The Hidden Feline Virus That Can Affect Joints and Kidneys

Analysis by Dr. Karen Shaw Becker

feline foamy virus

Story at-a-glance -

  • Feline foamy virus (FeFV) belongs to the same family of retroviruses as feline immunodeficiency virus (FIV); it is thought to be transmitted primarily through intimate social contact among cats
  • The disease is most often seen in free-roaming cats and is typically asymptomatic, but co-infections with FIV are relatively common
  • FeFV may be linked to myeloproliferative disease, chronic progressive polyarthritis, chronic kidney disease, and exacerbation of FIV
  • There is no treatment for FeFV; infected cats should be routinely monitored for early signs of FIV and progressive polyarthritis and receive treatment for those diseases as necessary

Here's a weird disease in cats you've probably never heard of — feline foamy virus (FeFV or FFV). FeFV belongs to the family of retroviral Spumaviruses to which the feline immunodeficiency virus (FIV) also belongs.

FeFV is what's known as a complex retrovirus, meaning it uses RNA as its DNA. Most strains of the virus don't cause disease; however, certain strains can cause differentiated lymphocytes, indicating a potential effect on the immune system. There is much yet to learn about this curious disease.

Prevalence of Feline Foamy Virus

FeFV occurs most often in free-roaming cats. The likelihood a cat has been exposed increases with age, as is also the case with FIV (and co-infections with both viruses are fairly common). According to one study, over 70% of cats older than 9 years have been exposed (are seropositive) for FeFV.1

Interestingly, FeFV exposures and infections are much more common in both pumas and domestic cats in Florida, Colorado, and Southern California,2,3 and cross-species transmission of the virus between domestic cats and pumas is also common in those areas.4

Of the domestic cats admitted to shelters or involved in trap-neuter-return programs in Southern California, 75% were found to be seropositive for FeFV. In Colorado, the percentage was 52%; in Florida, 42%. Among pumas, 69% in Southern California were seropositive, 77% in Colorado, and 84% in Florida.5

How FeFV Is Transmitted

Scientists haven't yet settled on the most common route of transmission of FeFV. It can be transmitted through bites, but the gradual increase in infected animals and the fact that so many older cats are seropositive suggests it's more likely transmitted by intimate social contact (typically mutual grooming) than aggressive behavior. The virus can also be passed from a mother cat to her kittens, but this route of transmission may be uncommon.


Most FeFV-positive cats are free of symptoms and in good health, however, changes in tissue in the lungs and kidneys over time have been reported. It isn't yet definitively known whether the virus can cause longer term symptoms or adverse effects. However, some experts suspect FeFV may be linked to myeloproliferative disease (disorders of the bone marrow and blood), chronic progressive polyarthritis, chronic kidney disease, and exacerbation of FIV.


Your veterinarian will take a history of your cat's health, including when you noticed symptoms (if any) and whether there has been close contact with other kitties. He or she will perform a physical exam, a complete blood count (CBC), biochemistry profile, urinalysis, and electrolyte panel.

These tests will reveal if there's an FeFV and/or FIV infection. The presence of feline foamy virus is confirmed using a serum antibody test — infected cats will test positive for antibodies specific to the virus. Since this test is expensive and not readily available, if your cat has no symptoms, it's unlikely your veterinarian will order it.

However, if your cat is exhibiting possible symptoms of FIV (more about this below), your veterinarian may order a more common, less expensive test for those antibodies. Your vet may also examine joint fluid if your kitty has chronic progressive polyarthritis.

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FeFV Treatment Options

There is no treatment for feline foamy virus, and interestingly, if future studies continue to indicate that FeFV is nonpathogenic, the virus could potentially play a role in the treatment of pathogenic cat viruses (e.g., FIV).

If a kitty goes on to develop progressive polyarthritis, a treatment protocol will be necessary to address it. If a cat has FIV along with FeFV, it must be addressed.

Asymptomatic cats infected with FeFV typically live normal lives and require little treatment or management. Since the virus is still being researched and questions remain, your veterinarian may suggest you keep your cat indoors or away from other cats as much as possible to prevent transmission.

Regular wellness veterinary visits are essential to check for the development of FIV or progressive polyarthritis. Undoubtedly, functional medicine veterinarians will trial novel therapies for addressing FeFV, including hyperbaric oxygen and ozone therapy, which have demonstrated beneficial outcomes when treating human viral infections.6,7,8

Feline Immunodeficiency Virus (FIV)

FIV is seen most often in free-roaming, aggressive male cats. Indoor kitties are much less likely to be infected. The average age at diagnosis is 5 years, and the risk of infection increases with age. There is no genetic predisposition for the condition, although genetics may play a role in disease progression and severity.

FIV is transmitted primarily through bite wounds. Unlike feline foamy virus, casual social contact doesn't appear to spread the virus, which is why friendly kitties in stable multi-cat households are at little risk of acquiring FIV. Rarely, an infected mother cat can transmit the virus to her kittens either during passage through the birth canal or while nursing. Sexual contact is not considered a major means of transmission.

A cat with an FIV infection can appear normal for years. But eventually, the disease creates a state of immune deficiency that leaves the kitty susceptible to other infections. This means that everyday bacteria, viruses and fungi that cause no problems for healthy animals can cause serious illness in kitties with compromised immune systems. Secondary infections are responsible for many of the diseases linked to FIV.

Early in an FIV infection, lymph nodes throughout the body are affected, resulting in temporary enlargement of the nodes, and often, a fever. This stage of infection often passes unnoticed unless the lymph nodes grow markedly enlarged.

The course of FIV can cause a progressive deterioration of health, or recurrent bouts of illness interspersed with periods of relatively normal health. Symptoms of immunodeficiency can occur anywhere in a cat's body and include:

Recurrent minor illnesses, often involving the upper respiratory tract and gastrointestinal tract

Persistent bacterial or fungal infections of the ears and skin

Inflammation of the gums is seen in 25% to 50% of cases

Fever and wasting, especially in the later stages

Upper respiratory tract disease is seen in 30% of cases

Cancer, especially lymphoma

Chronic eye problems, glaucoma

Chronic kidney insufficiency

Persistent diarrhea is seen in 10% to 20% of cases

Poor coat condition

Nervous system abnormalities, including disruption of normal sleep patterns, behavioral changes (e.g., pacing and aggression), changes in vision and hearing, disorders affecting the nerves in the legs and paws

You can find my treatment recommendations here.

Prognosis and Prevention

Cats who acquire one or more serious virus-related illnesses, kitties with persistent fevers and weight loss, and those with cancer, can be expected to have a much shorter survival time. The only foolproof way to keep your cat safe from these viruses is to prevent exposure to them. This obviously means keeping him away from potentially infected cats.

If he goes outdoors, it should be under your close and constant supervision, or in a safe, secure outdoor enclosure — one that prevents other cats from getting in, and as much as possible, prevents them from being able to make contact with your kitty through the sides or top of the enclosure.

If you have an uninfected cat, never allow untested or at-risk kitties to mingle with yours. House viral-positive cats separate from viral-free cats. FIV-positive and negative cats can live under the same roof as long as they don't fight or bite.

I don't recommend the FIV vaccine as it is often ineffective and has been linked to the development of vaccine-associated sarcomas in cats.