Feline Infectious Peritonitis (FIP) is an infection caused by a virus in cats. It is a very unique and complicated infection that is dependent on several factors involving the viral strain and the host itself. Many years of study and research went into uncovering this strange viral infection and understanding how it happens, but much is still unknown and there is no known effective therapy.
FIP starts as a relatively simple Feline Coronavirus. This is a type of virus/infection that can happen in many different animals with viruses that are often specific to that particular species. Typically, the coronavirus causes diarrhea and gastrointestinal signs and is self-limiting. Meaning, most animals recover from the infection without intervention or consequence.
But in some cats, some viral strains are able to transform into FIP. The virus is still a coronavirus so testing for these viruses will often still give a positive result. But the FIP strains take on new properties and abilities that allow them to infect the white blood cells that would normally kill them and clear the infection. This allows the FIP strain to survive past the host's initial disease state and GI signs and evade destruction by the immune system.
The next step in the evolution of FIP as a disease can take place months to years after the initial infection and can come on with no warning.
FIP hiding in the immune system itself causes inflammation that begins to damage the host's own body. There are two ways that this can happen. Depending on what type of inflammatory response takes place, a cat can develop FIP as a 'wet' or a 'dry' form. In cases of neurologic disease, the 'dry' form is the more common of the two.
In 'wet' form FIP, antibodies and inflammation due to the virus begins to irritate the blood vessels of the body. This irritation leads to the vessels becoming 'leaky' and fluid will begin to infiltrate into the body cavities. This fluid has very typical characteristics and can cause abdominal swelling and difficulty breathing due to the abdominal cavity or the chest becoming filled with fluid.
In the 'dry' form of FIP, the infected white blood cells begin to coalesce into clumps of cells called granulomas. These granulomas can settle out into any of the body's tissues and begin to cause organ damage. In the brain, the most common place to find them is the coverings of the brain (meninges) and the lining of the fluid filled spaces inside the brain itself (the ventricular system). In fact, there is a very common MRI finding in cats with FIP that shows marked inflammation of a particular part of the ventricular system called the mesencephalic aquaduct or Aquaduct of Sylvius.
An MRI image of a cat as if the cat were looking out of the screen at the viewer. The bright lines outline the ventricles (normal fluid filled spaces in the brain). But the ventricles are swollen and dilated from the disease. The lining is bright white because of the inflammation or granuloma formation secondary to the virus.
The prognosis for cats with FIP regardless of the form it takes, wet or dry, is guarded to grave. There are medications that can be helpful for short periods of time, but there is no way to clear the virus from the affected pet. Diagnosis is sometimes easy and at other times requires advanced testing and ruling out a multitude of other diseases.