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What is feline leukemia anyway? Does my cat need the vaccine?

09/26/2009

Many cat lovers are aware of feline leukemia, but aren't exactly sure what it is. They know it's bad, but is it an infection, or cancer, or...?

Here's the story. There is a virus called the feline leukemia virus, often abbreviated FeLV. It's in the same family of viruses as the human HIV virus, and the feline FIV virus. They are all in the family called retroviruses.

It's confusing to folks because leukemia is a kind of cancer (cancer of the bone marrow), so when we say a cat has feline leukemia, it seems like the cat has cancer. Really it's more correct to say that a cat is "feline leukemia virus positive" which, like a person being HIV positive, means the cat is infected with the virus. The reason the virus is confusingly called the feline leukemia virus is because it can cause cancer in cats, particularly lymphoma. In fact, it is thought that lymphoma in even feline leukemia negative cats can be caused by prior exposure to the virus, and the virus has been found in feline tumors. So maybe they should have called it the feline lymphoma virus.

If possible, cats who are positive for feline leukemia on the kind of test called an ELISA should then be tested with another test called an IFA. ELISA tests can have false positives, or can be positive in cats who are infected but are going to fight off the infection. If the IFA test is positive, it means the cat is permanently infected.

The feline leukemia virus is more severe than FIV. It causes serious health issues more quickly and is more infectious. FeLV positive cats often develop anemia, infections, cancer, and other problems. The best living situation to keep an FeLV postive cat alive and healthy as long as possible is for the cat to be an only cat who is kept indoors. Unfortunately, it's hard to find homes for FeLV positive cats, much less homes where they are the only cat and never go outside. I had a friend who had a cat who was both FeLV and FIV positive; she kept him inside and he was her only cat, and he lived for many happy years before eventually developing lymphoma.

It's important to realize that while FeLV is more infectious than FIV (which is mainly transmitted by bites during fighting in unneutered males), FeLV is only transmitted by close contact such as grooming, licking, or sharing food or water. It won't be brought in on the owner's shoes or clothing, for example, or on the air, and it doesn't live a long time in the environment or on surfaces. By contrast, the panleukopenia virus (feline distemper) lives a very long time in the environment, up to a year or more, and can easily be transmitted through contaminated objects; there doesn't have to be contact between the cats themselves.

Because actual contact between cats is required for transmission of the feline leukemia virus, there is little point to vaccinating an indoor cat who never meets new untested cats. If you have a completely indoor cat who is negative for the feline leukemia virus, she just can't get it (unless you bring a strange cat home without testing him first). So this vaccine only needs to be given to cats who go outside or who will be exposed to untested cats. You should also be aware that the vaccine is not 100% protective - a vaccinated cat can still get the virus. The only way to really protect your cat from FeLV and FIV is to keep her inside.

The American Association of Feline Practitioners does recommend that vets vaccinate all kittens against feline leukemia; they are concerned that it is hard to predict what a kitten's lifestyle will be throughout his entire life. They're worried about a scenario where the vet doesn't vaccinate the kitten against feline leukemia because the kitten is being kept indoors, and then the owner gives him away to someone else who lets him go outside. From your perspective as a cat owner, you need to understand that indoor cats don't need the vaccine but if your cat's lifestyle changes at some point the vaccine may become necessary.

Unfortunately, the feline leukemia vaccine is one of those that has been associated with the development of tumors at the vaccine site. The new transdermal vaccine has been formulated to combat this problem. The other vaccine that is implicated in tumor development in cats is the rabies vaccine; the newer Purevax vaccine does not contain the ingredients that are thought to cause these tumors. It is also thought that long-acting steroid injections may cause tumors at the injection site in cats; another reason to avoid long-acting steroids when possible.

The most current thinking on vaccination of pets is that protocols should be tailored to the indiviual pet's situation. All pets should receive the core vaccines for diseases that are highly infectious and devastating, such as panleukopenia, and the other vaccines should be chosen based on the particular animal's risk factors. Pets should not routinely be given every vaccine on the market whether they need it or not; I am happy to see this happening less and less. Before your cat receives a feline leukemia vaccination, discuss with your vet whether it is necessary based on your cat's living situation.

Here is some information about the feline leukemia virus and FIV that you may find helpful.

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