Feline Leukaemia Virus - What You Need To Know


Chris Hoare, M.A. Vet M.B., MRCVS

Received: January 1997

Which infection kills more cats than any other?

Feline Leukaemia Virus. It also seems to create more confusion in peoples' minds than any other! This article highlights the major facts about Feline Leukaemia Virus (known as FeLV) that affect all cat owners.

I thought Leukaemia wasn't infectious

You're right. Leukaemia and other cancers normally are one-off events that affect an individual only. (See the section on cancers below.) But cats can get cancers as a result of being infected by Feline Leukaemia Virus. It's not the cancers that are infectious, but the virus that causes them.

What is FeLV?

It's a virus that affects only cats. It got its name because it can cause Leukaemia (see the glossary below), but in fact the virus can cause many other different diseases too.

How do cats get FeLV?

They get it from close contact with other cats that are infected.

How do I know if a cat is infected?

It is impossible to tell without a blood test. The virus can be present for a long time before a cat shows any symptoms, so an apparently healthy cat is not necessarily free of infection.

What will happen if my cat is exposed to FeLV?

The virus will start to multiply in your cat. Fortunately most cats then make antibodies to the virus and reject the infection. But about 30% of cats - especially younger ones - cannot reject FeLV and they become permanently infected. These cats shed FeLV and are the source of infection for other cats.

What happens to permanently infected cats?

The virus keeps on multiplying inside them until it damages their white blood cells (see the glossary below). When this happens the cat can get infections that it would normally fight off easily. These infections eventually kill the cat. Most permanently infected cats die within three years or so of becoming infected.

Sometimes the virus causes cancer of the white blood cells before it has caused other major damage. These cats die of the cancer.

How can my vet tell if my sick cat has FeLV?

The only way to tell is by doing a blood test. The symptoms in infected cats are so variable that they are not a reliable way of identifying FeLV infection. But if your vet finds signs like a fever that won't go away, or persistent infections of the eyes, nose or mouth then a blood test may be recommended to see if FeLV could be the cause.

How can you treat cats with FeLV?

Unfortunately there is nothing we can do to halt the FeLV infection. We can help to treat some of the symptoms, but infected cats usually die quite soon after symptoms are seen. Very often the kindest thing is to put affected cats to sleep rather than prolong suffering.

So if there's no treatment, how can I stop my cat from getting FeLV?

There are two ways:

  1. Stop your cat from meeting infected cats. Since you don't knowwhich cats in your neighbourhood are infected, the only way to be sure of this is to prevent your cat from ever meeting any other cats (except ones that are clear on the blood tests for FeLV). For most pet cat owners this is not practical.
  2. Vaccinate your cat. Fortunately this is a highly effective way of preventing FeLV infection. It can be done at the same time as cat 'flu vaccination.

How common is FeLV?

Uncomfortably common. Estimates vary between 1% and 10% of UK cat deaths being due to FeLV. The variation in these figures is because the true incidence is quite hard to measure.

What about what my Aunt read in the Bolivian Cat Gazette?

As shown above, FeLV is unlike most other infections. This has commonly led to people misunderstanding the disease, which in turn results in inaccurate 'facts' being reported about FeLV. These cause a lot of confusion. The only way to be absolutely sure that what you are hearing is true is by checking with a vet who deals daily with cats.

Anything that damages genes can cause cancer. Examples include ultraviolet light and some chemicals. Normally quite large amounts of damage from these are needed to make cancer more likely.

Cats have one extra major cause of damage to genes. Feline Leukaemia virus actually lives in the cat's genes inside cells. Since it may eventually infect most of the cat's white blood cells, it often damages the genes of these cells. So FeLV can often cause cancer of the white blood cells in cats. When these cells are in the blood this is called Leukaemia.

Cancer is a disease where a part of the body starts to grow wrongly.

In healthy animals cells (see the glossary below) divide often to replace ones that are worn out or lost. For example some of our white blood cells only last a day, and those lining our intestines are worn away after only four days.

The genes (genetic code) of each cell decide how much it needs to divide, and when it must NOT divide.

If the genes are damaged a cell may start to divide non-stop. More and more of that type of cell are made. They may also spread to other areas of the body. This is cancer. Animals die of cancer when the cancer cells have damaged too many of the normal cells, which then stop working.

Causes of cancer


Antibodies: special proteins that the body makes to target and attack specific infections.

Cells: All animals are made of tiny building blocks called cells. Each type of cell has a different job. e.g. brain cells store messages and white blood cells attack bacteria and viruses.

Genes: These are the 'computers' that control the behaviour of cells. They dictate whether cells will grow or divide, or even whether they will become a heart cell, a white blood cell or some other sort of cell.

Leukaemia: cancer of the white blood cells. In people and dogs this is usually NOT caused by viruses. Cats are a special case because only they can get FeLV.

White blood cells: the cells that defend the body against disease-causing organisms such as bacteria and viruses.

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