Low neutrophils in cats, called neutropenia, results from one of three basic problems: the bone marrow isn’t producing enough neutrophils, the body is using them up faster than it can replace them, or the immune system is destroying them. A healthy cat’s absolute neutrophil count falls between 2,300 and 11,600 per microliter of blood, based on Cornell University reference ranges. When the count drops below that floor, the cat becomes increasingly vulnerable to infections it would normally fight off easily.
The causes range from common viral infections to medication side effects to rare bone marrow cancers. Understanding which category your cat falls into matters because it shapes the treatment path and the outlook.
Viral Infections Are the Most Common Cause
Three viruses account for a large share of feline neutropenia: feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and feline panleukopenia (parvovirus). All three can both slow the production of new neutrophils in the bone marrow and increase how fast existing neutrophils are consumed in the body, creating a double hit.
Panleukopenia is especially aggressive. The virus directly targets rapidly dividing cells, including the precursor cells in the bone marrow that would normally become neutrophils. Research has shown that feline parvovirus actually blocks the effect of the body’s natural growth signals that stimulate bone marrow production, which is why the resulting neutropenia can be so severe and resistant to treatment. Kittens and unvaccinated cats are most at risk, and a plummeting white blood cell count is one of the hallmarks of this disease.
FeLV causes neutropenia through a different route. Certain strains of the virus suppress bone marrow function broadly, which can also lead to anemia alongside the low neutrophil count. FIV works more gradually, weakening the immune system over months or years, and neutropenia may develop as part of a slow decline in overall blood cell production.
Severe Bacterial Infections and Sepsis
This one seems counterintuitive: a bacterial infection should raise neutrophil counts, not lower them. And initially it does. The bone marrow ramps up production and pushes neutrophils into the bloodstream to fight the invader. But in overwhelming infections, the body burns through neutrophils faster than it can make them. The marrow eventually can’t keep pace, and the circulating count drops.
Cats have an unusually large pool of neutrophils that sit along the walls of blood vessels (a “marginated” pool) rather than freely circulating. During severe inflammation or sepsis, these marginated neutrophils get pulled into tissues to fight infection, which can make blood test results look even more dramatically low than they would in dogs or humans with the same level of infection. A very low neutrophil count during an active bacterial infection is a serious finding that signals the body’s reserves are depleted.
Medications and Toxins
Several drugs commonly prescribed to cats can suppress the bone marrow and cause neutropenia as a side effect. The most notable is methimazole, widely used to treat hyperthyroidism. In some cats, methimazole causes severe neutropenia (agranulocytosis), sometimes accompanied by anemia. This is a recognized side effect, which is why cats on methimazole need regular blood work monitoring, particularly in the first few months of treatment.
Chemotherapy drugs are the other major culprit. They work by targeting rapidly dividing cells, which means they hit the bone marrow’s neutrophil-producing cells as collateral damage. Vets expect neutrophil counts to dip after chemotherapy cycles and typically time treatments to allow counts to recover between doses.
Estrogen exposure is a less common but well-documented cause. Whether from estrogen-containing medications, accidental ingestion of human hormone therapy pills, or (rarely) estrogen-producing tumors, the hormone can directly suppress bone marrow stem cells. The pattern is distinctive: an initial rise in white blood cells followed by a crash into pancytopenia (low counts across all blood cell types) around 21 days after exposure. Mild cases may resolve within 30 to 40 days, but severe cases can progress to fatal aplastic anemia, where the marrow essentially shuts down. The response varies between individual cats, and repeated exposures carry more risk than a single one.
Bone Marrow Disease
When abnormal cells crowd out the normal blood-producing cells in the bone marrow, the result is called myelophthisis. In cats, the most common culprits are lymphoma, leukemia, mast cell tumors, and multiple myeloma. These cancers physically replace the healthy marrow tissue, leaving less and less space for normal neutrophil production. The neutropenia in these cases tends to show up alongside anemia and low platelet counts because all blood cell lines are affected.
Myelodysplastic syndrome (MDS) is a precancerous bone marrow condition where the marrow produces defective blood cells. It frequently progresses to acute myelogenous leukemia. Myelofibrosis, where scar tissue replaces functional marrow, is rare in cats but has the same end result: the marrow can no longer do its job. In many of these cases, the underlying cause may be linked to chronic FeLV infection, which ties back to the viral category.
Immune-Mediated Neutropenia
In this condition, the cat’s own immune system mistakenly tags neutrophils as foreign and destroys them. It can be primary (the immune system attacks neutrophils for no identifiable reason) or secondary (triggered by an underlying infection, drug reaction, or other disease).
Primary immune-mediated neutropenia is rare in cats. In one documented case, an 18-month-old Himalayan cat presented with recurring fevers, lethargy, and eye inflammation over several months. The neutropenia kept returning, and secondary bacterial infections developed as a consequence of the depleted immune defenses. Treatment requires immunosuppressive therapy to stop the immune system from destroying neutrophils, but this creates its own risks since a cat with already-low neutrophils becomes even more vulnerable to infections when the immune system is deliberately dampened.
Signs That Point to Low Neutrophils
Neutropenia itself doesn’t produce obvious symptoms. What you’ll notice instead are the consequences of a weakened immune defense: recurring fevers, persistent lethargy, decreased appetite, and infections that seem to come back or won’t resolve. These infections can show up anywhere. Mouth ulcers, skin wounds that won’t heal, eye infections, and respiratory infections are all common presentations.
Because neutrophils are the body’s first-response cells against bacteria and fungi, a cat with very low counts can develop serious infections from organisms that would normally be harmless. A fever of unknown origin in a cat, especially one that keeps returning, is one of the more reliable clues that something is wrong with the immune system.
How Neutropenia Is Diagnosed
The starting point is a complete blood count (CBC), which reveals the absolute neutrophil count and whether other cell lines are also low. A blood smear, where a drop of blood is examined under a microscope, adds important detail. It can reveal abnormal-looking cells that suggest leukemia or myelodysplasia, immature “band” neutrophils that indicate the marrow is being pushed to its limits, or parasites inside red blood cells.
If the CBC points to a production problem rather than a consumption problem, a bone marrow aspirate or biopsy is typically the next step. This involves taking a small sample from the marrow (usually from the hip bone under sedation) and examining it for cancerous cells, fibrosis, or evidence that the marrow has simply shut down. Testing for FeLV and FIV is standard in any cat with unexplained neutropenia, since these viruses are both common and treatable to some degree.
The diagnostic process also involves ruling out drug reactions by reviewing any current medications, and checking for signs of overwhelming infection that could explain high neutrophil consumption. In cases where no cause is found after thorough testing, immune-mediated destruction becomes the working diagnosis, sometimes confirmed by the cat’s response to immunosuppressive treatment.