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Has your dog been acting sluggish, extra thirsty, or breathing heavily? While these could be signs of many common ailments, they might also indicate something more serious: polycythemia. Polycythemia in dogs is when the body produces too many red blood cells, thickening the blood and potentially leading to serious health problems, such as blood clots, which we will discuss in this article.
Polycythemia is a condition where your dog’s body produces an excessive amount of red blood cells. These extra cells thicken the blood, making it flow less efficiently. This can lead to a range of health issues, as the thickened blood struggles to pass through small blood vessels, thereby delivering oxygen and nutrients less effectively and potentially forming clots.
While the normal red blood cell count for dogs can vary slightly depending on the breed and individual factors, a significant elevation above the normal range signals a potential problem. It’s important to remember that polycythemia itself is not a disease, but rather a symptom of an underlying issue.
Polycythemia in dogs isn’t a single, straightforward condition—there are actually a few different ways it can develop. Below, I have broken down the different types for review:
This type happens when the liquid part of the blood (plasma) decreases, making the existing red blood cells appear to increase. In other words, the actual number of red blood cells doesn’t increase, but the blood becomes more concentrated.
Common causes include dehydration from not drinking enough water or fluid loss, such as vomiting and diarrhea. Because the red blood cell increase is mild, most signs come from whatever is causing the dehydration rather than the raised red blood cell count itself.
Some dogs experience a temporary boost in red blood cells when their spleen contracts under stress, excitement, or seizures. The spleen stores red blood cells and can release them into circulation all at once. This usually doesn’t cause symptoms and is often short-lived.
Also called polycythemia vera or primary erythrocytosis, this somewhat rare disorder occurs when the bone marrow creates too many red blood cells independently of the hormone erythropoietin (EPO). A genetic mutation is known to play a role in many cases. Because the body ends up with a persistently high red blood cell count, the blood becomes thicker, potentially leading to fatigue, increased thirst, or other issues.
In secondary polycythemia, also known as secondary erythrocytosis, the body produces extra red blood cells because EPO levels are abnormally high. Sometimes this makes sense—like when there is low oxygen due to heart or lung issues.
Other times, it happens “inappropriately,” such as when tumors or kidney problems trigger excess EPO release. The main approach to managing secondary polycythemia is to find and address the underlying cause.
When polycythemia develops—no matter the cause—the blood becomes thicker and harder to push through tiny blood vessels. This slows blood flow to major organs (especially the brain), leading to low oxygen delivery and clumping of blood cells (sometimes called “sludging”).
Once the packed cell volume (PCV) reaches around 70%, the blood can be about twice as thick as normal. This increase in thickness may also set off the body’s clotting mechanisms, raising the risk of harmful clot formation.
Because polycythemia makes the blood thicker and harder to circulate, it can affect many different body systems. Dogs may appear weak, tired, or depressed, and some develop difficulty walking (ataxia) or experience seizures.
An increase in white blood cells can also affect circulation and lead to health complications.
You might notice changes in their behavior or mental state, such as confusion or circling. At times, the gums and other mucous membranes look unusually red or even bluish due to abnormal blood flow. Other common symptoms include:
Some dogs also drink and urinate more than usual (polydipsia and polyuria) or show signs that resemble heart or lung disease. If you notice any of these symptoms, please reach out to your veterinarian for an evaluation.
When I suspect polycythemia in a dog, I start with a thorough physical examination. I then run a blood test called the complete blood count (CBC) to measure the packed cell volume (PCV). If the PCV is higher than normal, I need to figure out whether the dog has a true increase in red blood cells (absolute polycythemia) or simply a decrease in the fluid part of the blood (relative polycythemia).
If the PCV is higher than 60%, this gets my attention and I start investigating further. Additionally, clinical signs of polycythemia in dogs will often occur when the PCV is greater than 60%.
I can check levels of the hormone erythropoietin (EPO) next. If EPO is elevated, it usually suggests that a tumor is present (absolute polycythemia). I may also recommend X-rays, ultrasounds, or more specialized tests to search for underlying issues. By combining these results, I can pinpoint the exact cause of your dog’s polycythemia and plan the right treatment.
When treating polycythemia, I always start by looking for the underlying cause. If there’s a way to address that root issue—like dehydration, a tumor, or heart/lung disease—that’s the first step.
Treatment also differs based on whether the polycythemia is “relative” (caused by low fluid in the blood) or “absolute” (caused by too many red blood cells). Treatment approaches aim to lower red blood cell numbers to enhance blood flow and improve overall health.
Myelosuppression (hydroxyurea): If repeated phlebotomy alone doesn’t keep red blood cell levels under control, or we can’t fix the underlying problem, I might use a medication called hydroxyurea. It slows down bone marrow production of red blood cells. Possible side effects include bone marrow suppression, which can lead to low platelet or white blood cell counts, hair thinning, and stomach upset. Another medication option is chlorambucil, which works similarly and can be adjusted to find the lowest effective dose.
Some dogs—especially those with primary absolute polycythemia—may need iron supplements if their iron levels run low. I also keep an eye on any other symptoms. For example, dogs with eye problems or clotting issues may need separate treatments to help manage those signs.
Overall, my goal is to keep the blood at a healthy thickness and prevent complications from happening.
When I talk to pet owners about prognosis, I explain that every dog’s outlook varies based on how severe the polycythemia is and whether I can treat the underlying cause. Some dogs stay symptom-free for quite a while, and many do well with ongoing management for years.
For dogs needing a medication like hydroxyurea to slow the production of red blood cells, reported survival times usually range from about 12 to 36 months. Of course, every dog is different, and regular monitoring and follow-up care play a big part in keeping them comfortable and healthy for as long as possible.
In conclusion, polycythemia in dogs means there are too many red blood cells, making the blood thicker and potentially stressing the body. It comes in several forms—relative, transient, primary absolute, and secondary absolute—each with a different reason behind the high red blood cell count. Identifying the right type usually starts with a blood test, followed by more specific checks to uncover any underlying issues. From there, treatment often focuses on addressing that root cause while managing symptoms through fluid therapy, blood removal (phlebotomy), or medications like hydroxyurea. With consistent care and monitoring, many dogs with polycythemia can enjoy a good quality of life over the long term.
