Feline leukemia virus (FeLV)

Feline leukemia virus (FeLV) is probably the most important virus in cats. About one in three cats that come into contact with the virus develop a permanent infection which is almost always fatal. FeLV infection causes a wide range of symptoms and by weakening their immune defenses it can also make cats more susceptible to other infections. The effects of the virus on the immune system are similar to those that occur in humans with AIDS, but FeLV only affects cats. It cannot affect humans or other animals, such as dogs.

©Waltham Petcare Science Institute

The Feline Leukemia virus is present in the bodily fluids of affected cats, most commonly their spit (saliva). The virus may be spread when cats groom each other, share the same food bowls or litter tray, sneeze on or bite another cat. Infection may also occur during sexual relations, across the placenta or in milk from a mother cat to her kittens.

About one in every 100 cats has a persistent infection in which the active virus is permanently present in their body. When several cats live under the same roof, there is more chance of the virus passing from one animal to another and more cats may be affected. The chances of being exposed to the virus rise with increasing age. However, it is young animals (under six months of age) that are most likely to be infected with the virus, and one in three of these will go on to develop disease.

In half the cats that die of FeLV the problem is a direct result of destruction of the white blood cells that are one of the main bodily defenses against disease. This leaves the cat wide open to infection from any one of a range of other germs. The next most important problems are anemia and cancer. In the remaining cases the symptoms can include infertility, abortion, stillborn or very weak kittens, inflammation of the eyes, rapid weight loss, gut disease or nerve damage. An infected cat may appear healthy for several months but about eight out of ten cats are dead within three years of being infected by the virus.

FeLV must be suspected when your cat gets one illness after another. There is a simple blood test to show whether your cat has had contact with the virus. However, a positive result is not necessarily disastrous – it may just mean that your cat has been infected but is now immune to the virus. Similarly, a negative result is not an all clear – if the infection was recent your cat may not have reacted (produced antibodies) to the virus yet. Veterinarians usually like to take two tests a few weeks apart to give more reliable information about your cat’s health.

There is no way to stop an infection once it has become established. Medical treatments may make your cat more comfortable or help treat other infections which may occur as a result of FeLV. FeLV vaccines can prevent disease in about eight out of ten vaccinated cats exposed to the virus. The first in a series of vaccinations should be given to kittens at about nine weeks old and annual booster vaccinations are needed to maintain protection. If your cat has already been infected by the virus, it is too late to vaccinate it. FeLV vaccination may not be included in the routine vaccination given by your veterinarian. If you would like your cat to be protected against this disease, contact your veterinarian for advice.

The vaccines for FeLV are very safe but a reaction, such as a lump forming at the injection site, may occur in rare cases. If your cat is the only one in the household and spends all of its time indoors, there is no risk of contact with the virus and vaccination is probably unnecessary. When there are several cats in the house, or when a new one is about to arrive, it is worth having them all tested for FeLV and those which are free of FeLV antibodies should be vaccinated in good time before the new cat is introduced.

If two blood tests taken a few weeks apart show antibodies to FeLV it is safe to say that your cat is infected with the virus. The infected cat must be kept away from other cats in your home (even if these are vaccinated) and should not be allowed outside. It may be necessary to re-home your cat to a new home where it cannot pass on the infection to other cats. Sadly, in some situations you may have to think about having the infected cat put to sleep euthanized).

Your new cat is unlikely to be at risk from infection lingering in your home. The virus does not live long outside the body. However, to be safe all feeding bowls, litter trays, etc should be washed with hot soapy water and surfaces rinsed with a weak solution of bleach (about 5 tablespoons of bleach in a gallon of water) before introducing a new cat into the household.