FeLV is endemic in the feral cat population worldwide, has begun to infect large cat species that had not before shown signs of vulnerability (e.g. the Florida panther and the endangered Iberian lynx). It also remains a problem for pet owners because of the potential infection of outdoor cats. A great deal of research has been directed toward understanding FeLV since its discovery in 1964 (aa). Due to these efforts, much more is known about the virus, and it even became the first retrovirus for which a vaccine was developed (ag). Despite this, there is still no effective treatment or cure.
Exposure to infectious virus typically occurs via saliva during mutual grooming, fighting, or sharing a food bowl (ab,ac,ad) , though it can also be transmitted in other body fluids and feces (ae). Following exposure, the virus infects and replicates in the lymph tissues in the head and neck. If a cat has an immune response sufficient to overcome the virus at this time, the virus is cleared from the system, and the cat is not persistently infected (af, ag). It is currently unclear how many cats are able to completely overcome the virus at this stage, as new methods of analysis have found that cats formerly thought to have this abortive infection actually harbor proviral DNA
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If the virus is not cleared, it spreads to other lymph tissues throughout the body, moves into the bone marrow, and then to other tissues, where it remains for the lifespan of the host (1, ai, ae). A small number of cats (1-10%) develop a non-productive infection (regressive, or latent). After a short period of time during which these cats are contagious, they stop producing infectious virus, and do not have detectable levels of antigen. However, they still carry the integrated provirus in their genome, with all of the associated genes for viral While these cats do not pose a risk to other cats, some association has been found between regressive infection and health problems that include anemia, panleukopenia (feline distemper), and suppurating inflammation. Current studies disagree, however, regarding the involvement of regressive FeLV infection with the development of tumors in cats with no detectable levels of antigen, and there have been documented cases in which viral production was reactivated after a long period of latency.2
Many cats exposed to FeLV develop progressive infection, and are thus vulnerable to FeLV-associated diseases, including other viral or bacterial infections, cancers such as lymphoma and leukemia, blood disorders including leukopenia (reduced white blood cell count) and thrombocytopenia (low platelet count), and myeloproliferative diseases (when the bone marrow makes too many of one type of blood cell). These cats produce infectious virus on a continuing basis, and are contagious for the remainder of their lives.
