The story of FIP begins with a far more common and usually harmless virus: feline coronavirus (FCoV). This virus belongs to the Coronaviridae family and is extremely widespread in cat populations globally. In most cats, FCoV exists as the feline enteric coronavirus (FeCV) biotype, primarily infecting intestinal cells.

The vast majority of cats infected with FeCV show no symptoms at all or experience only mild, temporary signs like diarrhea or minor upper respiratory issues that resolve on their own.

Once FIPV starts replicating in macrophages and spreading, the cat's immune system recognizes the infected cells and mounts an intense inflammatory response. However, in FIP, this immune response becomes dysregulated and excessive.

Instead of clearing the infection effectively, the interaction between virus-infected macrophages and the immune system triggers severe inflammation, particularly around small blood vessels (vasculitis). This damages blood vessel walls, causing them to leak fluid and proteins into body cavities and forming inflammatory lesions in various organs.

A critical point for cat owners to understand is that while the initial FeCV is highly contagious via feces, the mutated FIPV that causes FIP is generally not considered contagious between cats. FIPV is thought to arise independently within each affected cat due to the internal mutation event.

Research suggests FIPV is not effectively shed in feces or other secretions in a way that readily infects other cats. While some studies have detected FIPV in the feces of affected cats, making horizontal transmission theoretically possible, it appears to be an uncommon or inefficient route in natural settings.

While FeCV infection is common, the development of FIP is relatively rare. Only about 5-10% of FeCV-infected cats progress to clinical FIP, highlighting that factors beyond simple exposure are critical for FIP development.

Age is a significant risk factor, with the highest incidence occurring in cats between 3 months and 2-3 years old. Genetic predisposition also plays a role, with purebred cats appearing to be generally overrepresented compared to domestic mixed-breed cats.

Diagnosing FIP can be challenging because its symptoms are often non-specific and can mimic other feline diseases. In the early stages, regardless of the form FIP eventually takes, the signs are often subtle and general.

Owners might notice lethargy, loss of appetite, weight loss or failure to grow in kittens, fluctuating fever that doesn't respond to antibiotics, and poor hair coat or unkempt appearance. Sometimes mild, initial signs of FeCV (diarrhea, respiratory signs) may have occurred weeks, months, or even years prior.

Wet FIP is often considered the more common form, especially in younger cats, accounting for perhaps 60-80% of cases. It tends to progress more rapidly than the dry form. The hallmark is the buildup of protein-rich, often yellow, sticky fluid due to virus-induced inflammation and damage to blood vessels.

Fluid can accumulate in the abdominal cavity (ascites), causing a characteristic swollen appearance, or in the chest cavity (pleural effusion), making breathing difficult. Less commonly, fluid can accumulate around the heart. Cats with wet FIP also exhibit general signs of illness, and jaundice may occur due to liver involvement.

In dry FIP, significant fluid accumulation is absent. Instead, the disease manifests as inflammatory lesions, called granulomas or pyogranulomas, forming in various organs. Dry FIP often develops more gradually than wet FIP.

Symptoms depend entirely on which organs are affected and can be extremely varied. Common sites include kidneys, liver, lymph nodes, intestines, eyes, and the central nervous system. Each affected organ produces different clinical signs, making diagnosis challenging.

Because the eyes and central nervous system are common targets in dry FIP, these forms deserve special attention. Ocular and/or neurological signs are the main presenting complaint in a large percentage (up to 70%) of dry FIP cases. These forms often occur together due to the close anatomical relationship between the eye and brain.

The brain and eyes have protective barriers that normally limit the entry of pathogens, but FIPV-infected macrophages can cross these barriers, establishing infection in these "privileged" sites. This also makes treatment more challenging, as antiviral drugs must effectively penetrate these barriers.

Obtaining a definitive FIP diagnosis remains one of the biggest challenges for veterinarians. This is due to several factors: the clinical signs are often vague and overlap with many other diseases; there is no single, simple, 100% accurate test for FIP in a living cat; and the causative virus, FCoV, is widespread.

Because no single test is perfect, veterinarians diagnose FIP by assembling evidence "brick by brick." This involves considering multiple factors together to build a strong index of suspicion.

While no single blood test confirms FIP, certain patterns are common and increase suspicion. Key indicators often include lymphopenia (low lymphocyte count), neutrophilia (high neutrophil count), and mild to moderate non-regenerative anemia.

Protein changes are particularly important, with high total serum protein due to elevated globulins (hyperglobulinemia) being a classic finding. This results in a low Albumin:Globulin (A:G) ratio, which is highly suggestive of FIP when below 0.8, and especially below 0.6 or 0.4.

For decades, FIP was considered untreatable. Supportive care could provide temporary comfort, but the disease inevitably progressed. The development of specific antiviral drugs has revolutionized this outlook, offering a high chance of cure.

The game-changers are primarily two closely related nucleoside analogs: GS-441524 and its prodrug, Remdesivir (GS-5734). Both were developed by Gilead Sciences initially for human viral diseases, and research led by Dr. Niels Pedersen at UC Davis demonstrated their potent activity against FIPV.

Treating FIP with antivirals requires adherence to specific protocols developed through research and clinical experience. It is absolutely essential to work with a veterinarian to determine the correct protocol for an individual cat.

The standard recommended treatment duration is a minimum of 12 weeks (84 days) of continuous daily therapy. This duration was established based on early trials showing relapses with shorter courses and the time needed for clinical and biochemical parameters to normalize.

Different forms of FIP require different dosages of medication. Cats with neurological or ocular FIP require higher doses to ensure adequate drug penetration into these protected sites. These forms may also have a higher risk of relapse.

Oral bioavailability is lower than injectable, requiring higher mg/kg doses. Some oral products may be labeled with SC equivalent doses but contain higher actual amounts. Twice daily oral dosing may be beneficial to maintain consistent blood levels.

FIP medications can be administered through injectable (subcutaneous) or oral routes, each with specific considerations. Working closely with your veterinarian is essential for proper administration and monitoring.