Feline leukaemia virus (FeLV) is what is known as a retrovirus. Retroviruses are a group of viruses which are able to hide in the host body and lead to prolonged infection. This is because the viral DNA is inserted into the DNA profile of the host as part of their replication process.
The main transmission of FeLV is in saliva. Most infections are acquired by prolonged close contact – by mutual grooming and by sharing food and water dishes. However, the virus can also be passed on through bite wounds acquired by fighting.
Kittens are most vulnerable to infection, though adult cats can also be infected. This occurs particularly if their immune function is reduced, or they are in close contact with a cat that is shedding the virus.
When initially infected with FeLV, cats generally don’t appear unwell. After infection, the virus travels to the lymph nodes and multiplies. In some cats, this viral multiplication continues, the virus circulates in the blood and they become unwell some time later. These cats remain infectious.
In other cats, the virus appears to be cleared from the body after infection. However, in these cats the virus is actually hiding in the bone marrow (latent infection) and can be reactivated later in life.
Feline leukaemia virus can cause a variety of clinical syndromes. The most serious is, as the name suggests, the development of certain cancers of the white blood cells or lymphoid tissues (lymphoma). These tend to be aggressive and poorly responsive to chemotherapy. The virus can also cause anaemia (lack of red blood cells), immunosuppression or auto-immune disease.
FeLV infection can be very serious. The average survival after diagnosis is 2.4 years. However, we need to bear in mind that this figure comes from a mixture of unwell cats, and healthy cats that were diagnosed on routine screening (for example before breeding or rehoming). In some cases, especially in kittens, the course of disease is often more rapid. In cats that develop lymphoma or leukaemia, average survival times from diagnosis are between 2 and 6 months.
Vaccination against FeLV should be considered in all cats, especially young kittens – as they are most vulnerable to infection. Annual re-vaccination would be advised in cats that have outdoor access, or in multi-cat households.
There are no other reliable ways to prevent infection with FeLV. However, if a cat is diagnosed with FeLV on a screening test and is otherwise unwell, they should be kept indoors due to the risk of transmission to others.
MSD Veterinary Manual: Feline Leukaemia Virus and related diseases in cats
BSAVA Manual of Canine and Feline Infectious Diseases: The Haemopoietic and Lymphoreticular Systems.