Feline Leukemia In Your Cat – FeLV+, FLV+

Ron Hines DVM PhD


Veterinarians have no effective treatments for FLV. A drug screening trial was halted due to lack of funding. Read about that here

Feline leukemia (FeLV) is the most common cause of serious disease in young cats. It is caused by a retrovirus that is spread from cat to cat through prolonged contact, bites and from parent to offspring. This virus, and its cousin retrovirus, the feline Immunodeficiency Virus (FIV), cause a slow, generalized decline in your cat’s health. Human AIDS is also a retrovirus and the two diseases share many similarities.

Feline leukemia is a misnomer in that fewer than half the cats infected with FeLV develop blood leukemia cancer. When they do, it is not actually the FeLV virus that is the cause. It is that the cat’s own cancer-fighting mechanisms have been crippled by the FLV virus.

Recent estimates are that 2-5% of the cats in the United States carry this disease. The incidence is likely to be greater in urban areas where cats live in high density than in rural areas where they do not. Feral cat colonies also give the FLV virus an excellent environment to multiply due to the ease with which the virus moves from cat to cat in those high density, high stress environments. 

How Did My Cat Catch Feline Leukemia?

If your cat is over eight months old, it was probably due to being bitten by an infected cat.The feline leukemia virus was present in the other cat’s saliva – although other body secretions can pass the disease as well.

If you have several cats, there is a slight chance the positive pet caught the virus from another one of your pets through mutual grooming or other intimate contact. So be sure to have all your cats tested by your veterinarian. Never introduce an untested cat of any age into your feline household until you know for certain that it does not harbor this virus or the FIV virus. 

However, if your cat is younger than six months old, its infection was probably passed down from its mother before it was born or while it was nursing. Basically, the younger a cat is, the more easily it can become infected.

The Leukemia virus does not survive outside of the cat’s body for more than a few minutes. So unless you own another infected cat, this pet did not catch FeLV from contact with contaminated places or objects.

Most cats that become infected with Feline Leukemia conquer the virus and recover. Their bodies mount an immune attack against the virus, producing antibodies that cure them or keep the virus in check. Veterinarians used to believe that in some of these cats the virus was completely eliminated from the body – read the paragraphs below that are in red to see why that is now in doubt. In other cats, the virus appears to be held in check throughout the cat’s normal life.

But in a few cats, the virus spreads to their bone marrow and persists in a dangerous form. These cats will eventually develop a fatal disease at a period in their life when stress or a compromised immune system allows the virus to proliferate.

What Are The Signs Of Feline Leukemia I Might See In My Pet?

In the earliest stage, your pet will usually not show any signs that it has been infected. But anywhere from a month to years later, owners will notice that the cat’s health is in decline. At first, the cat’s health problems appear to be minor – perhaps less appetite, some weight loss, less time spent grooming, more time spent sleeping.

I often notice that the third eyelid of these cats is extended farther over the eye than normal. Some of these cats develop enlarged lymph nodes that one can feel as lumps under the skin. Many develop abscesses or run explainable   fevers . Ear infections, bad breath and mouth infections are common – as is a persistent anemia.

Some cats develop persistent or reoccurring respiratory tract infections or relapses of previous respiratory virus (Herpes 1). Some of these cat develop persistent or intermittent diarrhea. A few even develop inexplicable neurological problems, personality changes or seizures.

How Will My Veterinarian Diagnose Feline Leukemia?

Alarms that the FeLV virus may be present go off when healthy cats begin to have too many minor medical problems or when problems do not respond as they should to treatment.

There are two very accurate tests for the presence of the virus.

The first is an ELISA blood test (Idexx Snap Test ) that can be performed in a few minutes in your veterinarian’ office. (This test also detects the FIV virus.)

If that test is positive, your veterinarian may send off a blood sample for an IFA test. The IFA test will determine if your cat is in an early stage of the disease when it might still conquer the virus, or in the later stages when it will probably not.

Along with those tests, they may send out or run some blood chemistry and blood cell tests to see what damage the virus has already caused. Sometimes, more complicated tests like bone marrow biopsies are also indicated.

What Does A Negative Feline Leukemia SNAP test mean?

Cats that have a negative SNAP result have either:

1) Not been exposed to FeLV

2) Were just exposed but it is too early to find the virus

3) Have been exposed but have conquered the virus

Most cats that have been bitten or exposed to an infected cat will test positive within 1-2 months of the incident. But to be certain, 90 days need to pass before a negative SNAP result can be relied on.

About ten years ago, veterinarians became more aware that, as Dr. Pedersen explained, there is a 4th possibility. An Australian study found that up to 10% of the cats that tested negative on in-office veterinary tests for FeLV, still had “echoes” of the virus or pro-virus present in their bodies (hidden deep within their genetic code). Veterinarians do not know the true significance of this finding yet or if it could have deleterious effects on your cat’s health as time goes by. You can read that study here, and a more detailed review of the problem here. Because of this new information, the American Association of Feline Practitioners has modified their treatment guidelines to now state: ” Similar to FIV, it is now thought that cats that become infected with FeLV remain infected for life.” (ref)

I personally tend to agree more with Dr. Hartmann ’s 2012 review – that veterinarians really do not know what, if any, affect these residual provirus particles might have on to your cat’s health.

What Does A Positive Feline Leukemia SNAP Test Mean?

Cats that have a positive SNAP result are either:

1) Early in the disease and may still overcome and eliminate the virus.

2) Able to live indefinitely or for a long period with the virus but without disease.

3) Later in the disease and will not overcome the virus. If cats show signs of ill health , they are most likely in this group.

What Does An Inconclusive Feline Leukemia Test Mean?

Occasionally, the test results will be somewhere between positive and negative. When that occurs, a blood sample needs to be sent to an outside laboratory for an IFA test. That test usually resolves the question. It is also possible to just re-run the SNAP test in a few months. Many owners decide to have any positive SNAP results confirmed by an outside laboratory using IFA or PCR tests.

Vaccination for FeLV does not influence the test results.

What About Testing Kittens For Feline Leukemia ?

The SNAP test can be used on cats of any age. However, little kittens may give a false negative test result for many months – even when they are infected. For you to feel entirely safe, the test should be re-run on kittens that test negative when they are 6 months old.

When Office Tests For Feline Leukemia Are Negative, Can I be Certain That My Cat Is Safe?

When the test is negative, your cat does not have the feline leukemia virus in its complete form. But there is some evidence that portions of the virus might linger in cats that have successfully conquered FeLV exposure (remember that most cats overcome the feline leukemia virus). There are veterinarians who think that certain diseases are more common in cats because of portions of the FLV virus that remain locked deep within the genes of their body. (ref). The veterinarian who has spent the most time studying the dynamics of feline leukemia is Dr. Niels Pedersen. He explained it in this way:

“After primary infection there is a brief and often non-detectable viremic stage where the replicating virus can be found all over the body.  The cats then build up immunity and this replicating stage ceases.  However, during the course of this replicative stage of infection the genetic material of the virus is inserted randomly in many places in the chromosomes of the cat.  This genetic material can be reactivated in some cases by stress and other factors, but after a few months the inserted FeLV genetic material becomes more and more fragmented and reactivation becomes impossible in most cats.  However, the insertion of this viral genetic material (and later, pieces of viral genetic material) can alter the cat’s normal genes in such a manner as to cause disease later in life.  This is usually in the form of a several fold increase in blood and lymph  node cancers.   The importance of this event to cats is largely unknown, “

If this aspect of feline leukemia is of concern to you, have you veterinarian discuss it with specialists at the Davis TaqMan Lab.

Are There Any Effective Treatments To Help My Cat?

There is no scientifically proven treatment for the feline leukemia virus. No medication has been found that kills it.

In 2006, a company called Imulan was granted a conditional license by the USDA to market a product for FLV+ and FIV+ cats called Lymphocyte T-Cell Immunomodulator (LTCI). To be granted such a temporary license, a company is supposed to convince the USDA that their product is safe, and has a “reasonable” chance of turning out to be effective. It would be marvelous if this product did turn out to be effective. However the verdict is not in yet. The only article I know of on this product was written by the scientific officer of the Company that makes and sells the product – something akin to having a politician count his own election ballots. The article was not published in a Journal I have learned to trust, such as JAVMA, CJVR or the BVJ and in reviewing the brief history of this journal, it is overloaded with articles touting the benefits of products produced by the various authors’ companies. Nor has this journal met the selection requirements of the National Library of Medicine (ncbi).

It would be a relatively “no-brainer” experiment for Imulan® to have a disinterested party (such as  Cornell) supply the product free to one group of cats and not the other and see which group lived longer. Imulan ‘s, patent-medicine -like website also troubles me. Why not invest that kind of money and effort in some straight-forward research to determine if their product works?

I don’t find anything wrong with the theory behind the product (LTCI) and perhaps it will help your cat. Given a few more years, human trials on the effectiveness of immunomodulators will become more apparent and veterinarians will have used this one enough to evaluate its true worth. That said, nothing would delight me more than that this product actually helped your cat.

These reservations apply equally well to an option European cat owners have that we in the US do not have, Virbagen Omega®. They also use the IJAR (Int. J. Applied Research) as a promotional conduit.

What your veterinarian can do is give your cat general support. That includes antibiotics, transfusions and other supportive care as it is required. Good nutrition and vitamin supplementation help cats that no longer eat well. A low-stress, indoor lifestyle and TLC that minimizes stress, is also important.

Cancer (lymphoma) caused by the FeLV virus responds better to treatment. It can often be held in check with chemotherapy and cortisones.

Alternative And New Age Medicine:

Rumors abound that one or another novel herb or medication cures or helps FeLV cats. If those rumors were true, word would spread very rapidly and every caring veterinarian in the World would be using these treatments immediately.

How Long Is My FLV+ Cat Likely to Live?

Even with treatment, 85% of positive cats brought to animal hospitals die within 3 years. However, your cat might be among the fortunate 15%. Fortunate cats can live indefinitely without symptoms but with the virus and the 85% might be inaccurately high because people tend to only take their cats to be tested when they notice they are already ill. But I do know that if your cat is already showing symptoms of FeLV, it is very unlikely that it will be among the fortunate few.

What Steps Can I Take To Keep My Cat From Catching Feline Leukemia?

1) Unless your cat was unfortunate enough to arrive with the feline leukemia virus, the most certain way to protect your pet is to keep it indoors. It is fine to take cats out-of-doors on a leash, or confine them to a wire-mesh folding kennel while you are in the area. 

2) Even indoors cats can escape. So you should also have your cat vaccinated against feline leukemia. But before the initial vaccinations, your pet should be tested to be sure it does not already harbor the feline leukemia virus. Vaccinating a FeLV+ cat probably won’t hurt it – but it will not help it either.

If you have obtained a kitten, give the first of two feline leukemia vaccinations when the kitten is 12 -14 weeks of age. It needs a second booster vaccination 3-4 weeks later.

Because fibrosarcoma tumors  have been associated with leukemia vaccines, many veterinarians now prefer using the Purevax® non-adjuvant leukemia vaccine. This vaccine was specially designed to try to avoid some of the factors that we think are responsible for these tumors. This includes a new needle-less transdermal administration. This vaccine has not been available long enough for us to know if it will actually reduce the frequency of these tumors. If it does, other vaccine producers will offer similar products soon. 

As cats get older, they develop a natural resistance to feline leukemia. So after the two vaccinations they receive as a kitten and a vaccination a year from that date, many veterinarians only give a booster vaccination against feline leukemia every three years. Some veterinarians give booster leukemia vaccinations to adult cats even less frequently – or never. Others only give the boosters to cats that live with FeLV positive cats or roam outdoors. It is likely that in the past, we gave these vaccinations too frequently.

Can The Feline Leukemia Virus Survive In My House Or Yard?

The virus can not live for more than a few minutes outside of a cat.

To be prudent, you can wash everything that was in contact with an infected cat and then dip the cleaned objects into ordinary household bleach diluted one part bleach in twenty parts water.

Can The Feline Leukemia Virus Infect Me?

Feline leukemia does not affect human beings and is not related to the leukemias that affect man. It does not infect dogs. It has been reported in lions, tigers and other large cats.

What Are The Special Care Needs Of My Feline Leukemia-positive Cat? What Things Can I Do To Help?

The most important thing you can do for FLV-positive cats is to minimize stress in their lives.

FeLV-positive cats do best in single-cat households. They do better as strictly indoor cats rather than cats that are allowed to roam out of doors. They also do better when they are fed a high quality diet that is not based on fish.

Cats with feline leukemia should receive a vitamin supplement when their appetite is poor or they are loosing weight.

The toenails of your cat should be clipped blunt with a human finger nail clipper and then smoothed with an emery board. This is because scratch wounds in FeLV-positive cats tend to become infected.

FeLV-positive cats are also more susceptible to ear , gum and eye infections. So you need to check these areas regularly and have any problems checked out with your veterinarian.

Your pet’s body temperature should be monitored periodically with a pet or child’s ear thermometer. If the cat’s temperature is persistently above 102.4 F/39C when it is resting and unstressed, tests or antibiotics may be warranted. It is important that antibiotics not be given for less than 7 – 14 days or they will loose their effectiveness the next time they are needed.

Do not feed your pet uncooked foods like raw or poorly cooked meat, eggs or fish. Uncooked foods contain bacteria that are more likely to cause problems in your cat because of its weakened immune system.

As frequently as every six months, your veterinarian can examine your pet for any new problems that might have occurred. Sending a blood sample off for analysis will also catch problems before they get out of hand. If your cat’s condition changes for the worse, set an appointment immediately.

When Would Euthanizing My Cat Be The Kindest Thing To Do?

Clients frequently ask their veterinarians “when is it time to euthanize my pet?”. You are more likely to recognize that time when it comes than they are. It is a terribly difficult decision for most of us to make and the answer is never the same for any two families. I tell my clients – “let your cat be, as long as it appears to be happy”. Many of my client’s FLV+ cats remain healthy for a long time when they are kept in a sheltered environment and provided with good nutrition, health care and love.

There will always be veterinarians, herbalists, chiropractors, naturopaths, homeopaths, nutritionists, acupuncturists and practitioners of other obscure arts who will claim to be able to cure your cat when it is dying of feline leukemia. Not everyone is honest and above board – perhaps less so than before the internet and personal and corporate greed dominated American society as it intensely as it does today. They will get your hopes up by telling you what they know you are desperate to hear. Pay no attention to any of them.

Leaves have their time to fall,
And flowers to wither at the north-wind’s breath,
And stars to set; but all,
Thou hast all seasons for thine own
John Milton, Mid 1600s

You find a flower half-buried in leaves,
And in your eye its very fate resides.
Loving beauty, you caress the bloom;
Soon enough, you’ll sweep petals from the floor.
Han Shan, about 630 AD

Creating Antibiotic Resistant Bacteria:

Periodic antibiotics can sometimes prolong the lives of cats with FeLV. But the long term use of antibiotics in a household, produces races of bacteria immune to antibiotics. If those bacteria should later infect you or your family, those infections may be difficult or impossible to control. This is a rare occurrence – but I have seen it happen to my clients. This is particularly true if you yourself are not in the best of health.

Cats with FeLV also often have unhealthy mouths (periodontal disease). Should these cats bite or scratch you when you are attempting to medicate them, those bite wounds can become infected. So be careful when attempting to do so. I know it hurts and stings – but 70% isopropyl alcohol rubbed vigorously into a shallow wound kills most every pathogen. Ask your physician.

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