Feline leukemia virus (FeLV) is a gamma‐retrovirus of domestic cats that was discovered by William Jarrett at the University of Glasgow in 1964. Interestingly as a side note, this major discovery lead Robert Gallo to hunt for T-cell cancer-causing viruses in humans, eventually leading to the discovery of HIV.  Some cats infected with FeLV may develop a ‘progressive’ infection resulting in persistent viremia, immunodeficiency, tumors, anemia and death, with over 80% of cats with persistent infection dying within ~ 3.5 years. However, majority of cats mount a protective immune response that suppresses the presence of the virus in the blood; these cats develop a ‘regressive’ infection characterized by the absence of viral replication and the presence of low levels of proviral DNA integrated into the feline genome. The biological importance of these latter provirus carriers is largely unknown.

Research from the University of Zurich in Switzerland showed that 10 cats developed active FeLV infection after receiving blood transfusions from donor cats that were aviremic but provirus carriers (i.e., regressive infection). The results of this study have profound implications for feline medicine; namely, the requirement for testing blood donors and blood products for FeLV provirus by sensitive polymerase chain reaction (PCR). In addition, if an aim is to eliminate a retroviral infection from a population, then provirus carriers must be considered. [GO]