Feline Leukemia Virus – Cat Fanciers

(dedicated to Oliver and countless other cats whose lives have been mercilessly shortened by this virus. We will all meet them again at the Rainbow Bridge.)

PLEASE NOTE: This FAQ is dated 1995. There are no plans to update this reference. I believe the materials to still be useful to cat owners, but I recommend you do an Internet search, such as at www.google.com on Feline Leukemia Virus to gather more current information. A good place to start is http://web.vet.cornell.edu/public/fhc/felv.html

Contents:

Last updated April 20, 1998.

Authors:

ermiller@dgsys.com

] with help from James Golczewski, PhD; Edwin Barkdoll, DVM-to-be; Cindy Tittle Moore; Jeff Parke, DVM; C.M. Newell, DVM; Barb French; Richard Kinoshita; and Ann Huber. Additional thanks to Wilf Leblanc, Dan Kozisek, Karen Kolling, Isako Honshino, Patty Winter, Harold Lemon, Jill Kronstadt, and many members of the Cat Fanciers’ Internet Mailing List for their comments.

The purpose of this FAQ is to answer frequently asked questions about the Feline Leukemia Virus (FeLV). The Feline Leukemia Virus is a virus which suppresses your cat’s immune system. A cat that becomes infected with the virus becomes susceptible to many ailments or breakdowns in its system. The virus was somewhat inappropriately named because it is different than the disease Feline Leukemia. A cat that tests positive for the virus will not necessarily contract the disease Feline Leukemia. However, cats that do test positive for the virus are more likely to catch any one of a number of diseases including, but not limited to, leukemia, lymphoma or opportunistic infections. This FAQ is divided into two parts; the first consists of information every cat owner should know or should ask about the virus. The second part is information for people whose cats have tested positive for the virus.

PART I: General Information about FeLV

What is Feline Leukemia Virus and can I catch it?

Feline Leukemia Virus is a virus that is specific to cats only. It is considered to be the most common cause of serious illness and death in domestic cats. It causes a breakdown in your cat’s immune system causing your cat to become susceptible to many diseases which it might otherwise be able to fight off. It CANNOT be transmitted to humans (including children) nor can it be transmitted to other species such as dogs. The National Cancer Institute and the National Institutes of Health have written a CancerNet Factsheet which is available via gopher. See the References section for more information on this.

I’ve heard FELV is like AIDS. Can I catch AIDS or anything else from it?

FIP FAQ

for more information on this disease).

What is a retrovirus?

Retroviruses carry with them an enzyme that causes a process to occur in the DNA known as “reverse transcription.” RNA normally pairs up with DNA, copies itself, and thus increases/replicates itself. When an RNA retrovirus does this, it fools the DNA to copy *it*, instead of the normal RNA, thus causing even more of the retrovirus to be created. So as long as a particular cell is affected with the retrovirus, that cell will be affected for its whole life. One would have to kill the cell before it reproduces to eliminate any chance of that cell making any more FeLV or FIV RNA. This is one of the reasons why it is so difficult to get rid of, because you must kill all the cells which contain the virus, and attempting to do this may either kill the animal itself, or it is simply impossible to tell which cells have the virus in the first place. Basically, a retrovirus is a parasite at the genetic level, a DNA-tapeworm, if you will.

How is it transmitted?

FeLV is transmitted via saliva, mucus, urine, feces and blood. This means mutual grooming and biting/fighting are the most likely methods of transmission, although sneezing, hissing, sharing food/water bowls and sharing litter boxes are also possible means of transmission.

Will I infect my healthy cat if I pet an FeLV+ cat, then pet my cat?

No. Not unless you were to go immediately from one cat to another, and have wet saliva, urine or blood on your hand. Even then the chances would be slim. FeLV needs to be transferred through the media above, and will not live long outside the host (the infected cat). Warm, dry environments will deactivate the virus and common household detergents and disinfectants will eliminate it from any contacted surfaces. If you wash your hands with soap after touching an infected cat, you will not infect an FeLV negative cat.

What does the virus do to a cat?

In sum, once the virus gains entry (usually via saliva or mucus membranes) it will reproduce in the lymph tissue which is your cat’s first system of immune defense. Some cats are able to mount a successful immune response against it and defeat the virus at this stage. In cats who don’t successfully destroy the virus here, the virus will then move into the bone marrow where red and white blood cells are produced. It may stay latent in the marrow for many years. After that it will attack other tissues, including possibly causing a breakdown in several stages of the immune response system.

Is there a vaccine?

Since writing the first version of this FAQ, I’ve had a number of people tell me that a cat has only a 30% likelihood of catching the disease, and even then if only in persistent close contact. I have found nothing in the recent literature to support this claim, and every veterinarian I have spoken with states that the likelihood of becoming infected depends on the level of exposure, which vary in different locales and situations. Since most tests are done in controlled situations, it would be impossible to come up with true “real world” statistics. In one vet’s words “But what if it is only 30%? Would you stop wearing seat belts if there was only a 30% chance of serious accidents if you commuted in a car every day? I hope the odds are far less than that … and yet I still hope you are taking the appropriate safety cautions. Unless the vaccine has been proven to cause serious harm, it is a good precaution to take, regardless of the percent chance of disease transmission.”

If I don’t get my cat vaccinated, what are its natural defenses against the virus?

According to the literature, neonatal kittens are 100% susceptible to catching the virus from one exposure. 8-week-old weanlings are 85% susceptible from one exposure. There is serious debate over the likelihood of non-vaccinated, healthy adult cats becoming infected with the virus but it seems that ~40% of cats exposed become immune, ~30 percent become persistently infected (show acute signs of a related-disease) and ~30 percent become infected, but the virus is latent in their system. (Hardy, et al, 1980).

Is the vaccine expensive and how often do my cats need to be vaccinated?

After your cat has gone through its more elaborate set of kitten-shots, the FeLV vaccine should be given once a year. If you adopt an adult cat and don’t know if it has been vaccinated for FeLV, start it immediately on its annual shots. If you are unsure if your adult cat needs be vaccinated, talk to your vet. Prices vary between vets, locally and nationally. I surveyed readers of rec.pets.cats and contacted vets around the country and found that prices for the vaccination alone cost between $9 and $30 dollars. The average was $18 (not including the vet-visit charge). It seems that the FeLV test costs about the same, but I didn’t do a full-scale survey.

If I get my cat vaccinated, isn’t there a chance that it will catch the virus from the vaccine?

No. As of July 1992, all of the vaccines which are approved for sale in the United States are incapable of causing a positive FeLV test result. (Sorry, but I don’t have figures for vaccines in other countries. If anyone does, feel free to append them onto this FAQ.)

Is there any risk in getting my cats vaccinated?

There is also some evidence that vaccinations in general may be the cause of tumors (known as postvaccinal sarcomas or fibrosarcomas). The chance of this happening is estimated to be approximately 1-2 in 10,000, but you should be aware of it nonetheless. This has not been limited to FeLV vaccines, in fact it was originally thought to pertain solely to rabies vaccines, but this is not thought to be the case any more. Since this form of cancer seems to have a high recurrence rate, and little is known about it, if you have *strictly* indoor-only cats, you may want to discuss with your vet if the risk of fibrosarcoma is greater than the risk of being exposed to FeLV if the cat gets out. This is an individual decision that will be different for each household. You should contact your vet immediately if you notice any lumps in the vaccine injection area.

Do I have to get my cats vaccinated?

No one can force you to vaccinate your pets, though there are laws in some areas regarding certain diseases like rabies. Check with your vet to see what vaccines are required in your area.

My cats are indoors-only. Why should I bother getting them vaccinated?

But, some people feel the risk of adverse reaction and possible fibrosarcomas from vaccinating are not worth the risk if the cat is not likely to go be exposed to FeLV+ cats, even if it did get outside for a short period. What YOU as the cat’s owner (not your vet, not your cat’s breeder, not your friend of a friend who knows a lot about cats, and not someone who wrote something you read on the internet) must decide is how much risk is there for your cat getting out and being exposed, and is that risk worth the other risks associated with the vaccine?

My cat is a purebred, and I’ve heard the vaccine should not be given to purebreds. The breeder I bought the cat from discourages getting the vaccine.

she

NEVER

you

exactly why

the breeder does not recommend the vaccine as it may very well be that the breeder has had numerous negative experiences with the vaccine in his/her particular line of cats, and that your cat may be genetically predisposed to have a bad reaction. If this is the case, you should still discuss the matter with your vet, and it may be best not to get the vaccine.

My cat gets sick after it gets vaccinations. Why should I put my cat through that?

Some cats do have bad reactions to vaccines. However, it is better to have a cat sick for one day per year from being vaccinated than to have it die a miserable death from an FeLV-related disease. If your cat has a bad reaction to a shot, ANY shot, and the reaction lasts more than 12-24 hours, you should immediately bring your cat to the vet. Even if your cat has a mild reaction, you should at least discuss the matter to see what are the best options for next year’s vaccinations. It may be best to spread your cat’s annual vaccinations out over a few months, or have them all administered at once. If your cat has had a *very* bad reaction, it very well may be best to discontinue vaccinating for FeLV. Definitely discuss this with your vet.

I already have a cat(s) and I found another which I want to bring home. What precautions should I take regarding FeLV (and other diseases)?

One of the most unfortunate situations that occurs far too often is when someone, out of the kindness of their heart, takes in a stray or unwanted cat — either permanently or in the hopes of finding it another home. This is often done spur-of-the-moment, and unfortunately sometimes has dire consequences. If you find a cat in a bad situation and you want to help it, keep it isolated or ask your vet to board it until all the test results have returned. It is never worth the lives of your current pets in an attempt to save another.

How is FeLV detected?

- If positive:
  - whether healthy or sick, confirm by Indirect Fluorescent Antibody test (IFA)
- If negative:
  - if healthy and not exposed to a positive cat then no need to retest
  - if healthy but exposed to a positive cat then retest in 3 months because the healthy cat may be incubating the virus.
  - if sick then confirm the ELISA test with an IFA test

So some cats who test positive can later test negative?

Yes. If you have a cat which tests positive on the ELISA test, you should immediately have an IFA test done. If it tests negative on the IFA test, you should have your cat retested with the ELISA test in 3 months. If a cat does not test negative again in roughly three months, chances are it will always test positive. Vets and virologists have devised an entire classification scheme of the different types of infected cats (transiently infected, persistently infected, etc.) based on the ELISA and IFA tests. It can be very confusing and if you are interested in learning the details you should consult with your vet regarding your particular cat’s status.

Is it possible for a cat to test negative when it really is positive?

Unfortunately, yes. Although false negatives are not very common, they do occur, especially if you are dealing with a young kitten. Sometimes the cat has been recently exposed to FeLV, so the antibodies have not yet had enough time to build up enough of a response to appear on the test. To be absolutely sure a cat is not FeLV+ you should test it twice, a few weeks apart (the cat should remain isolated from other cats during this period, too, otherwise there is little point in doing a second test). The likelihood of getting a false negative is increased depending on the nature of the test. The ELISA test will show more false negatives than the IFA test. Some mail-order catalogues now offer FeLV testing kits using saliva or tears as the medium. These are more likely to trigger a false response than the ELISA test which uses blood. Fortunately, the false negative rate on the ELISA test (which is what most vet offices use for standard FeLV tests) is low enough that most people don’t bother with the second test.

How long does a cat who tests positive have to live?

There is no set time period for how long an FeLV+ cat will live. One person on the internet said they had a cat which lived for 20 years with the virus, while others have given dates as long as 10 or 12 years, although these are probably extremes. I have not found any truly long-term studies to document, but it seems that of the studies done, 83% of FeLV+ cats do not live beyond 4 years. (Hardy, et al 1980). All cats which do not later test negative, but in all other ways are healthy, are carriers for the virus. Even though they do not have acute symptoms, they can still spread the virus to cats which are not infected. Often people do not have their cat tested for the presence of FeLV until the cat is noticeably sick, and by this time the FeLV-related disease may have progressed too far for the cat to recover.

What are symptoms for which I should be on the lookout?

Unfortunately, since FeLV is a retrovirus that attacks your cat’s immune system, your cat can become ill from many things as a result. This makes looking for a ‘sure sign’ very difficult. Often the immune system is weak so your cat will become chronically infected with certain conditions such as stomatitis, gingivitis, oral ulcers, abscesses and non-healing wounds of the skin, upper respiratory infections or FIP. Some cats whose digestive tracts are affected have been described as staring at their food bowl seemingly unable to remember how to eat, or their breathing will be very difficult and loud. Basically, whenever your cat shows chronic, peculiar and/or unhealthy behavior, take it to a vet to be examined.

My cat recently passed away from FeLV. How long should I wait before getting another cat and are there any special precautions I should take?

PART II: If your cat has tested positive

Some of these points are mentioned in the general section above, but here are more specific questions geared to people whose cats have tested positive. The most important point to stress is that FeLV+ cats *MUST* be made indoors-only. This needs to be done for two reasons. First, the more you expose your cat to outside ills, the more likely it is to contract an FeLV-related disease or infection. The second reason is that FeLV+ cats are like Typhoid-Marys to any other cat they meet. As noted above, the vaccine is only 75-85% effective, so any vaccinated cat that your cat encounters is at risk, as well as any unvaccinated cat. If these cats are then infected and they continue to interact and infect other cats, then you could give rise to an epidemic in your area. If you cannot or will not keep your FeLV+ cat indoors, than the only humane thing to do is find a home for it with someone who will (ways to do this are suggested below), or have your cat put to sleep. This may sound extreme, but it is extremely selfish to allow your cat to roam the neighborhood possibly infecting all the local outdoor cats just because you refuse to keep your cat indoors. Keeping your cat indoors is one of the responsibilities of owning an FeLV+ cat.

My cat has tested positive. Should it be put to sleep?

are

FELINE LEUKEMIA VIRUS WILL NOT GO AWAY IF YOU JUST IGNORE IT!

What will happen to my cat now that it has tested positive?

Because there are so many different ways to respond to the presence of the virus, it is impossible to predict exactly what will happen (and when it will happen) to your cat. I’ve noticed that any 5 vets you ask will give you 5 different answers to this question. The scientists who experiment and publish on the disease are also in disagreement. I have presented two different (published) viewpoints below. According to Hardy, et al (1980), (and these people seem to be the authorities on the disease) roughly half of all cats who test positive (and do not test negative again within a three month period) are persistently infected and show acute signs of FeLV-related diseases. The other half are latently infected and are in all other aspects ‘healthy’ although they are still carriers. 17% of all cats which test positive (and do not later test negative) will live past 4 years. The next point of view is taken from a much more recent article and has radically different statistics. According to Loar (1993), of cats which test positive for the disease only 5% will immediately become infected with an FeLV-related disease. The other 95% will enter the latent phase which will last for months to years. These 95% are still carriers for the disease and can infect other cats.

Although my cat has tested positive, it is healthy in all other respects. How can I prevent an FeLV-related disease from becoming active in its system?

There is no sure way to keep your cat healthy. Eventually, an FeLV-related disease will probably develop no matter what you do. However, one way in which a disease is likely to develop is if you stress your cat’s system. If a cat’s system is stressed, its body can’t put as much energy into fighting off illnesses (just like you always seem to get sick during exams or when you have to make a presentation to your boss). “When all my cat does is eat and sleep 20 hours a day, how can it possibly be stressed?” you may ask. Anything which is upsetting or unpleasant for your cat may stress it, such as going for very long periods without food or water, overcrowding, movement to new territory, territorial conflicts, sending your cat to be boarded for long periods of time, or pregnancy and lactation. (Of course one should NEVER breed a FeLV+ queen as it will expose not only the tom, but all the kittens will be FeLV+. There is also an indication, although no proof, that FeLV causes abortions in queens. For the same reasons, neither should FeLV+ toms be bred.) Basically, keep in mind things which you have noticed in the past that seriously upset your cat. These are things which are more likely to lower your cat’s natural immune system and give a disease the chance to attack.

I have an FeLV+ kitten that I have decided to keep. Should I have it neutered/spayed?

I’ve only encountered one case of a vet recommending not to spay a kitten because it was FeLV+. Unfortunately, going into repeated heat cycles was very stressful on her, and that owner believes it caused her to suffer ill effects of the disease sooner than she may have otherwise (of course there is no proof to this). It is true that there is a small added risk to anesthetizing an FeLV+ cat, but that risk will well worth the alternatives. Especially since a whole cat will have a very strong desire to roam, thus further spreading the disease. In my personal experience, my vet did a very careful screening to make sure my FeLV+ cat could handle the anesthesia before operating. It really should not be much of a problem, but always make sure if your cat needs surgery that you remind the vet and staff that your cat is FeLV+.

Should I continue to vaccinate my cat if it is FeLV+?

The literature recommends against continued FeLV vaccinations. Other feline vaccinations (panleukopenia, rabies, etc.) should be continued.

I have had several cats for a long time. One of them recently tested positive, but the others have not. Do I need to get rid of the FeLV+ one?

This is a tough situation, for which there is no pat answer. You should discuss the matter with your vet. One vet I spoke with felt that chances are the other cats have already been exposed and it is probably best to just keep them up on their vaccinations and not change the household drastically. One study (Barlough, 1984) says that in a survey of 45 households from which FeLV+ cats were removed, 99.5% of the FeLV- cats remained negative. However households in which the FeLV+ cats were not removed had infection rates 40 times greater. It is probably best to remove the infected cat if it can be sent to a good home without causing too much disruption in your household, and your mental psyche.

I have an FeLV+ cat and I want to find it a playmate. What should I do?

dog FAQs

Another option is getting a second FeLV+ cat. The obvious down side is that you not only have twice the vet bills, but you also put yourself at risk for twice the heart-ache when one or both become ill. However, if you are willing to take the risk you can search for FeLV+ cats by putting an ad in the newspaper and calling all your local vets and animal shelters and explaining your situation. They may put you on a list and should they receive any cats which test positive, they may give you a call. Make sure you give your current vet as a reference as most shelters and other vets will want to make sure you understand the responsibility of owning an FeLV+ cat or to make sure that you are not some psychopath looking for sick kitty-cats to do nasty things to (of course most people, especially your relatives, will think you are a psychopath anyway, for keeping and seeking out more FeLV+ cats).

I have an FeLV+ cat that is otherwise healthy, I do not want to put it to sleep, but I can’t keep it. What can I do?

are

people who will lie to get cats to feed to other pets, to sell to labs or to abuse. Think about this as you interview each potential candidate.

References

CancerNet Factsheet is available via gopher to gopher.nih.gov following the menu path Health and Clinical Information/CancerNet Information/Fact Sheets from the NCI/Risk Factors and Possible Causes. Or you can call the Cancer Information Service toll free at 1-800-422-6237 (1-800-4-CANCER).
Hardy, William D., Essex, Myron, and McClelland, Alexander J. (eds). Feline Leukemia Virus. Elsevier/North-Holland, Inc. New York, 1980.
Loar, Andrew S. “Feline Leukemia Virus: immunization and prevention” in Veterinary Clinics of North America: Small Animal Practice. 23(1):193-211, 1993.
Barlough, J.E. “Seriodiagnostic aids and management practice for retrovirus and coronavirus infections” in Veterinary Clinics of North America: Small Animal Practice 14(5):955-969, 1984.
Olsen, R.G. et al “Oncogenic viruses of domestic animals: in Veterinary Clinics of North America: Small Animal Practice 16(6):1129-1144, 1986.

Additional Readings

Hardy, W.D. Jr, et al. “Biology for Feline Leukemia Virus in the natural environment” in Cancer Res. 36:582, 1976.
McClleland, A.J. et al. “Prognosis of healthy Feline Leukemia Virus infected cats” in Rev. Cancer Res. 4:121, 1980.
Rojko, J.L., and Hardy, W.D. Jr. “Feline Leukemia Virus and other retroviruses” in The Cat: Diseases and Clinical Management, NY: Churchill Livingston, 1989.
Rojko J.L. et al. “Reactiviation of latent Feline Leukemia Virus infection” in Nature (Lond.) 198:385, 1982.
Also, the Journal of the Am. Vet. Med. Association, 199(10), Nov. 15, 1991 is devoted entirely to feline viral diseases.