Lymphoma in Cats: Understanding Feline Lymphoma

Lymphoma is the most common cancer diagnosed in cats, accounting for approximately 30% of all feline tumours and roughly 90% of haematopoietic (blood cell) cancers. It arises from lymphocytes — white blood cells that are essential to the immune system — and can develop in virtually any organ in the body, which is why its presentation varies so widely.

"Lymphoma in cats is not a single disease — it is a spectrum. A twelve-year-old cat with low-grade intestinal lymphoma and a three-year-old FeLV-positive cat with mediastinal lymphoma have very different diseases with very different prognoses. Accurate diagnosis and classification are essential because they determine the treatment approach and expected outcome." — Dr. Sarah Chen, DVM

The landscape of feline lymphoma has shifted significantly over the past three decades. In the 1980s and 1990s, when FeLV was far more prevalent, mediastinal and multicentric lymphoma in young cats predominated. Thanks to widespread FeLV testing and vaccination programmes, FeLV-associated lymphoma has declined dramatically. Today, gastrointestinal (GI) lymphoma in older, FeLV-negative cats is the most common form — a shift that has also brought changes in treatment approaches and outcomes.

The encouraging news is that some forms of feline lymphoma — particularly low-grade GI lymphoma — respond remarkably well to treatment, with many cats enjoying good quality of life for years after diagnosis. Understanding the type of lymphoma your cat has is crucial to understanding what lies ahead.

Feline lymphoma is classified by its anatomical location, and each type has distinct characteristics, associations, and prognoses. The type your cat is diagnosed with significantly influences treatment decisions and expected outcomes.

Gastrointestinal (Alimentary) Lymphoma

GI lymphoma is now the most common form of feline lymphoma, accounting for 50–70% of all cases. It primarily affects the small intestine, though the stomach and colon can also be involved. Mesenteric lymph nodes (the lymph glands near the intestines) are frequently affected as well. GI lymphoma is most common in older cats, typically over ten years of age. A crucial distinction exists between small-cell (low-grade) and large-cell (high-grade) GI lymphoma, which dramatically affects prognosis and treatment.

Mediastinal Lymphoma

Mediastinal lymphoma affects the thymus and mediastinal lymph nodes in the chest. It is strongly associated with FeLV infection and tends to occur in younger cats — often between two and five years of age. The growing mass compresses the lungs and can cause fluid to accumulate around them (pleural effusion), leading to severe breathing difficulty. This form has historically carried a poorer prognosis, though FeLV-negative cats may respond better to treatment.

Renal Lymphoma

Renal lymphoma involves one or both kidneys and accounts for approximately 5% of feline lymphoma cases. It often presents with signs of kidney failure — increased thirst and urination, weight loss, and decreased appetite. Both kidneys are affected in the majority of cases. Renal lymphoma is frequently associated with central nervous system (CNS) involvement, which worsens the prognosis. Prophylactic treatment of the CNS is sometimes recommended even when no neurological signs are present.

Nasal Lymphoma

Nasal lymphoma accounts for roughly 3–5% of feline lymphoma and is typically a localised disease. Affected cats present with chronic nasal discharge (often from one nostril initially), sneezing, facial swelling, and sometimes noisy breathing. Despite its alarming appearance, nasal lymphoma is one of the more treatable forms — radiation therapy can achieve long remission periods, with median survival times of one to two years.

The relationship between retroviral infection and lymphoma is one of the most important aspects of feline oncology. Understanding this connection informs both prevention strategies and treatment expectations.

FeLV and Lymphoma

Feline leukaemia virus remains the single greatest risk factor for lymphoma in cats. FeLV-positive cats are approximately six times more likely to develop lymphoma than their FeLV-negative counterparts. The virus acts as a direct oncogenic (cancer-causing) agent by inserting its genetic material near genes that control cell growth. This can activate oncogenes or disrupt tumour suppressor genes, leading to uncontrolled lymphocyte proliferation.

FeLV-associated lymphoma typically affects younger cats (under five years old) and is most commonly mediastinal or multicentric in form. These tend to be high-grade (aggressive) lymphomas that require intensive chemotherapy. The decline in FeLV prevalence due to testing and vaccination has been one of the greatest success stories in feline medicine — it has directly contributed to the overall shift toward GI lymphoma in older cats as the predominant form.

FIV and Lymphoma

Feline immunodeficiency virus also increases lymphoma risk, though by a smaller margin — FIV-positive cats have approximately two to five times the lymphoma risk of FIV-negative cats. Unlike FeLV, FIV does not appear to cause cancer directly. Instead, it weakens the immune system's ability to detect and destroy abnormal cells. FIV-associated lymphoma tends to be more varied in location and can occur at any age.

Prevention Through Vaccination and Testing

FeLV vaccination is highly effective and is recommended for all kittens as part of their primary vaccination course. Testing all new cats before they enter a household prevents transmission to existing pets. Keeping cats indoors eliminates the primary route of FeLV and FIV exposure — contact with infected cats. For cats already positive for either virus, regular veterinary monitoring (every six months) allows for early detection of any emerging health issues, including cancer.

The symptoms of feline lymphoma depend heavily on which form of the disease is present. Because cats are exceptional at masking illness, symptoms may be subtle initially and easily attributed to other conditions or simply to ageing.

GI Lymphoma Symptoms

• Chronic vomiting — intermittent vomiting that gradually increases in frequency over weeks to months is the most common presenting sign. Owners often dismiss early vomiting as hairballs or dietary indiscretion
• Diarrhoea — may be intermittent or persistent, and can vary from soft stools to watery diarrhoea. If you're unsure when to worry about vomiting and diarrhoea, our guide covers the key warning signs
• Weight loss — often significant and progressive despite a maintained or only slightly decreased appetite. This is one of the most reliable indicators of GI disease in cats
• Decreased appetite — may range from mild pickiness to complete food refusal
• Thickened intestinal loops — your vet may feel thickened bowel loops during abdominal palpation

Mediastinal Lymphoma Symptoms

• Breathing difficulty — progressive respiratory distress is the hallmark sign, caused by the mass compressing the lungs or fluid accumulating around them
• Open-mouth breathing — cats should never breathe with their mouths open at rest; this is always an emergency
• Coughing — less common in cats than dogs but can occur with chest masses
• Regurgitation — the mass may compress the oesophagus, causing difficulty swallowing

Other Forms

Renal lymphoma causes increased thirst, increased urination, and signs of kidney-related discomfort. Nasal lymphoma presents with persistent nasal discharge (often bloody and from one nostril), sneezing, and facial deformity. Cutaneous lymphoma causes skin nodules, plaques, or ulcers. Regardless of the form, any cat showing persistent weight loss, appetite changes, or behavioural withdrawal should be examined promptly.

Accurate diagnosis is critical in feline lymphoma because the type and grade directly determine the treatment approach and prognosis. The diagnostic process typically involves several complementary tests.

Blood Work and Viral Testing

Initial assessment includes a complete blood count, biochemistry panel, and FeLV/FIV testing. Blood work may reveal anaemia, elevated liver or kidney values, or abnormal white blood cell counts. In some cases, malignant lymphocytes may be seen in the blood (leukaemic phase). FeLV and FIV status is essential for prognosis and treatment planning.

Abdominal Ultrasound

Ultrasound is arguably the most valuable single diagnostic tool for feline GI lymphoma. It can reveal thickened intestinal walls (the normal feline small intestinal wall is approximately 2.5 millimetres; lymphoma often causes diffuse thickening to 4 millimetres or more), enlarged mesenteric lymph nodes, and involvement of other abdominal organs. However, ultrasound cannot definitively distinguish lymphoma from inflammatory bowel disease (IBD), which can appear identical — tissue sampling is essential.

Biopsy

Tissue biopsy is the gold standard for diagnosing feline lymphoma and distinguishing it from IBD. Endoscopic biopsies are less invasive and can sample the stomach and upper small intestine, but they only obtain superficial tissue. Full-thickness surgical biopsies provide deeper tissue and are often necessary for a definitive diagnosis, particularly for small-cell lymphoma where the malignant cells may infiltrate the deeper layers of the intestinal wall.

Immunohistochemistry

This laboratory technique identifies specific proteins on the surface of cancer cells, determining whether the lymphoma is B-cell or T-cell in origin. This distinction is critically important in cats. Most GI lymphoma in cats is T-cell, while most mediastinal lymphoma is also T-cell (and FeLV-associated). B-cell lymphoma is less common but may be found in certain extranodal locations. The immunophenotype influences treatment selection and carries prognostic significance.

In some cases, additional molecular testing (such as PCR for antigen receptor rearrangement) may be needed to distinguish between lymphoma and severe IBD when histopathology alone is inconclusive.

The most important prognostic distinction in feline lymphoma — particularly GI lymphoma — is whether the disease is small-cell (low-grade) or large-cell (high-grade). This classification fundamentally shapes treatment approach, expected side effects, and survival times.

Small-Cell (Low-Grade) Lymphoma

Small-cell lymphoma is the more common form of feline GI lymphoma and carries a significantly better prognosis. Treatment typically involves oral medications given at home — chlorambucil (a mild chemotherapy agent) combined with prednisolone. This protocol is well-tolerated, with the vast majority of cats experiencing minimal side effects. Response rates are excellent, with 85–96% of cats achieving remission. Median survival times range from two to three years, and some cats live considerably longer. Many owners find it difficult to believe their cat has cancer because the treatment is so gentle and their cat feels so well.

Large-Cell (High-Grade) Lymphoma

Large-cell lymphoma is more aggressive and requires more intensive treatment. Common chemotherapy protocols include COP (cyclophosphamide, vincristine, prednisolone) and the Wisconsin-Madison protocol (a CHOP-based protocol adapted for cats). Treatment involves regular veterinary visits for intravenous chemotherapy, typically weekly during the induction phase. Response rates are approximately 50–70%, with median survival times of six to nine months for cats that achieve remission. Cats that do not respond to initial treatment have a poorer outlook.

Other Forms

Nasal lymphoma responds well to radiation therapy, with median survival times of one to two years in many studies. Mediastinal lymphoma response depends heavily on FeLV status — FeLV-negative cats generally fare better. Renal lymphoma carries a more guarded prognosis, particularly when the central nervous system is involved.

Quality of Life and Monitoring

Throughout treatment, quality of life is the guiding principle. Regular monitoring — including physical examinations, blood work, and periodic ultrasound — tracks response and detects relapse early. Most cats on small-cell protocols need check-ups every one to three months, while those on intensive protocols are seen more frequently. If your cat's lymphoma relapses after initial treatment, rescue protocols are available, though response rates are generally lower with subsequent treatments.

Working closely with your veterinary team ensures that treatment decisions always prioritise your cat's comfort and wellbeing. No one knows your cat better than you, and your observations about appetite, energy, and behaviour at home are invaluable in guiding treatment.