Veterinarian checking dark red gums in cat — sign of polycythemia too many red blood cells

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Polycythemia (Too Many Red Blood Cells) in Cats: Signs, Causes, and Treatment

Jun 08 • 10 min read

Your cat's blood report shows a haematocrit of 68%. The reference range tops out at 45%. Your vet uses words like "polycythemia" and "hyperviscosity." You know the gums should be pink yours are a dark, brick red. And your cat has been having what look like small seizures.

This is polycythemia in cats. It is serious, it is treatable, and it is genuinely one of the more interesting blood disorders in veterinary medicine because unlike almost every other serious blood problem, this one isn't about having too little. It's about having too much.

Key Takeaways

Polycythemia (or erythrocytosis) means too many red blood cells. The blood becomes too thick for the heart to pump efficiently, and oxygen delivery to organs actually worsens despite the surplus of red blood cells.
There are two main types: primary (the bone marrow produces red blood cells abnormally on its own) and secondary (another disease is driving excess red blood cell production through elevated erythropoietin).
Secondary erythrocytosis is more common and is caused by conditions like chronic kidney disease, severe lung disease, heart defects, tumours that secrete erythropoietin, hyperthyroidism, and acromegaly.
A third type relative erythrocytosis is not a true blood disorder at all, but simply the result of dehydration concentrating the blood. This resolves with fluid treatment.
The Merck Veterinary Manual notes that clinical signs of absolute erythrocytosis are generally not observed until the haematocrit (PCV) exceeds 60%. Signs include brick-red gums, neurological changes, bleeding tendency, increased thirst and urination, and occasionally blindness.
Treatment involves drawing blood to reduce the red blood cell count (phlebotomy), replacing the volume with fluids, and giving drugs like hydroxyurea to suppress overproduction. For secondary erythrocytosis, treating the underlying disease is essential.

What Is Polycythemia in Cats?

Erythrocytosis — more commonly called polycythemia means an abnormally high number of red blood cells in the bloodstream. According to the Merck Veterinary Manual's Erythrocytosis (Polycythemia) in Cats, having too many red blood cells causes the blood to become too thick, making it hard for the heart to pump circulating blood and to deliver oxygen to cells throughout the body.

It is the flip side of anaemia and in some ways it is the more counterintuitive problem. People instinctively assume that more red blood cells means more oxygen. But beyond a certain point, the blood becomes so viscous think syrup instead of water that it moves sluggishly through blood vessels. Organs start receiving less oxygen, not more, because the thick blood can't circulate fast enough.

This thickening is called hyperviscosity, and it is the common thread behind most of polycythemia's symptoms: neurological signs, bleeding tendency, organ stress, and the distinctive dark-red gums that are often the first sign owners notice.

The Merck Veterinary Manual defines the condition under the Red Blood Cells of Cats framework simply: a lack of red blood cells is anaemia, too many is erythrocytosis. But the clinical consequences of the "too many" side are just as serious and often overlooked.

The Paradox: How Too Many Red Blood Cells Causes Oxygen Problems

Diagram comparing normal blood flow vs polycythemia hyperviscous blood in cats

Here is the clearest way to understand what hyperviscosity does to the body.

Imagine a water tank with a pump. The pump is the heart. The water is blood. Now imagine the water gradually thickening first to the consistency of juice, then to a milkshake.

The pump has to work much harder. The flow slows down. Parts of the system that should get regular water delivery start receiving less not because the tank is low on water, but because the thick fluid can't reach the far ends fast enough.

In the body, those "far ends" are the small blood vessels: in the brain, the retina, the kidneys, and the extremities. When hyperviscous blood slows through tiny capillaries, two things happen. First, cells in those areas get less oxygen than they need. Second, the slow, thick blood is more likely to form clots fibrin catches more easily in sluggish flow. And where clots form in small vessels, cells die.

The Merck Veterinary Manual's professional erythrocytosis section confirms: hyperviscosity slows blood flow and may increase the likelihood of thrombosis and rupture of small blood vessels. This is why polycythemia cats can present with neurological signs that look like stroke because the mechanism is essentially similar.

Relative Erythrocytosis: The False Alarm Caused by Dehydration

Before diagnosing a cat with true polycythemia, vets first rule out something much simpler.

Relative erythrocytosis is not an increase in the total number of red blood cells in the body. It is an apparent increase because the liquid part of blood (plasma) has decreased, concentrating the red blood cells that are already there.

The most common cause is dehydration from vomiting, diarrhoea, heat exposure, or simply not drinking enough. The professional Merck Veterinary Manual explains: relative erythrocytosis is an increase in red blood cell numbers without an increase in total red blood cell mass, usually caused by loss of plasma water with resultant haemoconcentration.

A second, mild, transient form can occur when fear or excitement causes the spleen to contract, releasing sequestered red blood cells into circulation. This is most obvious in horses but can occur to a mild degree in other species.

Both of these resolve easily rehydrate the cat, collect the next blood sample under calmer conditions, and the haematocrit normalises. No treatment for the blood itself is needed.

The practical implication: if your cat's blood report comes back with a high haematocrit but the cat is also clearly dehydrated or was extremely stressed during the blood draw, your vet may retest before investigating further. A single high reading in a dehydrated or frightened cat is not a polycythemia diagnosis.

Primary Erythrocytosis: When the Bone Marrow Goes Rogue

Primary erythrocytosis also called polycythemia vera is a bone marrow disease. The Merck Veterinary Manual describes it as a myeloproliferative disease resulting from the autonomous clonal expansion of haematopoietic progenitor cells. In plain language: a group of bone marrow cells starts multiplying independently, producing red blood cells at a rate that has nothing to do with what the body actually needs.

Crucially: erythropoietin (EPO) the hormone that normally controls red blood cell production is normal or low in primary erythrocytosis. The marrow has become independent of its usual regulatory signal. It is producing on its own schedule.

Primary erythrocytosis has been reported in cats, though it is rare. It tends to cause severe, sustained, progressive polycythemia with all the symptoms of hyperviscosity.

There is no upstream disease driving it the bone marrow itself is the problem.

Secondary Erythrocytosis: When Another Disease Is the Driver

Three types of erythrocytosis in cats — relative primary and secondary polycythemia classification

Secondary erythrocytosis is far more common than primary. Here, the bone marrow is working correctly it is responding to a signal from somewhere else in the body that is incorrectly elevated.

That signal is erythropoietin (EPO). The Merck Veterinary Manual explains the two paths this takes:

Appropriate secondary erythrocytosis — the body responding to real hypoxia

The tissues are genuinely oxygen-deprived. The kidneys detect this, produce more EPO, and the bone marrow ramps up red blood cell production in response. This is the body's correct emergency response trying to carry more oxygen with more red blood cells.

Causes of genuine tissue hypoxia include:

Severe lung disease when the lungs cannot oxygenate blood properly, the oxygen content of circulating blood drops. Chronic bronchitis, pleural effusion, or lung tumours can do this.

Heart defects with right-to-left shunting the Merck Veterinary Manual specifically names conditions where blood bypasses the lungs entirely (reversed patent ductus arteriosus, tetralogy of Fallot). Blood circulates without getting oxygenated. EPO rises in response.

In both of these cases, the erythrocytosis is physiologically appropriate the body is correctly trying to compensate for a real oxygen deficit. But if the underlying lung or heart condition cannot be fixed, the polycythemia eventually becomes as damaging as its cause.

Inappropriate secondary erythrocytosis — EPO produced without genuine hypoxia

EPO is elevated but not because tissues are truly oxygen-deprived. Instead, something is producing EPO abnormally.

Kidney disease certain kidney tumours (renal cell carcinoma) and non-cancerous kidney disorders can cause local hypoxia within the kidney tissue itself, triggering EPO production even when systemic oxygen levels are normal. This is one of the important overlaps between renal disease and polycythemia.

EPO-secreting tumours tumours of the kidneys or occasionally other organs can secrete EPO independently of oxygen status. The Merck Veterinary Manual names EPO-secreting tumours as a specific cause of inappropriate secondary erythrocytosis.

Endocrinopathy-associated erythrocytosis — hormones other than EPO

A third sub-type exists that the Merck Veterinary Manual names specifically for cats: endocrinopathy-associated erythrocytosis. This occurs when hormones other than EPO including thyroxine (thyroid hormone) and growth hormone directly stimulate erythropoiesis (red blood cell production) beyond normal levels.

The Merck Veterinary Manual states explicitly: cats with hyperthyroidism or acromegaly might have mild erythrocytosis without obvious symptoms.

This is mild compared to primary erythrocytosis or oxygen-driven secondary erythrocytosis. Hyperthyroidism-associated polycythemia rarely crosses the threshold where hyperviscosity symptoms appear. But it is important to recognise because treating the hyperthyroidism typically resolves the erythrocytosis.

The India Angle: Hyperthyroidism and CKD as Common Triggers

In Indian cats, two of the secondary erythrocytosis triggers deserve special attention.

Hyperthyroidism is one of the most common hormonal disorders in cats over 10 years old. An overactive thyroid produces excess thyroxine, which directly stimulates red blood cell production. Most Indian cat owners are familiar with hyperthyroidism as a weight-loss disease the wasting, the hyperactivity, the ravenous appetite. What is less known is that the elevated thyroid hormone also quietly drives up the haematocrit. In a hyperthyroid cat, a moderately elevated red blood cell count may be a simple side effect of the thyroid disorder, resolvable once the thyroid is brought under control.

Chronic kidney disease (CKD) is extremely common in cats over 7 years old. While CKD more often causes anaemia (by reducing EPO production), certain forms of kidney pathology tumours or localised hypoxia within kidney tissue can cause the opposite: inappropriate EPO secretion driving erythrocytosis. This is less common than CKD-related anaemia but is a documented and important phenomenon.

Understanding CKD fully including its trajectory, how it progresses, and what each stage means for your cat matters here. Our guide on why your cat's urinary issues keep coming back covers CKD in depth. For cats already managing CKD who develop a paradoxically high haematocrit on their routine blood work, this secondary polycythemia mechanism is worth discussing with your vet.

What Are the Symptoms of Polycythemia in Cats?

The Merck Veterinary Manual lists the following symptoms of erythrocytosis:

Very red (brick-red or dark-red) mucous membranes
Tendency to bleed
Increased thirst and urination
Seizures
Behavioural changes
Poor coordination (ataxia)
Weakness
Occasionally: blindness

The professional Merck Veterinary Manual section adds an important clinical threshold: clinical signs of absolute erythrocytosis are generally not observed until the PCV (haematocrit) exceeds 60%. Below that level, even genuinely elevated red blood cell counts may cause no visible symptoms.

This is why polycythemia is often an incidental finding on a blood test a vet checking routine bloods on a senior cat finds a HCT of 63% in a cat that the owner describes as "just a little slower than usual." The symptom threshold is high, but once crossed, signs can escalate quickly.

The gum sign

In anaemia, pale or white gums warn you that oxygen delivery is failing. In polycythemia, the opposite sign appears: gums that are too red dark brick-red, almost purple-tinged rather than the normal salmon-pink.

This happens because the blood is so loaded with red blood cells that the capillaries in the gums are visibly congested. The haemoglobin colour is saturating the tissue.

Press your finger on the gum briefly and release. In a healthy cat, pink returns in 1–2 seconds. In a polycythemic cat, the colour may actually darken temporarily when pressure is applied the vessel is already overfull.

Neurological signs

These are the most alarming and often what brings owners to the vet urgently. Seizures, confusion, walking in circles, sudden loss of coordination, or unexplained behavioural changes in an otherwise middle-aged or senior cat all of these can be polycythemia presenting through hyperviscosity-driven neurological compromise.

When the brain's small capillaries are sluggish with thick blood, neurons get less oxygen. When micro-clots form in cerebral vessels, the result is clinically indistinguishable from small strokes.

Bleeding tendency

Paradoxically, polycythemia can cause bleeding even though the blood has too many cells. The thick blood distends small blood vessels, which can rupture. Platelet function is also impaired in severe polycythemia the clotting mechanism becomes dysregulated in the hyperviscous environment. This may present as spontaneous bruising, nosebleeds, or prolonged bleeding from minor wounds.

Increased thirst and urination

The kidneys are directly affected by hyperviscous blood slowed renal blood flow alters kidney function and can increase urine output as the kidneys attempt to compensate. This polydipsia/polyuria pattern (increased drinking and urinating) mirrors what you see in CKD, diabetes, and hyperthyroidism one more reason polycythemia can be missed without a blood test.

The Retinal Sign Your Vet Will Look For

One of the most specific clinical findings in polycythemia and one that owners cannot check at home is visible on examination of the retina at the back of the eye.

The Merck Veterinary Manual's professional section notes: on retinal examination, dilated, tortuous (twisted) blood vessels may be visualised. These blood vessels look engorged and curving abnormally because the retinal capillaries are under pressure from hyperviscous blood that cannot move through them freely.

If your vet examines your cat's eyes with an ophthalmoscope and describes "dilated, tortuous retinal vessels," that finding is a strong positive indicator for polycythemia and the reason the Merck Veterinary Manual also lists occasional blindness as a symptom. The tortuous vessels can rupture or clot within the retina itself, causing acute vision loss.

How Is Polycythemia Diagnosed?

Infographic showing causes types symptoms diagnosis and treatment of polycythemia in cats

The Merck Veterinary Manual states that erythrocytosis is diagnosed via blood and urine tests and sometimes via X-rays or other imaging techniques.

Step 1 — Confirm it is true (absolute) erythrocytosis

The first step is ruling out relative erythrocytosis. Does the cat look dehydrated? Was the blood draw stressful? The vet will check for clinical dehydration signs and assess other blood parameters elevated total protein concentration and elevated urea (prerenal azotemia) support dehydration as the cause. If dehydration is found, treat with fluids and retest.

Step 2 — CBC and haematocrit

A complete blood count confirms the HCT, red blood cell count, and haemoglobin. It also checks white blood cells and platelets a true polycythemia vera often shows elevated red blood cells with normal or elevated white cells.

Step 3 — Urine analysis

Urinalysis helps assess kidney function and checks for EPO-secreting renal tumours affecting kidney structure.

Step 4 — Arterial blood gas

This is the key test for distinguishing appropriate from inappropriate secondary erythrocytosis. The Merck Veterinary Manual professional section specifies: arterial blood pO2 below 80 mmHg and pulse oximetry saturation below 90–95% are consistent with hypoxaemia meaning true tissue oxygen deficiency is present, and the erythrocytosis is an appropriate compensatory response. If oxygenation is normal despite the elevated red blood cell count, the erythrocytosis is inappropriate pointing to EPO-secreting tumour or primary disease.

Step 5 — Imaging

Chest X-rays assess for lung disease or heart abnormalities causing hypoxia. Abdominal ultrasound checks for kidney tumours or other EPO-secreting lesions. Cardiac ultrasound (echocardiography) evaluates for structural heart defects with shunting.

Step 6 — Serum EPO measurement

The Merck Veterinary Manual notes that serum EPO measurement could potentially differentiate primary from secondary causes primary erythrocytosis typically shows low or low-normal EPO (the marrow is working independently). Secondary shows elevated EPO. However, the Merck Veterinary Manual cautions: considerable overlap exists among healthy animals and animals with primary or secondary erythrocytosis. Furthermore, validated EPO assays for companion animals have limited availability in India this test is not yet standard practice at most Indian veterinary hospitals.

Step 7 — Thyroid function test

Given the frequency of hyperthyroidism-associated mild erythrocytosis in older cats, a T4 (thyroxine) measurement should be part of the workup for any senior cat with elevated haematocrit.

Why Bone Marrow Biopsy Is NOT the Answer

A common assumption is that the way to tell primary from secondary erythrocytosis is to biopsy the bone marrow and see if it looks abnormal.

The Merck Veterinary Manual's professional section gives a clear, definitive caution against this: routine examination of bone marrow is NOT useful to distinguish primary from secondary erythrocytosis, because both conditions show erythroid hyperplasia (increased red blood cell precursors in the marrow). The marrow is active in both it just doesn't tell you why.

As a result, primary erythrocytosis is diagnosed by eliminating secondary causes. It is a diagnosis of exclusion. If a thorough workup finds no lung disease, no cardiac shunting, no kidney tumour, no EPO-secreting mass, no hyperthyroidism, and no other identifiable driver and the EPO level is low then primary erythrocytosis (polycythemia vera) is the most likely conclusion.

This means the investigation process cannot be short-circuited. All secondary causes must be genuinely ruled out.

Treatment: Phlebotomy, Fluids, and Hydroxyurea

Treatment depends on the type and severity of erythrocytosis. The Merck Veterinary Manual describes the approach: remove excess red blood cells by drawing blood and replacing fluid, and give drugs such as hydroxyurea to decrease red blood cell production. For secondary erythrocytosis, treating the underlying disease is also essential.

Phlebotomy — the immediate intervention

Phlebotomy means drawing blood removing a measured volume from the cat to physically lower the red blood cell mass. The Merck Veterinary Manual's professional section specifies the dose: 10–20 mL per kg of body weight, with simultaneous replacement of the drawn volume with IV fluids.

The target is to bring the PCV down to approximately 50–60% not to normal, but to a level where blood viscosity is no longer causing acute clinical signs. This is done urgently in cats presenting with neurological symptoms or severe clinical signs.

A single phlebotomy session usually gives significant immediate relief. For primary erythrocytosis, phlebotomy needs to be repeated periodically as the overactive bone marrow continues producing. For secondary erythrocytosis while the underlying disease is being treated, phlebotomy provides symptomatic control until the driver is resolved.

Hydroxyurea — the long-term suppressor

Hydroxyurea is a drug that suppresses bone marrow activity reducing red blood cell production at its source. The Merck Veterinary Manual's professional section gives the dosing protocol for cats: 30 mg/kg once daily for 7–10 days (loading phase), then reduced to 15 mg/kg daily, titrated to the lowest effective dose and frequency.

Hydroxyurea is monitored carefully because it also suppresses white blood cell and platelet production. The Merck Veterinary Manual notes: RBC, WBC, and platelet counts should be monitored during hydroxyurea treatment. Regular CBCs (every 2–4 weeks initially, then monthly) are part of management on this drug.

Hydroxyurea is the cornerstone of long-term medical management for primary erythrocytosis in cats that cannot be managed with phlebotomy alone.

For relative erythrocytosis

Treat the dehydration with IV or subcutaneous fluids. No specific blood treatment is needed. The haematocrit will normalise once fluid balance is restored.

For secondary erythrocytosis — treat the cause

Hyperthyroidism methimazole (a thyroid-suppressing drug), radioactive iodine therapy, or surgical thyroid removal. Once thyroid function normalises, the erythrocytosis-driving thyroxine excess resolves.
Lung disease oxygen therapy, treat the underlying pulmonary condition, manage respiratory symptoms.
Heart defects surgical correction where possible; medical management otherwise.
Kidney/renal tumour surgical removal of the EPO-secreting tumour if operable; if inoperable, manage erythrocytosis symptomatically with phlebotomy.

Living With a Cat on Long-Term Polycythemia Management

Cats with primary erythrocytosis require ongoing management for life it is a condition to be controlled, not cured. Understanding what this long-term picture looks like matters for owners.

Routine blood monitoring

A cat on hydroxyurea needs a CBC every 2–4 weeks initially to monitor that bone marrow suppression is not going too far. Once stable, monthly CBCs are typical. Over time, dose adjustments will be needed.

Hydration matters

Keeping a polycythemic cat well-hydrated is important. Any episode of dehydration from illness, hot weather, reduced water intake concentrates the blood further. Indian summers are relevant here: a cat on hydroxyurea who stops eating and drinking during a hot day can slide from managed to symptomatic within hours. Always ensure fresh water is available. If your cat is also managing CKD alongside polycythemia which is possible since CKD can trigger secondary erythrocytosis a wet food diet helps maintain hydration.

For cats managing CKD as an underlying driver of secondary erythrocytosis, a renal diet that reduces phosphorus load and supports kidney function is part of the management plan. VETLIFE RENAL CAT DRY FOOD by Vet Life is a veterinary renal diet formulated specifically for cats with chronic or temporary renal insufficiency low phosphorus, high digestibility, complete nutrition. Used under veterinary prescription as part of a CKD management protocol.

For cats managing ongoing CKD with phosphate control as a priority elevated serum phosphorus accelerates kidney damage IPAKITINE POWDER by Vetoquinol is an oral phosphate binder that mixes into food. It is indicated from IRIS CKD Stage II onward, reducing intestinal phosphate absorption. When the underlying erythrocytosis trigger is CKD, slowing that CKD's progression with dietary phosphate management directly supports the whole polycythemia management picture.

Cardiovascular and general health support

Cats on long-term polycythemia management particularly those with concurrent hyperthyroidism or cardiac concerns benefit from maintaining overall nutritional completeness. Taurine sufficiency matters for heart muscle function; an overstressed heart managing hyperviscosity through repeated phlebotomy cycles needs its cardiovascular support intact. Our guide on the missing nutrient that can blind your cat covers taurine's role in cardiac and retinal health both particularly relevant when retinal vessels are already compromised by polycythemia.

TOP 10 CAT TABLET by Beaphar provides vitamins A, B-complex, D, and E, taurine, minerals, and trace elements in a palatable shrimp-flavoured tablet formulated specifically for cats. It supports general vitality, cardiac and ocular health maintenance, and nutritional completeness during the long management trajectory of a chronic condition. Dose: 1 tablet per 2.5 kg of body weight per day.

Managing fever

Fever is both a symptom to watch for (indicating secondary infection, particularly in immunosuppressed cats on hydroxyurea) and a potential sign of underlying disease activity. Our guide on how to prevent fever in your cat gives context for when fever in a cat on long-term medication warrants immediate veterinary attention.

FAQ

My cat's gums are dark red, not pale. Should I be worried?
Yes dark red or brick-red gums in a cat are just as alarming a sign as pale gums, and in the opposite direction. Pale gums mean anaemia; dark red gums can mean polycythemia. In a healthy cat, gums should be salmon-pink. Dark congested gums suggest the blood is too thick and the capillaries in the gum tissue are engorged. Combined with any neurological signs, increased thirst, or weakness, this warrants same-day veterinary attention and a CBC.

Can polycythemia kill a cat?
Yes, if untreated. The most dangerous acute risk is from hyperviscosity-driven thrombosis blood clots forming in small vessels of the brain, heart, or other organs. A clot in the brain presents as a stroke-like event. Chronic untreated polycythemia also causes progressive organ damage from sustained low oxygen delivery despite the red blood cell excess. Treatment is effective, but it requires diagnosis and ongoing management.

Is polycythemia the same as leukaemia?
No. They are both bone marrow disorders, but they involve different cell types. Leukaemia involves abnormal proliferation of white blood cells. Polycythemia vera involves abnormal autonomous overproduction of red blood cells. They are distinct conditions, though both are classed as myeloproliferative disorders.

My senior cat has hyperthyroidism. Could she also have polycythemia?
Possibly, mildly. The Merck Veterinary Manual specifically notes that cats with hyperthyroidism might have mild erythrocytosis without obvious symptoms. This endocrinopathy-associated erythrocytosis is typically not severe enough to cause hyperviscosity signs. However, on routine blood work, a mildly elevated haematocrit in a hyperthyroid cat may simply reflect the elevated thyroxine stimulating red blood cell production. Treating the hyperthyroidism usually resolves this. If the haematocrit is very high or neurological signs are present, investigate further.

What is the difference between primary and secondary erythrocytosis and does it change treatment?
Significantly. In primary erythrocytosis (polycythemia vera), the bone marrow is producing red blood cells abnormally on its own EPO is normal or low, there is no upstream driver. Treatment is phlebotomy plus hydroxyurea for life. In secondary erythrocytosis, another disease is the driver lung disease, heart defect, kidney tumour, hyperthyroidism. If that underlying disease can be treated, the erythrocytosis resolves. Phlebotomy manages symptoms while the primary cause is addressed. The entire investigation process aims to tell these two apart because treating secondary erythrocytosis with hydroxyurea alone without finding and treating the cause would be incomplete.

How often does a cat with polycythemia need blood tests?
During the initial workup and first weeks of treatment frequently, sometimes weekly. Once stabilised on hydroxyurea, monthly CBCs to monitor red blood cell, white blood cell, and platelet counts are standard. For secondary erythrocytosis managed through the underlying disease, the monitoring schedule depends on the underlying condition. Long-term monitoring is not optional it is what keeps the treatment safe and the cat stable.