Feline Infectious Peritonitis (FIP): The Mystery Illness of Cats

Feline Infectious Peritonitis, commonly known as FIP, it is one of the most feared diseases among veterinarians and pet parents. Caused by a mutation of the feline coronavirus (FCoV), it is an enveloped, positive-sense, single-stranded, non-segmented RNA virus. FIP is a rare but often fatal condition that affects cats worldwide. FIP was first recognized as a distinct disease in 1963 by Dr. Jean Holzworth and her team at Angell Memorial Animal Hospital in Boston, USA. The reasons behind the mutation and why some cats develop the disease while others do not are still not fully understood. FCoV is transmitted via the faecal-oral route.

Most cats will encounter FCoV at some point in their lives—especially in multi-cat environments like shelters or catteries. In its usual form, this virus causes only mild gastrointestinal symptoms, or none at all. However, in a small percentage of cats, the virus mutates inside the body, triggering a severe immune response that leads to FIP.

Two Faces of the Disease

FIP appears in two main forms: wet (effusive) and dry (non-effusive).

• Wet FIP causes fluid buildup in the chest or abdomen, resulting in symptoms such as difficulty breathing or a swollen abdomen.

• Dry FIP affects organs like the kidneys, liver, eyes, and brain, causing a variety of signs—weight loss, fever, neurological problems, or vision changes.

In some cases, cats may show symptoms of both forms, leading to what is sometimes called a “mixed” presentation. This “mixed” form of FIP is described as a transitional state from one of the effusive or non-effusive types to the other, in which symptoms of both types are observed at the same time.

Why Is FIP So Hard to Diagnose?

One of the biggest challenges with FIP is that no single test can confirm the disease in all cases. Its symptoms often mimic other illnesses, and diagnosis typically relies on a combination of clinical signs, bloodwork, imaging, and, in some cases, fluid analysis or biopsies. This uncertainty often delays treatment and leads to devastating outcomes, as the disease progresses rapidly once symptoms appear.

A Ray of Hope

For many years, FIP was considered untreatable, and a diagnosis was essentially a death sentence. However, new antiviral therapies, such as GS-441524, have shown remarkable success in clinical trials and compassionate use cases. Novel antiviral therapies such as GS-441524 and remdesivir demonstrated high efficacy. While these drugs are not officially approved in all countries, they offer new hope for cats diagnosed with FIP.

Prevention Begins with Awareness

Currently, no reliable vaccine exists to prevent FIP. The best preventive measures include –

• Maintaining a clean environment

• Reducing stress in multi-cat households

• Minimizing exposure to infected faeces

FIP remains one of the most complex and heartbreaking diseases in feline medicine. But with growing research, improved diagnostics, and emerging treatments, there is real hope on the horizon. Increased awareness and continued support for FIP research are key to turning this once-fatal disease into a manageable condition—and giving more cats a fighting chance.

(Dr. Tarun Kumar and Dr. Ankit Kumar – Department of Veterinary Clinical Complex, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana

Dr. Sukhbir and Dr. Ratneshvar Singh – Department of Veterinary Medicine, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana)