Feline leukaemia virus (FeLV) is a retrovirus infection that causes immunosuppression in cats. FeLV is transmitted horizontally (from cat to cat) primarily through saliva, which contains high concentrations of the virus. While biting, licking and grooming are the most common routes of infection, kittens may be infected transplacentally or by nursing from infected mothers. However, virus contact does not ensure persistent viremia or disease. Stages of Progression of FeLV

Stage 1. Replication of virus in lymphoid tissue (1-4 days post-infection)
Stage 2. Small number of B-lymphocytes and macrophages infected (2-14 days p.i.)
Stage 3. FeLV replication in spleen and other lymphoid tissues (3-12 days p.i.)
Stage 4. Peripheral viraemia
Stage 5. Epithelial and glandular infection of virus causing excretion from saliva (28-56 days p.i.)


Possible Outcomes of Exposure to FeLV

  • Virus neutralisation: the cat mounts an immune response, making the cat resistant to infections for an undetermined period
  • Persistent viraemia
  • Latency: the cat fails to produce immunity but does not become viraemic
  • Immune carrier: the virus is sequestered in epithelial tissues and is replicating, but it does not leave the cells because of antibody production
  • Disease – Leukemia, lymphoma/lymphosarcoma, immunosuppression, FeLV enteropathy, myelodysplasia (bone marrow disorders) and reproductive disorders

FeLV-infected cats are found worldwide, but the prevalence of infection varies greatly depending on their age, health, environment, and lifestyle. In the United States, approximately 2 to 3% of all cats are infected with FeLV. Rates rise significantly—13% or more—in cats that are ill, very young, or otherwise at high risk of infection.


Virology of FeLV

All retroviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV), produce an enzyme, reverse transcriptase, which permits them to insert copies of their own genetic material into that of the cells they have infected. Although related, FeLV and FIV differ in many ways, including their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein constituents are dissimilar in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.

There are three antigenic types of feline leukaemia virus – A, B and C – based on differences in envelope antigens. Cells transformed by either FeLV or feline sarcoma virus express a novel viral antigen (FOCMA) in their plasma membrane, antibodies to which protect the cat against the disease. The serum of persistently infected cats lacks both neutralising and FOCMA antibodies. Strains of virus from immunosuppressed cats infect CD4+ and CD8+ T-lymphocytes, B-lymphocytes and myeloid cells..


Clinical signs

FeLV is the most common cause of cancer in cats. FeLV-associated diseases are detected most often in young cats. Ocular disease can occur alone or in combination with systemic signs of illness. Ocular disease only occurs when FeLV infection is associated with lymphosarcoma or by immune suppression and increased susceptibility to other infectious diseases such as Toxoplasma gondii and systemic mycoses. The incidence of ocular disease among clinically affected cats FeLV-positive cats is reported to be low (2% or less); however, FeLV has been shown to be a common cause of uveitis in enucleated eyes.

It may cause various disorders including FeLV-induced anaemia, and it may lead to a state of immune deficiency that hinders the cat’s ability to protect itself against other infections. The same bacteria, viruses, protozoa, and fungi that may be found in the everyday environment—where they usually do not affect healthy animals—can cause severe illness in those with weakened immune systems. These secondary infections are responsible for many of the diseases associated with FeLV.

During the early stages of infection, it is common for cats to exhibit no signs of disease at all. However, over time—weeks, months, or even years—the cat’s health may progressively deteriorate or be characterized by recurrent illness interspersed with periods of relative health. Signs can include: loss of appetite, slow but progressive weight loss, followed by severe wasting late in the disease process, poor coat condition, enlarged lymph nodes, persistent fever, pale gums and other mucus membranes, inflammation of the gums (gingivitis) and mouth (stomatitis), infections of the skin, urinary bladder, and upper respiratory tract,persistent diarrhoea, eizures, behaviour changes, and other neurological disorders, a variety of eye conditions and in unspayed female cats, abortion of kittens or other reproductive failures.



Two types of FeLV blood tests are in common use. Both detect a protein component of the virus as it circulates in the bloodstream. IFA and/or ELISA tests detect FeLV p27 protein. The p27 protein is one of several core proteins that are products of the gag gene. The IFA test may detect p27 in neutrophils and platelets from blood or bone marrow. The ELISA test detects p27 in whole blood, plasma, serum, saliva, or tears. Serum is the fluid of choice for ELISA testing because saliva and tears may not contain sufficient p27 for viral detection. Antibody titers to FeLV envelope antigens and FOCMA antibodies may be used for diagnostic purposes, but are of little clinical relevance.

There are 6 stages of FeLV infection as follows:

  • Stages 1-3 – FeLV is disseminated through the lymphoid tissue.
  • Stage 4 – FeLV infects the bone marrow.
  • Stage 5 – Viral-infected neutrophils and platelets are released from the bone marrow.
  • Stage 6 – FeLV appears in epithelial tissues (e.g., salivary glandular epithelium).

ELISA testing can detect p27 at stages 2-6, while IFA detects FeLV at stages 5-6. Therefore, ELISA testing is the first diagnostic assay to detect a positive result. Cats seropositive following ELISA testing should be evaluated by IFA or retested by ELISA 4-6 weeks later. Some seropositive cats can revert to a negative status due to the latency of the virus in the bone marrow. However, the presence of FeLV can still be detected by virus isolation to prove latency. IFA-positive cats correlate with viremia as a positive IFA will always be ELISA positive. Cats that are ELISA positive but IFA negative are called discordant, meaning that there is either a false positive ELISA, false negative IFA, or the virus is yet not in stages 5-6 of infection. The most specific way to identify latent infections is by viral isolation or PCR amplification of FeLV sequences from cells of the bone marrow.

Gross pathological findings depend on the type of disease that occurs. Bone marrow hypercellularity often accompanies leukemias. Lymphoplasmacytic infiltrates of the gingiva, lymph nodes, spleen, kidney, and liver may be seen. Intestinal lesions similar to those seen in feline panleukopenia may occur with enteric FeLV infection. These gross changes include an edematous, turgid small intestine with petechial to ecchymotic hemorrhages on the serosal and/or mucosal surfaces of the jejunum and ileum.

ELISA (enzyme-linked immunosorbent assay) and similar tests can be performed in your veterinarian’s office. ELISA-type tests detect both primary and secondary stages of viremia. IFA (indirect immunofluorescent antibody assay) tests must be sent out to a diagnostic laboratory. IFA tests detect secondary viremia only, so the majority of positive-testing cats remain infected for life.



Treatment of FeLV-infected cats is symptomatic and supportive.

1. Use of antibiotics and nutritional support is important to establish good clinical health.

2. Treatment of anaemia associated with bone marrow erythrocyte suppression may require blood transfusion.

3. Omega interferon can be used to reduce viremia and stimulate the cat’s immune system, however it is not virucidal.

4. Chemotherapy is the principle therapy for addressing secondary lymphoma.



The only sure way to protect cats is to prevent their exposure to FeLV-infected cats. Keep cats indoors, away from potentially infected cats that might bite them. If you do allow your cats outdoor access, provide supervision or place them in a secure enclosure to prevent wandering and fighting. Adopt only infection-free cats into households with uninfected cats. House infection-free cats separately from infected cats, and don’t allow infected cats to share food and water bowls or litter boxes with uninfected cats. Consider FeLV vaccination of uninfected cats. (FeLV vaccination of infected cats is not beneficial.) Discuss the advantages and disadvantages of vaccination with your veterinarian. FeLV vaccines are widely available, but since not all vaccinated cats will be protected, preventing exposure remains important even for vaccinated pets. FeLV vaccines will not cause cats to receive false positive results on ELISA, IFA, or any other available FeLV tests.


Managing FeLV in a cattery

Confine FeLV-infected cats indoors to reduce their exposure to other infectious agents carried by animals, and to prevent the spread of infection to other cats in the neighbourhood.

Spay or neuter FeLV-infected cats.

Feed nutritionally complete and balanced diets.

Avoid uncooked food, such as raw meat and eggs, and unpasteurized dairy products because the risk of food-borne bacterial and parasitic infections is much higher in immunosuppressed cats.

Schedule wellness visits with your veterinarian at least once every six months. Although a detailed physical examination of all body systems should be performed, your veterinarian should pay special attention to the health of the gums, eyes, skin, and lymph nodes. A complete blood count, serum biochemical analysis, and a urine analysis should be performed at every examination. Additionally, your cat’s weight should be accurately measured and recorded, as weight loss if often the first sign of deterioration.

Closely monitor the health and behaviour of your FeLV-infected cat. Alert your veterinarian to any changes in your cat’s health immediately.

There is no scientific evidence that alternative, immunomodulator, or antiviral medications have any positive benefits on the health or longevity of healthy infected cats.



1.Peiffer, RJ et al (1991) Histopathologic study of uveitis in cats: 139 cases. JAVMA 198(1):135-138

2. Cornell Feline Health Center, Cornell University, College of Veterinary Medicine, Ithaca, New York; http://www.vet.cornell.edu/fhc/resources/brochure/felv.html


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