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Transmission of infectious disease in cats - FeLV and FIV
Question: Dr Richards,
I hope you can help us with our situation. We have an indoor cat who is female and 7-8 years old. We got her as a kitten from an outdoor litter, and had her immunized and she has always been indoors. She also got all her tests when we took her in and was fine. Over the years we let her immunizations lapse because she never came into contact with any other cats.
This weekend we were out of state, and were befriended by an outdoor, stray male cat. Naturally we adopted him. We went to the vet and the poor little guy (between 1 and 2 years of age) tested positive for feline leukemia as well as fiv.
We have the cats totally separated, different food, different litter etc. Tonight our indoor cat will go to the vet and get her vaccinations, including those against fiv and feline leukemia. The vet we saw out of state said that the cats should remain separated for about a month so the vaccinations can kick in.
Doing research on line suggests that these 2 cats should never, ever come into contact with each other. I would be so happy if they could keep each other company.
Can the indoor cat still contract a virus even if properly vaccinated? What do you suggest about having the two of them interact? We really look forward to hearing from you.
Thanks, Petra and Barry
Answer: Petra-
There are several things that you need to consider at this time. I'll try to put them in an order that is understandable.
Is the testing accurate?
The first thing to consider is the accuracy of the feline leukemia test. A positive ELISA (the usual "in house" test run at vet clinics) is approximately 50% accurate when a cat tests positive and appears to be healthy. It is much more accurate in cats who appear ill.
It would be best to have the FeLV test repeated using a more specific test procedure, either an IFA test or a Western Blot test. These usually have to be done by commercial labs.
The FIV test is more likely to be a true positive but even so, it is also a good idea to confirm this test using a different testing method. The Western blot test would be a good choice for confirming this positive. From a practical standpoint, with both an FIV and FeLV positive test result the odds of both being wrong are not too high, so if retesting is prohibitive you probably should put the money into the FeLV test as it is more likely to be a false positive.
Testing your female cat before vaccinating
The American Association of Feline Practitioners recommends testing for FeLV and FIV prior to vaccination. This would be especially important for your female cat in regards to FIV, as it will be impossible to test her accurately after vaccination. While the odds are very low that she would be FIV positive it would be important to know that she was actually positive in the future when secondary illnesses occurred as they must be treated more aggressively in a FIV positive cat than in a cat with a normally functioning immune system. Again, if cost is a major factor this test could be skipped on the presumption that she is most likely negative but it is best to test before vaccination if possible.
What are the risks of FIV and FeLV to an adult cat housed with a cat who is positive for either of these illnesses?
This is a good web site for information on FIV http//www.vet.cornell.edu/fhc/resources/brochure/fiv.html In this information there is a line stating that the risk of FIV transmission between stable housemates is low. Unfortunately, the situation you have is not stable at this time. If over time it becomes evident that the male cat is not likely to be aggressive towards your female cat, so that bite wounds are unlikely, you may decide that the risk of FIV transmission is low enough that you don't have to worry over it. On the other hand, if either cat shows aggression towards the other one it is better to keep them apart since it doesn't matter who started it if a fight occurs and the female ends up being bitten by the male.
It is hard to get a really firm estimate of how much risk of feline leukemia infection is present for an adult cat living in a household with an FeLV positive cat. This has been studied in a small number of studies and it looks like the risk is about 11% for infection over the course of the life of the cat but the studies were done in multiple cat households (greater than 5 cats in most cases) so the risk may be less or more when only two cats are present. Whether or not to take this risk would have to be a decision that you make after some thought.
Vaccinating for feline leukemia virus and FIV
The risk of using an adjuvented vaccine in cats is the possibility of inducing a vaccine associated sarcoma. This is a cancer that occurs at vaccine sites. It appears that the risk of causing cancer is mostly associated with adjuvents, substances in vaccines that cause irritation and make the body respond to the vaccine. I am almost certain that the FIV vaccine is an adjuvented vaccine because it is a killed whole virus vaccine and almost all killed vaccines require adjuvents in order to work. The risk of causing cancer with an adjuvented vaccine is thought to be about 1 in 3000 ( one case of cancer for every 3000 vaccinations administered).
There is one FeLV vaccine that works without adjuvents, Purevax (tm) from Merial. It is administered through a vaccine "gun" that shoots the vaccine through the skin and into the subcutaneous tissues in a manner that causes sufficient reaction without adjuvents. It is currently thought that this reduces the risk of vaccine associated sarcoma to very low levels -- or possibly eliminates it. At the present time it is too early to tell if the vaccine will actually eliminate the risk of vaccine associated sarcomas but if I was going to vaccinate my own cat for feline leukemia this is the vaccine that I would use at the current time. It is more expensive that the other vaccines and due to the need to purchase the vaccine gun not all veterinarians use it.
Feline leukemia vaccination does not protect all exposed cats from the virus. It is thought to be about 80% effective in protecting cats from infection. This is a factor to consider but given the high natural protection in adult cats combined with the vaccine's efficiency, the risk of FeLV infection would be very low for your female cat if she is exposed to the male cat after vaccination, especially if you are trying to keep them separate that the contact is incidental and infrequent.
The feline immundeficiency virus was 77% effective at protecting exposed cats from FIV infection in the initial studies. (reference for this http//www.sciencedaily.com/releases/2002/03/020325080206.htm )
Since the FIV virus is transmitted primarily through bite wounds, if the cats are compatible the risk to your female cat after vaccination would be very low but if the cats fight it may be close to a 25% chance of infection.
Quarantine periods
It is a good idea to quarantine a new cat from other cats in the household for at least a month and two is better. So even if the male had no known problems this would be the best course of action. It is sufficient to keep the cats in separate areas of the house. It would be OK to keep one cat in a room for a day and then let it have free roam of the house the next day while keeping the other cat confined. This would allow the cats to get used to the idea that another cat lives in the house, gives you an idea of how they respond to that knowledge and some idea of how much trouble it might be to put them together. If one of the cats is obviously making an effort to find and fight with the other cat, you'd know it wasn't going to be likely they could ever live together, for instance. This also gives you time to get more certain test results and to evaluate how healthy the male cat really is. It gives you time to complete the vaccination process for your female cat and for the vaccines to take effect (maybe not quite enough time for the FIV vaccine to take full effect --- it takes about 1 month after the last vaccination in three vaccination protocol).
To answer your questions specifically, in case the information above was just confusing
1) Yes there is a risk that your female cat could contract either FeLV or FIV after vaccination. The risk for an adult cat of contracting FeLV after vaccination while living with a cat with FeLV should be very low but is still likely to be about 2 to 5%. The risk of contracting FIV is also low -- very low if the cats get along but up to 23% (based on the one study) if they fight.
2) If after further testing it still seems likely that the young cat actually has FeLV and FIV, I think that deciding whether to let the cats interact should be based on how they respond to each other, to the best of your ability to judge that after watching them during a quarantine period. The idea situation would be for them to establish a peaceful coexistence in which they share the house without interacting too much -- which isn't an uncommon way for two cats in a house to act. A less than ideal situation but acceptable one would be if they really like each other -- this almost eliminates the risk of FIV but increases the risk of FeLV if they engage in friendly acts like mutual grooming. Still, the risk is pretty low and some people would find it acceptable, especially if having a friend seemed to enhance your older cat's life. An unacceptable situation would be aggression between the cats over the household territory leading to bite wounds. You would have to avoid this and if this appears very likely during the quarantine period based on the way either cats is acting you should make plans to keep them completely separate or find an alternative home for the male cat, if possible.
I would be happy to try to clarify any points that are confusing in this note. Don't hesitate to ask if there is some confusion.
Mike Richards, DVM
8/24/2005
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Last edited 01/12/08