The search for an animal reservoir of the virus that causes severe acute respiratory syndrome (SARS) has turned to wild mammals after the isolation of a virus “almost identical” to SARS-associated coronavirus (SCV) from six masked palm civets (Paguma larvata) and one raccoon dog (Nyctereutes procyonoides) in a market in Shenzhen, southern China, in May. The research team, led by Guan Yi (University of Hong Kong), also detected antibody to SCV in a Chinese ferret badger (Melogale moschata) in the market. All three species are sold as delicacies for human consumption throughout southern China.
“The homology between SCV and the animal virus is extremely high at 99%”, says Guan. “The major difference is a 29 nucleotide deletion in SCV, which suggests transmission from animals to humans rather than a human virus that has jumped to animals. There are also consistent differences in the surface protein gene”.
“The study provides the first indication that the SARS virus exists outside a human host”, says team member Malik Peiris (Hong Kong University). All six civets in the market harboured the virus, which Peiris suggests points to recent infection. “It is possible that they got infected from the primary animal reservoir and are not themselves the true reservoir in nature.”
Trevor Drew (Veterinary Laboratories Agency, Weybridge, UK) thinks it is too early to say whether any of the species studied is the natural host of the virus. “It is quite possible that civets represent another susceptible species, infected by a virus very similar to SCV, rather than a reservoir of this virus and of the SARS virus.”
Any lingering doubts about the cause of SARS have been swept away with the demonstration that SCV meets all six of Koch's postulates for viral diseases. Three criteria–isolation of virus from diseased hosts, cultivation of the virus in host cells, and proof of filterability–had already been met. Albert Osterhaus (Erasmus Medical Centre, Rotterdam, Netherlands) and colleagues have now proved the last three by successfully infecting macaques (Macaca fascicularis) with SCV, reisolating the virus from diseased animals, and showing that the monkeys had mounted an immune response against SCV (Nature 2003; 423: 240). The authors say their findings do not exclude the possibility that other pathogens, such as human metapneumovirus and Chlamydia pneumoniae, may have exacerbated the illness in some SARS patients.