What’s the Difference Between FIP and Lymphoma in Cats?

Veterinary professionals often face challenges when diagnosing cats with vague but serious symptoms. Feline Infectious Peritonitis (FIP) and feline lymphoma are two conditions that can look strikingly similar in clinical presentation. This overlap often leads to confusion and even misdiagnosis of cat lymphoma, where cats are incorrectly assumed to have one disease over the other.

At CureFIP, we specialise in helping cat owners and vets worldwide, including in the UK, accurately identify and treat FIP. Knowing the difference between FIP and lymphoma in cats is crucial for ensuring the right treatment and giving your cat the best chance at survival.

Understanding FIP (Feline Infectious Peritonitis)

Causes

FIP is caused by a feline coronavirus (FCoV) mutation, a common virus especially prevalent in multi-cat households or shelters. In rare cases, the virus mutates within the cat’s body, leading to an aggressive and often fatal immune response.

Common Symptoms

• Persistent fever that doesn’t respond to antibiotics

• Loss of appetite and significant weight loss

• Lethargy

• Accumulation of fluid in the abdomen or chest (wet FIP)

• Neurological or ocular symptoms (dry FIP)

These signs are non-specific and often mistaken for cancer or other chronic diseases.

Understanding Lymphoma in Cats

What Is Feline Lymphoma?

Feline lymphoma is a type of cancer affecting the lymphatic system. It can occur in cats of any age but is more common in older cats or those positive for FeLV (feline leukaemia virus).

Symptoms and Forms

Lymphoma in cats is classified by its location:

• Intestinal lymphoma: vomiting, diarrhoea, weight loss

• Mediastinal lymphoma: difficulty breathing due to fluid or a mass in the chest

• Renal lymphoma: kidney enlargement, increased thirst and urination

• Multicentric lymphoma: affects multiple lymph nodes or organs

• General signs can include fever, lethargy, and inappetence—very similar to FIP.
  

Key Differences Between FIP and Lymphoma

Bloodwork

• FIP often presents with:

  • Non-regenerative anaemia

  • High total protein with low albumin-to-globulin (A:G) ratio

  • Elevated white blood cells and bilirubin
    

• Lymphoma may show:

  • Mild anaemia

  • Elevated calcium levels (less common)

  • Changes that are less definitive compared to FIP
    

Ultrasound and Imaging

• FIP may show fluid accumulation with fibrin strands, enlarged mesenteric lymph nodes, or granulomas.

• Lymphoma is more likely to show well-defined masses or diffuse thickening of intestinal walls.
  

Response to Steroids

• Lymphoma often improves temporarily with corticosteroids such as prednisolone.

• FIP rarely shows a positive steroid response, and symptoms usually progress.
  

Diagnostic Tools and Tests

When to Request Further Testing

If your cat is not improving or shows overlapping symptoms, it’s critical to pursue additional diagnostics.

Role of Biopsy, PCR, and Fluid Analysis

• Fluid analysis: FIP-related fluid is yellow, sticky, and high in protein; lymphoma-related fluid may contain malignant cells.

• PCR (Polymerase Chain Reaction): Confirms the presence of mutated FCoV in fluid or tissue samples (highly specific for FIP).

• Biopsy: Especially helpful in suspected lymphoma cases; histopathology can confirm cancerous cells.

Advanced diagnostics are available at veterinary referral centres or through collaboration with global resources like CureFIP.

When to Suspect One Over the Other

Suspect FIP if:

• The cat is young (<2 years)

• There's significant abdominal or thoracic fluid

• Bloodwork shows a low A:G ratio

• No improvement with antibiotics or steroids
  

Suspect Lymphoma if:

• There's a visible mass on imaging

• The cat is middle-aged to senior

• There's some response to steroids

• The cat is FeLV-positive
  

Conclusion: Getting an Accurate Diagnosis Matters

Distinguishing between feline lymphoma vs FIP can mean the difference between effective treatment and missed opportunities. While lymphoma may respond to chemotherapy, FIP now has effective antiviral treatment through GS-441524, which is offered globally by CureFIP.

If your cat has been diagnosed with FIP or lymphoma and you’re unsure of the diagnosis, don’t hesitate to seek a second opinion. A growing number of cases reported to CureFIP involve misdiagnosed cat lymphoma later confirmed as FIP, or vice versa.

Visit CureFIP to learn more about FIP symptoms, diagnostics, and treatment options. If you're based in the UK, our support team is ready to help you find the right diagnostic path and offer access to life-saving antiviral therapies.