http://www.retrovirology.com/content/7/1/14 Dominance of highly divergent feline leukemia virus A progeny variants in a cat with recurrent viremia and fatal lymphoma A Katrin Helfer-Hungerbuehler1 email, Valentino Cattori1 email, Felicitas S Boretti2 email, Pete Ossent3 email, Paula Grest3 email, Manfred Reinacher4 email, Manfred Henrich4 email, Eva Bauer1 email, Kim Bauer-Pham1 email, Eva Niederer5 email, Edgar Holznagel1 email, Hans Lutz1 email and Regina Hofmann-Lehmann1 email 1 Clinical Laboratory, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland 2 Clinic for Small Animal Internal Medicine, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland 3 Institute of Veterinary Pathology, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland 4 Institute of Veterinary Pathology, University of Giessen, Giessen, Germany 5 Institute of Biomedical Engineering, University of Zurich and ETH, Zurich, Switzerland Retrovirology 2010, 7:14doi:10.1186/1742-4690-7-14 The electronic version of this article is the complete one and can be found online at: http://www.retrovirology.com/content/7/1/14 In a cat that had ostensibly recovered from feline leukemia virus (FeLV) infection, we observed the reappearance of the virus and the development of fatal lymphoma 8.5 years after the initial experimental exposure to FeLV-A/Glasgow-1. The goals of the present study were to investigate this FeLV reoccurrence and molecularly characterize the progeny viruses. Results The FeLV reoccurrence was detected by the presence of FeLV antigen and RNA in the blood and saliva. The cat was feline immunodeficiency virus positive and showed CD4+ T-cell depletion, severe leukopenia, anemia and a multicentric monoclonal B-cell lymphoma. FeLV-A, but not -B or -C, was detectable. Sequencing of the envelope gene revealed three FeLV variants that were highly divergent from the virus that was originally inoculated (89-91% identity to FeLV-A/Glasgow-1). In the long terminal repeat 31 point mutations, some previously described in cats with lymphomas, were detected. The FeLV variant tissue provirus and viral RNA loads were significantly higher than the FeLV-A/Glasgow-1 loads. Moreover, the variant loads were significantly higher in lymphoma positive compared to lymphoma negative tissues. An increase in the variant provirus blood load was observed at the time of FeLV reoccurrence. Conclusions Our results demonstrate that ostensibly recovered FeLV provirus-positive cats may act as a source of infection following FeLV reactivation. The virus variants that had largely replaced the inoculation strain had unusually heavily mutated envelopes. The mutations may have led to increased viral fitness and/or changed the mutagenic characteristics of the virus.