Feline leukemia is most common in young cats who go outdoors. Learn to recognize the signs and symptoms of feline leukemia, and how to care for an infected cat.

By Kate Eldredge Basedow, LVT
November 03, 2020
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Credit: VioletaStoimenova / Getty

Feline leukemia, frequently abbreviated FeLV, is a retrovirus that affects cats. According to the Cornell Feline Health Center, Feline leukemia virus (FeLV) is one of the most common infectious diseases in cats, affecting between 2% and 3% of all cats in the United States.

FeLV is also the most common cause of cancer in cats, predisposing infected cats to lymphoma and leukemia. The condition suppresses the immune system, making cats who are affected more susceptible to a wide range of illnesses and even to infection from normally benign bacteria and other microorganisms. It can also cause anemia.

The 2020 AAFP Feline Retrovirus Testing and Management Guidelines released by the American Association of Feline Practitioners (AAFP) breaks feline leukemia infections into three categories: abortive, regressive, and progressive. 

  • Abortive FeLV infection: An abortive infection is when the cat is exposed to and contracts the virus, but is able to fight it off with his immune system and clear the infection. Current data suggests that most cats exposed to FeLV fall into this category. 
  • Regressive FeLV infection: In regressive infections, the cat’s immune system succeeds in clearing the virus from the bloodstream, but not the bone marrow. These cats are not contagious most of the time, but the virus can be reactivated in the future and cause the cat to become symptomatic and to shed the virus.
  • Progressive FeLV infection: A progressive infection is what cat owners fear most—when the cat is unable to fight off the infection, and gets progressively more sick as time goes by. Unfortunately, about 85% of cats with progressive infections die within three years of diagnosis.

Is Feline Leukemia Contagious?

Yes! Feline leukemia is contagious between cats, and is spread from one cat to another in the blood, saliva, nasal secretions, tears, milk, urine, and stool. Cats typically transmit this virus through bite wounds and from grooming each other, and mother cats can infect their kittens both in utero and while nursing. Less commonly, the virus can be spread through shared resources such as food and water bowls and litter boxes. Cats can still spread the virus even if they appear healthy. But if your cat has FeLV, it cannot be spread to humans, dogs, or other animals. The virus is only transmissible from cat-to-cat. So it won’t harm you or any of your other pets, as long as they are not cats.

Kittens and young adult cats are at the highest risk for contracting FeLV, but cats at any age can become infected. Cats who go outdoors and interact with other cats or who live in a high-density environment such as a shelter or cattery are at increased risk for FeLV. Cats with a weakened immune system also have a higher risk of becoming infected.

The good news is that this virus is not very hardy, and doesn’t last long in the environment. It can also be easily killed with standard cleaning measures.

Because of how contagious it is, all cats should get the feline leukemia test before they are added to a new home. If you are taking in a stray and don’t know his background, keep him separate from your other cats until he can be tested.

Signs and Symptoms of Feline Leukemia

Signs of feline leukemia may include:

  • Lethargy
  • Failure to thrive (unkempt coat, weight loss)
  • Poor appetite
  • Fever
  • Pale gums
  • Persistent diarrhea
  • Icterus (yellow tinge to skin, often most visible around mouth, ears, and eyes)
  • Enlarged lymph nodes
  • Difficulty breathing
  • Recurring infections (including upper respiratory, bladder, and skin)
  • Stomatitis (inflammation in the mouth)
  • Behavioral changes, seizures, and other neurological disorders
  • Eye problems
  • Sterility, abortion of kittens

Feline leukemia symptoms may take a while to develop. Many cats live normal, healthy lives for weeks, months, or even years after infection. When symptoms do appear, they often get gradually worse over time, but can come in cycles where the cat appears healthy between periods of illness.

How to Test Cats for FeLV

Thankfully, there are several tests available to check for FeLV infection. All cats should be tested for FeLV before being added to a home with other cats to prevent spreading the disease. The AAFP also recommends testing all sick cats, as feline leukemia can appear similar to a variety of health conditions.

The most common test used in clinics is an enzyme-linked immunosorbent assay (ELISA) that checks for the protein FeLV P27 in a small blood sample. These tests only take about ten minutes to run, are very sensitive, and can pick up early infections before the cat shows any symptoms. Ideally, the test should be done 30 days after a known or possible exposure to FeLV to prevent a false negative result.

If your cat tests positive for feline leukemia on the in-house ELISA test, your veterinarian will likely recommend sending out samples for additional testing to confirm whether or not your cat truly has FeLV and to get an idea of what type of infection he has. Confirmation tests include polymerase chain reaction (PCR) testing, virus isolation, and indirect immunofluorescent antibody assays (IFA). The IFA detects the presence of FeLV P27 in white blood cells, which indicates a progressive infection and a poorer prognosis.

If your cat comes up positive for feline leukemia but clears the infection on his own, he will test negative in the future.

Feline Leukemia Treatment Options

Unfortunately, there is no cure for feline leukemia. Your veterinarian will work with you to treat symptoms as they occur. FeLV-positive cats are vulnerable to other infections and illnesses, so they need to be kept up to date on all preventive care. Strategies to keep your cat healthy include the following:

  • Keep up to date on recommended vaccinations
  • Use parasite preventives for both internal and external parasites
  • Keep your cat indoors
  • Get a wellness exam with your vet every six months to catch any developing problems early
  • Regular blood work to watch for changes in organ function or signs of secondary infections
  • Spay or neuter (prevents reproduction-related stress and fighting, as well as the risk of passing the disease to kittens)
  • Monitor weight, eating habits, behavior changes
  • Check eyes and mouth regularly for sores or signs of inflammation

Catching any problems early will make them easier to address and help to prolong your cat’s life. The development of widespread lymphoma or bone marrow suppression both have a very poor prognosis.

Zidovudine (ZDV), also known as azidothymidine (AZT), is an antiviral compound that can be used to reduce the viral load in affected cats. The AAFP notes that it is particularly helpful for cats that have stomatitis or neurological signs. If your FeLV-positive cat fits this profile, ask your vet about this medication.

Preventing FeLV

The best way to protect your cat from feline leukemia is to prevent exposure to infected cats. Keep your cats indoors or only let them outside on a leash or in a secure enclosure to prevent interactions with other cats. Remember that young, sick, and immunosuppressed cats are at the highest risk for contracting FeLV, so these cats in particular should be kept away from cats with an unknown health history.

When adding a new cat to your family, have the cat tested for FeLV before allowing him to interact or share bowls with your other cats. 

There is a feline leukemia vaccine. Cats should be tested for FeLV before receiving the vaccine, as vaccinating after the cat has been infected will not provide any benefit. This vaccine is not considered a core vaccine for cats because it is usually given based on whether or not a particular cat is at risk for contracting FeLV. The AAFP recommends vaccinating all kittens under two years of age starting with a series of two vaccinations given three to four weeks apart. The first vaccine can be given as early as eight weeks of age. After the initial series, the AAFP recommends boosting one year later and then determining further vaccinations based on the cat’s lifestyle. Indoor cats usually do not require additional FeLV vaccinations, while cats who go outdoors benefit from boosters annually or every other year. Discuss with your veterinarian whether the feline leukemia vaccine is needed for your cat.

It is important to note that while the FeLV vaccine will help to protect your cat from feline leukemia, it is not 100% effective, and preventing exposure to infected cats is still the best way to keep your cat healthy. The AAFP cautions that it takes two to three weeks to develop protective immunity, so newly vaccinated cats should be kept indoors and away from potentially infected cats for at least those first few weeks to give the vaccine time to stimulate an adequate immune response.

Many owners wonder if a cat with feline leukemia can live with other cats. If you have an FeLV-positive cat, adding a new cat to your household can create stress that may further compromise his immune system, so adding a new cat should be done with care. It is also best to keep positive and negative cats separate to prevent the disease from spreading. If you have multiple cats and one tests positive, all of the cats should be tested and then separated accordingly. Have separate food and water dishes and litter boxes for the FeLV-positive and FeLV-negative cats and prevent them from interacting with cats from the opposite group. Separate bowls and litter boxes are also a good idea if you have multiple cats in the house and the feline leukemia status of one or more is unknown.