October 01, 2018

A Practical Guide to Feline Retrovirus Testing

Knowing when and how to test for feline leukemia virus and feline immunodeficiency virus will enable optimal care for feline patients.
Cat Retrovirus TestingTesting and segregation form the bedrock of feline retrovirus control in veterinary clinics, shelters, and homes, according to Susan Little, DVM, DABVP (Feline Practice), who co-owns 2 feline practices in Canada. “These retroviruses are not going anywhere,” she emphasized, “so we need to be continually vigilant.”

In a recent webinar hosted by Idexx, Dr. Little discussed recommendations for retrovirus testing in cats in various situations.

Testing For Feline Immunodeficiency Virus

Currently available feline immunodeficiency virus (FIV) tests detect either anti­bodies against the virus or the presence of viral nucleic acid, Dr. Little said. Point-of-care (POC) test kits used in clinics and shelters detect soluble antibodies that target various FIV antigens.

Referral laboratories also conduct enzyme-linked immunosorbent assay (ELISA) as a screening test, as well as antibody-based Western blotting (targeting more antigens than the in-clinic test targets) and poly­merase chain reaction (PCR) testing as follow-up tests to detect viral proviral DNA, viral DNA, or both.

Point-of-Care Test Kits for FIV
Various POC tests can now be used in the clinic to detect FIV, and all perform well. One recent inde­pendent study compared performance among 4 POC tests and found no significant differences for FIV diagnosis—all showed reasonably good sensitivity and specificity for FIV.1 These tests use ELISA and rapid immunomigration technology.

When using a POC test to screen for FIV in an adult cat, if the result is positive, Dr. Little advised retesting immediately using a different kit, especially if the cat has received an FIV vaccination. If possible, choose a kit that is more likely to discriminate between FIV vaccine antibody and natural infection, she recom­mended. Alternatively, veterinarians could just use a validated PCR test, she said.2

How FIV Vaccination Affects FIV Testing
Even though an FIV vaccine is no longer available in the United States or Canada, differentiating between antibodies resulting from natural infection and those arising from vaccination remains a problem, according to Dr. Little. “Some cats with FIV-vaccine–induced anti­bodies will continue to test positive for FIV antibodies for years after their last vaccination,” she said, under­scoring the need to determine whether the cat was vaccinated for FIV.

One study indicated that some POC antibody test kits may better discriminate between FIV-infected and noninfected (vaccinated) cats, she said.2 As long as the cat was vaccinated more than 6 months before testing, the Witness FeLV-FIV (Zoetis) and Anigen Rapid FIV Ab/FeLV Ag test (BioNote) kits seemed to differentiate between antibodies from FIV vaccina­tion and those from natural infection better than the SNAP FIV/FeLV Combo (Idexx) could. “Veterinarians should be aware of these differences when using POC tests,” Dr. Little said.

Testing Kittens for FIV
Spread predominantly through fighting and biting, FIV is shed in high concentrations in the saliva, which contains infected white blood cells.3

However, kittens can have FIV antibodies without being infected, because they can acquire the anti­bodies from their mother if she was either vaccinated against FIV or naturally infected with the virus.

“There is a misconception that you can’t test kittens for FIV until they reach 6 months of age because of the effect of maternally derived antibodies [MDAs] against FIV,” Dr. Little said. By a few months after birth, however, kittens lose MDAs and are less likely to be seropositive for FIV antibodies. “In 1 study, 60% of 8-week-old kittens born to vaccinated queens were seropositive,” Dr. Little said, “but by 12 weeks of age, all kittens were seronegative.”4 The updated retrovirus management guidelines will address the mistaken idea that kittens younger than 6 months should not be tested.

Kitten FIV testingWhen kittens older than 12 weeks test FIV positive, Dr. Little tends to believe that this signals infection and is not simply the result of vaccine-induced MDAs. These kittens should be retested with another methodology, such as a validated PCR assay, to determine infection status.

The bottom line: There is no magic age at which to start testing for either virus, according to Dr. Little—kittens should be tested for both FIV and feline leukemia virus (FeLV) promptly. “My goal is to test as many kittens as possible for both viruses, so I never delay testing them,” she said. “I test them as soon as I see them in the clinic.”

Most kittens do test FIV negative, Dr. Little said, which is a relatively reliable result. Because this virus does not easily transmit vertically, kittens typically do not acquire infection from the mother. For kittens that do test FIV positive and are not sick, she advised veterinarians simply to retest using a different POC test at the next vaccination visit 3 or 4 weeks later. “They usually test negative on the next visit,” she said. Alternatively, the kitten can be tested immediately using a valid PCR test.

Western Blot Testing for FIV
Because the consequences of a positive FIV-screening test are significant, follow-up testing should be performed.3 Although veterinarians can choose simply to use a different POC soluble antibody test, they could send blood samples to a laboratory for Western blot analysis to detect antibodies against various FIV antigens.3 However, Western blotting has been shown to be less sensitive and specific than POC screening tests.5

PCR Testing for FIV
Dr. Little reminded veterinarians that PCR testing by a referral laboratory should be used only in specific situations. These include a second-tier assessment for a kitten that tests FIV positive using a POC test, as well as in cats that received the FIV vaccine or in kittens with vaccine-induced MDAs. “This test is much less likely to pick up a positive result from an FIV vaccine,” she noted.

PCR is also useful in situations where a kitten has tested FIV positive and is not sick, but a follow-up test should not wait until the next vaccination visit. For example, if an owner has more kittens at home, veter­inarians can retest immediately using a validated PCR test. Dr. Little stressed the importance of using a PCR test that has been evaluated independently and has good sensitivity and specificity, such as the Idexx FIV RealPCR.2

“A positive result on a validated PCR test is very reliable,” Dr. Little said, noting the substantial genetic diversity among FIV subtypes. “And a negative result probably rules out infection, but we cannot be 100% sure about this, because sometimes no FIV virus is in circulation and thus it will not be detected by the PCR test.”

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