Feline leukaemia virus (FeLV) is an enveloped RNA virus of the family Retroviridae that has a worldwide distribution and is associated with immune suppression, haematopoietic neoplasia and bone marrow disorders.
Transmission mostly occurs through contact with saliva (licking, bowl sharing, etc). After infection, the virus enters the host cell where RNA undergoes reverse transcription and a DNA copy of viral genome is integrated to the host genome (provirus). The provirus can remain latent (ie not be transcribed), or can be transcriptionally active (synthesis of new virions). The viral genome encodes for gag proteins (group-specific antigens, which includes the capside protein p27), pol (reverse transcriptase) and env (envelope).
There are three subtypes of FeLV:
- A, which is present in all FeLV positive cats and is the least pathogenic, although it is the only one that is transmitted from cat to cat
- B, which results from recombination between FeLV-A and endogenous retrovirus sequences, and is associated with lymphoma and neurological signs
- C, which results from mutations of FeLV-A and is associated with non-regenerative anaemia
The overall prevalence ranges from 1 to 6 percent. Risk factors for infection include male gender, adulthood, co-infection with FIV, outdoor access and aggressive nature, as well as intact status.
After infection, the virus replicates in oral lymphoid tissue, after which seroconversion and viraemia may occur. Four clinical outcomes exist after seroconversion:
- Abortive infection (no viraemia)
- Focal infection (rare, proviral DNA present in localised tissues, but not blood or marrow)
- Regressive infection (latent provirus, no virion production or shedding)
- Progressive infection (bone marrow infection, transcriptionally active provirus and persistent viraemia)
Progressive infection due to lack of effective immune response and virus replication leads to FeLV-related disease and ultimately death. Cats with regressive infection, if immunosuppressed, can also develop progressive infection.
The most common clinical signs are due to myelosuppression, most notably non-regenerative anaemia due to pure red cell aplasia, but also thrombocytopaenia and granulocytopaenia.
Neoplasia is also a feature of FeLV infection, especially of lymphoid and haematopoietic origin. Figure 1 shows the ultrasonographic appearance of the small intestine in a cat affected with intestinal lymphoma. Progressive FeLV infection is associated with a 60-fold increase in risk of lymphoma (commonly the mediastinal form as seen in Figures 2 and 3, except in Siamese cats, in which an FeLV-negative mediastinal form exists) and acute leukaemias. These can be seen in 25 percent of FeLV antigen positive cats. Other signs, such as opportunistic infections due to myelosuppression and acquired cell-mediated immunodeficiency, immune-mediated diseases and neurological and gastrointestinal signs can be seen.
Diagnosis of FeLV infection
The test of choice is an ELISA assay to detect soluble (circulating in blood) antigen p27, correlating with viraemia. False negatives may occur in the first 30 days after infection. A recent study suggested superior performance of the IDEXX SNAP Combo FeLV Ag/FIV Ab Test compared to similar point of care antigen tests.
Confirmation with immunofluorescent antibody (IFA) to detect fixed p27 antigen in blood or marrow cells; PCR to detect proviral DNA; or RT-PCR to detect FeLV RNA is recommended. This is particularly important in a clinically well cat, given implications such as cat segregation or euthanasia. Maternal antibodies or vaccination against FeLV do not interfere with ELISA testing.
Recommendations after a positive ELISA result are:
- Repeat the ELISA test (using a different manufacturer) at the time of the first positive, and then again six months after. Regressive infection may be positive initially and negative after subsequent testing, and progressive infection will commonly remain positive
- Perform IFA on blood or bone marrow smears. IFA does not detect infection until the bone marrow is infected. Therefore, outcomes other than progressive infection should test negative. False negatives are possible in leukopaenic cats if testing peripheral blood
Recommendations after a negative ELISA result are:
- Perform blood PCR to detect provirus. PCR is usually positive sooner than p27 antigen detection. Some cats with bone marrow infection may be infected without circulating soluble antigen
- Repeat ELISA testing after 30 days (if testing for FIV, it may be more practical to retest for both 60 days after, as false negative results for FIV can occur during the first two months of infection)
Management of infected cats
Infected cats should be confined indoors and remaining cats from the same household should be tested and segregated appropriately. Cats that have been residing together long term may be less likely to develop progressive infection if they have not already.
If owners decline segregation, uninfected cats should be vaccinated against FeLV. Infected cats should not be vaccinated against FeLV; core vaccinations are recommended, although they may have an inferior response, and it is recommended to use inactivated vaccines, although there is little evidence to support this.
Progressively infected cats should have checks and bloodwork performed every six months to detect signs of clinical illness.
When hospitalised, infected cats should be kept in normal wards individually, and not alongside sick cats. The virus has a short survival outside the host, so transmission among cats usually requires direct contact.
Treatment of progressively infected cats
Clinical illness in cats with FeLV infection may be due to: direct effect of retroviral infection (lymphoma or pure red cell aplasia); a secondary disease due to immune dysfunction (opportunistic infections or stomatitis); or may be unrelated to the viral infection. Secondary or unrelated infections should be excluded first and treated aggressively, sometimes requiring long-term antibiotics.
If regenerative anaemia is documented, Mycoplasma infection should be excluded or treated with doxycycline. Immunosuppressive drugs should be avoided when possible. Antiviral drugs and immunomodulators have limited value. However, feline interferon omega improved clinical scores and reduced mortality. The nucleoside-analogue reverse transcriptase inhibitor, zidovudine (AZT), led to decreased antigenaemia and stomatitis scores, although a more recent study did not support this finding.
Neutering is recommended to reduce behaviours associated with transmission.
Vaccination against FeLV is important in prevention, but the cornerstone of control is diagnosis and segregation of infected cats. Immunity can persist for one to three years, or more, and reduces the risk of progressive infection; however, regressive infection may occur. Only at-risk cats should be vaccinated and should be tested prior to vaccination, as there is no value in vaccinating infected cats. Vaccination has been associated with injection-site sarcomas; non-adjuvanted vaccines might reduce this risk.
Long-term prognosis for progressively infected cats is guarded, as most cats will develop FeLV-related disease. Median survival times in infected cats have been shown to be 2.4 years versus 6.3 years in control cats. However, with supportive management, infected cats may live for years.
A full reference list is available on request.