Page 110 - Speedhorse April 2021

IN THE
BLOOD
A complete blood count (CBC) gives veterinarians a snapshot of what is happening during illness
by Denise Steffanus
An essential part of the clinical workup for a sick horse is the complete blood count, usu- ally referred to as a CBC. During the workup, the veterinarian is on a fact-finding mission. He or she will check the horse’s vital signs, palpate any areas of heat or tenderness, observe the appearance of the animal’s eyes and mucous membranes, and note any symptoms present, as well as abnormal behavior. The veterinarian will then draw blood for a CBC that will be used to support (or refute) the clinical findings.
“It’s like having a picture window into the blood stream at that moment in time, but that moment in time is right then when you put
the needle into the horse’s vein. Three or four hours later, things could change,” explained Dr. Fairfield Bain, associate director of Equine Life Cycle Management (Innovation and Discovery) for Merck Animal Health. Bain is triple board- certified in internal medicine, veterinary pathology, and emergency and critical care.
“It is very common in a hospital setting or even out on the farm, if you have an illness, to evaluate the CBC daily or every few days, depending on the type of disease process, to make a determination as to how the horse is doing,” he said.
When performing a CBC, either a laser flow cytometer electronically identifies and counts
White blood cells are the disease fighters in the body, but they also respond to heal injuries, cool inflammation, and react to allergens.
the number of different cells that comprise
the blood sample or, less commonly these days, a laboratory technician performs the task manually. Whole blood is made up of white cells, red cells, platelets, and plasma.
Each individual laboratory establishes its own range of what it believes to be normal for each parameter, based on a statistical calculation of the blood assays it has performed. So normal ranges tend to vary slightly from laboratory to laboratory, Bain said.
WHITE BLOOD CELLS
White blood cells are the disease fighters in the body, but they also respond to heal injuries, cool inflammation, and react to allergens. An important part of the CBC is a differential, which identifies the types of white blood cells present in the sample and how many there
are of each type. The most significant types of white blood cells in the horse are neutrophils, lymphocytes, and monocytes.
“Neutrophils are your most active white blood cells as far as fighting infection. These are the rapid-responding white blood cells that fight off inflammation and infection right away,” Bain said. “They eat up bacteria and release little packets of chemical mediators of inflammation that destroy invaders, as well as their own tissue, so they also may become self-destructive in some excessive states of inflammation.
“Lymphocytes are the immune responders. Certain types of lymphocytes produce antibodies; certain types are responsible for killing virus-infected cells or tumor cells.
“Monocytes are present in much smaller numbers than neutrophils or lymphocytes. Similar to lymphocytes, they participate in inflammatory and immune responses. By migrating into the tissues, they become a
cell called a macrophage that eats up foreign material and bacteria. They also release chemical mediators of inflammation.
“Neutrophils are the quick responders; monocytes are the slow responders. Neutrophils
rush in within seconds to minutes; monocytes come in hours to days later.”
Bain said that when a veterinarian is looking at a differential, he pays particular attention to the ratio of neutrophils to lymphocytes. In a healthy, normal horse, more than 50% of the white blood cells should be neutrophils and less than 50% should be lymphocytes.
Most horsemen have learned that a high white count means a severe infection, but the opposite may be true in some cases.
“In a very rapidly occurring inflammatory process, such as a developing diarrhea, all
those neutrophils that are normally in the circulation are leaving so fast and entering the area of inflammation that they will decline in the blood,” Bain said. “So, instead of a normal percentage of 55% to 60% neutrophils, they may drop rapidly to less than 30% or maybe almost zero, say in a foal that is severely septicemic. In those cases, as the bone marrow starts to respond, you may be able to see the immature neutrophils called bands start to appear. That implies such a severe inflammatory process is present that the bone marrow is trying to spill more immature cells into the blood [because] it can’t keep up with the body’s demand and it is kicking them out early.”
RED BLOOD CELLS
The primary function of red blood cells is to deliver oxygen to the body and carry away waste, such as lactic acid and carbon dioxide, that is produced when the body uses oxygen to convert nourishment to energy.
The heart pumps the blood through
the lungs where a portion of the red blood
cell called hemoglobin exchanges carbon dioxide for oxygen. When there is insufficient circulating hemoglobin because red blood cells have been destroyed, lost, or not produced
in adequate numbers, the horse becomes anemic. Electronically measuring the size
of the circulating red blood cells will help
the veterinarian determine what underlying
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