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Feline Leukaemia (FeLV)


Infection Routes
Signs & Symptoms
Testing & Treatment
Vaccinations & Antibodies

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What is Feline Leukaemia (FeLV)?

Firstly, FeLV (Feline Leukaemia) should NOT be confused with FIV (Feline Immunodeficiency Virus) - they are two very different viruses. They are often mentioned together due to the 'snap' tests carried out by vets, but they differ greatly in how they affect a cat, and it's expected lifespan. FeLV is a serious risk to a cat's health and longevity, whereas FIV is not. To find out about FIV go here »» FIV information

Like FIV, the feline leukaemia virus (FeLV) belongs to a group of viruses called 'retroviruses', but to different families within the group. Both are long-term viruses which compromise a cat's immune system, leaving him more susceptible to other infections. In addition, FeLV also causes tumours in 20-30% of infected cats.

How does a cat catch Feline Leukaemia?

Cats become infected with FeLV in two different ways:
(1) The unborn kittens are infected while in the uterus if the mother is carrying the virus. They may also be infected from her milk. This is different from FIV, where kittens are not infected by the mother, although they may receive antibodies.
(2) Cat-to-cat infection occurs only where there is prolonged, intimate contact between cats. The virus is shed in the saliva of infected cats and dies very rapidly once outside the body, so it is only likely to be transmitted as a result of close contact with an infected cat. This means mutual grooming, sharing of feeding bowls or actual biting are necessary for the infection to be passed on. Note the difference from transmission of FIV, where an actual bite is necessary for the infection to be passed to another cat - sharing of feeding bowls and mutual grooming is not considered to be a channel of infection for FIV.
FeLV is also found in tears, nasal secretions and urine from infected cats, but this is much less likely to be a source of infection to another cat than saliva. Blood is another source of infection and blood donors should always be screened for both FeLV and FIV.

What happens when a cat is exposed to infection?

This depends on the age of the cat. Resistance to infection increases with age.
Most new-born kittens will receive maternal antibodies to FeLV, which protect them if they are exposed to the virus. However, if they have not, (because their mother has not encountered it) and they are exposed to it, they will be permanently infected. In kittens between six weeks and four months (whose maternal immunity has waned), 85% exposed to the virus will be permanently infected .
Of kittens over four months or adult cats, only 15% become permanently infected. The other 85% will produce antibodies to the virus and recover from the infection. These cats will have a life-long immunity to FeLV, which can be assessed by the Virus Neutralising Antibody (or VN) test. (This test is only carried out at the Feline Virus Unit at the University of Glasgow, and the vet will send a blood sample to them.). However, many vets do not appear to have heard of this test, so you may need to ask for it specifically.

It will be obvious from this, that the most susceptible age-group to infection with FeLV is the six weeks to four month-old kitten. Of those which become permanently infected, 80-85% will die within 2-5 years. Kittens infected before birth will also die within this time-scale. The long-term outlook is not good for kittens infected at this age.

What are the signs of Feline Leukaemia?

Two to four weeks after infection the cat may be off-colour and run a temperature for a short while. This may be so slight as to be unnoticeable. After he recovers from this there are no further signs for a variable period of time - ranging from a few weeks to five years. When (and if ) signs do eventually develop, one third of the affected cats will develop tumours - either in the chest (thymus gland), bowel, lymph nodes, kidney, liver, spleen or bone marrow (true leukaemia). The remaining two-thirds develop a variety of other problems - anaemia, infertility, abortion and various infections due to the destruction of the cats natural immunity by the virus. These include Feline Infectious Peritonitis (FIP),Toxoplasmosis, Feline Infectious Anaemia and various chronic infections, such as gingivitis, skin infections and flu. Since all these conditions can cause a variety of symptoms and all the symptoms could be caused by other diseases, feline leukaemia can only be diagnosed by a blood test.


The initial test is usually a blood test. Veterinary surgeons may buy kits to carry out these tests on their premises or the blood may be sent to a laboratory. These tests are known as ELISA tests, although they have a variety of brand-names. They detect antigen (part of the virus). There is also a test for use with saliva.
Whichever of these tests is used, they are all highly sensitive and sometimes give 'false positive' results. Possible causes of false positives are cross-contamination of samples or operator error in failing to follow the instructions absolutely accurately. However, these tests are useful in that they are quick to perform, giving an instant result if the cat is to be vaccinated provided he tests negative.
If the ELISA test is positive, then a further test should be carried out to determine whether it is a true positive i.e. whether the cat really is infected. There are two tests which may be used. The Virus Isolation Test is carried out commercially in the UK only at the University of Glasgow Feline Virus Unit or the University of Bristol, Langford Veterinary Diagnostics. As Glasgow make no extra charge for carrying it out on any samples which are positive to the ELISA test, some vets prefer to send the first sample to them, to avoid possibly having to take a second blood sample. However, as most samples will be negative anyway, other vets use the test-kits first, only sending away samples from cats which are positive. The IFA (ImmunoFluorescent Antibody) test is used by Idexx Laboratories, and TDDS (click for details) which is more reliable than ELISA tests and almost as reliable as Virus Isolation (It is accurate 98.3% of the time). As it is quicker than Virus Isolation it could be used as a screening test. However, the Virus Isolation Test is usually preferred for confirmation.
Cats which are positive to the Virus Isolation Test may have been recently infected. 85% of cats will develop Virus Neutralising Antibodies, eliminate the virus and be immune to further infection. This may take up to 8 weeks. Therefore all positive cats should be re-tested after 8-12 weeks (and, of course a Virus Isolation Test should be carried out again if the test is still positive to ensure the accuracy of the result).
A positive FeLV test does not prove that a disease which a cat has is caused by FeLV. It could be an incidental finding. Therefore if a cat is found to be positive, tests should still be carried out for other possible causes of the illness.


One third of cats which become ill from the effects of feline leukaemia virus develop tumours (lymphosarcoma). Although there is no specific treatment for the virus, some forms of tumours are treatable with cytotoxic drugs. While these do not produce a cure, they may achieve variable periods (possibly several years) of remission (i.e. a period when the cat appears to be cured.) Combinations of drugs are used for the best results.
Two-thirds of cats with leukaemia suffer, not from the direct effects of the disease, but from other infections associated with the destruction of their immunity by the leukaemia virus. Treatment is available for these conditions, but some cats will die despite it because they are not able to fight the infection for themselves. Vets frequently suggest that a cat is tested for leukaemia if he fails to respond to treatment for a chronic infection.
Anaemia is a frequent occurrence in cats with leukaemia. It is said that FeLV + cats are more prone to develop anaemia caused by a blood parasite which gains a hold because of the cat's inability to shake it off. This disease is treated with doxycycline and steroids, with variable success. Or anaemia may be due to the direct effect of the virus on the bone marrow. This sometimes responds to treatment with steroids or other immuno-suppressive drugs. It is certainly worth trying.
Infertility and abortions are present in 9 out of 10 FeLV positive queens. The remaining queens will produce infected kittens and perpetuate the disease. There is no treatment for the kittens which will almost certainly die within 2 years. The queen should be spayed to prevent her producing more infected kittens.


Feline leukaemia virus is passed from cat to cat either from the infected queen to her offspring or by close contact between kittens under 4 months of age. Only 15% of cats over 4 months will become infected and then only if exposed to heavy doses of the virus.
Therefore if all breeding queens were screened for FeLV and spayed if positive the disease could be eliminated within a generation. Unfortunately, this is not going to happen! Leaving aside pedigree cats (which are generally blood-tested anyway) most feline pregnancies are unplanned.
Because of the long period between cats contracting the virus and becoming ill (up to five years in 85% of cases and even longer in 15% of cases), there is a considerable length of time when apparently healthy cats may pass on the infection. The main method of control is to utilise the available tests to identify carriers of the virus and prevent them from passing it on to other cats. Since the virus does not live for long outside the cat's body, intimate contact is required for it to be passed from cat to cat. In practice, this means mutual grooming, eating out of communal feeding bowls and bites inflicted during fighting or mating. Therefore sensible precautions should be taken to avoid this happening. There is no risk of cats passing on the virus to other cats merely because they live in the same house if they studiously avoid each other! Equally, there is no risk of cross-infection in a properly run boarding cattery. Litter trays are a potential source of infection if they are not changed after use, since the virus is present in urine. However, it is much less likely that a cat will obtain a high dose of the virus this way than through being licked by a cat which has the virus in his saliva.

There are several vaccines against Feline Leukaemia on the market in the U.K. These vaccines are in the order of 70-80% effective, so it is important to realise that some cats will still not be protected even though they are vaccinated. It is recommended that cats should be tested for FeLV before vaccination, because the vaccine will be ineffective if the cat is already infected. However, the cat will not come to any harm if it is carrying the virus and is vaccinated, so some vets do not feel that the additional expense to the client and stress to the cat is justified. If a test for FeLV is carried out however, and found to be positive, the test will need to be repeated 12 weeks later. If it is then negative, it can be assumed that the cat has overcome the infection and developed its own life-long immunity. This can be checked by the Virus Neutralising antibody test. If the cat is found to be immune then the cost of annual vaccinations will be saved, (which is something to bear in mind when you are paying out for repeat blood tests!)

What to do with an FeLV positive cat

Once a cat has reacted positively to two virus isolation tests 12 weeks apart, he is certainly permanently infected.
If he is healthy no treatment is necessary, but it has to be faced that 85% of positive cats will die within 5 years (as compared to 15% of the general cat population). For the rest of his life he will be a potential source of infection to other cats if he comes into intimate contact with them. Generally speaking, if anyone who owns a FeLV positive cat is thinking of acquiring another cat this should be tested for FeLV Virus Neutralising antibodies. If it has antibodies it is immune to leukaemia and it is safe to mix it with a positive cat. The mere fact that the new cat has been vaccinated is not enough - it may be one of the 20% who fail to respond to the vaccine. But if circumstances arise when it is unavoidable for an owner in this position to take in another cat (as happens frequently in cat rescue situations), remember that the virus is actually very difficult to pass on to other adult cats. Simple precautions like separate feeding dishes and cleaning the litter tray out as soon as it has been used are sensible.
If the infected cat is one of a multi-cat household, the others may either carry the virus or have Virus Neutralising antibodies, which means they are immune (this is very likely if they are all adults) or if they have no intimate contact with the infected cat, they may have neither. Remember, the highest risk of infection is to kittens under 4 months.

The FeLV positive cat in a rescue centre

These cats pose a great problem to rescue centres as relatively few people are willing to adopt them. As a general principle, healthy FeLV positive cats should be re-homed to households without other cats (certainly without kittens). However, this should not be adhered to so strictly that the welfare of the individual cat is forgotten, since the risk of transmission between adult cats is actually quite small. 85% develop their own immunity to Feline Leukaemia, but if they have not been tested for this, or they are tested and found to have no immunity, they may be vaccinated against it, and that apart, they are simply not easily infected with the virus.
FeLV positive feral cats pose a particular problem. If they are prone to fight they should be neutered as this may decrease their aggression, (in practice they are unlikely to be tested unless they have been trapped to be neutered.) There is not a great deal else that can be done, although an aggressive feral cat might be 're-located'. In a colony situation, provided the entire colony has been neutered, so that no more infected or vulnerable kittens are born, it is just as well to leave them all together. The adult cats which are not FeLV positive will probably be immune.
Unfortunately, it is pointless to think we shall ever succeed in eliminating FeLV from the cat population because there is a large pool of cats who are never seen by a veterinary surgeon. Therefore the practice of killing individual healthy cats who happen to have been tested and found to be positive, with the object of preventing them spreading the infection is not only callous but pointless. We should do our best for each individual cat in the circumstances.

Testing Labs

University of Glasgow, Veterinary Diagnostics Services:
University of Exeter, TDDS (Torrance Diamond Diagnostic Services), Exeter:
University of Bristol, Langford Veterinary Services:
Idexx Laboratories:

Further Information

Why neuter? / Low cost neutering »» Further neutering information
Post your query under 'Health & Behaviour' on our Feline Forum »» the Cat Chat Forum

help prevent the spread of feline leukaemia - please neuter your cat

Our grateful thanks go to Maureen Hutchison BSc, BVMS, MRCVS for supplying the information for this page.
For information on many other feline medical conditions - visit the Cats Protection website »
UK Animal Rescue

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