Oct. 26, 2007 Researchers at the University of Leeds have made a breakthrough in understanding a virus which poses one of the greatest global disease threats to wild carnivores including lions, African wild dogs and several types of seal.
The discovery of how canine distemper Virus (CDV)1 jumps across and infects different species of carnivores could lead to a more effective monitoring and control of the virus.
Whilst these ‘pathogen jumps’ across species are quite common, very little is known about the process of how viruses takes hold and become established in new host species.
CDV is passed through close contact from domestic and feral dogs causing epidemics that often result in mass mortalities – and is pushing some species to the brink of extinction2.
“The virus needs to bind to a specific receptor on cells in the host in order to infect it,” explains lead researcher, PhD student Alex McCarthy, from the University’s Faculty of Biological Sciences. “But the sequences of receptors vary between species, so a virus from one species shouldn’t be able recognise and infect the cells of other species.”
By analysing the virus’ genetic sequence in both dog and wild carnivore species, the research team were able to prove that two key parts of a CDV protein specifically involved in receptor recognition had evolved during the host jumps, where as the rest of the protein showed very few changes among viruses from different species.
“It was a very satisfying moment when our ideas proved correct,” says McCarthy. “The results really screamed out at us. They were so clear-cut, we think it’s highly likely that pathogen evolution is a much more general mechanism in cross-species transmission of viruses than anyone imagined.”
The findings could lead on to new antiviral therapies that are targeted at the binding mechanism, to prevent the virus from taking hold, rather than trying to eradicate it once it’s in the host’s system.
Current conservation policies include vaccination of wild animal populations, but this is not appropriate for logistical or biological reasons for most species. Alternative strategies include vaccination of neighbouring domestic and feral dog populations, which prevents CDV circulating in dogs and therefore limits transmission to wildlife. Culling the surrounding domestic reservoir species, such as dogs, is also an option, but this is more controversial because its efficacy can be questionable and because of the impact this can have on support from local communities for wider conservation efforts.
The spread and incidences of CDV epidemics are increasing, due to globalisation and the rise in the domestic and feral dog populations associated with growing human populations, especially where these impinge on previously undisturbed habitats.
“CDV in wild carnivore populations is usually fatal” says McCarthy. “So as well as the possibility of developing new therapies, the techniques used in this research offer a way of predicting when local virus isolates may become capable of causing a full blown epidemic in species of conservation concern.”
The research could have similar implications for developing new therapies for pathogenic diseases that have successfully crossed species to humans. Additionally, the team believes that the phenomenon may be involved in the emergence of many new, previously species-specific diseases that have been able to infect new host species, such as SARS, Hendra and Nipah Virus.
The research paper is published online in the Proceedings of the Royal Society B. Pathogen evolution and disease emergence in carnivores by Alex J. McCarthy, Marie-Anne Shaw and Simon J. Goodman, Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds.
1. Species affected by CDV include lions, spotted hyenas, African wild dogs and several species of seals. The study included virus sequences from a range of carnivore species including ferrets, leopard, racoons, mink, panda, red panda, javelina, foxes, Baikal & Caspian seals.
2. In the case of Caspian seals (the world’s smallest seal) a series of CDV epidemics over the last 10 years has each seen up to 10,000 animals die – out of a total population of only 120,000. CDV is also thought to have been involved in the extinction of the Tasmanian tiger and the rare black-footed ferret (later successfully re-introduced to the wild from a captive breeding programme).
CDV is in the same family of viruses as measles. It is also closely related to, but distinct from, Phocine Distemper virus (PDV) which has caused two mass die offs of harbour seals in the North Sea (1988 and 2002).
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