China is enduring its fifth epidemic in four years of an avian influenza strain that has killed hundreds of people, according to the World Health Organization.
Recent isolates of the influenza A H7N9 virus, found in humans and environmental samples, have genetic sequences characteristic of highly pathogenic avian influenza viruses, which cause disease in birds. While viruses with those changes could cause severe disease in birds, WHO officials reported in March that they had no evidence the changes affect pathogenicity or transmissibility in humans.
The current epidemic is the largest of five, with 460 reported human infections since Oct. 1, 2016, according to a March 3 report from the U.S. Centers for Disease Control and Prevention. Most infections in humans have been connected with exposure to poultry, especially on farms, in live bird markets, and at slaughter locations.
CDC officials are developing a vaccine against the H7N9 virus and working with counterparts in China in response to the latest epidemic.
The virus is transmitted from birds to humans, and the risk of sustained human-to-human transmission is low, according to the WHO. But the organization’s report notes that constant change is the nature of influenza viruses.
In a recording from a March 1 WHO press briefing in Switzerland, Jacqueline Katz, PhD, deputy director of the CDC’s influenza division and director of a CDC collaborating center with the WHO, said sporadic infections can occur in humans while avian influenza viruses circulate in poultry. Avian influenza viruses can acquire genetic changes that would let the viruses infect people more easily or spread person to person, which could lead to a pandemic, she said.
A negative-stained transmission electron microscopic image of the A H7N9 avian influenza virus. (Courtesy of Cynthia S. Goldsmith and Thomas Rowe/CDC)
The CDC report—an early release from the Morbidity and Mortality Weekly Report—indicates 453 of the 460 laboratory-confirmed human infections since October 2016 occurred in mainland China. Four others were associated with travel from the mainland to Hong Kong, one each involved travel to Macao and Taiwan, and one involved an asymptomatic poultry worker in Macao.
Of the 800 known human infections from the prior four outbreaks, 88 percent of victims developed pneumonia, and 41 percent died, the CDC reported.
The CDC Influenza Risk Assessment Tool, which is used to assess the pandemic risk of influenza A viruses not circulating in people, lists the low-pathogenic H7N9 avian influenza virus as having the highest risk of emergence and highest potential impact among 12 influenza viruses that are public health concerns. It also has the highest listed pandemic risk: moderate to high.
But the MMWR article notes the public health threat remains low.
A. Danielle Iuliano, PhD, an epidemiologist in the CDC’s influenza division and one of the MMWR article authors, said the general risk of a person becoming infected with the H7N9 virus is low, and she noted that most of the H7N9 infections in humans have followed exposure to poultry in specific areas of China. The assessment of a virus’s pandemic potential considers factors such as epidemiologic characteristics, virus characteristics, and clinical presentation, and the findings from that assessment become more important if the virus becomes transmissible among humans.
“Right now, we think the general risk to the public is low because it’s not transmissible person to person,” she said. “But we’re monitoring things very closely because of the anticipated pandemic risk.”
The virus is circulating among poultry populations and is endemic in some areas.
| ||Source: The World Health Organization
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As for the avian influenza isolates with characteristics of highly pathogenic viruses, Dr. Iuliano noted that clinical signs—including die-offs—among birds infected with highly pathogenic avian influenza viruses can help alert people that the virus is present before anyone becomes infected. Identifying the presence of low-pathogenic avian influenza often involves looking for the sources of infections in humans, she said. She was not aware of any bird die-offs reported in connection with the H7N9 epidemics.
U.S. Department of Agriculture officials also announced March 7 that a highly pathogenic H7N9 avian influenza virus found in a Tennessee chicken breeding flock was different from the H7N9 virus in China. On March 5, officials with the USDA Animal and Plant Health Inspection Service said the farm had the first commercial flock of 2017 with confirmed infections with a highly pathogenic avian influenza virus, then an unknown variety of H7 influenza.
The farm had 73,500 chickens. Birds on the property were depopulated and buried, APHIS information states.