An unusual influenza virus has caught the attention of public health officials because it has rapidly spread from pigs at U.S. county fairs to children. The virus, a variant of an H3N2 strain that seasonally infects humans, has not caused severe disease. Scant evidence exists that it can spread between people, and the vast majority of cases are in two states.
Epidemiologist Joseph Bresee of the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta explained in a teleconference today that 145 cases have been confirmed since 12 July, a big jump from last week when CDC tallied only 16 confirmed cases since that date. “This is clearly a significant increase since last week’s total,” he said, adding that this reflects increased testing and the large number of county fairs now taking place.
Bresee said that more than 90% of the cases occurred in children, most of whom had direct or indirect contact with pigs, and all but two were in Indiana and Ohio. “At this point, there’s no evidence of sustained, efficient, human-to-human spread in the community,” he said. “Our seasonal influenza systems are active and have not shown any signs in an increase in influenza activity. This is not a pandemic situation. But of course CDC is continuing to monitor the situation closely.”
H3N2v first surfaced in July 2011 and infected 12 additional people before disappearing this past winter. Researchers were intrigued because the virus has an internal “M” gene that matches one found in the pandemic H1N1 virus that rapidly infected humans throughout the world in 2009 and 2010. CDC virologist Michael Shaw says he suspects the M gene, which affects how well the virus grows in its host, plays a role in the current transmissions. “There’s something about that gene that gives it an advantage for circulating in pigs,” Shaw says. “And if it becomes more common in pigs, it gives more of a chance to jump into humans.” But Shaw says there’s no evidence that this M gene by itself leads to human-to-human transmission.
Excluding this outbreak and the pandemic H1N1 virus, CDC has recorded only 35 total cases of influenza viruses stemming from pig-to-human transmission in the United States during the past 7 years.
Bresee said that two people were hospitalized because of H3N2v infections but were released. He urged people who visit animals at county fairs to wash their hands frequently and to not eat or drink around pigs. Studies have shown that the current flu vaccine, which contains an H3N2 component, likely will not protect against this strain, but adults presumably have immunity from exposure to “wild type” strains of influenza and previous vaccines that provide some protection against the new variant.
CDC plans to release an update about the outbreak tomorrow in its issue of the Morbidity and Mortality Weekly Report.