...but what about the test?
FIV antibody testing has become the hallmark of serologic tests used to identify cats infected with FIV. In fact, FIV testing
has become so widely used in practice that an advisory panel organized by the American Association of Feline Practitioners
and Academy of Feline Medicine recently published revised guidelines on feline retrovirus testing and implications for managing
cats determined to be positive for FIV antibody. [REF: Guidelines]. In summary, the advisory panel recommended that all cats
be tested for both FeLV and FIV infection. With respect to FIV antibody testing, emphasis is placed on the importance of testing
all sick cats regardless of negative results of previous FIV tests.
Regarding test selection, the advisory panel has published that ELISA and other immunochromatographic tests available in clinical
practice, are the preferred screening tests to be used in the initial assessment of any sick cat. It is further recommended
that all positive screening test results be confirmed by the Western blot test.
Here's the problem...all cats vaccinated with the killed FIV vaccine are expected to develop FIV antibodies following administration
of the first dose. Antibodies are known to persist for at least 1 year. Vaccine-induced antibodies interfere with all antibody
tests commercially available in the US and Europe:
In addition, kittens of vaccinated queens are likely to have a positive test result due to passively acquired vaccine-induced
antibody. Negative test results for antibody may still be interpreted as negative for exposure and infection.
With the introduction of the FIV vaccine, and loss of the ability to identify FIV-infected cats in clinical practice, a substantial
effort is underway to identify an alternative diagnostic test that is reasonably priced and accurate.
Virus isolation (VI) has been suggested as possible means of distinguishing vaccinated cats from infected cats. However, virus stability during
transport, availability, and cost are such significant limiting factors that
VI is not a reasonable consideration for veterinarians in clinical practice. Isolating FIV from infected cats is well suited
to experimental laboratories where the sample collection and virus isolation methods can be highly controlled.
On the other hand, polymerase chain reaction (PCR)-based tests for identification of RNA and proviral DNA have received considerable
attention, subsequent to the release of the killed FIV vaccine, as "the" alternative test for detecting infected cats...whether
or not they've received prior vaccination. While it is possible to identify FIV, in both vaccinated and unvaccinated cats,
using PCR technology, the ability to provide widespread diagnostic services to practitioners through commercial laboratories
has not yet been accomplished. In the long run, this may prove to be quite problematic given the nature of PCR technology.
PCR technology must not be viewed as simply another "new and improved" means of detecting FIV antibody. In fact, it doesn't
detect antibody at all...but that's just the beginning. The "family" of feline immunodeficiency viruses, is varied and their
expression, once they've infected a cat, is quite complex. That, combined with the inherent sensitivities of PCR-based test
methods make turning a PCR test into the replacement test for FIV antibody, and doing so at the levels of reliability and
consistency we have enjoyed, a major technological challenge.
Although most clinicians will not be especially interested in all of the technical and methodological issues pertaining to
PCR testing, it is important to understand that PCR, discovered just within the last 20 years, is an exceptionally accurate
method for rapidly manufacturing unlimited copies of DNA. In effect, PCR has made it possible to identify unique sequences
of DNA even when the sample size is miniscule. Obviously, such technology would be of considerable value in diagnosing infection,
particularly viral infections, where the virus quantity can be quite small and the genetic features of the virus quite distinct.
Recently, the ability of a novel quantitative polymerase chain reaction (qPCR) method to detect proviral DNA in FIV-infected
cats was described in 1999. While using the Taqman® PCR to detect FIV provirus is a significant fact, incorporating the technology
into routine use in the clinical setting will take time.
Even if PCR testing for FIV does become commercially available in the near future, the clinician must understand and appreciate
the fact that the incredible sensitivity of PCR method has direct and important implications on test results. For example,
in-hospital PCR testing for FIV, or anything else, is simply not feasible today. An outside laboratory must analyze all specimens.
In addition to contending with the risk of sample transport and contamination (with extraneous nucleic acid), it will become
critical (at least it should be) for all laboratories offering PCR testing for FIV to use standardized, validated reagents
and testing protocols. The process of validating test methods will require documenting the accuracy of PCR against various
FIV field strains (variants of virus subtypes) seen in the United States. At this writing, the means of standardizing PCR
tests for veterinary medicine simply doesn't exist. Then, with all that said, it's a matter of price.