FeLV and FIV in Cats


If your cat becomes unwell, seems to have trouble recovering from an infection or develops certain types of tumour your vet may want to check if there could be a viral infection causing the problem. There are two types of virus which cats can become persistently infected with and carry inside them for some time without initially showing any signs. One is the Feline Immunodeficiency Virus (FIV) and the other is the Feline Leukaemia Virus (FeLV). Both these viruses can interfere with a cat’s immune system and reduce their ability to fight off infections. They can also, less commonly, cause tumours, or cancer, to form. If vets are testing a cat for the presence of one virus they will usually test for both since the symptoms shown by infected cats can be pretty similar. Whilst there are commonalities in the way that these viruses cause disease in cats there are also significant differences.

FIV is equivalent to HIV in humans and so is sometimes called Feline AIDS. It’s not that easy to catch and requires very close contact. Although it can spread through sexual contact in cats it is most likely to be passed from cat to cat during fights. The virus is present in the saliva of infected cats and if they bite another cat they can introduce it into their blood stream. Cats who are more likely to fight therefore; males (especially if not neutered) and cats who live in crowded or overpopulated situations are at higher risk of catching the virus.

FeLV doesn’t really have a human equivalent. The virus is easier to catch than FIV, it is also present in saliva but can also be in other bodily fluids. Licking, grooming and sharing food bowls may well be enough to transmit the infection between cats as well as fighting and sexual contact. So again cats coming into contact with many other cats are more at risk of the disease as are males.

Once a cat has become infected with FIV it will remain that way for the rest of its life. Up to 5% of apparently healthy cats can be carrying the virus, and amongst high risk cats it may be up to 20%. Conversely some cats which encounter FeLV will be able to fight off the infection and become immune but most will become persistently infected. About 1-2% of cats who appear healthy may be carrying FeLV.

In the UK there is currently no vaccine available against FIV, like HIV it is a hard virus to produce an effective vaccine against. The best way to prevent the disease is by neutering cats to reduce sexual contact and fighting and by testing those cats who are more likely to be carrying the virus and limiting their ability to go on spreading it. For this reason it is a good idea to test cats who are being re-homed, have been living as strays or have had higher risk lifestyles.

Fortunately, since FeLV is a more contagious disease, there is a widely used and effective vaccination against FeLV in the UK. Kittens can be vaccinated once they are no longer protected by their mother’s immunity and regular boosters given for the rest of their life. Again, if a cat is being re-homed and its vaccination history is unknown it can be a good idea to test for FeLV infection, especially if you are going to be introducing it into a group of other cats. If a cat is already carrying the virus, vaccination will not help the cat to become immune.

Cats infected with either virus can carry the virus buried in the cells of the body for some time before showing any signs. FIV may lie dormant for many years and it is possible that your cat will live its normal lifespan without ever showing signs of the disease. On average, once cats have been diagnosed with the virus (often it is picked up in middle age) they will live another 5 years as a ‘healthy’ cat. Cats carrying FeLV however will tend to develop signs of the disease more quickly. Some cats will live for another 2.5 years after the virus is detected but it may well be less than this.

Because the most common effect of both viruses on cats is to interfere with the immune system the signs which indicate their presence can start with what appears to be a ‘normal’ infection. Without a functioning immune system the cat is unable to fight off this infection adequately. Signs may well include a persistent fever, lethargy, reduced appetite, weight loss, swollen lymph nodes (or glands), anaemia (lack of red blood cells), red and sore gums, respiratory or gut problems. Less commonly the viruses may have caused tumours to form. Certain collections of symptoms or poor responses to treatment may prompt your vet to do a blood test looking for evidence of these viruses. Quick in house tests are widely available and can give a result in less than half an hour. In the event of a positive FIV or FeLV result they may send a blood sample away to a more specialist laboratory for confirmation.


If you find out that your cat is carrying FIV but is healthy you may need to do very little for a while. There is nothing you can do to prevent them developing ‘Feline AIDS’ in the future as currently there are no medications available which have been shown to help. Because your cat is potentially contagious to other cats you will need to think about whether they are likely to be in situations where this is a problem. If there are lots of cats in your area and your cat is prone to fighting you may need to think about keeping them indoors for example.

There are more difficult issues to deal with if a cat is found to be carrying FeLV. Even if they are currently well there is a high chance that the virus will make them unwell. They are also a significant risk to other cats since the virus is relatively easy to catch. Cats they come into contact with outside may not be vaccinated and there is always a chance that even a vaccinated cat could still be infected by them since no vaccine is 100%. FeLV infected cats really have to be kept indoors at all times and this might not suit every cat. Some people therefore will make the decision to put a cat to sleep once it has been confirmed that they are carrying the virus.

If your cat is already unwell and the vet thinks that either FIV or FeLV is the underlying cause there is fundamentally little that can be done. Any treatments are likely to give minimal or temporary relief and so you will need to consider euthanasia to prevent your cat deteriorating and suffering any further.

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