Feline Viral Disease Testing in General Veterinary Practice

Although a feline SNAP^® test or other in-house enzyme-linked immunosorbent assays (ELISA) seems simple enough, obtaining a positive result for feline leukemia (FeLV) or feline immunodeficiency virus (FIV) on a client’s new kitten in the middle of a busy day can present a challenge. Do you tell the client that her new kitten is positive, or do you need to run a confirmatory test? How will you explain the test results and your diagnostic plan to the owner?

These simple, straightforward tests can rapidly become complicated in a “real world” setting. For this reason, every veterinarian and veterinary technician should periodically review feline viral testing guidelines, to ensure that they are making the best possible recommendations for their patients and clients. At this time, the most recent guidelines are those published by the American Association of Feline Practitioners (AAFP) in 2020.1

Feline Leukemia Virus

A traditional in-house FeLV ELISA test looks for the presence of the FeLV p27 antigen. The AAFP recommends testing all cats for FeLV when they are first acquired, prior to initial FeLV vaccination, following potential exposure to infected cats, or if any clinical signs of illness are displayed.¹ Kitten can be tested at any age.

A negative test result is generally reliable, although false negatives may occur within the first 30 days of infection. For this reason, the AAFP recommends retesting high-risk cats 30 days after potential virus exposure.

When a cat tests positive on an in-house test, however, the results are less reliable. False positives can occur; therefore, confirmatory testing is recommended. Confirmatory FeLV tests typically include polymerase chain reaction (PCR) or immunofluorescent antibody testing (IFA), although a microtiter antigen testing performed at a reference laboratory may also be utilized. There is no true “gold standard” test for FeLV, which makes things more complex. A positive result on any confirmatory test (following a positive result on an in-house test) confirms the diagnosis of FeLV. A negative result, however, only means that the cat’s status is unclear and further testing may be negative.

Understanding the results of confirmatory testing requires understanding the possible outcomes of FeLV infection:

• Abortive infection (formerly referred to as ‘regressor cats’): The cat’s immune system clears the FeLV infection. Tests for viral DNA and RNA will both be negative.
  
  
• Regressive infection (formerly ‘latent infection’): The cat’s immune system contains, but does not entirely eliminate, the virus. These cats do not shed the virus, but FeLV is integrated into the cat’s genome and proviral DNA can be detected by PCR testing.
  
  
• Progressive infection (formerly ‘persistent viremia’): FeLV replication is uncontained, resulting in virus shedding and clinical disease.
  
  
• When testing for FeLV, it’s not as simple as determining whether the cat is positive or negative. The differences between progressive, regressive, and abortive infection have clinical implications, as well as diagnostic implications.
  
  
• PCR testing for FeLV looks for proviral DNA in the cat’s bloodstream. A positive PCR result indicates current infection with FeLV. However, this infection could have a number of potential outcomes, including regressive infection. Therefore, cats that test positive on a FeLV PCR test should be retested after approximately 4 months, to determine whether their infection has been cleared. A persistent positive result indicates regressive or progressive infection.
  
  
• A positive result on IFA testing will only be obtained when FeLV infection affects the bone marrow. A positive IFA result indicates progressive infection, while a negative IFA can indicate regressive infection, abortive infection, or a true FeLV-negative cat.
  
  
• In many cases, repeated testing is necessary to determine a cat’s true FeLV status. Results may change over time, and monitoring these results is essential to determining a true diagnosis. Regardless of which confirmatory testing you elect to use, it is important that you and your staff never make life-or-death decisions based solely on an in-house ELISA test.

Feline Immunodeficiency Virus

An in-house FIV ELISA looks for the presence of antibodies directed against FIV. The AAFP recommends testing all cats when they are first acquired, following potential exposure to infected cats, or if clinical signs of illness are displayed.¹ Kitten can be FIV-tested at any age, although false positives may occur in kittens that received maternal antibodies. For this reason, kittens with a positive FIV test should be retested every 60 days until they reach 6 months of age.1 If a kitten that has tested positive later converts to negative, this indicates that the positive result was simply due to maternal antibody transmission.

A negative test results in an in-house FIV ELISA is generally reliable, although false negatives may occur within the first 60 days of infection. For this reason, the AAFP recommends retesting high-risk cats 60 days after potential exposure.

When a cat tests positive, however, further confirmatory testing is indicated. Although in-house FIV tests are highly specific, false positives can occur. Additional tests may include Western blot, PCR, or a point-of-care test produced by another manufacturer. Although Western blot has typically been regarded as the “gold standard” for FIV diagnosis, some studies indicate that current in-house ELISA tests have a higher sensitivity and specificity than a Western blot.2 A positive result on a Western blot confirms the diagnosis, while a negative result means that the cat’s disease status is unclear. PCR can also be beneficial, especially if the results confirm the results obtained on an in-house test, but false negatives can occur during times when there is little circulating FIV viral material.² Therefore, discordant results present a diagnostic challenge and indicate a need for further testing.

Testing in Shelter Cats

Given the diagnostic challenges posed by feline viral disease testing, an increasing number of shelters are choosing to forego testing cats in their possession.³ Therefore, it is not uncommon to see newly adopted cats with unknown FeLV/FIV status. Shelters cite two primary reasons for this decision:

The incidence of FeLV and FIV in shelter pets is low. Approximately 3.1% of North American shelter cats are positive for FeLV antigen and 3.6% are positive for anti-FIV antibodies.4 A low incidence of disease increases the likelihood that a positive test result is a false positive. Confirmatory testing requires financial resources and requires cats to spend longer periods of time in the shelter, with questionable benefits.

A negative result is simply a snapshot in time. Cats do not typically remain in shelters for the entire 30 to 60-day period that would be required for serial testing needed to reliably rule out FeLV or FIV disease. Even cats that test negative upon intake may still carry the virus to their new homes.

Despite these limitations, the Association of Shelter Veterinarians (ASV) recommends testing of all shelter cats that are entering a group-housed environment.5 Cats that test positive may or may not actually be carrying FeLV or FIV, but they should be treated as if positive until more definitive results can be obtained. These cats should be sent to well-informed foster homes, adopters, or partner organizations, with instructions to pursue additional testing as recommended by their veterinarian.

Summary

All cats, regardless of age, should be tested for FeLV and FIV when first acquired, as well as following potential exposure to infected cats or if any clinical signs of illness are displayed. Negative results can be regarded as trustworthy, although cats that are at high risk should be retested after 30 days (FeLV) or 60 days (FIV). A positive result in an in-house FeLV or FIV ELISA test, however, requires confirmatory testing. Confirmatory testing for FeLV often involves a PCR or IFA test, while confirmatory testing for FIV may include PCR or Western Blot. If a kitten under six months tests positive for FIV, this may be due to maternal antibody interference and periodic retesting (until negative or six months of age) is recommended.