Like feline immunodeficiency virus (FIV), feline leukaemia virus (FeLV) belongs to the retroviruses.
Prevalence of FeLV in Germany is less than 1%, with regional differences. Not only cats, but also other felids are susceptible to FeLV. In particular, kittens and cats from multi-cat households are affected, as FeLV is transmitted directly from cat to cat. The main source of transmission is saliva, but other secretions and excretions can also be infectious. In most cases, cats develop an oropharyngeal infection. The virus penetrates the mucous membranes and multiplies there as well as in the tonsils and the retropharyngeal lymph nodes. While some cats with abortive infection are able to fight the virus in a way that no viraemia occurs, other cats develop viraemia which can be detected by an antigen test. If it can be overcome by the cat, it is called transient viraemia. It is therefore always advisable to retest a cat that is positive in the antigen test at a later time. However, some cats do not succeed in fighting off the virus sufficiently, so that the bone marrow becomes infected as well. As a result, provirus PCR is positive, regardless of whether the virus continues to circulate in the blood (progressive infection) or not (regressive infection).
Cats with progressive infection usually have the worst prognosis, younger animals are more often affected by progressive infection than older ones.