September 01, 2020
Influenza virus in China’s pigs may have pandemic potential

An influenza virus circulating among Chinese pigs has the genetic potential of a pandemic virus in humans, according to a scientific report.
The virus appears to have already spread from pigs to people who work with or live near them. The authors wrote that further mutations could increase the risk to humans.
The article, published in late June in the Proceedings of the National Academy of Sciences, describes an H1N1 influenza virus different enough from today’s seasonal influenza viruses that people would be unprotected by vaccines or prior illnesses.
The H1N1 influenza virus emerged during 2013 in southern China and became the predominant cause of influenza in the country’s swine in 2016. Known as Genotype 4 or just G4, it carries genetic features from human-, swine-, and avian-type influenza viruses.
The article shows no evidence the virus has spread person to person. But, under laboratory conditions, the authors found the virus replicated well in human airway epithelial cells and spread with ease among Angora ferrets, which are used as animal models for human infections with influenza viruses.
Dr. Richard Webby, director of the World Health Organization Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds and a faculty member in the infectious diseases department at St. Jude Children’s Research Hospital, said the virus is concerning but not “head-for-the-hills” alarming. It’s one of many influenza viruses worth studying, and more research is needed to understand how swine influenza strains become more dangerous to humans, he said.
Prior research gives some idea which mutations help avian influenza viruses replicate or transmit well in humans, Dr. Webby said. But swine influenza viruses share many properties with human-type influenza viruses, and researchers are trying to figure out which changes are important.
“We would assume that only a few changes would be necessary, but we don’t really know what those changes are and where they would be,” he said.
The institutions behind the research paper—the China Agricultural University College of Veterinary Medicine, Shandong Agricultural University College of Animal Science and Veterinary Medicine, the Chinese Center for Disease Control and Prevention, the Chinese Academy of Sciences WHO Collaborating Center for Influenza Research and Early-Warning, and the University of Nottingham School of Veterinary Medicine and Science in the United Kingdom—collected about 30,000 nasal swab samples from slaughterhouse pigs from 2011-18 and another 1,000 nasal swabs or lung tissue samples from pigs with signs of respiratory disease during that period.
They found swine influenza viruses in only about 0.5% of pigs in slaughterhouses. They also collected nasal swabs and lung tissue samples from about 1,000 pigs that had signs of respiratory disease and were brought to a veterinary teaching hospital, and they found the portion positive for influenza grew from 1.4% in 2011 to 8.2% in 2018.
The surveillance also included collecting blood samples from 338 people who worked on 15 swine farms—10% were seropositive, with younger workers twice as likely to have antibodies against the virus. The prevalence of antibodies also rose over time, from 7% of swine workers tested in 2016 to 12% in 2018.
Among 230 members of the public, 10 people—about 4%—had antibodies against the virus.
Blood and serum tests can indicate people were exposed to the target virus—or a similar virus—but those tools cannot show how people became exposed to the virus, Dr. Webby said. They also cannot show whether people became ill or infectious or how much the virus replicated within their bodies.
With the results so far, most people assume the virus spread from pigs to humans, with very limited amounts of human-to-human spread, he said.
Dr. Amy Vincent, who is a research veterinary medical officer for the Agricultural Research Service of the U.S. Department of Agriculture, expects the influenza surveillance programs already used in U.S. swine herds likely would find the G4 virus or its close relatives. As for risks to humans, she also said the reported 10% overall seropositivity rate among swine workers suggests the virus did not spread efficiently to people from pigs and there was no evidence of human-to-human spread among those people tested.
“Although the viruses reported in the PNAS paper showed elements of risk for the human population, the results are within comparable ranges to other swine viruses that have been evaluated in a similar way,” Dr. Vincent said. “There was no evidence for human-to-human transmission, a key element of concern for novel influenza viruses from animal sources.
“Surveillance in animals, research, and cooperation among animal and human health sectors are critical for the early identification of pathogens that have the potential for human zoonoses.”
Pigs can be mixing vessels for influenza viruses. The swine-origin 2009 H1N1 influenza virus, for example, developed through a combination of genes from avian, human, and swine viruses.
Dr. Vincent said biosecurity is important to keep novel influenza strains out of U.S. swine herds as well as reduce the risk that a variant strain could develop and spark a zoonotic event. She noted that China’s CDC, in collaboration with WHO authorities, has helped prepare for a pandemic by submitting a candidate vaccine virus from a closely related H1N1 influenza virus.
The PNAS article also arrived almost two years after African swine fever began killing millions of pigs and devastating herds in China, which is the world’s largest pork producer. Dr. Webby noted those outbreaks and said he wants to see how widespread influenza viruses remain in herds today.