In answer to Michelle's question... A while back we had someone post on this list who was subsequently "kicked off" because his style of expressing himself rubbed some listmembers the wrong way. I thought that was unfortunate because this person has a really awesome personal library, is well-read and evidently rubs shoulders with vets and/or researchers and might have had much useful information to share with us. This person provided the first, and best, explanation I've found so far to explain why some cats can live with the virus long term, while other cats develop fatal anemia, lymphomas/other types of cancer, which are fatal. It seems that all cats who become infected with the FeLV virus are infected with the FeLV-A subgroup. This manifestation of the virus is the only one that is passed from cat to cat, and in the scheme of things is less serious in that it causes significant immunosuppression, but not much else. Provided a cat's FeLV infection does not mutate beyond this state, there is a reasonable prognosis for long-term survival, even if the infection has progressed to stage 5, at which point the virus has settled in the bone marrow and the cat will thus remain infected for the rest of his/her life. It is in stage 6 that the mutation and development of fatal symptoms develops. An ELISA+ can indicate even the earlier stages of infection, while an IFA+ correlates with stage 4 on.
Other subgroups of the FeLV virus develop within an individual cat through mutation and recombination with that cat's DNA. The FeLV-A+B subgroup is associated with the development of various types of lymphoma or other cancers, while the FeLV-A+C subgroup is the one associated with severe/nonregenerative anemia. (Within each subgroup there is more than one "strain," but that seems to be considered useful info only to researchers.) It is possible for both the B and C subgroups to manifest in a particular cat either simultaneously or in tandem so that the cat will have FeLV-A, B+C, which I suppose is the worst case scenario as a cat with some form of lymphoma or cancer might still be considered to have some sort of chance to respond to treatment, but one which also has nonregenerative anemia and can no longer prodcue red blood cells has no chance at all. It IS possible, in research settings to determine which subgroup(s) of the virus a cat may be infected with, but, since there is yet no known way to prevent a mutation from occurring or forestall the development of symptoms once it has, that info is not considered to be useful info except to researchers. It becomes obvious when the serious symptoms of lymphoma/cancer, and/or nonregenerative anemia present that the infection in a particular cat has progressed beyond a simple FeLV-A state to one of its more serious manifestations. Once a cat has "crashed" and presents with such symptoms, there is, so far, no known treatment that can reliably affect the eventual outcome, which is death. The wild card in all of this seems to be the relative strength of each individual cat's immune response, or whatever immune response he/she still has after becoming infected with FeLV-A. ( Wouldn't it be great if there were some way to test and measure this?!) In this regard it must be assumed there is a genetic component. Some cats are blessed and initially come into this world with a stronger immune system than others. Another factor is how much and what kinds of stressors a cats's immune system may be subjected to, in terms of poor nutrition, other diseases, environmental toxins/pollution, the over-use of vaccines, emotional stress and deprivation, etc. Kittens are most at risk, with geriatric cats whose immune function is declining at an increased-if-not-high risk. Most healthy, adult cats are thought to be naturally and relatively resistant to infection by the FeLV virus and if exposed and transiently "infected" are able to clear the virus even up to stage 4 of the infectious process. Kittens are most at risk because they can become infected in utero from an infected mom, or soon after birth through nursing. After weaning, young kittens are still at high risk because their immune systems are immature and still developing. For whatever reason, 50 times more FeLV virus replicates in the lymphocytes of kittens than in the lymphocytes of adult cats. It is not so unusual, then, for an adult cat to be exposed, become infected to the extent an ELISA will yield a positive result, or even an IFA positive if the cat is still in stage 4, and for the infection to be only transient and the cat able to seroconvert back to negative status. There is also the possibility, however, that the infection could have become latent and later be triggered back into active status by vaccination or some other type of stress to the immune system. For kittens, however, it is all but unheard of for one who initially tests ELISA+ that is IFA confirmed to be able to seroconvert. Such an instance would be highly suspect, with the need to test for a latent infection indicated. I have not yet come across a valid explanation as to why a kitten/cat might initially test negative, then later test positive, with no interim exposure to an FeLV+ cat. I speculate that the infection might have been in the incubation process when the test was performed. Or, is it possible for an infection to be initially latent?? These are two questions to which I would like to find the answer. For reasons that I still do not understand, some young kittens initially test ELISA+ but later retest negative. I have one such kitten who at 6 weeks tested positive and was going to be PTS if I had not rescued him. Because of the cost, I did not have that result confirmed with an IFA, but just assumed he would remain positive for however long he lived. When he was neutered at 5-6 mos. old, he retested NEGATIVE on an ELISA. Is he truly a miracle baby who has cleared the virus, or has the infection become dormant? Without doing an expensive, invasive and stressful bone marrow reactivation test, it seems I won't know for sure. I was hoping that there was some other less invasive test, such as the PCR test, which might provide the necessary answer. But, according to Dr. Susan, unless that test is meticulously performed and interpreted, it would not be reliable for that purpose. It is said that "false negatives" on an ELISA would only be due to error on the part of the test administator, so are rare. A true "false positive" on an ELISA would be for FeLV antigen to be detected when none was actually present, so that term is a bit of a misnomer. What is more likely is that because the ELISA test is 100 times more sensitive at detecting the antigen than is the IFA, a positive result can be obtained when only minute amounts of antigen are present and perhaps if a particular kitten is fortunate enough to have a particularly strong immune response, and the exposure was not intense, the infection might not become permanent. That, of course, is speculation on my part and this is an aspect about the testing and infectious process about which I seek to learn more. I had previously (and erroneously) thought such transient positives in kittens were because when tested before weaning or soon after, whatever antibodies were passed from the mom to her kittens were still in play and until the kitten was old enough and far enough past weaning for his/her own immune function to be starting to mature, it was not possible to get a truly indicative reflection of a kitten's FeLV status. But it has since become clear to me that it is not antibodies, but the antiGEN produced by the replicating virus that is being detected by the ELISA and IFA tests. So, it would seem that, for now, the best we can do for the FeLV+ kittys who find their way to us is to feed them the best possible diet we can -- preferably a raw one (Dr. Ihor Basko mentioned in Dr. Goldstein's book claims to have a good rate of success in that regard and has devised a special diet for FeLV+ cats) -- with immunosupportive supplementation, keep stressors in their lives to a minimum and protect them from pathogens that might overwhelm a compromised immune system. My limited personal experience, and from reading about the experiences of other listmembers in the almost two years I've been a member, is that once a cat begins to develop and present with major and serious symptoms characteristic of the B or C subgroups, it is not a matter of if, but when, the end will come. There seems to be no way to predict which cats will remain in the FeLV-A state, and which ones are so ill-fated the virus will mutate into the B, C or B+C subgroups. All we can do is provide the best supportive care and diet and love them one day at a time. Sally in San Jose I hope and pray that one day it will be understood what it is that causes the FeLV virus to mutate from the less severe FeLV-A form to one of its more virulent subgrous. Perhaps then a viable form of treatment can be formulated to prevent this from occurring and give more kittys a better chance for long-term survival.
