American Association of Feline Practitioners
_____________________________________
Academy of FelineMedicine

RECOMMENDATIONS FOR FELINE LEUKEMIA VIRUS TESTING

1. The Feline Leukemia Virus ( FeLV) status of all cats should be known. FeLV infection is global in occurrence with prevalence rates varying by location.¹ FeLV is transmitted contagiously among cats and is associated with the illness and death of more cats than any other pathologic condition.²

2. Testing and identifying positive cats is the mainstay of managing FeLV infection; this is not supplanted by vaccination. The best means of preventing disease is by preventing exposure to FeLV-infected cats.¹

3. Testing should occur in the following:

• a. new kittens or cats before introduction into a household to prevent exposing existing cats
• b. newly adopted kittens or cats, even if they are the only cat in the household, for the following reasons:
  • i. the strong emotional bond that forms between pet owners and pet justifies knowing the FeLV status because of future ramifications
  • ii. statistically, most cats do not remain the only cat in a household so future exposure is very likely to occur
  • iii. even cats that are meant to be kept indoors may escape and expose other cats
• c. cats in existing households where the FeLV status is not known because carriers can remain asymptomatic for years and expose all other cats in the household
• d. cats in which a recent exposure (known or potential) occurs - regardless of previous negative test results - because the FeLV status can change
• e. ill cats because FeLV has been associated with a great variety of illnesses in cats
• f. cats presented for FeLV vaccination should have a known FeLV status prior to vaccination because:
  • i. vaccination does not affect the carrier state or the development of disease in cats with existing infection
  • ii. existing carriers remain an exposure risk
  • iii. an existing carrier can subsequently become ill and appear to be a 'vaccine failure'

4. The principle of FeLV testing is the detection of p27 core antigen or other antigens of the virion. The ELISA detects p27 in whole blood, serum, plasma, tears or saliva, whereas the IFA detects antigen within leukocytes and platelets.^a

5. Kittens can be tested at any age; maternal immunity in young kittens does not interfere with FeLV diagnostic tests.¹

6. Vaccination with FeLV vaccines does not interfere with FeLV diagnostic testing. The diagnostic tests assay for viral antigens; so, immune response to the vaccine is not detected.¹

7. The ELISA test is recognized as the preferred screening test for FeLV.¹ The IFA test is more appropriate as a confirmatory test for FeLV.¹ The ELISA kit tests are accurate and allow for rapid in-clinic testing.²

8. After screening by use of an ELISA test, a positive or equivocal result should be repeated.1 The FeLV ELISA systems are most reliable when serum or plasma is tested.¹ If an ELISA test is positive using whole blood, saliva or tears, the test should be repeated using a serum or plasma sample.

9. ELISA tests using saliva or tears have a disproportionate number of both false positive and false negative results.¹ Therefore, these tests cannot be recommended for routine screening of individual cats.

10. One should always run a validated IFA test to confirm results on a healthy positive cat before considering it persistently viremic.¹

11. No test is 100% accurate at all times and under all conditions; therefore, a critical decision about the care of a patient - whether healthy or ill - should never be based solely on a single test result.³

12. All kittens or adult cats which test negative by the first ELISA screening test - but with a known or suspected exposure to FeLV - should be retested. This is done to rule out possible negative results obtained during incubation of the FeLV virus. Although the majority of cats will test positive within several weeks, final retest of negative cats should be no sooner than 90 days post-exposure. Clients should be counseled on the potential risk of FeLV exposure when adding a cat with one negative test result to an FeLV-negative household.

13. Discordant results are defined as conflicting test results, usually being an ELISA positive and IFA negative result.¹ Discordancy can occur due to testing in the early phase of infection; antigenemia without viremia (no intact virus); or a false positive ELISA due to faulty technique or cross-reactive antigens. These cats should be monitored by both ELISA and IFA assays at 4-8 week intervals for at least 90 days. ²

14. Periodic (e.g., annual) testing of cats 'at risk' is justifiable. Cats 'at risk' are defined as those with known or potential exposure to FeLV. These include: outdoor cats; fighting cats; strays; cats with bite wounds; escapees; recently mated females if the FeLV status of the male is unknown; cats in open multiple-cat households; cats in closed multiple-cat households with any other cats of unknown FeLV status; cats in households having a known FeLV positive cat.

15. All FeLV negative exposed cats in a multiple-cat household (in which an FeLV carrier is found) should be retested at 3 month intervals until all cats within the household or facility test FeLV negative on at least 2 consecutive tests and the risk of exposure to FeLV infection is past. All FeLV positive cats should be immediately isolated or removed at the time of diagnosis.^1,4

16. Other confirmatory tests, such as methodologies utilizing polymerase chain reaction (PCR), may be useful but are currently unvalidated. Although PCR offers a promising approach to FeLV testing, currently neither production of PCR reagents (primers) nor testing protocols is standardized or consistent.

17. Latent infections with FeLV are undetectable with ELISA or IFA testing. PCR testing can offer a new means of detecting these cases.5 Shedding of FeLV or development of FeLV-related diseases can be a consequence in latently infected cats, but is felt to be a rare clinical occurrence. ²

18. FeLV-positive healthy cats may live for months to years. The question of euthanizing a positive cat is one that must be addressed in each individual case, in consultation between veterinarian and client. Effective FeLV case management involves measures aimed at preserving the health of the infected cat; preventing the spread of FeLV infection; and early recognition and aggressive treatment of FeLV-associated disease. The quality of life and clinical status of FeLV-positive cats can be enhanced through the concerted efforts of the pet owner and the attending veterinarian.

a. All recommendations regarding IFA testing are based on studies done by a VALIDATED IFA test run by National Veterinary Laboratories.

References

1. Colloquium on FeLV/FIV: Tests and Vaccination, Journal of the American Veterinary Medical Association, Vol. 199, No. 10, November 15, 1991, pp1275-1276.

2. Loar, Andrew S., Feline Leukemia Virus Tests: Evaluation and Interpretation, in Consultations in Feline Internal Medicine, Editor, John August, W.B. Saunders Company, Philadelphia, 1991, pp. 535-541.

3. Barr, Margaret C., Feline Immunodeficiency Virus Tests and their Interpretation, in Feline Health Topics for veterinarians, Cornell Feline Health Center, Volume 8, Number 3, 1993.

4. Rojko, Jennifer L and Hardy, William D. Jr., Feline Leukemia Virus and Other Retroviruses, in The Cat: Diseases and Clinical Management, Editor: Robert Sherding, Churchill Livingstone, New York, Second Edition, 1994, pp. 361-375.

5. Kremer, KA and Thompson, JP, Polymerase Chain Reaction Detection of Feline Leukemia Virus in Naturally Infected Cats, in ACVIM abstract, Proc. 12th ACVIM Forum, San Francisco, CA, 1994, P. 1007

_________________________________________