Common Misdiagnoses: Is It Really FIP or Something Else?

Feline Infectious Peritonitis (FIP) is a progressive, often fatal disease caused by a mutated form of the feline coronavirus (FCoV). It primarily affects cats with weakened immune systems and is characterized by inflammatory responses in the abdomen, chest, eyes, and brain. FIP can present in two forms: wet (effusive) and dry (non-effusive), each with varying clinical signs.

Why Accurate Diagnosis Is Critical

Because FIP shares symptoms with many other diseases and is notoriously difficult to diagnose definitively without invasive procedures or post-mortem examination, misdiagnosis is common. An incorrect diagnosis may delay proper treatment, especially if the actual disease is treatable, or it may lead to unnecessary emotional distress and euthanasia decisions.

Overview of the Problem: Many Cats Misdiagnosed or Overlooked

Veterinarians often diagnose FIP based on symptoms like fever, weight loss, and abdominal fluid—yet these signs overlap with numerous other feline diseases. Some cats with lymphoma, FeLV, or bacterial peritonitis may initially be diagnosed with FIP, and vice versa. The lack of a single, definitive diagnostic test adds complexity to the process.

Challenges in Diagnosing FIP

• No single test confirms FIP with 100% certainty in living cats

• Symptoms vary widely, especially in the dry form

• Overlap with other diseases leads to confusion

• Diagnosis often relies on a combination of clinical signs, lab tests, imaging, and exclusion of other conditions
  

Conditions Commonly Misdiagnosed as FIP

Below is a list of diseases that mimic FIP and how to differentiate them.

A. Lymphoma

• Overlapping Symptoms: weight loss, poor appetite, lethargy, abdominal or chest fluid

• Diagnostic Clues: Biopsy or cytology can confirm neoplastic (cancerous) cells. Lymphoma may respond to steroids or chemotherapy, whereas FIP typically does not.
  

B. Toxoplasmosis

• Overlap with Neuro FIP: Causes seizures, ataxia, and other neurological signs

• How to Tell: Antibody testing (IgG and IgM), response to Clindamycin treatment, and radiographic imaging can help distinguish between the two.
  

C. Feline Leukemia Virus (FeLV)

• Similar Features: Lethargy, chronic infections, anemia, fever

• Diagnostic Tip: A simple ELISA or IFA test can confirm FeLV status
  

D. Feline Immunodeficiency Virus (FIV)

• Mimics Dry FIP: Chronic inflammation, immunosuppression

• Diagnosis: FIV antibody test; unlike FIP, FIV progresses more slowly and presents with recurring infections
  

E. Fungal Infections (e.g., Cryptococcosis)

• Confused with Dry or Ocular FIP: Neurological signs, eye inflammation

• Diagnosis: Blood or CSF culture, antigen testing, and fungal titers
  

F. Bacterial Peritonitis

• Resembles Wet FIP: Abdominal swelling, fever, fluid in the peritoneum

• Distinguishing Factor: Fluid analysis shows high neutrophils; bacteria are visible on stain or culture
  

G. Congestive Heart Failure (CHF)

• Common Overlap: Ascites (fluid in the abdomen), labored breathing

• Key Differentiator: Echocardiogram reveals heart disease; thoracic fluid in CHF is often non-inflammatory
  

H. Pancreatitis or GI Inflammation

• Mimics Dry FIP: Vomiting, diarrhea, weight loss

• Diagnostic Approach: Ultrasound, specific pancreatic enzyme tests (fPLI), and dietary response
  

Red Flags and Differential Diagnosis Tips

• Lack of fever despite other FIP-like symptoms

• Non-progressive symptoms (FIP often worsens rapidly)

• Clear, straw-colored abdominal fluid points more strongly to FIP

• A positive response to antibiotic or steroid treatment suggests an alternative diagnosis
  

How to Ensure Accurate Diagnosis

1. Request full blood panels, including A/G ratio, globulins, and hematocrit

2. Conduct imaging (ultrasound, X-rays, echocardiography)

3. Rule out infectious diseases (FeLV, FIV, toxoplasmosis, fungal infections)

4. Fluid analysis—cytology, protein content, and PCR for FCoV

5. Monitor clinical progression—FIP typically escalates quickly

6. Seek second opinions and specialist input when in doubt
   

When It Is FIP: What to Do Next

Once FIP is confidently diagnosed:

• Begin antiviral treatment such as EIDD-1931 (Molnupiravir) immediately

• Supportive care may include fluids, anti-inflammatories, and nutritional support

• Monitor symptoms daily and consult a veterinarian with experience in FIP treatment

🔗 MolnuFIP treatment guide

Conclusion

FIP is a complex and heartbreaking condition, but not every case that resembles FIP is actually FIP. Cat owners can avoid misdiagnosis and pursue the right care path for their feline companions by understanding the common differential diagnoses and advocating for thorough testing.

At MolnuFIP, we’re committed to providing science-backed education and access to safe, effective antiviral treatments. Your cat deserves clarity—and hope.

For more expert resources and access to Molnupiravir-based FIP treatments, visit MolnuFIP today!