One of the most dangerous infectious diseases in cats today is caused by the feline leukemia virus (FeLV). FeLV is a retrovirus that may cause suppression of the immune system, impairing your cat's ability to fight infections. It may also cause anemia, leukemia and some forms of cancer.

The signs of FeLV, like other viral infections, are very diverse. Symptoms may include:

Pale gums
Weakness
Lethargy
Diarrhea
Weight loss
Anorexia
Breathing difficulties
Increased thirst and neurologic abnormalities

The virus is contagious among cats. It is spread by contact through the saliva, tears and urine. Outdoor cats are especially at risk. There is no evidence that FeLV causes disease in dogs or people.

FeLV is easily diagnosed by a simple blood test. We strongly recommend all new cats be tested for this virus. Once a cat tests negative, a vaccine is available in an attempt to prevent infection with the virus. Although it is not 100 percent effective, the vaccine does offer immunity to most cats and has minimal side effects. Unfortunately, there is no cure for cats that are infected with FeLV.

It is recommended to vaccinate all cats ten weeks or older who are likely to be at increased risk of becoming infected. This includes cats that spend any time outdoors, in multiple-cat households, or at catteries and cat shows.

If your cat is strictly indoors, you may want to discuss the need for FeLV vaccine with your veterinarian. FeLV and rabies vaccinations have been implicated in the very rare inject-site sarcoma syndrome.

Feline Immunodeficiency Virus (FIV)

The Feline Immunodeficiency Virus (FIV) was first identified in 1986. As its name implies, the virus may cause a syndrome in cats very similar to AIDS in people. Like HIV, FIV may lie dormant for years before symptoms occur. An FIV-positive cat may even live out his natural life without developing symptoms of the disease.

If the virus does become active, it may suppress the immune system, allowing infections that can normally be controlled by the body to cause severe, debilitating disease.

Cats can transmit FIV to other cats only through bites, not through urine or casual contact. No evidence exists to show that FIV is transmissible to people or dogs. A blood test, often combined with the test for FeLV, can identify infection. This test is recommended for all cats who are tested for FeLV.

Recently, a vaccine has been developed to help reduce the risk of contracting FIV. It is about 84 percent protective. However, it is not effective against the strains of FIV found in the NY area. Unfortunately, there is no cure for cats that are infected with FIV.

The best protection from FIV is prevention. Keep your cat indoors and have him or her neutered, since neutered cats tend to fight less. Most importantly, have all new cats or kittens tested for the virus before introducing them to your home

Controversy About Vaccination against FeLV

There is some controversy about whether or not it is prudent to vaccinate your cat against FeLV. For this reason we have decided to present to you two arguments, one for vaccination, and one against vaccination. The choice is ultimately yours, as your cat’s guardian. A similar situation may be found in human medicine with the new vaccines against Chickenpox and Human Papilloma Virus. Parent must decide for themselves the best course of action, and likewise, in this case, you must decide whether or not you should vaccinate.

The Case FOR Vaccination (by Dr. Phillip Raclyn)

We all must make medical decisions for ourselves and our animal companions based on a simple premise…. “What is the risk versus the benefit?” In this case, concerning Feline Leukemia, the risk can be summed up as… ”What can happen to my cat if I don’t vaccinate him or her against FeLV?”

Briefly, the answer is that FeLV is a fatal disease that is easily transmitted by saliva, tears, and urine. There is no cure for cats that are infected with FeLV. Should your cat come in contact with a cat that has FeLV, it could result in the infection and ultimate demise of your pet.

This is the risk. But you may ask.. “What are the chances of my cat coming in contact with an infected cat if my cat lives in my apartment?”

The answer is that nobody really knows. I was living in an apartment on the second floor here on the Upper West Side. I came home one day and my door was open, the apartment was ransacked, and the cat was gone. I found the cat in the alley three weeks later, but in the meantime he was in contact with all the alley cats. I’ve also had clients that had indoor cat that unexpectedly brought new cats into their household. It may be a new kitten that hasn’t been tested, or a stray that winds up on your doorstep, or a friend who suddenly has to go on vacation. Nobody thinks at this moment “Oh, my cat hasn’t been vaccinated against FeLV, I should keep them separated. Or if they do think of this, keeping cats separated in an apartment is next to impossible.

I also recently had a client whose cat had died. He had two other cats who were not vaccinated against FeLV. He went to the shelter to get a new kitten and came home with a beautiful 7 week old kitten. He had been told this adoptee was vaccinated and checked for FeLV as well as FIV. He integrated the new kitten with his cats, and finally brought the new kitten in to see us three weeks later, when other vaccines were due. However, the shelter neglected to inform him that the FeLV and FIV test are not accurate until kittens are 9 weeks old. So our client had put his own cats, that he has had for over a decade, in harms way by letting them intimately socialize with a kitten they may have been carrying FeLV or FIV. This is just one example, of many different circumstances that i have seen where unprotected cats come in contact with cats that may be carrying this fatal disease.

I spoke to Dr. William Hardy, the pioneer of FeLV research and world renowned expert in feline viral diseases, about this controversy. He told me simply “The virus is fatal, and the vaccine is safe. Why wouldn’t you want to recommend it? What would you tell your client if their cat actually became infected by the FeLV vaccine and you had counseled them against using the vaccine, or did not counsel them to use it?”

I agree wholeheartedly, and strongly recommend vaccinating against Feline Leukemia, even for indoor cats.

What is the risk if I choose to vaccinate against FeLV?

Cats can get a tumor at the site of injection of a vaccine. This tumor is called a VAS (Vaccine Associated Sarcoma) or a Fibrosarcoma. This tumor does not spread to other parts of the body, but can grow dramatically large and invade surrounding tissue, requiring surgery (sometimes multiple surgeries). Even when it is removed, it can grow back in the same spot. The incidence of the occurance of these vaccines is unsure. At The World Veterinary Congress in 2001, a research paper by Dr. Glenna Maudlin said “ The estimated incidence of vaccine-associated sarcomas varies significantly from source to source, but is likely 1 to 10 in 10,000 vaccines administered.”

Three years ago we began using a new type of FeLV vaccine that is not injected UNDER the skin, but rather is blown into the skin (ala Star Trek) without a needle. This new technology should decrease even further the odds of VAS.

In my opinion, the risk of not vaccinating against FeLV is greater than the risk of vaccinating against it.

The Case AGAINST Vaccination (by Dr. Steven Kasanofsky)

Actually I am not against vaccination for Feline Leukemia. It can be and is a deadly disease. I just don’t feel that every cat needs to be vaccinated. Why? Feline Leukemia is a viral disease that does not last long in the environment and requires direct contact with a feline leukemia positive cat to possibly become infected. The answer whether or not to vaccinate is not crystal clear – I have sat at many a table with groups of veterinarians, and believe me, the conversation can become very heated. Over the years, I have made the decision to follow the protocol of the American Association of Feline Practitioners, a group of veterinarians that have put together guidelines regarding the Feline Leukemia vaccination.

Vaccination Guidelines, AAFP

Vaccination of all kittens is highly recommended.
Vaccinate when cats that have direct contact with cats of known positive or uncertain status, such as outdoor cats and group housing foster or shelter situations.

I do recommend vaccinating kittens, cats that reside in a foster or kennel situation with other cats, and cats that are indoor/outdoor or outdoor residents. Most of my NYC clients have indoor only cats that never go out unless they are visiting the veterinarian’s office. For them, I do not usually recommend the vaccine. In addition, adult cats develop natural immunity to the disease. An adult cat that is inadvertently exposed to the virus has a very low risk of contracting feline leukemia, as shown by many researchers including the ones cited below:

“Most adult cats are highly resistant to FeLV infection whether vaccinated or not”.

Craig Datz, DVM, DABVP
University of Missouri
College of Veterinary Medicine
Consultant, Infectious Disease/Immunology, Parasitology

“Depends on the risk, but for most (those at relatively low risk for repeated/continual exposure), vaccination as an adult is not really necessary. If you do, be sure to use the intradermal version (as opposed to the killed adjuvanted vaccine).”

Melissa Kennedy, DVM,PhD,DACVM
Clinical Virologist
University of Tennessee
College of Veterinary Medicine

What are the down sides to vaccinating? Let’s be clear. Any vaccine can cause a reaction. They can tax the immune system. They add cost to veterinary care. The vaccine is not 100% effective. Most importantly, there are other vaccines that are urgently needed, so if my clients need to make choices, my job is to help them prioritize.

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