Regressive and progressive feline leukemia virus infections – clinical relevance and implications for prevention and treatment

Introduction Pathogenesis of feline leukemia virus (FeLV) infection has always been subject of discussions and intensive research. Sensitive polymerase chain reaction (PCR) assays have provided new data on the courses of FeLV infection. In light of these new data, new aspects of FeLV pathogenesis have been detected. Tests that detect genome-integrated FeLV provirus, e.g. PCR, reveal a higher number of FeLV-infected cats than tests routinely used for detection of FeLV infection (i.e. ELISA detecting free FeLV p27 antigen). These FeLV proviruspositive, however FeLV antigen-negative cats are called “regressively infected cats”. Although regressive FeLV infections are frequently detected, their role is still not well understood. It is currently subject of discussion, how common these regressive infections are, how often they are reactivated, and which clinical signs they can cause. Today, FeLV infection is less commonly diagnosed than in the previous 2 decades. Prevalence of FeLV antigenpositive cats has decreased in most countries. Still, the importance of FeLV as a pathogen is likely underestimated as it has been shown that regressively infected cats (that are negative in routinely used antigen FeLV tests) can also develop clinical signs.