Feline leukaemia virus is a gamma retrovirus affecting domestic and small wild cats. As with all retroviruses, FeLV is an enveloped RNA virus, which relies for its replication on a DNA intermediate. Using its reverse transcriptase activity, the RNA genome is reverse-transcribed into a DNA, which is usually integrated into the host's cell genome. FeLV incorporated into the host's DNA is called provirus. Synthesis of viral proteins occurs according to the conventional mechanisms of transcription with assembly of the virus near the cell membrane and budding from the cell. Usually, infection of a cell by a retrovirus does not lead to cell death. The FeLV genome contains three genes: envelope (env) coding for the glycoprotein 70 (SU) and the transmembrane protein p15 (E) (TM); the polymerase (pol) gene coding for reverse transcriptase, protease and integrase; and the group specific antigen (gag) gene coding for the structural proteins of the virus. Besides this so-called exogenous FeLV, two forms of endogenous (enFeLV) gamma retroviruses are known in the domestic cat: the endogenous feline leukaemia virus and the RD114 virus. The enFeLV are not pathogenic and not of veterinary importance. FeLV exists in four immunologically closely-related subtypes: A, B, C, and T.
In Europe, the prevalence of FeLV infection in cats that are kept individually is very low, usually in the range of below 3%. In shelters and in large multi-cat households where no specific preventive measures against introduction of FeLV are taken, the prevalence may go up to over 20%. Over the last 25 years, the prevalence and importance of FeLV infection in Europe has greatly diminished due to the availability of reliable tests, introduction of test-and-removal programmes, the better understanding of the pathogenesis, and the introduction of highly efficacious FeLV vaccines.
Transmission and Pathogenesis
FeLV is transmitted mostly by direct contact via saliva, nasal secretions, and milk in the context of mutual grooming or occasionally by bites. In addition, as demonstrated recently, it can be transmitted indirectly via contact with faeces from FeLV-viremic shedders. Transmission from mother to offspring occurs occasionally via the mammary gland where the virus can remain latent until the mammary gland develops during the last period of pregnancy.12 Young kittens are especially susceptible to FeLV infection while with increasing age cats become more resistant to infection.
FeLV infection starts in the oropharynx and spreads to the bone marrow. Once the bone marrow is infected, viremia develops within a few weeks of infection. In turn, viremia leads to the infection of salivary glands and intestinal linings from where it is shed in large quantities by saliva and faeces. In most cases, viremia is overcome by a functioning immune system resulting in a transient or not detectable viremic phase. Cats that overcome viremia are almost not at risk for disease. The typical clinical signs of FeLV infection are observed in viremic cats usually after several years of viremia.
Most cats that have overcome FeLV viremia exhibit high antibody titres--mostly of virus-neutralizing quality- to the virus. In addition, immune cats usually also develop cytotoxic T-lymphocytes (CTL's), an observation that suggests an important role for CTL's in FeLV immunity.1
Persistently FeLV viremic cats suffer from three major disease complexes (listed according their frequency): Immune suppression, anaemia, and lymphoma. Independently of the presence of recognisable clinical signs, every FeLV viremic cat is immune depressed. Immune suppression results in protracted healing of wounds, diarrhoea, and chronic infections.
Today, FeLV infection is usually diagnosed by the detection of the FeLV core protein p27 in the peripheral blood by double-sandwich antibody ELISA10 or by Immunochromatography.6 In ELISA, presence of antigen leads to binding of an enzyme-conjugated antibody and after the addition of a substrate, to colour formation in the ELISA well or on the membrane of the test. Colour is therefore indicative of the presence of p27, which is a marker of infection but not always of viremia as in many cats soluble p27 occurs in the absence of infectious virus.11 In the immunochromatography tests, presence of antigen leads to binding of an antibody conjugated to small beads which become visible by the naked eye in the form of a band. In the past, the immunofluorescence test and virus isolation in cell culture were important tools to detect FeLV infection.5,8 Both procedures have some disadvantages (VI requires specialized laboratories, virus may be inactivated during shipping, IFA has a low sensitivity and occasionally gives rise to false-positive results) and have therefore lost in importance. In the last years, PCR became quite attractive to detect provirus present in peripheral blood cells. The development of real-time PCR made it possible to quantitate the proviral load in blood.7 Viremic cats usually have very high proviral loads while immune cats exhibit low loads. Importantly, cats that have overcome viremia remain provirus-positive for life.7 Therefore, presence of provirus in blood cells is solid evidence of past infection. Several studies have shown that up to 10% of the cats tested by veterinarians are PCR-positive with low proviral loads but p27 negative.7 These cats have undergone infection, have developed immunity but FeLV DNA remains incorporated in the bone marrow cells. To what degree this reflects latent (i.e., dormant) infection and whether these cats can be used as blood donors, is still unclear. PCR has proved useful for the confirmation of questionable p27 tests.9 If e.g., a healthy cat is weakly positive by p27 detected by ELISA or immunochromatography and also positive for provirus, the p27 result is likely to be correctly positive. In addition to FeLV DNA, reverse-transcription PCR can detect FeLV RNA. Almost 100% of cats being positive for p27-positive in serum or plasma are also positive for FeLV RNA in blood and in saliva.3 Therefore, detection of FeLV RNA in saliva can be taken as a marker for viremia and has almost the same diagnostic significance as a positive p27 test. RT-PCR using saliva as substrate is expensive to run and therefore may not be used frequently in individual cats. However, based on its extremely high sensitivity, RNA PCR is ideally suited to test pooled saliva samples for presence of viral RNA. If one of up to thirty cats is infected, the pooled saliva will turn positive in RNA PCR. Thus, in spite of the fact that RNA PCR is expensive it provides an efficient measure to check for FeLV shedding in multi-cat household situations.4
The different assays for the detection of FeLV infection provide us with different information. When the course of experimental infection is followed by subsequent testing, the first parameter that becomes positive is usually virus isolation, followed within a few days by DNA and RNA PCR and later by ELISA. Persistently viremic cats are regularly positive by all parameters including the IFA test which is less sensitive and therefore hardly used any more. Cats that overcame viremia completely will usually be negative by ELISA, immunochromatography, IFA, and RNA PCR, but positive by PCR DNA. The mean proviral load in cats that overcame viremia is several hundred times lower than in cats with persistent viremia. After viremia has been overcome, 2 to 3% of all cats remain positive by ELISA and immunochromatography. These cats (ELISA and immunochromatography positive but IFA and VI negative) represent animals with foci of infection outside of the bone marrow from which soluble p27 is released into the circulation. In summary, ELISA and immunochromatography are ideally suited to test for FeLV infection. The predictive value negative is very close to 100%. The predictive value positive for infection is lower and depends on the tests used and the prevalence of viremia in a given area.
Recently, an additional pathogenesis of FeLV infection has been recognized: cats that are exposed to very low levels of FeLV, may seroconvert without showing any other signs of infection; i.e., in these cats antibodies to FeLV can be detected but at the same time, FeLV provirus is detected only in some organs or not at all.2 This means that after infection, FeLV replication must take place somewhere in the cat's body where it cannot readily be detected by PCR. The viral load produced at the site of replication must be high enough to drive the immune system. The biological significance of this observation remains to be elucidated.
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