Feline Leukemia Testing

Susceptibility to feline leukemia virus (FeLV) infection is highest in young kittens.  AlthoughFeline Leukemia Testing the risk of infection is reported in cats of all ages, the rate of infection declines steadily as cats age.  The first vaccine for FeLV was produced in 1985, but there appeared to be a decline in the overall infection rate prior to that time.  The decline in FeLV is due in part to feline leukemia testing and the ability to remove infected cats.

FeLV is shed in high levels in the saliva, urine and other body secretions.  Sharing of food and water dishes, mutual grooming and use of common litter boxes can contribute to the spread of virus from cat to cat.  Infected cats can also spread the virus to offspring in utero.

Feline Leukemia Testing as Diagnosis of FeLV infection

Routine screening for FeLV infection became available in 1973 with the development of an immunofluorescence assay (IFA) for the virus.  This test requires special handling and processing and must be performed by a qualified reference laboratory.  Years later, an enzyme-linked immunosorbent assay (ELISA) test was developed, which allowed for a rapid and reliable in-house testing procedure that could be done easily in any veterinary hospital.  Both tests detect the FeLV core protein p27 (referred to as an antigen), which is produced abundantly in most infected cats. FeLV p27 antigen is usually detected by 4 weeks after infection, but may take up to 12 weeks after infection for positive test.

ELISA tests detect viral antigen p27 circulating freely in blood, whereas, the IFA test detects the viral antigen p27 in infected white cells (neutrophils) and platelets. Some tests have been developed for use on tear or saliva samples, instead of blood, but these tests are not as accurate.  Vaccination does not interfere with ELISA or IFA testing, because those tests detect the p27 antigen, not antibody that the vaccine is designed to stimulate the production of for protection against the virus.

ELISA test may detect infection a few weeks earlier than the IFA test and is considered moreCat Leukemia Testing sensitive at detecting infection.  Since some cats may clear the infection in the early stages of infection with an adequate immune response, they may revert back to ELISA negative status within a few weeks to months.  A single positive test cannot predict which cats will be persistently infected, so if IFA test is not performed, a follow-up ELISA test should be performed in 12 weeks.  IFA test indicates advanced infection in the bone marrow with FeLV virus and IFA positive cats will be persistently infected for life.

American Association of Feline Practitioners recommendations for feline leukemia testing

The American Association of Feline Practitioners (AAFP) recommends testing all cats in a household and any new kittens or cats before they are introduced in to the household.  Any cat should be tested as well before being adopted prior to a pet-owner bond becoming established.  Any cat that becomes ill with clinical signs of disease consistent with FeLV infection should be tested.  Testing should also be performed before FeLV vaccination.

ELISA is the preferred screening test.  IFA testing is used to confirm infection if a cat tests positive for FeLV with the ELISA test.  Because cats may be early in the course of infection, it is recommended that a follow-up test be performed at least 90 days after the initial test or after a potential exposure to a FeLV positive cat.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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