SARS may have originated in wild bats in China, an international team of scientists report this week in Science. The family of bats carrying the virus is widespread in Asia and is distributed across Europe and Australia, "and we just don't yet know if the viruses are as well," co-author Peter Daszak, executive director of the Consortium for Conservation Medicine based at the Wildlife Trust in New York, told The Scientist.
These findings spotlight how future research into emerging diseases needs multidisciplinary studies across "virologists, ecologists, wildlife biologists and veterinarians" to understand what specific factors make pathogens more likely to jump across species when humans encroach upon wild habitats, Daszak added.
In 2003, investigators found that masked palm civets and two other species harbored the SARS coronavirus. However, subsequent research showed there was no widespread infection in wild or farmed masked palm civets, suggesting the disease jumped to civets from another species. Bats are reservoirs of zoonotic diseases such as Nipah and Hendra, and can have many chronic infections while only rarely displaying clinical symptoms. They are also increasingly present in food and traditional medicine markets in southern China and elsewhere in Asia, co-author Linfa Wang of Australian Animal Health Laboratory in Geelong told The Scientist in an Email.
As part of the study, Wang and his colleagues in China, Australia and the United States tested blood, throat and fecal swabs from 408 wild horseshoe bats (genus Rhinolophus), representing nine species, from four locations in mainland China. Polymerase chain reaction (PCR) analyses revealed SARS-like coronavirus in five fecal samples from Rhinolophus pearsoni, Rhinolophus macrotis and Rhinolophus ferrumequinum, and none from five other genera tested. Nucleocapsid protein sequences from the bat-derived viruses revealed greater genetic variation than that seen in human or civet SARS, and SARS coronaviruses appear phylogenetically related to those from bats. "It's pretty clear from that phylogeny that this is the origin of SARS isolate in the past outbreaks," Daszak said.
Independently, Kwok-Yung Yuen of the University of Hong Kong and his colleagues this week in PNAS reported that reverse-transcriptase PCR analysis of blood samples and nasopharyngeal and anal swabs from Rhinolophus sinicus from Hong Kong revealed coronavirus in 23 of 59 anal swabs. Genome sequencing showed the coronavirus was closely related to human and civet SARS. They noted the virus could only be detected in anal swabs, suggesting it might grow intestinally, similar to findings in SARS.
"It's essential to know the disease reservoir if future outbreaks are to be stopped. This research allows us to focus on finding pragmatic ways of minimizing contacts between bats and humans," Andrew Dobson of Princeton University, who did not participate in either study, told The Scientist.
Donald Burke, at the Johns Hopkins University Bloomberg School of Public Health in Baltimore, who was also not a co-author of either study, told The Scientist he's "inclined" to accept that bats are the "most likely natural reservoir" of SARS, based on the findings. The report shows that different bat species each have different coronavirus variants, suggesting each virus underwent some adaptation, "which in turn suggests a longer term relationship as a reservoir for that virus."
Bats, which apparently show little symptoms of disease from infection with these viruses, make "more sense" as reservoirs for SARS than civets did, which "were frequently ill" when infected with SARS, W. Ian Lipkin of Columbia University, who did not participate in either study, told The Scientist. "In the most successful host-parasite relationships the host has no or only minimal disease."
Daszak added bats should be protected since they play critical roles in pest control, pollination and seed dispersal, and not culled as civets were by the Chinese government in Guangdong. "We need to know why bat immunology is different from that of other mammals. They show little or no pathology to these pathogens. This knowledge could help us develop new antiviral drugs," Dobson said.
Future experiments to confirm the link between bats and SARS "should take the bat virus and see if it can infect other species and see if SARS can infect bats," Burke said. Researchers should also survey more bat species to find the genetic variants more closely related to human SARS, Wang said.
"There is a possibility that the bat species harboring the virus most closely related to SARS coronavirus may not be inside China," and instead in countries bordering China, Wang added. "So we need international cooperation in future studies."