Selecting and interpreting diagnostic tests for FeLV is complicated by the fact that cats are quite variable in their response to FeLV+. Following initial exposure to the virus, some cats have a robust immune response, completely eliminate the virus, and are cured and immune from future infection. This is called abortive infection, and cats will test negative for FeLV after eliminating the virus. If cats have a very weak immune response, the virus will replicate out of control with high levels in the blood. This is called progressive infection, and cats are at risk for FeLV-associated diseases like anemia and lymphoma and shortened lifespan. And in between are cats whose immune response is not strong enough to completely eliminate the virus, but strong enough to keep it suppressed to very low levels in the blood. This is called regressive infection, and cats are unlikely to shed the virus to other cats and may live a normal lifespan.
The two main types of tests for FeLV in the US and their interpretation are:
- Antigen: These tests detect FeLV proteins (p27) in the blood. Examples of this are the commonly used point-of-care tests and laboratory plate tests.
- PCR: These tests detect viral DNA (provirus) in the blood. A special type of PCR called quantitative PCR reports on the amount of DNA in the blood. We recommend the quantitative PCR because it provides more important information than the PCR that provides only a positive/negative result.
- Screening and follow-up: The antigen tests are most commonly used for screening for FeLV because they are inexpensive, fast (10 min), require only a few drops of blood, and can be performed in the clinic or shelter. Depending on the brand selected, they can be highly accurate. PCR tests are more expensive and must be sent to a reference lab. The PCR can be helpful for confirming the antigen results, when the antigen results are unexpected, or when information about the proviral load is important.
- Concordant and discordant results: When both the antigen and proviral tests agree (concordant), there can be high confidence in the results. However, when they disagree it can be challenging to determine which is correct. This occurs most commonly when the proviral load or antigen concentration is low and near the lower cutoff limits for FeLV detection. To add to the confusion, test results may not be the same on different days or may even have times when both tests are negative, since the level of immune suppression of the virus may vary over time.
Zorya: A case study in challenging diagnosis
Zorya came to an animal shelter as a young kitten and had a positive result in her intake FeLV antigen screening test (IDEXX SNAP). She was enrolled in our FeLV Lifetime Study aimed at determining the best test and ideal sample for FeLV testing. She had a panel of blood tests performed every month for 6 months. In addition to using whole blood, the samples were also separated into plasma and serum.
Zorya is a great example of a cat with inconsistent test results over time. There are a few important findings to note:
- The SNAP test on whole blood was positive most of the time and the PCR test was negative most of the time. However on Month 4, the pattern reversed. If her screening test had occurred on that day, the FeLV infection would have been missed.
- The accuracy of the antigen test was highest in whole blood, followed by plasma, and then by serum. That may be due to cell-associated virus (virus stuck to the outside of cells). Whole blood is cell-rich, plasma is cell-poor, and serum is nearly cell-free. If she had ever been screened with a serum sample, she would have been misdiagnosed as FeLV-negative.
- We classified her as FeLV Low Positive with a good prognosis for long-term survival. She is still alive today, at the age of 7.
- Zorya is a good example how many different factors can affect the accuracy of FeLV diagnosis. The good news is that failure to detect FeLV is most common in cats with a low level of FeLV, low chance of shedding FeLV to other cats, and a low level of FeLV-related morbidity and mortality. However, her case also illustrates that even with exhaustive testing, being certain of a cat’s FeLV status is not always possible.
The UF Shelter Medicine Program FeLV and FIV testing protocol
Given what we’ve learned about the brands, types, and costs of FeLV tests in combination with how they perform with different sample types and viral loads and that we can never rule out FeLV infection with 100% certainty, the Shelter Medicine Program has developed a testing protocol that maximizes efficiency, accuracy, and cost-effectiveness.
- In the “one-and -done” protocol, routine screening of cats with low risk of infection can be performed once with a single antigen test. This is often done when costs and time for testing are limited. We recommend the IDEXX SNAP test with anticoagulated whole blood. Results are accepted as the most likely correct status with the recognition that the FeLV status of some cats may be misinterpreted.
- In cases in which the antigen test does not match expectations, more certainty of screening test results is desired, or in cats that may have had a high-risk recent exposure, the PCR test can be added as a follow-up. We recommend the panel from the IDEXX reference lab that includes quantitative PCR in combination with the PetChek p27 ELISA for its utility in providing prognostic information for long-term survival. If the tests agree, the status is confirmed. If the tests are discordant, the true status of the cat may remain undefined.
- Cats maintained as blood donors should have a PCR panel for blood-borne pathogens including FeLV and FIV to reduce the risk of using a cat with regressive FeLV infection that is not detectable with the antigen test.
