Feline leukaemia virus
Profs. Regina Hofmann-Lehmann and Margaret Hosie
Published September 2015
Reviewed November 2019
Feline leukaemia virus (FeLV) is an oncogenic gammaretrovirus that infects domestic cats and some wild felids, including the wildcat Felis sylvestris, the Florida panther Puma concolor and European and Iberian lynxes (Lynx lynx and Lynx pardinus respectively). Infection with FeLV is the primary cause of oncogenesis in domestic cats worldwide
The course of infection with FeLV may follow either of two paths. Most often, exposed cats recover and become solidly immune to subsequent infection. In some infected cats, the virus infection becomes established and the cat becomes persistently viraemic, with an increased likelihood of developing severe and ultimately fatal disease. The diseases observed in viraemic cats include lymphomas and leukaemias, immune suppression and anaemia.
The identification of viraemia by virus isolation represents the gold standard of FeLV diagnosis, since this is the only test that detects replication-competent virus. However, FeLV infection is generally diagnosed by the detection of circulating p27 capsid antigen using in-practice tests. Following the implementation of test and removal schemes and the production and widespread use of effective vaccines, the prevalence of FeLV infection has greatly decreased over the past twenty years; associated with this decreasing prevalence is an increased risk of false-positive results and therefore the positive predictive value of in-practice tests detecting FeLV antigen is low. However, the reliable identification of FeLV-infected cats is a priority, especially for cat shelters, as the prognosis for infected cats is poor and the virus is easily spread amongst cats by close contact and in multi-cat environments.
It is possible to test kittens for FeLV infection early after birth, since maternal antibodies do not interfere with the antigen test (so false positives due to maternal antibodies do not occur), but it may take many weeks for infected kittens to test positive.
Indeterminate in-practice test results should always be confirmed by laboratory testing; in Zurich, samples are first tested by quantitative FeLV ELISA and Glasgow confirms positive results using virus isolation, but PCR can also be used to detect proviral DNA. Test results are interpreted according to the different outcomes of FeLV infection which are dependent on the efficacy of the host immune response following exposure to FeLV. If an effective immune response occurs in the early stages of infection, replication is halted, cats test p27 negative (abortive infection) and such cats are rarely diagnosed. If an effective immune response does not occur until the later stages of infection and FeLV proviral DNA integrates into the bone marrow, cats may take months to fully recover from viraemia (regressive infection). If immunosuppression occurs, integrated provirus may reactivate and viraemia and antigenaemia reappear. A poor or absent immune response leads to progressive infection, with continuous viraemia and the subsequent development of FeLV associated diseases.
In cats with regressive infections, it is generally accepted that the levels of p27 antigen in blood decline over time and proviral DNA remains detectable by PCR, although sometimes at very low levels. Research underway at Glasgow will determine whether current diagnostic tests are failing to detect some regressive FeLV genotypes, or other novel retroviral genotypes, similar to those described recently in Japan.
Only cats testing FeLV-negative should be introduced into catteries; cats that might have been exposed to FeLV within the previous few weeks should be housed separately and should test negative before being allowed to mix with other cats.
FeLV-infected cats should be kept separate from uninfected cats and protected against exposure to other infectious agents. If cats are sick, many respond well to early intervention and treatment. Many cats will require fluid therapy and blood transfusions can benefit anaemic cats, providing the donor blood tests negative for FeLV proviral DNA. Some cases of lymphoma respond well to chemotherapy, although the prognosis for FeLV-infected cats is poor. Feline interferon omega has been shown to extend the survival times of some FeLV-infected cats and zidovudine can reduce the plasma viral burden and improve clinical parameters. Studies conducted in Zurich have shown that the HIV integrase inhibitor raltegravir inhibits FeLV replication in vitro and is well tolerated, reducing viral loads. However, the drug does not completely control viremia and must be administered over long periods to maintain low viral loads and prevent disease.
Rather than trying to cure FeLV-infected cats, it is better to protect from infection by vaccination. There are highly effective vaccines that provide good protection against progressive infection and disease and FeLV vaccination is highly recommended for any cats at risk of infection. Several FeLV vaccines are available in Europe, based on conventionally prepared FeLV antigens, recombinant viral envelope glycoprotein, or a recombinant canarypox FeLV vaccine. Testing cats, to ensure that they are uninfected with FeLV, is recommended prior to vaccination.
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