Feline Leukaemia Virus (FeLV)

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What is FeLV?

FeLV is an acronym for Feline Leukaemia Virus, in addition to Leukaemia it also weakens the immune system, causes anaemia and causes certain kinds of cancer.

80% of cats with FeLV die because of their weakened immune system and 85% will die within 3 1/2years from infection, a very nasty virus.

How do cats catch FeLV?

FeLV is usually transmitted from cat to cat in saliva. Cats who live together and enjoy washing each other are at particularly risk if one of them catches FeLV, as the infection is ingested. A cat bite would also have the same effect.

Mother cats will usually pass on the infection before the kittens are born, or from ingesting milk.

Kittens under 4 months old are at higher risk of catching FeLV. After 4 months they will build up a natural resistance, though this resistance is not strong enough to completely protect them if they enter a house with lots of FeLV infected cats or are continually exposed to the virus.

FeLV is a fragile virus that doesn't survive long on hands, bowls or carriers and is easily killed with disinfectant

Symptoms and diagnoses

FeLV infected cats don't show many clear signs of sickness. After they are first infected the cat may have raised lymph nodes (glands), raised temperature, loss of appetite and general lethargy.

Diagnosis is usually made with a simple blood test at the vets. The blood test detects the part of the virus called the p27. About half of in-house vet tests come back with false positive results and should be confirmed by virus isolation (growing the virus in a cell culture) This will be done at Glasgow or Bristol university veterinary schools.

In around 5-10% of cats with positive p27 results, no virus can be detected by virus isolation. Cats with negative virus isolation results are not infectious to other cats.

Clinically well cats should not be euthanased on a p27 result alone in case the result is a false positive.

Recently it has been recognised that some p27 tests on sick cats have given false negative results. Such cats should also be retested by virus isolation

It can take up to 8 weeks from infection for the virus to show up in a blood test, so it is often recommended to take 2 tests 12 weeks apart to be sure of getting the correct result.

Another good reason to do the two tests is that sometimes a cat can develop immunity to the virus. If the cat is in the process of developing this immunity when they are tested it creates a false positive result. If a second test 12 weeks later still shows up positive then the cat is infected, if not the cat is immune.


There are several different types of FeLV vaccines available. None of which are 100% effective.

Kittens can be vaccinated after 9 weeks, all the vaccines consist of an initial course of 2 doses three to four weeks apart, with yearly boosters recommended.

Sick cats should never be vaccinated.

You might also be interested in our article on FIV (Feline Immunodeficiency Virus)

This article was first published in The Purr Company's regular Mews-letter, visit us for more cat stories and articles, a gallery of our visitors cats , cat videos and our online shop.

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