develop serious bacterial infections during the periods when neutrophil numbers are low. They also develop amyloidosis, an accumulation of an abnormal protein called amyloid (see Disorders Affecting Multiple Body Systems of Dogs: Amyloidosis in Dogs).

Your veterinarian can diagnose the disease based on the signs and results of blood tests. Treatment by bone marrow transplantation at an early age may be effective.

Pelger-Huët Anomaly

This inherited condition is caused by the failure of white blood cells to mature normally. White blood cell function is normal, and most animals do not have any signs of illness. A rare form of the disease also causes skeletal deformities and increased susceptibility to infection. This type of the disease is fatal. Pseudo-Pelger-Huët anomaly is an acquired maturation defect of white blood cells that may follow longterm infection, viral disease, drug therapy, and some tumors; it has been reported in dogs.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Timothy M. Fan, DVM, PhD, DACVIM; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM