Therapeutic vaccinations have a potential application in
infections where no curative treatment is available. In contrast to HIV, efficacious
vaccines for a cat retrovirus, feline leukemia virus (FeLV), are commercially available. However, the
infection is still prevalent, and no effective treatment of the
infection is known. By vaccinating persistently FeLV-infected cats and presenting FeLV
antigens to the immune system of the host, e.g., in the form of recombinant and/or adjuvanted
antigens, we intended to shift the balance toward an advantage of the host so that
persistent infection could be overcome by the infected cat. Two commercially available FeLV
vaccines efficacious in protecting naïve cats from FeLV
infection were tested in six experimentally and persistently FeLV-infected cats: first, a canarypox-vectored
vaccine, and second, an adjuvanted, recombinant envelope
vaccine was repeatedly administered with the aim to stimulate the immune system. No beneficial effects on
p27 antigen and plasma
viral RNA loads, anti-FeLV
antibodies, or life expectancy of the cats were detected. The cats were unable to overcome or decrease
viremia. Some cats developed
antibodies to FeLV
antigens although not protective. Thus, we cannot recommend vaccinating persistently FeLV-infected cats as a means of improving their FeLV status, quality of life or life expectancy. We suggest testing of all cats for FeLV
infection prior to FeLV vaccination.