Feline Leukemia, Symptoms and an Extremely Important Lesson about Diagnosis

As a feline veterinarian, I have had experiences with Feline Leukemia many times over the last 20 years. I also was well aware of the FeLV virus in the years prior to becoming a veterinarian when I was rescuing cats. Every multi-cat household owner’s worst nightmare is finding out a member of their feline family has tested positive for the leukemia virus. Fortunately, I learned early on how important it was to test every new cat for Feline Leukemia before bringing them home.

Recently, however, I had a unique experience involving cat leukemia testing that I had never experienced before. A very sad, then frustrating, then somewhat infuriating experience, but one with a happy ending. This is a story I have to pass on to as many cat owners as possible due to the impact it could have on other cats.

First, I’d like to review the important points about the Feline Leukemia virus for you.

Feline Leukemia is one of several viruses that can affect cats. It is one of the more well-known viruses among cat owners, largely due to its potentially devastating consequences. There's little that can surpass the nightmare a multi-cat owner faces when one of the cats is diagnosed with the feline leukemia virus.

That is because cat leukemia, unlike human leukemia, is highly contagious. If you find out that one of your cats is infected with the feline leukemia virus, not only are you faced with the likely possibility that it will be fatal, but you also have to worry that your other cats will test positive for FeLV also. Large, multi-cat households have been completely wiped out by feline leukemia.

The feline leukemia virus is found worldwide to varying degrees. Some estimates place the occurrence of FeLV at around 3% while others suggest it is as high as 10%. Really, it depends on the location you are discussing. If you are talking about the indoor population, there is less incidence than the outdoor population of cats. If you compare urban cats with rural cats, the incidence is greatly decreased in rural cat populations. At any rate, it is such a deadly, horrible disease that any incidence at all is too much.

Cats pass the feline leukemia virus to other kitties through saliva, nasal secretions, urine, feces, prior to birth from an infected mom or after birth from her milk, through bite wounds, via grooming of each other, and by sharing food dishes and litter boxes. Opinions about the length of time the virus survives in the environment have changed from time to time over the years. Currently thinking is that the virus can only survive in the environment a few hours at the most.

The cats that are most likely to acquire feline leukemia are:

(1) Cats that live with leukemia positive cats

(2) Outdoor Cats, especially males who have more fighting encounters and can transmit the virus by biting each other.

(3) Kittens who have acquired FeLV while in the mom cat’s uterus or from her milk after birth.

Kittens are much more likely to catch the virus than are adult cats, due to their immature immune systems. But adult cats can absolutely acquire cat leukemia also.

There are many different ways the feline leukemia virus can affect a cat. Most commonly, it suppresses the immune system which leaves the cat vulnerable to serious disease from normal organisms in the environment that would not ordinarily cause problems. This includes certain viruses and bacteria as well as some fungi and protozoa. There are also some types of cancer in cats that are caused by the leukemia virus. Various blood disorders can result from the virus as well.

Symptoms of feline leukemia

There are usually no feline leukemia symptoms early in the disease. Over time, however, health may deteriorate gradually. There can be periods of good health that alternate with periods of illness.

Feline Leukemia Symptoms to watch for include:

- decreased appetite

- weight loss

- fever that doesn't go away

- unhealthy feeling or appearing fur

- pale gums

- swollen lymph nodes

- extremely inflammed gums and mouth

Just about any body system can be affected. There may be skin infections, upper respiratory infections, bladder infections, eye problems, diarrhea, neurological problems such as seizures and behavior changes. Pregnant infected females may abort their kittens or have other problems related to pregnancy. Cats with Feline Leukemia are often anemic.

The decreased appetite will lead to a slow but steady loss of weight which becomes a severe wasting away state as the disease process becomes worse.

Unfortunately, the symptoms of feline leukemia can mimic those of many other diseases.

Diagnosis of Cat Leukemia

Diagnosis of FeLV is made through blood tests. There is a blood test that is usually done first and is often performed right in your vet's office. It may also be that the blood is sent to an outside lab and tested there. If the test is positive, a second different test should be performed by the lab to verify the positive test and to ascertain if the cat is going to stay positive for life.

The test most commonly used in your vet’s office to test for the feline leukemia virus is called the Elisa test. The letters stand for Enzyme Linked ImmunoSorbent Assay. The procedure for running the test is a lot more simple than its name. It simply involves combining a few drops of your cat’s blood with a special solution and watching for a color change. Much like a pregnancy test.

If the Elisa test is positive, a confirmatory test must be performed. It is called the IFA test which stands for indirect ImmunoFluorescent Antibody Assay, also known as the Hardy test after its maker, Dr. Hardy. If both the Elisa and the IFA are positive, then you can be almost 100% certain that the cat has Feline Leukemia and is not going to fight it off and will succumb to the disease and can transmit it to other cats.

There are certainly false positive Elisa tests. Your cat may have a positive Elisa and a negative IFA. This can mean that the Elisa was a false positive or that it was a true positive and that the cat has the virus in the blood, but not in the bone marrow. In that case, the Elisa should be repeated to see if it was a false positive. If it is still positive, the cat must be sequestered away from other cats and retested in a month.

The Two Stages of FeLV Infection

To understand the difference between the two tests a bit better, it is helpful to know that FeLV infection has two stages. In both stages, the virus is present in the blood. In the first stage, it is only in the blood and some cats can fight off the virus with their immune system and rid their blood of the virus.

However, those who cannot fight off the virus go on to the second stage in which the bone marrow is infected with the virus. At this point, almost all cats will remained infected with the FeLV virus for the rest of their lives.

- If a cat has Feline Leukemia, the Elisa test will be positive in both the first and second stages.

- The IFA test only detects the latter stage and indicates that the infection is in the bone marrow and is there to stay.

At some point, and this is very important, both of the test results must match. They must both be positive or both negative before you have a definitive answer.

persian cat

The frightening experience I was referring to earlier concerned Neko, the Persian cat in the above picture. He appeared outside my daughter's apartment building one evening, thin and hungry and covered with fleas. We rescued him and I temporarily fostered him in a spare bedroom in my home. Of course, I immediately tested him for Feline Leukemia and Feline Immunodeficiency virus. The in-house test that I ran came up with a very faint possibly positive result for leukemia.

We were devastated, but sent blood to the outside lab to have the test repeated. First, I had them run the same type of test I had run, the standard initial test, and they reported a negative result. Due to my own faint positive result and the fact that I wanted to be absolutely sure this kitty was negative before placing him with another cat, I then had the lab run the Hardy test discussed above. We held our breath. When the test result came back a few days later, we were again devastated. The lab reported a positive Hardy (IFA) test. Our hearts were broken. This was one of the most adorable cats you could ever hope to find. So sweet and innocent and so beautiful.

I was also confused. It is impossible to have a negative Elisa test (the initial test) and a positive IFA. The two tests have to agree. So I had the lab repeat the Elisa test. Again, it was negative. In desperation, we prepared to send the prospective housemate's blood off to the University of Glasgow, the only place I could find that could do a special test that would see if she was a cat that was immune to feline leukemia and could safely live with Neko.

Then, I learned that Dr. Hardy, the originator of the IFA test, was at his lab in New Jersey and would run the IFA test there. He was incredibly gracious and helpful and got right on the phone with me even though we had never met and assured me that the IFA test run by the outside lab was most likely inaccurate.

To make a long story short (and I guess I've already violated that intention!) or to keep it as short as possible, Dr. Hardy ran the IFA test, it was negative, Neko did not have Feline Leukemia and is living happily ever after in a wonderful, loving home.

However, I learned a scary lesson which, unfortunately, many other veterinarians and cat owners still do not know. The lesson is that all IFA positive test results performed by commercial labs MUST be confirmed by Dr. Hardy's lab. I cringe when I think of the number of positive IFA test results that I personally know about that led to the end of a cat's life. And that's just the ones I have heard of. Which, no doubt, is an infinitely small number compared to the actual number of cats this has happened to.

Philli's FeLV Story

In February of 2013, a young woman wrote with a similar shocking story about her own cat, Philli. Philli had tested positive for FeLV on an in-house ELISA test run on whole blood, then negative on the same test run on serum, and then received a positive IFA test. Philli, at only 6 months old, was given a death sentence prior to his neuter procedure. But his mom, loving him as much as she did, refused to accept this result. Philli was not symptomatic, seemed otherwise so healthy, and she couldn’t believe that these tests didn’t match up.

Fortunately, Philli’s mom did her research online and really educated herself about her options. She wrote to us, looking for advice on how to contact Dr. Hardy at National Vet Laboratory, and looking for reassurance that pursuing this additional test was worthwhile. Just as most veterinarians do, her vet was informing her that the IFA she had sent to the outside lab was 98-99% accurate and that there was no point in contacting Dr. Hardy’s lab, but Philli’s mom pursued this anyway, trusting her gut that he did not have feline leukemia. And it’s a good thing that she did, because Dr. Hardy’s lab confirmed that the IFA was in fact a false positive, just as I had experienced with Neko.

The moral of these stories...

Please spread the word! Not only should all Elisa positive test results be confirmed by an IFA (Hardy) test, but all Elisa negative, IFA positive tests run by commercial vet labs other than NATIONAL VETERINARY LABORATORY in Franklin Lakes, NJ should be repeated by National Veterinary Laboratory. Here's the address and phone number:

P.O. Box 239, 1Tice RoadFranklin Lakes, NJ 07417877-NVL-LABS (877-685-5227)www.natvetlab.com

Prevention of Feline Leukemia

Theoretically, preventing your cat from acquiring the feline leukemia virus is actually pretty simple. In a nutshell, test any new cat before you bring him home. Keep your cat indoors always. Test any additional cats for FeLV before you bring them home. Never mix your cat with your neighbor's cat, your best friend's cat, your daughter's cat......any cat, without a negative leukemia test first.

You cannot tell if a cat is FeLV positive or not just by looking at it. I have had many clients tell me that they "just knew their cat didn't have it" or they "just knew the stray outside their door didn't have it." You CANNOT possibly tell the difference between positive and negative cats early in the disease if they both look healthy and have no symptoms of feline leukemia.

FeLV positive cats can look perfectly healthy initially. That's why you can never be comfortable letting your cat be around another cat just because it looks healthy, because its owner says the cat is well, or even if the other cat has never been outdoors. The blood test is the only way to be sure.

Please don't take risks with your cat's life. Test all cats, keep them indoors, and take new cats straight to your vet's office before taking them to your home.


Ok, I would love to stop this page right there, but I know many cat owners are not comfortable with keeping their cats indoors. So ... if your cats go outside, speak to your veterinarian about vaccinating your cats with the feline leukemia vaccine. It doesn’t provide 100% protection, but it is much better than not giving it at all. The vaccine does not cause cats to test positive on the FeLV tests as does the FIV vaccine.

What To Do If You Find Out Your Cat Is FeLV Positive

If you suddenly find out that one of your cats in a multicat household tests positive, you should immediately test all cats. Then positive cats should be kept in separate living quarters from negative cats.

Keep positive cats indoors so they cannot infect other cats outside and for their own health and safety. They will be exposed to infections outdoors that are harder for them to fight with their FeLV positive status.

Of course, do not let positive cats have kittens.

FeLV positive cats need good veterinary care, good nutrition, and constant monitoring for any signs of illness. At the very first symptom of anything, a positive cat needs to be taken immediately to the veterinarian.

Meticulous dental care is essential. It is important to have regular blood work and to keep a close eye on your cat’s weight.

How Long Can My Kitty Live with Cat Leukemia

Clients often ask me how long their FeLV positive cats will live. Unfortunately, there’s no way to predict. Some live a few weeks after diagnosis, some a few months and, more rarely, some live a few years.

Can Cat Leukemia Be Transmitted To People?

Another frequent question I hear is concerning the transmission of FeLV to people. There is no evidence that Feline Leukemia can be transmitted to people. The type of leukemia that cats have is not at all the leukemia from which people suffer.

However, positive cats can have other types of infections that could be dangerous to individuals with weaker immune systems. It is recommended that the elderly, babies, pregnant women and the immunosuppressed (such as a person with HIV or someone on chemotherapy) not be around FeLV positive cats.

Treatment of the Feline Leukemia Virus

Attempts have been made at treatment with various agents including AZT, SPA, ImmunoRegulin, Interferon, and Acemannan. There is not enough information available due to an insufficiency of controlled studies. At this time, there isn’t a cure for FeLV.


Out of all this information, the most important thing for you to take away with you: ALL cats should be tested for FeLV, sometimes more than once. Negative cats that should be re-tested are all sick cats, cats who have been exposed, cats that are about to receive the feline leukemia vaccine, and any cat that is about to be adopted.


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