What is Feline Leukaemia Virus (FeLV)?

FeLV is an incurable viral infection found only in cats. There is no risk to humans since the virus is not transmissible to any other species. FeLV is caused by a type of virus called an RNA retrovirus and was first described in veterinary medicine in 1964. The virus infects the cat's cells and reprograms them to continually reproduce itself, resulting in what is called 'persistent viremia', meaning that the virus persists in the bloodstream, making the disease impossible to cure.

Around one to two per cent of cats in Britain are infected with FeLV and the majority will die within four years of being diagnosed.

The virus adversely affects bone marrow and compromises the immune system. Initially, FeLV causes only mild symptoms of fever and lethargy but as the disease progresses, over the coming months and even years, the symptoms become more severe. As the immune system becomes compromised (via damage to the white blood cells) the cat becomes more susceptible to catching, and less able to fight off, even routine infections. Hence, as with Feline Immunodeficiency Virus (FIV), FeLV can have many different symptoms, in the form of the many different secondary infections that the cat might pick up.

In general, then, symptoms indicative of FeLV would include:

  • frequent infections of any kind
  • jaundice
  • anaemia (which causes lethargy, weakness & very pale colouring of the gums and tongue)
  • enlarged lymph nodes
  • weight loss
  • fever
  • neurological symptoms e.g. difficulty walking
  • recurrent diarrhoea

The effect of the virus on the immune system can also result in cancers of the white blood cells, called 'lymphosarcomas'. This type of cancer can affect many different organs, starting with the lymph nodes and sometimes spreading to the liver, spleen, intestines and brain. About one-fifth of FeLV infected cats die from cancer (source: wikipedia).

How is FeLV transmitted?

Infected cats carry the virus in all bodily fluids - blood, saliva, mucus, nasal discharges, urine and faeces. The virus cannot survive easily outside of the cat's body but nonetheless does survive in the environment for a time (although experts disagree on how long for). Transmission can therefore occur not just by mutual grooming, biting or mating but also by sharing water or food bowls or litter trays. Some experts say that fairly long-term contact is needed for the virus to be transmitted via shared bowls or trays, but this has not yet been proven either way.

The virus can be transmitted from mother to unborn kittens, all of whom are usually infected at birth. Occasionally, an apparently uninfected mother can infect her kittens - presumably via a localised infection confined to the mammary gland, which is then passed to the kittens via the milk.

Kittens are more susceptible to the virus than adult cats because the body's resistance to FeLV increases with age. Cats over six months old are fairly resistant but high-level exposure (via direct contact such as biting or mating) with infected cats can still overcome this resistance.

What are the symptoms?

FeLV generally has 6 phases:

  • The virus enters the cat, usually through the pharynx - the tube that connects the mouth and nose with the oesophagus (which leads to the stomach) and the trachea (which leads to the lungs). Here, it infects the top-most layer of tissue and also the white blood cells in the tonsils. These white blood cells then travel to the lymph nodes and begin to replicate.
  • The virus now enters the blood stream and starts to distribute throughout the body.
  • At this point, the lymphatic system (the tissues and organs which produce, store, and carry the white blood cells that fight infection and disease) becomes infected. Since the lymphatic system is a network which carries lymph and white blood cells into all tissues of the body, this results in further distribution of the virus throughout the body.
  • At this crucial stage, the immune system will either fight off the infection or be taken over by it, resulting in viremia (the virus is now carried in the bloodstream). The intestines become infected.
  • The virus then spreads to the bone marrow (which is where the different types of white blood cells are produced). At this point, the FeLV infection takes hold and becomes permanent i.e. the cat becomes terminally ill with FeLV. In the bone marrow, the virus now replicates and infects various types of white blood cells, which are then released four to seven days later.
  • Eventually, the body is overwhelmed by infection and the protective tissue lining (epithelial cells) of hollow organs (mouth, nasal passages etc.) and glands become infected. The virus replicates in various epithelial tissues including the salivary glands, pharynx, stomach, oesophagus (leading to the stomach), intestines, trachea, renal tubules, bladder, pancreas, alveolar ducts (air sacs in the lungs), and sebaceous ducts from the muzzle.

Cats diagnosed as persistently infected with FeLV can die within a few months but might display no symptoms for up to 4 years.

There is some hope, however, since exposure to FeLV does not always result in permanent infection:

  • In approx. 40% of cats, the immune system successfully extinguishes the virus; analysis of their blood shows specific antibodies against FeLV, meaning that there was, at some point, exposure to the virus but that the body was successful in fighting it off.
  • In approx. 20% of cases, the immune system will force the virus into a latent stage, whereby the virus will remain dormant until a later point in time when perhaps the cat becomes stressed, causing the FeLV to re-emerge.
  • In approx. 10% of cases, the viremia is limited, intermittent, or absent altogether.
  • In the remaining 30% of cases, the cat will go through the disease from start to finish, normally resulting in death.

How is FeLV diagnosed

There are two types of blood test available:

  • the ELISA (or 'kit' test), which is likely to be the one used by your local vet, and
  • the Immunofluorescence Assay (IFA) test which is only done at veterinary diagnostic laboratories

Both types of test look for the actual protein of the FeLV virus circulating in the blood.

The blood tests can be carried out at any age from birth. However, the testing is not straightforward since both false positives and negatives can occur. Hence, a vet will often recommend that the result found by the ELISA test be confirmed by an IFA test at a commercial laboratory.

If a recently exposed, but healthy, cat tests positive with the 'in-house' ELISA test, it is recommended that you wait at least 12 weeks and then retest; this is because it is likely that their immune system will successfully fight off the infection. A retest will increase the accuracy.

If the cat then tests positive again, but remains healthy, your vet may recommend an IFA test be carried out by a commercial laboratory to confirm or disprove the ELISA test result.

The situation is different if the cat is ill because, statistically, a positive result in a sick cat with symptoms compatible with FeLV is likely to mean that the cat is indeed infected with FeLV.

With false negatives, the test comes back clear but the cat is actually infected. With a healthy cat that has been exposed, therefore, it is again recommended that a retest be done at least 12 weeks after the first test. This retest will pick up those cats with FeLV that were in the early stages of infection at the first test and did not then develop immunity.

A false negative can occasionally occur with a sick cat. If other symptoms point towards FeLV in a 'negative' sick cat, commercial IFA testing may be recommended.

Sometimes, a cat that has tested positive more than once on the ELISA test, will still test positive on the commercial IFA test. The explanation for this is thought to be due to a 'latent' FeLV infection, where the virus is hidden in the body (usually in the bone marrow). As long as the IFA test remains negative, there is no risk to other cats, but the situation should be monitored.

Is it possible to vaccinate against FeLV?

Yes. A vaccination for FeLV is available in the UK. You should consult with your own vet when deciding whether to vaccinate your cat against FeLV.

Vaccination is usually recommended for cats that go outside, especially younger cats. However, the chances of infection vary between geographical areas and cats that never go outside will probably not need vaccinating. If you are planning on adopting another cat, it may be a good idea to blood test the new cat before introducing him / her to your other cat/s, just to be sure.

There is always a slight risk of side effects with vaccinations - this is true for humans, dogs, cats, in fact, any animal. Some cats will develop a mild reaction to the vaccination and will be just a little bit off colour for a few days afterwards. Occasionally, a lump may develop where the vaccination was injected - if this lasts for more than a few weeks, you should take the cat back for a check-up.

The FeLV vaccine is as safe as other commonly used feline vaccines and is thought to be effective in preventing infection with the disease. However, it should be noted that no vaccine is ever 100% effective and there will always be a few cats that contract FeLV despite being vaccinated.

So how do you treat FeLV?

Unfortunately there is no cure for FeLV. Infection with the Leukaemia virus is a death sentence for a cat.

Importantly, though, this can be prevented by keeping your kitten indoors until he / she has been fully vaccinated and neutered/spayed. Although FeLV can be spread by other means, the majority of transmission will occur via mating or deep penetrative bites during fighting. Both fighting and mating are behaviours associated with un-neutered and un-spayed cats and therefore neutering and spaying (and, importantly, keeping the kitten/cat inside until they have had the operation) is one of the best ways to protect your cat. Remember too that although FeLV can be vaccinated against, Feline Immunodeficiency Virus (FIV) cannot and so neutering/spaying is the best protection you can give your cat. All of the cases of FeLV/FIV we see here at SHUA are simply because cats have not been neutered or spayed (let alone vaccinated) which means mating and territorial fighting, which spread disease. As contradictory as it sounds, neutering and spaying really does save lives.

FeLV undoubtedly reduces lifespan, but predicting how long an infected, but currently healthy, cat will live is impossible. Most survive less than two years after diagnosis but some will live longer. It should be noted that all infected cats, healthy or sick, can infect other cats and must therefore be kept indoors and away from other uninfected cats (including other cats in the same household). There is a test which can identify antibodies likely to give immunity to FeLV and this can be used to test other cats living with an infected cat.

FeLV suppresses the immune system and so the best treatment for an infected cat is to try to limit the chances of contact with infection and boost the immune system as much as possible. This will involve:

  • feeding the cat a high quality diet (without any foods that may carry bacteria such as raw meat or eggs)
  • minimal stress
  • regular vaccinations (although you should discuss with your vet whether it might be better to use a special 'killed' vaccine)

You must keep a close watch for the first sign of any illness, since prompt treatment of any secondary infections can help to prolong a cat's life. Regular veterinary check-ups are recommended. Antibiotic courses are likely to be needed for infections and your vet may recommend that the cat be put onto antibiotics indefinitely, to minimise the risk of catching secondary infections.

Cats with FeLV are also more susceptible to cancer. Chemotherapy treatment is available and can be attempted, however, this doesn’t work for all cats and the underlying viral infection remains. Chemotherapy can, at best, give a FeLV infected cat with cancer a further 6 months or so. You should discuss treatment with your own vet.

Mayhem's story

Little Mayhem was barely 7 months old when she came to SHUA in May 2006. Shortly after arriving, she was diagnosed with FeLV. Read about Mayhem.