Feline Leukemia Virus


Feline leukemia virus (FeLV) is a retrovirus present worldwide which primarily infects domestic cats. The virus is a member of the Orthoretrovirinae subfamily of retroviruses.1


Vaccines for FeLV are administered according to the following schedule:

Schedule Vaccine
(12 Weeks or Initial Vaccination) Initial FeLV Vaccination
Two (2) to Three (3) Weeks Later FeLV Vaccine Booster
One (1) Year Later Final FeLV Vaccination

The initial vaccine and booster must be administered two (2) to three (3) weeks apart; if not administered within six (6) weeks (42 days), the series must be restarted.


Transmission of FeLV occurs primarily via saliva, although the virus has also been shown to be transmitted via urine and feces.23 Therefore, transmission chiefly occurs through fighting and biting, as well as through social behaviors such as the sharing of food and water bowls or mutual grooming, and to a lesser extent from sharing a litter box.

Transmission of FeLV can also occur from mother to kittens.4


Symptoms of FeLV can vary greatly, depending on a variety of factors;5 however, age predominantly determines severity of infection.6 While the virus acquired its name from the contagious malignancy for which it was first recognized,5 anemia is most commonly associated with FeLV.7 Diseases secondary to immunosupression are also commonly linked to FeLV;8910 while less common symptoms include tumors, other hematologic disorders, reproductive disorders, and neuropathy.5


Testing for FeLV, as part of our in-house Laboratory service, indicates the presence of the feline leukemia virus antigen. The relationship between an antigen and antibody can be viewed as similar to that of a key and lock, where the interfacing of antigen and antibody triggers the immune system to attempt to identify and neutralize the bacteria or virus. Therefore, because the test detects antigen and not antibodies, it does not cross-react with either maternal antibodies, or those from previous exposure to FeLV or from vaccination,11 although the test may cross-react with antigens from the vaccine itself if blood was collected immediately following vaccination.12

All cats or kittens are tested for FeLV prior to FeLV vaccination. Kittens are tested at their first visit; while adult cats, regardless of current health, are tested if no previous test result can be verified. Additionally, any kitten younger than six (6) months of age testing positive will have follow-up testing performed at 60-day intervals, while any cats testing negative are retested if a cat within the household tests positive.


All outdoor-going cats, which are considered to be at-risk of exposure to FeLV,13 should be vaccinated for FeLV. “Strictly indoor” cats may not need yearly booster vaccinations following the initial vaccine series; determined by their risk of exposure to FeLV, which may require testing of other cats in their household. Please consult your veterinarian for vaccination recommendations based on your cat’s current and prospective environment and lifestyle.

We test all kittens (and any adult cat for which no previous test result can be verified) for feline leukemia virus and feline immunodeficiency virus (FIV) at their first visit.

Additional Resources

For additional information regarding FeLV, FIV, and their prevalence in your area, visit kittytest.com, presented by IDEXX Laboratories.


  1. ^ Hartmann K. Feline Leukemia Virus Infection. In: Greene CE, ed. Infectious Diseases of the Dog and Cat 4th ed. St. Louis, MO: Elsevier; 2012: 108.
  2. ^ Cattori V, Tandon R, Riond B, et al. The kinetics of feline leukaemia virus shedding in experimentally infected cats are associated with infection outcome. Vet Microbiol. 2009; 133: 292.
  3. ^ Gomes-Keller MA, Gönczi E, Grenacher B, et al. Fecal shedding of infectious feline leukemia virus and its nucleic acids: a transmission potential. Vet Microbiol. 2009; 134: 208.
  4. ^ Hartmann K. Feline Leukemia Virus Infection. In: Greene CE, ed. Infectious Diseases of the Dog and Cat 4th ed. St. Louis, MO: Elsevier; 2012: 113.
  5. ^ a b c Hartmann K. Feline Leukemia Virus Infection. In: Greene CE, ed. Infectious Diseases of the Dog and Cat 4th ed. St. Louis, MO: Elsevier; 2012: 118.
  6. ^ Hoover EA, Olsen RG, Hardy WD Jr., et al. Feline leukemia virus infection: age-related variation in response of cats to experimental infection. J Natl Cancer Inst. 1976; 57(2): 365-369.
  7. ^ Gleich S, Hartmann K. Hematology and serum biochemistry of feline immunodeficiency virus-infected and feline leukemia virus-infected cats. J Vet Intern Med. 2009; 23(3): 552. doi:10.1111/j.1939-1676.2009.0303.x.
  8. ^ Diehl LJ, Hoover EA. Early and progressive helper T-cell dysfunction in feline leukemia virus-induced immunodeficiency. J Acquir Immune Defic Syndr. 1992; 5(12): 1188-1194.
  9. ^ Ogilvie GK, Tompkins MB, et al. Clinical and immunologic aspects of FeLV-induced immunosuppression. Vet Microbiol. 1988; 17(3): 287-296.
  10. ^ Pardi D, Hoover EA, Quackenbush SL, et al. Selective impairment of humoral immunity in feline leukemia virus-induced immunodeficiency. Vet Immunol Immunopathol. 1991; 28(3-4): 183-200.
  11. ^ Hartmann K. Feline Leukemia Virus Infection. In: Greene CE, ed. Infectious Diseases of the Dog and Cat 4th ed. St. Louis, MO: Elsevier; 2012: 127.
  12. ^ Levy J, Crawford C, Hartmann K, et al. 2008 American Association of Feline Practitioners’ feline retrovirus management guidelines. Journal of Feline Medicine and Surgery. 2008; (10): 300.
  13. ^ Richards JR, Elston TH, Ford RB, et al. The 2006 American Association of Feline Practitioners Feline Vaccine Advisory Panel Report. J Am Vet Med Assoc. 2006; 229(9): 1405-1441.