Page 111 - Speedhorse April 2021

“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.”
condition may be causing anemia. These range from a common iron deficiency to neonatal isoerythrolysis, a rare condition in foals similar to RH factor in humans that causes their red blood cells to be attacked by the antibodies in the mare’s colostrum.
When looking at the CBC, a veterinarian first will note the packed cell volume or hematocrit. This is determined by placing the blood in a tube and spinning it until the cells are completely packed. The percentage of the red blood cells is the hematocrit.
“As it is spun, the red cells settle to the
very bottom. The number of white blood
cells is so small relative to the number of red blood cells that they only form a thin layer,” Bain explained. “Red blood cells should be
35% to 45% of the volume; a small layer of white blood cells will form above it, and that’s called the buffy coat. Then the rest above is plasma.” A component of plasma is the protein fibrinogen, which also is an indicator of severe inflammation. Serum amyloid A (SAA) is another protein that responds to inflammation, with a shorter timeframe of response than fibrinogen. Both have their place when assessing timeframe and severity of inflammation.
Plasma is the fluid component of a blood sample, and if a horse is severely dehydrated, such as in the case of profuse diarrhea, the amount of blood plasma will decline and
this will skew the hematocrit number, since
it is a percentage of the whole blood. “That hematocrit is going to rise from about 50% to sometimes 75%,” Bain said.
“The horse is unique in that it has a large spleen with a large network of capillary spaces and a very muscular structure,” he continued. “The horse can hold as much as one-third of its blood volume in red blood cells in its spleen.
A normal phenomenon during exercise stress
is that the spleen contracts and squeezes out a good portion of those red blood cells. So, with stress and exertion, a horse can inject more red blood cells into the circulating blood and cause
a relative increase in the hematocrit, and the veterinarian has to take that into consideration when evaluating it. The horse could have a 50% to 60% hematocrit and still be a normal animal.”
Another component of a red blood cell is
the platelet, which rushes in to stick to the site where a blood vessel has been injured and form the initial framework for the clot that forms. In general, the platelet count in the horse should range from 100,000 to 450,000, but it can fall as low as 40,000 when platelets leave the circulating blood to attach to leaking blood vessels.
INTERPRETING THE CBC
“When interpreting the results of a CBC, the veterinarian must consider the overall picture, not just the individual variations in components,” Bain said.
“If we’re looking at an animal in the clinic, we want to know how the CBC matches up with the clinical process. If we are looking
at a horse with diarrhea, we want to know how severe the intestinal inflammation is.
So, the first thing we go to is the total white blood cell count. If we see a horse with severe
diarrhea and the white blood cell count is less than 2,000, then we know this animal has
very severe inflammatory process in its large intestine. On the other side, if we see a horse with severe diarrhea but the white blood cell count is not that low, that might give you some level of comfort to know that the inflammatory process is not that severe based right at that point in time.
“We’re trying to look at patterns—low versus high white blood cell count plus low versus normal total protein. So, for the horse that has just walked in the door that already has a really low white blood cell count and really low protein, we know it is going to be pretty sick and the hospital course is going to be long and potentially expensive. So, the CBC gives us some prognostic picture.”
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.
When interpreting the results of a CBC, the veterinarian must consider the overall picture, not just the individual variations in components.
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