Everything You Ever Wanted to Know About Animal Blood

October 04, 2013
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When I was in vet school, I loved learning about hematology, which is the study of blood. I was amazed to learn all the things you could tell about a sick animal just by looking at its red blood cells under the microscope. I was even more fascinated to learn there were considerable differences in red blood cells (also called erythrocytes) between species. I’d like to share some of this cool stuff with you today.

When I see a really high quality photograph of a red blood cell, I am always reminded of a cherry Life Saver candy. Round in shape, red blood cells are referred to as “bi-concave,” meaning they are thin in the middle and chubby around the outside. This thinness in the middle is called “central pallor” and is most prominent in canine blood cells. Although I said red blood cells are round in shape, that’s not true for llamas and alpacas — these species have oval red blood cells. Another interesting fact about mammalian red blood cells is they lack a nucleus. Birds and reptile red blood cells have a single dark round nucleus.

Red blood cell size relative to the animal also differs between species. Although the red blood cell diameter is measured in micrometers, so actual measurements mean practically nothing to me, relatively it’s interesting to note that of our domestic species, dogs have the largest red blood cells (7 micrometers in diameter), while a cow’s red blood cells are roughly 5.5 micrometers in diameter.

Anemia, or the decrease in red blood cells in the body, is a common disease encountered in veterinary medicine. This is because it has numerous causes, from the most obvious being overt blood loss from an injury to the more insidious causes such as intestinal parasitism or chronic kidney disease. In large animal medicine, I frequently see anemia (and sometimes very severe anemia) due to intestinal parasites, most commonly due to a nasty worm called Haemonchus contortus, AKA the barber pole worm. This guy hangs out in sheep and goats, burrows into the lining of the stomach, and literally sucks the animal’s blood. If not caught early, animals will sometimes die from barber pole infections. Sometimes I am required to do a blood transfusion.

So, how does one do a transfusion in an animal? Naturally, the rules are different depending on the species.

Just as humans have different blood types, so do animals. Some species, such as the cat, have very few blood types (for cats there are three: type A is the most common; type B; and type AB, which is very rare). Other species have many blood types, such as the horse, which has seven different types but also 32 different antigens, creating a very complex system.

For this reason, horses should always be cross-matched before receiving a blood transfusion. The chance of giving blood of a different type or with a different antigen is greatly increased in a horse versus a cat, and this sort of procedure is performed in well-equipped veterinary hospitals, not on the farm.

In contrast, sheep and goats have seven blood types, but lack the number of antigens that horses have. In emergency situations, such as extreme anemia from barber pole infection, I will perform an on-farm transfusion in a sheep or goat, grabbing the healthiest comrade of the same species and volunteer it as blood donor. Here I undergo a risk-benefit decision: Is the chance of a reaction worth the blood transfusion for a severely anemic animal? Often, the answer is yes when it comes to small ruminants.

Of course, the blood transfusion is only the first step in getting the goat or sheep back on his feet. Lots of nursing care from the owners is also required for the animal to make a comeback. I’d say in my cases, chances are usually 50/50.

On that note, I’d like to leave you with a little hematology humor: A red blood cell walked into a bar. The hostess asked if it would like a seat. It said, “No, thanks, I’ll just circulate.”  

See you next week!

Dr. Anna O’Brien

Image: Baronb / Shutterstock