FeLV and FIV: testing... diagnosing... preventing (Proceedings) - Veterinary Healthcare
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FeLV and FIV: testing... diagnosing... preventing (Proceedings)


CVC IN SAN DIEGO PROCEEDINGS


...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.

Alternative Testing

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.


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Source: CVC IN SAN DIEGO PROCEEDINGS,
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