Determining Bacterial or Viral Infection By A Horse's White Blood Cell Count

The white blood cells (WBC), unlike the RBCs, do not permanently reside within the bloodstream but travel into the body tissues to perform their functions. There they fight infections and are involved in immunologic responses.

The WBC count is expressed as the number of cells per cubic mm of blood. The average normal value in hot-blooded breeds is about 9,000 cells per cubic mm, with a range of 5,000 to 14,000. In cold-blooded horses, the average is about 8,500 and the range is 6,000 to 12,000.

Changes in WBC counts are used to diagnose bacterial and viral infections. During the first day of a bacterial infection, transient leukopenia (decreased number of cells) may be detected, since circulating WBCs leave the blood to attack the bacterial invaders at the infection site. The WBC count quickly increases, however, as WBC production accelerates later in the infection process. A moderately severe bacterial infection may cause an increase in the WBC to 15,000 to 17,000 cells per cubic mm of blood. A leukocyte count of 35,000 or more may accompany a severe bacterial infection. An increased WBC is commonly referred to as leukocytosis. Viral infections are usually accompanied by leukopenia.