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Diagnosing the Forms of Feline Infectious Peritonitis (FIP) and Symptoms: A Guide for Concerned Owners

Updated: Mar 25

FIP presents in several insidious forms, categorized as Neurological, Ocular, and 'classic' Wet/Dry FIP. Understanding these nuances and seeking the correct diagnostics are crucial, particularly in successfully combating this disease using antivirals like EIDD-2801 (molnupiravir).

1. Neurological FIP

  • The Challenge: When the mutated FCoV attacks the nervous system, symptoms can span behavioral changes, incoordination, tremors, seizures, and partial paralysis. These overlap with many neurological disorders in cats, posing significant diagnostic hurdles. Diagnostic Measures:

  • Bloodwork: While rarely definitive for FIP on its own, changes align with inflammation and potential nervous system dysfunction.

  • Cerebrospinal Fluid (CSF) Analysis: Tapping into the fluid surrounding the brain and spinal cord allows for detection of FCoV viral material, increased protein levels, and inflammation markers. This carries some risk under anesthesia, and sensitivity may vary.

  • Advanced Imaging (MRI/CT Scans): Essential to reveal brain/spinal cord lesions consistent with FIP, but limited by availability and cost. (Citation 2:,had%20abnormalities%20on%20MRI%20evaluation.) Symptoms of Neurological FIP Behavioral and Mental Shifts:

  • Lethargy, depression

  • Confusion or disorientation

  • Personality changes, aggression, unusual vocalization

  • Incontinence


  • Movement and Coordination Issues:

  • Ataxia (loss of balance, wobbly gait)

  • Tail flicks (sudden, jerky flicks upwards)

  • Weak or dragging hind legs

  • Tremors or twitching

  • Head tilt

  • Difficulty walking, circling

  • Paresis (partial paralysis) or paralysis, usually

  • Seizures

2. Ocular FIP

  • The Telltale Signs: FIP affecting the eyes leads to inflammation within their structures (uveitis), changes in appearance (color shifts, deposits on the cornea), light sensitivity, and potential vision impairment. It's worth noting that not all feline eye inflammation stems from FIP. Diagnostic Tools:

  • Specialized Ophthalmic Exam: Veterinarians use advanced instruments to observe eye changes typical of FIP.

  • Aqueous Humor Analysis: Collecting a tiny sample of eye fluid can undergo PCR testing for the FCoV, offering high specificity.

  • Other Blood & Effusion Tests: As seen in Neurological FIP, these provide supplementary clues in conjunction with eye-specific findings.

Symptoms of Ocular FIP

  • Cloudiness or haziness in the eye

  • Changes in eye color (iris)

  • Inflammation of the eye tissues (uveitis, iritis)

  • Bleeding within the eye (hyphema)

  • Deposits on the cornea (keratic precipitates)

  • Unequal pupil size (anisocoria)

  • Excessive tearing or discharge

  • Sensitivity to light

  • Blindness

3. 'Uncomplicated' FIP (Wet & Dry Forms)


Wet FIP-specific:

  • Abdominal distention (fluid buildup)

  • Difficulty breathing (due to fluid in chest)


Dry FIP-specific:

Organ-specific symptoms depending on what's affected:

  • Liver: Vomiting, diarrhea, jaundice

  • Kidneys: Increased thirst and urination, decreased urine production

  • Skin: Lumps, lesions

Always Remember: Without a viable test for FIP antemortem, testing for FIP is often a process of elimination by ruling out similar diseases and labwork. If your vet deems that there is a strong suspicion of FIP, it is best to begin treatment as a diagnostic tool. If your cat responds to treatment within 14 days, that is a positive FIP confirmation on its own. If no response, then your vet would need to explore other diagnoses.

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