A Guide to the Feline Leukemia Virus: Why Vaccination is Important!

Reading Time: 4 minutes

drowsiness as one of the feline leukemia symptomsFeline leukemia virus FeLV is an infection passed between cats, which affects the immune system. Those cats at greatest risk of picking up infection often live in multi-cat households and have access to the outdoors.

The feline leukemia virus is highly infectious and doesn’t need direct cat-to-cat contact to pass between cats.  Always discuss your cat’s individual risk factors with your vet, especially for kittens. Young cats are particularly vulnerable to this infection.

Viral particles are present in body fluids such as saliva, thus one cat grooming another can spread infection. This also means good hygiene is essential, such as washing food and water bowls.

There is a highly effective vaccine that protects against feline leukemia infection. If your cat is at risk of encountering the feline leukemia virus then vaccination is strongly advised.

A Scary Infectious Disease: What is Feline Leukemia?

One of the scary things about FeLV is how common the virus is and how easy it is to acquire an infection. But it’s not all bad news. A healthy immune system can often fight off infection. This means older cats have often developed a natural immunity and a new diagnosis in an old cat is unusual.

This disease itself causes vague symptoms, which can easily be mistaken for other conditions. Indeed, one of the problems is FeLV infection weakens the white cells that fight infection. This makes the cat more vulnerable to picking up secondary infections which then make them sick.

For example, in a healthy cat as simple cold isn’t too much of a worry. But in a FeLV positive cat that head-cold could easily develop into life-threatening pneumonia. Thus, it’s important to recognize if your cat has FeLV so you can seek prompt treatment for even minor ailments.

An active infection can affect different body systems. Typically, the virus interferes with the production of white blood cells, lowering the cat’s defense against disease. The virus may also cause an unusual amount of one particular type of white blood cell to be manufactured, flooding the lymph nodes and causing them to enlarge.

Other times the virus can cause an enlargement of a gland within the chest, the thymus. Or it affects the gut in a form of cancer known as alimentary lymphosarcoma (although, it should be noted not all lymphosarcomas are caused by FeLV.)

A Viral Infectious Agent!

The feline leukemia virus belongs to a group of viruses called retroviruses.

FeLV infection can only pass between cats and does not infect other species such as dogs or humans. Although a highly successful virus at passing between hosts, it doesn’t survive above a few hours in the environment.

Thus, disinfecting wipeable surfaces with dilute bleach, and washing all food utensils and bowls with hot soapy water is sufficient to clean them. When bringing a new kitten into a home where a FeLV positive cat once lived, these common-sense hygiene actions are sufficient to eliminate an environmental infection.

What Causes Feline Leukemia?

FeLV is spread by close contact between cats. Activities such as mutual grooming can be enough to spread infection. Also, viral particles can survive for a few hours in saliva on food bowls or urine in litter boxes.

Those cats at risk from feline leukemia may also be at risk from the feline immunodeficiency virus FIV (which causes feline aids). However, the latter needs direct contact with blood to infect a cat, whereas FeLV does not. So whilst FIV is spread mainly by biting or mating, FeLV is passed on by grooming.

Those cats most likely to pick up FeLV infection are young, with developing immune systems. This said any cat that has prolonged contact with a FeLV positive cat or sustains bite wounds from an infected feline can get this disease.

healthier cat from the symptoms of feline leukemia vaccine

Most cats that are diagnosed with FeLV virus are less than six years old. Older cats may have encountered the virus and built up a natural immunity to it.

When cats or kittens contact the FeLV virus, either by licking or sniffing droplets infected with the virus, it enters their bloodstream. This stage lasts from 3 to 16 weeks. If the cat has a strong immune response, it can fight the virus and eliminate it from the body.

If the cat has a weak immune system, the virus circulates around unchecked. This is known as ‘active viremia’ and is linked to a sick cat showing clinical signs of  FeLV infection.

Also, as the virus circulates around the body in the blood, after about three weeks it enters the bone marrow. It can then lodge there and either set up a persistent infection or lie latent. With the latter, when the cat is stressed, sick, or given immunosuppressive drugs, this pushes the cat into developing an active felv infection.

Another problem for cats with a latent infection is they may not be ill themselves but do excrete virus. This makes them an infection risk to other cats.

From Seizures to Fevers: Feline Leukemia Symptoms

The symptoms of FeLV vary depending on which body system is affected. In the first instance, the clinical signs are often vague. Typically the cat loses interest in food and sleeps more than usual. They may also be feverish or have diarrhea. This combination often leads to weight loss and the coat being in poor condition.

Signs to watch out for that should trigger a trip to the vet include:

  • Poor appetite
  • Weight loss
  • Lack of energy
  • Sickness or diarrhea
  • Increased thirst
  • Swollen lymph nodes
  • Regurgitation.

Sometimes the signs are so vague it seems the cat is a ‘poor doer’ and fails to thrive.

Other times the cat does OK but has repeated secondary infections, such as rhinitis or diarrhea, which refuse to clear up completely.

Here are 6 Facts You Need to Know FeLV

  1. The feline leukemia virus can be destroyed on surfaces by dilute bleach or hot soapy water
  2. Some geographical areas have a higher incidence of FeLV than others. Speak to your vet to see if you live in such an areas
  3. A single positive blood test for FeLV doesn’t necessarily mean your cat will become sick with the condition.
  4. Additional testing is advisable for healthy cats that test positive.
  5. Young cats in multi-cat households that go outdoors are at greatest risk of picking up FeLV infection.
  6. You can choose to titer test your kitty for FeLV.

How Does My Vet Know if My Cat Has FeLV?

The vet will keep an open mind when examining the cat. Certain signs such as weight loss in the presence of pale gums and fever will make the vet’s thinking towards testing for FeLV virus infection.

Testing for FeLV is complex. It’s often not possible to make a definitive diagnosis that the cat is sick because of FeLV, based on a single positive blood test.

This is because the common in-house blood test (an ELISA  test) used by vets looks for the presence of proteins in the blood linked specifically to the feline leukemia virus. However, a positive test means the cat has encountered the virus, and not necessarily that it will make them sick.

As already mentioned, a healthy immune system can fight off infection, leaving the cat protected. These cats will have a positive  ELISA test initially, but when tested again three months later may be negative.

If a cat is poorly and has a positive ELISA test, the vet may not want to wait three months to discover if the problem is FeLV related. To get a quick answer, the vet can send blood away to specialist labs for a more sophisticated type of test. Known as IF, VI, or PCR tests, these look for the presence of feline leukemia virus with blood cells, which indicates active infection.

positive result of cat leukemia as the main sign for feline leukemia virus

Treatment of FeLV in Cats

There is no cure for feline leukemia virus infection in cats.

Treatment targets killing secondary infections, such as using antibiotics to clear up pneumonia.

Some forms of blood-related cancers, such as lymphoma or lymphosarcoma, may respond well to chemotherapy. However, this involves the use of drugs that further weaken the immune system. This means the additional complication of further reducing the cat’s ability to fight off regular infections.

Studies were cats were treated with interferon have given encouraging results. Interferon helps strengthen the body’s fight against viruses and seems to help keep FeLV positive cats healthy.

The take-home message with regards to FeLV in cats is to always see a vet promptly if the fur-friend becomes off color. Swift action can prevent serious complications from developing and keep the cat healthy.


For healthy cats at the time of diagnosis, with a vigilant owner and good care, many feline leukemia virus-positive cats go on to live for several years. However, the outlook is more gloomy if the cat was tested because they were unwell because there was a suspicion of this infection.

Life Expectancy

The long-term outlook isn’t great. Statistics show that of 85% of cats with an active feline leukemia viremia, have a life expectancy of fewer than three years.

No Place for Anti-Vaccination! The Feline Leukemia Vaccine can Save Your Kitty!

The feline leukemia vaccine is not considered a core vaccine. This means that not all cats need it. Your vet makes the call based on the cat’s age, lifestyle, and risk of exposure to the virus.

For example, an elderly lone cat that lives indoors is at low risk of meeting the feline leukemia virus. However, a young kitten in a multi-cat household where they have free access to the outdoors is at high risk.

If the vet advises vaccination then it is a good idea to heed this advice. It’s easy to forget just how many cats died when this disease newly emerged in the 70s and 80s. The vaccine was literally a life-saver back then, and it’s easy to take it for granted now.

But don’t fall into the trap of thinking FeLV is no longer common. It is. It’s just that the vaccine does such a great job of protecting pets that we don’t hear so much about it.

All product and Company names are Trademarks™ or Registered® trademarks of their respective holders.

Disclosure: Bear in mind that some of the links in this post are affiliate links and if you go through them to make a purchase CertaPet.com may earn a commission. Keep in mind that we link these companies and their products because of their quality and not because of the commission we receive from your purchases. The decision is yours, and whether or not you decide to buy something is completely up to you.