Retroviruses have a unique strategy that involves incorporation into the host genome as a routine part of the life cycle. As a result, most vertebrate genomes have incorporated a large number of retroviruses over the course of evolution, and approximately 8% of the human genome is retroviral in origin.2 This raises significant challenges when looking for novel retroviruses and establishing associations with disease. In the course of clinical investigation of a bottlenose dolphin with chronic lymphocytosis, a novel gammaretrovirus was identified using consensus PCR and sequencing.1 A quantitative PCR was developed. This virus was found at constant levels in dolphin DNA over time and was not identified from RNA, suggesting the virus was not actively being transcribed. This virus was also found in wild dolphins. Proviral DNA was further detected in representatives of each of eight delphinid species tested and in harbor porpoises, but not in Kogia spp. beluga whales, or fin whales. Further sequencing of a total of 4,874 base pairs of the virus was done for additional characterization, representing more than half of the expected genome. The presence of several degeneracies suggests that multiple copies are present with slight variations. The intact genome of this virus suggests a relatively recent insertion of the virus into the dolphin genome, and the presence/absence in different populations and species may be useful as a marker for population studies. While it is probable that there are retroviral disease etiologies in bottlenose dolphins, this case illustrates the challenges of retroviral diagnostics.
1. Martin J., E. Herniou, J. Cook, R.W. O'Neill, and M. Tristem. 1999. Interclass transmission and phyletic host tracking in murine leukemia virus-related retroviruses. J Virol 73(3): 2442-2449.
2. Mayer J., and E. Meese. 2005. Human endogenous retroviruses in the primate lineage and their influence on host genomes. Cytogenet Genome Res 110(1-4): 448-56.