What is it?
FeLV is Feline Leukaemia Virus, a retrovirus of cats that attacks their immune system. Although in some ways FeLV is similar to FIV ("Feline AIDS"), they are in fact completely different viruses.
What causes it?
The FeLV virus is relatively common in unvaccinated cats (perhaps as many as 8% of cats carrying it). When a cat is infected (by bites, shared feed bowls, litter trays or even mutual grooming), the virus starts to attack the white blood cells. The cat's immune system will usually stop this rapid replication, but eventually the immune system itself is damaged, and stops working properly. In addition, the virus can trigger infected cells to become cancerous, causing leukaemia, lymphoma and (occasionally) sarcomas.
What cats are at risk?
Unvaccinated cats who come into even casual contact with infected carriers. Indoor cats are relatively safe, unless infected animals are introduced (or come in of their own accord).
What are the symptoms?
In the early stages of the disease, there are usually no symptoms - only when the immune system has been severely damaged, months or years after infection, do symptoms become visible. Immunosuppressive disease (i.e. disease caused by collapse of the immune system) may present in a range of different forms, but often include runny noses, sore eyes, persistent diarrhoea, sores in the mouth or gums, chronic skin or ear infections, or an unexplained fever or weight loss. In all cases, minor low-grade infections develop rapidly and may even become life-threatening. Neoplastic Disease (due to the development of virus-induced cancers) typically cause weight loss, obvious masses (e.g. swollen glands), diarrhoea, or anaemia. Sometimes, there may also be neurological signs (wobbliness or even seizures) if a tumour forms in the nervous system.
How is it diagnosed?
There is a simple blood test that can be carred out at your vet's practice to confirm whether a cat is carrying FeLV (and they'll usually test for FIV at the same time).
How can it be treated or managed?
There is no cure for the disease. In the case of a cat who is incubating the disease but has not yet developed symptoms, it is really important to isolate them from at-risk cats - ideally by keeping them alone as indoor cats. This will also reduce their exposure to other diseases that may take advantage of their weakened immune system. The use of human anti-AIDS drugs may slow down the development of disease, but these drugs are difficult to dose safely in cats, and will not clear the virus completely.
Can it be prevented?
Yes, there is a highly effective vaccine available as an optional part of your cat's routine boosters.