Based on the results of genomic analysis of virus samples, pet hamsters are most likely to be the spreader of the Delta variant in Hong Kong and trigger the COVID-19 outbreak in humans.
The study confirms previous concerns that pet stores were the source of an outbreak that involved about 50 cases of infection, and led to the culling of about 2,000 hamsters across the city.
Hamsters are highly susceptible to SARS-CoV-2 and are becoming a popular model for studying it. But the Hong Kong study is the first to show that hamsters can become infected outside the laboratory, and that they can transmit the virus to other hamsters as well as humans.
Quoted from Nature, hamsters are another animal known to infect humans after cultivating minks. Previously, this animal sparked panic and mass extermination when it was found to have caused a small outbreak among Danish and Dutch citizens in late 2020.
"This latest study points to the pet trade as a route for the virus to spread. However, human-to-human transmission is still much more likely to be infected than from pets," said co-author Leo Poon, a virologist at Hong Kong University.
New route of virus spread
"Nevertheless, it is very important to monitor the pet trade closely," said Marion Koopmans, a virologist at Erasmus University Medical Center, in Rotterdam, The Netherlands.
He warned that SARS-CoV-2 could continue to circulate in animals, develop in unexpected ways, and then spread back to humans.
The study began when a 23-year-old female pet shop worker tested positive for Delta on January 15. Poon said it was a bit strange. The last time the city detected Delta was in October.
Within days, public health officials swab about 100 animals at the pet shop, and another 500 at the warehouse that supplied them.
They detected the presence of SARS-CoV-2 viral RNA, or antibodies to the virus, in 15 of 28 Syrian hamsters (Mesocricetus auratus). Viruses are not found in pygmy hamsters, rabbits, guinea pigs, chinchillas, or mice.
The researchers then analyzed the genome sequences of samples collected from the 12 hamsters and the first 3 infected people, including pet shop workers and shop visitors. All samples contained a previously undetected Delta variant in Hong Kong, and may have come from the same source.
The researchers also looked at some diversity in the sequences, and concluded that the hamsters may have been first infected in November, prior to their arrival in Hong Kong, and the virus had spread undetected among the animals, while accumulating multiple single-nucleotide mutations.
Jump between species
Pet shop workers and visitors may have been infected on two separate occasions, though Poon said there could be more jumps. What's most surprising, he says, is that even after replicating in hamsters, the virus can still be transmitted between humans quite effectively.
The warehouse imported the animals from the Netherlands, and further analysis of the genomes uploaded to a public global database identified their closest matches in sequences collected in people in eastern Europe.
Koopmans said he was confident in his analysis and conclusion that the animal imported the variant into Hong Kong. He said it was very important to trace the source of infection in hamsters.
But Arinjay Banerjee, a virologist at Saskatchewan University in Saskatoon, Canada, said the researchers couldn't rule out the possibility that the hamsters were first infected by someone in Hong Kong, not importing the virus into the country.
"There are so many people who handle hamsters during their transfer process. In conclusion, the risk of infection from hamsters appears to be low, but still something to watch out for," he concluded.