Posted by RJG on September 29th, 2013 • Where are the canine retroviruses?
The mysterious absence of canine retrovirusesIt is striking that no exogenous retroviruses have been identified in domestic dogs (Canis familiaris), or any canine species for that matter. Historically, research on mammalian viruses has tended to focus on domesticated species (or else on species that are vectors or reservoirs for human viruses). A precondition of discovering a new mammal virus is locating and sampling an infected animal, and viruses that cause chronic, lifelong infections, such as retroviruses, are often more easy to locate than those that cause transient infections. Accordingly, exogenous retroviruses have been identified in many livestock or companion animals, including cows, horses, sheep, goats and cats, but not dogs.
Exogenous and endogenous retroviruses of carnivoresThe domestic dog belongs to the same order of mammals (Carnivora) as the domestic cat (Felis catus), and the two species share many life history traits. But in contrast to canids, feline species (felids) are known to harbor at least two exogenous retroviruses. These include feline immunodeficiency viruses (FIVs), which belong to the same genus of retroviruses (Lentivirus) as the human immunodeficiency viruses (HIV-1 and HIV-2). FIVs are found in both wild and domestic felids, in which they cause immunodeficiencies. Domestic cats also harbor an exogenous Gammaretroviruses called feline leukemia virus (FeLV). FeLV commonly causes leukemias, lymphomas and immunodeficiencies in domestic cats. In fact, FeLV infection is second only to trauma as the leading cause of death in cats, killing 85% of persistently infected felines within three years of diagnosis.
The relatively high level of interest in canine health and disease suggests that if there were highly prevalent canine equivalents of FeLV and FIV, they would most likely have been identified by now. Leukemias and lymphomas are relatively common in dogs, but so far none of these symptoms have been conclusively associated with retroviral infection . There have been isolated reports of retroviral particles in canine lymphomas and cell lines, but these viruses were not fully characterized. Since retroviral contamination of in vitro cell culture systems is well-documented, it is premature to draw conclusions from these reports.
A red fox (Vulpes vulpes) hunting mice - canines are likely under constant challenge from retroviruses infecting prey species.Although no exogenous canine retroviruses have been described, studies of the retroviral fossil record leave little doubt that they have existed in the past. For example, the domestic dog genome contains a lineage of ERVs called CfERV , which is clearly derived from a ancient relative of FeLV. Highly related ERV sequences have been identified in the red fox (Vulpes vulpes) genome, indicating that this retrovirus entered the canine germline prior to the divergence of these two species, approximately 9 million years ago.
Overall the diversity and overall quantity of ERVs in canine genomes is comparatively low. Studies of the domestic dog genome have identified between 187-408 ERVs (estimates vary) [2-3] - considerably less than have been described in the genomes of other mammalian species. But it is difficult to draw general conclusions from this observation, since many factors effect the rate at which retroviruses enter the germline of host species, and the rate of ERV copy number increase following germline invasion.
The distribution and diversity of mammalian virusesThe broader question posed by the apparent absence of exogenous canine retroviruses relates to virus distribution and diversity across mammals; should we expect to find every major mammalian virus group in every mammalian lineage? Or might distributions in fact be much more patchy and variable, with entire groups of viruses being excluded from selected host lineages? Under the latter circumstance, approaches that extrapolate from a single species to estimate diversity across all mammals  may be prone to large errors.
1. Tarlinton RE, Barfoot HK, Allen CE, Brown K, Gifford RJ, Emes RD. (2013) Characterisation of a group of endogenous gammaretroviruses in the canine genome. Vet J. 196(1):28-33 [pubmed citation]
3. The first sequenced carnivore genome shows complex host–endogenous retrovirus relationships. (2011) Barrio ÁM, Ekerljung M, Jern P, Benachenhou F, Sperber GO, Bongcam-Rudloff E, Blomberg J, Andersson G. PLoS One 12;6(5):e19832 [full text]
3. Jo H, Choi H, Choi MK, Song N, Kim JH, Oh JW, Seo K, Seo HG, Chun T, Kim TH, Park C. (2012) Identification and classification of endogenous retroviruses in the canine genome using degenerative PCR and in-silico data analysis. J. Gen. Virol 422(2):195-204 [pubmed citation]
4. A strategy to estimate unknown viral diversity in mammals. (2013) Anthony SJ, Epstein JH, Murray KA, Navarrete-Macias I, Zambrana-Torrelio CM, Solovyov A, Ojeda-Flores R, Arrigo NC, Islam A, Ali Khan S, Hosseini P, Bogich TL, Olival KJ, Sanchez-Leon MD, Karesh WB, Goldstein T, Luby SP, Morse SS, Mazet JA, Daszak P, Lipkin WI. MBio 4(5). [full text]