Feline Leukemia Virus (FeLV) in Plain Language


As the name suggests, this is a disease caused by a virus that infects cats. It is a complicated disease. It can develop in various ways.  Sometimes an infected cat will cure himself. Sometimes it is a fatal disease as it can cause cancer. It also stops the cat’s immune system working which allows other serious illnesses to take hold. Some cats can partly cure themselves and carry the disease. There is no effective treatment.

FeLV Postive Cat Copper. Photo Rocky Mountain Feline Rescue

The Virus

The virus that causes FeLV is similar to the one that causes FIV (Feline Immunodeficiency Virus). They are both retroviruses. This sort of virus multiplies inside the cat’s cells as part of the cell’s DNA (DNA – molecules containing chromosomes and genes). The diseases caused by these viruses are similar. However, the ways in which they are caused are not the same.

How Common is FeLV?

This virus causes more diseases than any other sort of infection. It causes more cat deaths than any other illness other than “trauma” (injury). It is the biggest cause of cancer in cats. Its presence makes other disease worse. As you can see from the percentages, a lot of sick cats have FeLV.

  • About 1-2% of roaming cats are infected (USA).
  • In the UK probably less than 1% of healthy cats have FeLV.
  • Cornell Feline Health Center say that 2-3% of all cats in the United States are infected with FeLV.
  • In houses where there are several cats, and more, up to 30% of the cats can be infected (meaning the virus is in the blood of these cats). This figure is for the USA.
  • Of all feral and roaming cats up to 40% may have the infection (USA).
  • In the UK up to 18% of sick have FeLV.

How is it Passed from a Cat to Another Cat?

It is passed from cat to cat through saliva which contains the virus. So the infection is passed by:

  • cat bites
  • shared water bowls
  • shared food bowls and
  • one cat grooming another

Also the virus is passed by a mother to her kitten when pregnant or when the kitten drinks his mother’s infected milk. Sometimes the virus is transferred through urine and poop. If cats share a litter tray the virus could pass from one cat to another this way.

Infection in a stressed cat is more likely to happen. Cats living in overcrowded and dirty conditions are also more likely to catch the disease. Kittens are more likely to be infected because they have less resistance.

How Does the Disease Develop?

At the beginning, the illness last for up to about 16 weeks. The signs are vague, sometimes mild and include:

  • Weight loss
  • Fever
  • Apathy
  • Vomiting
  • Diarrhea
  • Constipation
  • Possible anemia

Afterward the disease can take one of four courses:

FeLV Chart

The cats that take the red path have an immune system that is much less effective which allows other diseases to take hold. These diseases are usually:

  • FIP – feline infectious peritonitis
  • Feline infectious anemia
  • Respiratory diseases
  • Toxoplasmosis
  • Cystitis
  • Gum disease
  • Bacterial infections

3 in 10 of the cats taking the red path develop cancer. The cancer can sometimes be felt in the abdomen area. The cancer may spread. Sometimes a cat in this group get leukemia which is cancer of blood.

Diagnosing FeLV

Your vet will probably use a test called the ELISA test. This test detects for antigens in blood and saliva. An antigen is a substance that instigates the production of antibodies that fight the disease. This test should be repeated to see if the cat has cured the disease.


There is no effective treatment. But cats that have FeLV (FeLV positive) are good cat companions and they need homes. Often rescue organisations advertise them for rehoming. Of course they should go to a home where there are no other cats. They require excellent care.


Stop the spread of the disease:

  • Remove infected cats from a group
  • Don’t introduce infected cats into a group
  • Seek veterinary advice
  • Clean the house. The FeLV virus is easily killed.
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Michael Broad

Hi, I'm a 71-year-old retired solicitor (attorney in the US). Before qualifying I worked in many jobs including professional photography. I have a girlfriend, Michelle. I love nature, cats and all animals. I am concerned about their welfare.

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