i thought i would add this info since i didn't see it in the archives (which
doesn't necessarily mean it's not there. it only means there was so much in the
archives i didn't check out every thing.) the following was emailed to me by my
sister in law who has a phd in veterinary medicine. (however, she does not deal
with living animals but does pathology on deceased animals for a large spca on
the other side of the country.) this is info about a positive ELISA followed by
a negative IFA (what my cat Sport produced) and also includes what causes a
false positive in the ELISA test. i hope it will be of some help to someone
here, be it now or in the future.....
If you follow ELISA positive, IFA negative cats over time, about 25% will go
positive on both tests (progressive infection), 25% will go negative on both
tests (regressive infection) and 50% will remain ELISA positive, IFA negative
(discordant infection). In discordant infection, the virus is sequestered
somewhere in the cat and virus protein is being leaked into circulation (hence
the positive ELISA) but infected cells are not being shed from the bone marrow
(thus, negative IFA).
>>> 1. Of the 50% that stay with discordant infection what % of these cats
>>> become clinically ill? <<< in my experience, not very many. Age matters -
>>> kittens with discordant results are more likely on their way to becoming
>>> IFA positive and coming down with clinical disease. Adult cats with healthy
>>> immune systems are more resistant to disease even if they stay ELISA +.
>>> 2. At what time intervals should I retest the cat? <<< if healthy, I do
>>> this once a year along with the annual wellness visit. If sickly or dealing
>>> with other diseases, it makes more sense to repeat both ELISA and IFA more
>>> often, say 2-3 times a year.
And remember that resistance increases with age; so a kitten less than about 16
weeks that is ELISA+, IFA- is more likely to become positive in both, while
adults, are much more likely to clear the virus and become - by both assays.
False positives can occur with ELISA, and in screening healthy cats, it can be
a significant percentage, as much as 10%. One way that this can occur is that
in some of these ELISA's, a mouse antibody is used to "capture" the viral
antigen in the cat's serum, then a soluble mouse antibody with the enzyme
attached binds the bound viral antigen. In cats that hunt, occasionally they
will have antibody to the mounse antibody - it binds both the anchored mouse
antibody (in the well, on the filter paper) and the soluble mouse antibody with
the enzyme attached - substrate leads to color change - false positive. This
happens also with nondomestic cats.
My long-winded point is that I would continue to recheck the IFA or perhaps try
a PCR to confirm the infection status, esp in an adult healthy cat.
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