Note for Pet Owners:

This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

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Cancer is one of the most common causes of death in cats and feline leukaemia virus (FeLV) is responsible for approximately 33% of all feline cancer-related deaths. In addition, many other cats with FeLV die from infectious diseases that they are unable to overcome because of immunesuppression caused by the virus.


The causative agent is feline leukaemia virus (FeLV) an oncornavirus belonging to the retrovirus group. It consists of a single strand of RNA protected by an envelope. Based on nucleotide studies it is believed to have evolved from a virus prevalent in an ancestor of the rat.

This virus does not kill cells that it infects, and it replicates by budding. During budding the viral particles assume some host antigen and so are protected from the hosts immune system because they are recognised as "self". The bone marrow, salivary gland and epithelium of the respiratory tract are just some of the tissues in which this virus can replicate.

Initial signs of FeLV infection include:

Later signs include :

  • weight loss
  • recurrent infections

There are numerous complications and sequelae following infection with FeLV including :

a) The development of cancer - lymphoma - often involving the intestine or the thymus region of the chest

b) Cancer of the blood cells (Leukaemia) - white cells and red cells

c) Anaemia due to bone marrow suppresssion

d) Reproductive abnormalities - including infertility, abortion, resorption

e) Osteochondromas (cancer of cartilage)



Close contact is needed and transmission from one cat to another occurs mainly via saliva. Apparently healthy cats can live for years and yet have FeLV virus in their blood (called viraemia) and shed virus all the time. Other routes of transmission are much less likely - in urine, faeces, other secretions , via flea bites, from contaminated hypodermic needles, surgical instruments, blood transfusions, infected environmental materials because in the environment the virus only survives up to 48 hours.

Feline leukaemia virus is not highly contagious , so infected individuals can be kept in a hospital environment - provided direct contact with other cats is prevented, and reasonable hygiene is practices - such a snot sharing feeding utensils, and washing hands by personnel after handling each individual cat.

Transplacental transmission does take place , and transmission can also occur in the immediate post-natal period when the infected queen grooms them. Kittens are more susceptible to infection than adults.


Any cat with enlarged lymph nodes (lymphadenopathy) should be tested to see if it has feline leukaemia virus.

One of the viral core proteins p27 is produced within infected cells and this protein is used in fluorescent antibody tests and ELISA tests to detect the presence of the virus.

Feline oncornavirus cell membrane antigen (called FOCMA) is present on malignant cell membranes but not other cells. Hence, cats with high FOCMA antibodies are resistant to the development of leukaemia and lymphoma - even if they are infected with the FeLV virus.

Prevention Routine vaccination is available and recommended for cats that are in regular contact with others. 

Provet comment : Expert committees have designated FeLV vaccines as non-core HOWEVER Provet does not agree with this categorisation. Although not highly contagious vaccination has significantly reduced the incidence of FeLV-related cancers in young cats since the 1970's and Provet does not wish to see a return to those days, and vaccination is safe and effective.

There is no specific treatment for cats infected with leukaemia virus. Supportive therapy is often helpful in providing an increased quality of life . However, treatment is controversial in confirmed cases because FeLV can be transmitted to other in-contact cats.

Always guarded, and often poor.

Long term problems

Updated January 2016