Feline leukemia virus is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved.
Diagnosing Feline Leukemia Virus. Your veterinarian can diagnose the disease by conducting a simple blood test called an ELISA, which identifies FeLV proteins in the blood. This test is highly sensitive and can identify cats with very early infections.
Feline leukemia and aids are complex diseases of viral origin, both somewhat contagious. The viruses that cause these illnesses are related. One is the feline leukemia virus (FeLV); the other is the feline aids virus (FIV). SYMPTOMS

Seroprevalences of feline leukemia virus and feline immunodeficiency virus infection in cats in the United States and Canada and risk factors for seropositivity. J Am Vet Med Assoc 2017; 251: 187194.
Feline leukemia virus (FeLV) is one of the most common and important infectious diseases in cats, affecting between 2-3% of all cats in the United States and Canada. Infection rates are significantly higher (up to 30%) in cats that are ill or otherwise at high risk (see below).

Not to be confused with FIV Positive, Feline Leukemia Positive, or FeLV Positive or Feleuks for short, is a much grimmer diagnosis. In our local stray cat population, we see maybe 1 or 2 cats out of 100 either test positive for Feline Leukemia or FIV.
Diagnosing feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) infection: an update for clinicians.Feline leukaemia virus (FeLV) can be a challenging infection to diagnose due to a complex feline host-pathogen relationship and occasionally unreliable test results.

Feline Leukemia Virus (FeLV) is a common but preventable virus that can affect almost all cats. Dr. Boldan shares how FeLV weakens a cat's immune system, symptoms, and how the feline leukemia virus vaccine can be life-saving.