Feline infectious peritonitis, or FIP, is one of the most challenging diseases veterinarians encounter in cats. This disease primarily affects cats under two years old or those with weakened immune systems, though it can appear in cats of any age. FIP is rare, but it’s important to recognize the signs and understand the available options if it affects your cat.

Causes of Feline Infectious Peritonitis

Feline infectious peritonitis, or FIP, is caused by a mutation of the feline coronavirus, a common virus that typically poses little to no threat to cats. However, under certain conditions, this harmless virus mutates into the deadly strain responsible for FIP. 

Although the exact reason why feline coronavirus mutates into the FIP-causing strain is still not fully understood, there are some observed factors that might contribute to the cause.

In most cases, the disease is transmitted through the cat’s feces. Because of this, contact with infected feces might cause feline infectious peritonitis.

Weakened Immune Systems

Cats with undeveloped and compromised immune systems have a higher chance of developing feline infectious peritonitis. This was observed in kittens whose immune systems are still developing and in senior cats whose immune systems may weaken with age.

Additionally, cats with underlying health conditions also have a higher chance of developing feline infectious peritonitis because of the effect on their immune systems.

Genetics

Certain cats may have a genetic predisposition that makes them more vulnerable to the mutation of the feline coronavirus, causing feline infectious peritonitis. However, because the subject is still relatively unknown, there is no definite conclusion yet regarding this factor.

Stress and Overcrowding

Stress was observed to be a factor in the development of feline infectious peritonitis in cats. Cats who had significant changes in their routine or those who had experienced stressful medical procedures might be a victim.

It was also observed that cats living in multi-cat households or overcrowded areas such as shelters are at a higher chance of contracting feline infectious peritonitis. Contact with infected feces is one of the more notable modes of transmission, and the chance increases with more cats around.

Types of Feline Infectious Peritonitis

FIP can be present in two primary types: wet (effusive) and dry (non-effusive). Each type behaves differently, but both are caused by the same mutated virus.

Wet (Effusive) Feline Infectious Peritonitis

Wet FIP is the more recognizable and rapidly progressing form of the disease. It’s characterized by the accumulation of fluid in body cavities. This fluid buildup results in noticeable physical changes. Bloating may affect certain organs, such as the heart and lungs. 

For example, a cat with wet FIP may develop a swollen, distended belly, which many pet owners initially mistake for weight gain. If the fluid accumulates in the chest, it can press on the lungs and make it hard for the cat to breathe.

Dry (Non-Effusive) Feline Infectious Peritonitis

Dry FIP is more subtle and slower to progress than its wet counterpart. It does not cause significant fluid buildup but instead leads to the formation of granulomas, which are masses of inflammatory cells. 

Cats with dry FIP may exhibit a range of symptoms depending on the organs affected. Neurological signs such as tremors, difficulty walking, or seizures can occur if the brain or spinal cord is involved. Eye inflammation, causing redness or cloudiness, is another common sign.

Since the symptoms of dry FIP overlap with other diseases, it’s often harder to diagnose. Veterinarians may need advanced diagnostic tests to confirm this form of FIP.

Mixed Feline Infectious Peritonitis

In some cases, the symptoms of feline infectious peritonitis might have a combination of both types. While this is less common, it’s also possible to happen. For example, a cat with neurological signs can also exhibit bloating.

Symptoms of Feline Infectious Peritonitis

Initially, cats with feline infectious peritonitis might not show any loud signs of illness, but subtle changes in their behavior and physical appearance are early clues. These symptoms are often attributed to other issues, which makes it hard to diagnose accurately.

Early General Symptoms

Persistent Fever

One of the earliest signs of feline infectious peritonitis is fever. This type of fever might fluctuate and may not be consistent when observed for long periods.

Loss of Appetite

Cats with FIP may start eating less or refuse food altogether. This symptom, combined with lethargy, is often mistaken for general stress and other less concerning conditions.

Weight Loss

Even if your cat appears to eat occasionally, they may begin losing weight. This is another change that will appear subtle at first but will be clearer as time passes by.

Lethargy and Weakness

A decline in energy levels is another early indicator. Cats with feline infectious peritonitis may become less playful, sleep more than usual, or appear uninterested in their surroundings. This lack of energy is usually misinterpreted as a normal sign of aging in senior cats, making it harder to confirm FIP.

Symptoms of Wet Feline Infectious Peritonitis

Wet FIP is the more rapidly progressing form of the disease. It’s characterized by an abnormal accumulation of fluid in the body’s cavities, which leads to physical and physiological changes.

• Abdominal Swelling

The most obvious symptom of wet FIP is fluid buildup in the abdomen, giving the cat a swollen, distended belly. This fluid, known as effusion, can be clear or cloudy, with a sticky and viscous consistency. Typically, these fluids contain high amounts of protein.

Many pet owners mistake this swelling for weight gain, but it’s critical to note that cats with wet FIP will often appear thin elsewhere on their bodies, especially around the ribs and spine.

• Breathing Difficulties

If the fluid accumulates in the chest cavity rather than the abdomen, it can put pressure on the lungs, making it difficult for the cat to breathe. You’ll notice the signs when your cat starts breathing or panting through its mouth. Breathing difficulties are always a concern, which is why it’s important to get urgent care.

• Other Signs

Cats with wet FIP may also display additional symptoms, such as a dull coat, vomiting, increased thirst, and urination.  

Symptoms of Dry Feline Infectious Peritonitis

Dry FIP is more challenging to identify because it doesn’t cause fluid buildup. Instead, the disease forms granulomas, which are clusters of inflamed immune cells in various organs. These granulomas can disrupt normal organ function, leading to a range of symptoms depending on the organs affected.

• Neurological Symptoms

One of the most concerning manifestations of dry FIP is when it affects the brain or spinal cord. Because of this, neurological symptoms such as tremors and seizures can be possible symptoms. Cats can have difficulty walking over time, as well as have changes in their normal behavior.

• Ocular Symptoms

Dry FIP frequently affects the eyes, causing inflammation that can be visibly noticeable. Cats with ocular symptoms may squint, paw at their eyes, or avoid bright lights due to discomfort. Their eyes can also change color and bleed. 

Diagnosing Feline Infectious Peritonitis

Diagnosing feline infectious peritonitis can be super challenging, as there’s no definitive way to determine if your cat has the disease. FIP’s symptoms are often similar to those of less concerning diseases, making it impossible to accurately diagnose. 

However, if the symptoms are specific to the types of feline infectious peritonitis, then additional testing could be done to confirm the diagnosis. Fluid build, neurological symptoms, and ocular symptoms are one of the main indicators.

Diagnostic imaging tools such as X-rays are common tools that can be used to detect the buildup of fluid inside your cat’s body. Additionally, they can check on the condition of your cat’s organs, helping detect any areas of concern.

Currently, the most accurate way to diagnose feline infectious peritonitis is through a biopsy and histological examination. Vets will usually obtain a sample of the affected tissue and take it to the laboratory for testing. 

Treatment for Feline Infectious Peritonitis

For many years, feline infectious peritonitis was considered untreatable, leaving cats with limited options beyond supportive care. However, with advancements in medication, new options, such as remdesivir and GS-441524, have been effective.

For cats unable to access antiviral treatments or those in advanced stages of FIP, supportive care can help improve their quality of life. Supportive treatments focus on alleviating symptoms and maintaining the cat’s comfort.

If your cat’s abdomen has become bloated so much to the point that certain movements are hard to do, then draining should be part of the treatment process. Draining the excess fluid from the abdomen or chest can temporarily relieve pressure and improve breathing. 

Additionally, supportive medications such as anti-inflammatory drugs can also be used to help manage the symptoms. These are usually used with drugs that can suppress the immune system, helping your cat manage the condition.

Conclusion

While feline infectious peritonitis is still a challenging diagnosis, the veterinary community’s growing understanding of the disease offers hope for improved treatments and outcomes. Emerging therapies and ongoing research continue to expand the possibilities for managing FIP, giving more cats a chance at recovery.