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Archive-name: cats-faq/leukemia
URL: http://www2.dgsys.com/~ermiller/FeLV.html
Last-modified: 13 Aug 1999

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==========


                         Feline Leukemia Virus FAQ
                                      
   (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.) 
   
   Contents:
     * General information about FeLV
     * If your cat has tested positive
     * References
     * Other FeLV sites
       
   Last updated April 20, 1998.
   
   Authors:
   Written by Erin Miller [ 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?
   People often use the "it's like AIDS" phrase to describe a number of
       illnesses in the animal (and human) community with the idea that
       most people know so much about AIDS that this analogy is useful.
       Unfortunately most people don't know much about AIDS and the
       resulting effect is to scare people out of their wits and have
       them dump their cats or dogs at the nearest pound because they are
       so deathly afraid of AIDS. They are similar to AIDS in that they
       affect and weaken the body's immune system. The ONLY similarity
       between FeLV, FIV (Feline Immunodeficiency Virus) and FIP (Feline
       Infectious Peritonitis) is in their genetic makeup. All are RNA
       (as opposed to DNA) viruses, and FeLV and FIV are what are known
       as "retroviruses." FIP is a "coronavirus." (See the 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?
   Yes, there are several commercial vaccines available. Unfortunately
       the USDA does not have standard requirements for FeLV vaccines, so
       different manufacturers can publish 'Effectiveness Ratings' which
       cannot be compared with each other due to a lack of uniformity in
       testing terms and requirements. Estimating the effectiveness of
       the vaccines is difficult and most vets I've spoken with estimate
       them to have ~75-85% effectiveness (this means cats challenged
       with the virus will fight off infection 7-8 times out of ten).
       Published studies show that there are irreconcilable differences
       and bias in Effectiveness Ratings put out by the manufacturers so
       currently it is impossible to estimate the actual effectiveness of
       and between vaccines. Suffice it to say that your cat is MUCH
       better off trying to fight the virus having received the vaccine,
       but the vaccine is not 100% effective so you should not expose
       your cat to undue risks. (Note: NO vaccine, whether it be for
       cats, dogs or people, is 100% effective. Some are about close to
       being as fully effective as possible, however the FeLV vaccine is
       not one of them).
       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 always a risk that your cat may have a bad reaction to a
       vaccine, ANY vaccine. It is a good idea to wait in your vet's
       waiting room for 30 minutes after receiving a vaccination, ANY
       vaccination, to make sure your cat does not have an adverse
       reaction, and if it does the vets will be on hand to deal with the
       situation.
       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?
   The answer to this question all boils down to a risk/benefit
       assessment.
       If you live in a high-rise, do not plan on moving in the next
       year, and do not plan on exposing your cat to other cats (such as
       getting a new kitten, or temporarily housing a stray), then there
       is really no need to get the cat vaccinated. If it is somehow
       possible for your cat to make it to the outside world, whether it
       be because of a break-in, or a landlord who forgets to close the
       window they just fixed, or a visitor doesn't realize the cat is
       not allowed outside (all of which are real-life cases of people I
       know whose indoor-only cats have gotten outside) then your cat is
       at SOME risk. Many people who lived through Hurricane Andrew or
       the LA Earthquake can tell you that some of their indoors-only
       cats ended up on the street for days. Fortunately disasters like
       these are infrequent, but the point is accidents can happen. In
       the few hours or days that your cat is outside it could come in
       contact with an infected cat, and it is better to give your cat
       that 75-85% boost to its natural resistance.
       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.
   This is a touchy subject because there is often the general feeling
       among vets that many breeders don't know what they are doing, and
       the feeling among breeders that vets don't know much about
       purebreds. There is probably a little bit of truth to both sides.
       It is true that some breeders may think they understand veterinary
       medicine better than they really do, and unfortunately rely on
       word-of-mouth advice of other breeders rather than vets (eg: "Jane
       Smith has been breeding for 15 years and she knows a lot so if she
       doesn't like this vaccine, it must be bad"). But it is also true
       that most vets do not deal specifically with purebred issues.
       There is no course in vet school called "Purebreds 101" and vets
       are often just as guilty as anyone in misidentifying mixed breeds
       as purebreds, of being too quick to diagnose a "breed specific"
       illness with less data than if they would if the cat were not a
       purebred, or of not being aware of conditions which may affect one
       breed more than another.
       Keep in mind that some veterinarians are also breeders, or work
       within the CFA, TICA, ACFA (AKC for dogs) or other purebred
       registries, and these vets are most likely going to be more
       knowledgeable about conditions which are more common to certain
       breeds. Also remember that not all purebreds are the same, each
       breed is different and has its own characteristics. Just like you
       can't say "don't vaccinate Europeans for smallpox," because there
       are *many* different cultural and ethnic groups in Europe.
       The immune system of purebred cats has NEVER been tested to
       determine if it is different than that of mixed breed cats. It is
       impossible to state one way or another if the purebred immune
       system, because of inbreeding, has any reason to adversely react
       to vaccines that are tested on a largely mixed-breed cat
       population. Some people feel there is a significant difference
       between the two based on antecdotal evidence and won't vaccinate
       for that reason, and some people feel this is nonsense.
       But that doesn't answer the question because this one is going to
       have to be answered by you and your own gut feeling. Who do you
       trust more with the combined necessary knowledge of vaccines, as
       well as about your purebred? Talk to your breeder and see if s/he
       is aware of the vet literature, or is repeating word-of-mouth
       arguments. Many of them are well aware, many read the literature
       more than vets do, so don't be afraid to ask them why they hold
       the opinions they do. Talk to your vet and ask how familiar they
       are with your partiuclar breed of cat as well as purebreds in
       general. Get a feel for both. Keep in mind that many breeders do
       not vaccinate their own breeding cats because FeLV vaccines can
       cause miscarriages and stillbirths, or it may just be cheaper for
       the breeder to test their cattery every year and every cat that
       comes into the cattery rather than to vaccinate, especially as
       cats from a cattery are less likely to escape to the outside world
       (remember the risk/benefit assessment above). Make sure you find
       out 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)?
   This depends on the environment the new cat comes from. If it is a
       stray, or from a shelter which does not routinely test for the
       viruses (make sure you ask this of any shelter you visit), or from
       a household where you have reason to doubt the person has had the
       cat tested/vaccinated, then keep the new cat separated from yours
       until you can have a vet examine it for many things. Keep it in a
       separate room and provide its own food dish, water bowl and
       litter. DO NOT let your cats share any of these things, or share
       the same space, until your vet checks out the new one. If it is a
       stray cat, it may never have been vaccinated against FeLV and
       Rabies, or if it was a housecat it may be past its time for an
       update and have been exposed. Cats which have been on the street
       may also have fleas, tapeworm, ringworm or other parasites which
       are transmittable to you and your pets.
       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?
   Your vet will do a blood test; there are two types of blood tests
       which can be performed. Some vets will automatically do one of the
       tests before vaccinating your cat to make sure it is not already
       positive for the virus. The first (ELISA test) is where the vet
       takes some of your cat's blood, mixes it with a chemical and
       watches for a color change. If the blood changes color then your
       cat has tested positive for the virus. False positives are not
       uncommon in this form of test, so if your cat tests positive it
       may be a good idea to have it retested. "Light positives" are
       where the treated blood only changes color slightly. This means
       your cat is infected with the virus, but the virus is not very
       active in its system. The second type (IFA test) involves sending
       the blood sample to a special lab. This lab tests to see if the
       virus is being produced in the bone marrow. If this second test is
       positive, it is unlikely that your cat will ever test negative.
       Below is a flow chart (from a lecture by Dr. William Hardy, U of
       Penn. Vet School) which depicts what you should do if your cat
       tests positive on the ELISA test:
       
     * 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?
   Well, your own grief issues aside, from a medical point of view you do
       not need to wait very long. The virus is fragile outside the host,
       and I know of one vet who has even stated that it would be safe
       the next day. Personally, I would wait a week at least. You should
       definitely discard the litter box and food/water bowls, or else
       clean both well with household detergents or a 1:32 bleach
       solution.
     _________________________________________________________________
   
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?
   The mere fact of testing positive is not enough to merit putting a cat
       to sleep, although there may be other significant factors involved
       which do make putting the cat to sleep the best option. There are
       vets who recommend putting all FeLV+ cats to sleep. If your vet
       recommends this and you feel comfortable with that decision, then
       that is the best solution. Sometimes putting an FeLV+ cat to sleep
       is the best option for the cat, especially if it has acute
       symptoms and is in pain. This is never an easy decision and one
       which should not be taken lightly. If you do not feel you are
       capable of emotionally dealing with having an FeLV+ cat (or cannot
       keep the cat indoors), but do not want to put the cat to sleep,
       there are other alternatives which are discussed below. The one
       thing you should not do is ignore the virus. If your cat has
       tested positive, then you have a responsibility to take some
       action. 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

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