With this year's big outbreak of swine-origin H3N2 influenza apparently over, a leading flu expert at the Centers for Disease Control and Prevention (CDC) says extensive contact with pigs seems to have been the key risk factor and that very few cases had clear signs of human-to-human transmission.

The CDC has recorded 306 cases of variant H3N2 (H3N2v) infection since July 12, almost all of them in people who participated in or visited county and state fairs where pigs were exhibited. The outbreak prompted warnings for people to avoid swine barns if they were at risk for flu complications or wanted to limit their risk of catching flu. The review of the CDC report was published Friday by the Center for Infectious Disease Research & Policy (CIDRAP) at the University of Minnesota.

H3N2 viruses contain the matrix gene from the 2009 pandemic H1N1, which experts have said may increase its transmissibility. Most of the illnesses have been described as mild.

Lyn Finelli, surveillance and outbreak response team lead in the CDC's Influenza Division, said the most recent H3N2 case reported to the CDC had an onset date of Sept. 7, suggesting that this year's outbreak has faded away.

And although a few fairs are still going on in Alabama, Arizona, and Louisiana, the agricultural fair season is pretty much over for this year, judging from schedules listed by the International Association of Fairs and Expositions.

Exhibitors
In discussing transmission patterns in the outbreak, Finelli reaffirmed the view that CDC officials expressed at its peak that most of the cases occurred in people, mostly children, who had plenty of contact with pigs at fairs.

“This year the vast majority of transmission was among exhibitors and their families. We didn't see much among fairgoers who were not directly associated with pigs,” she told CIDRAP News.

She said the CDC has a detailed exposure history on about 203 cases that occurred in August. “I think 98% had swine contact, and 77% had direct contact, and the majority were exhibitors,” she said. “Sixteen percent had indirect contact, and at least 50% of those went to the swine barn on multiple days.” (The CDC defines "direct contact" as touching pigs, and "indirect contact" as touching things that pigs touched, such as railings.)

She commented that a CDC analysis of an outbreak of H3N2 that was associated with a fair in Pennsylvania in August 2011 supported the conclusion that extensive contact with swine is the risk factor in most cases. The researchers identified three confirmed, four probable, and 82 suspected H3N2 cases in people who had attended the fair, according to a recent report in Emerging Infectious Diseases.

At that fair, “At least a third of the people [who attended] went through the swine barn, yet we had very few cases,” Finelli said. “We didn't have the feeling that people just walking through the swine barn were at risk.”

Few Human-to-Human Cases
This year, the CDC identified “just a couple” of  H3N2 cases of likely human-to-human transmission, in that the patients had no contact with swine, Finelli said. About a dozen other cases involved possible human transmission.

Finelli said she wasn't sure of the precise number of clear human-to-human cases. A CDC report in August cited three likely cases.

That's quite a different picture from 2011, when six of the 12 H3N2 cases involved patients who had no exposure to pigs, Finelli said. “We have no idea” what explains the difference, she commented.

For the dozen or so cases with possible human transmission, the picture is complicated, according to Finelli.

“In all of the cases that are suspected human to human, almost everyone had swine contact,” she explained.

“Attending a state or local fair is a family event, so families go together to fairs, and when there's a case, say in a child, it's hard to sort out whether a second case is from having attended the fair with the child or a secondary case from household transmission.”

She said the CDC looks at the incubation period to try to distinguish human-to-human from swine-to-human cases. The incubation period for most swine-origin flu cases is about 4-5, about the same as in ordinary flu cases, she explained.

Therefore, for patients who had swine contact, CDC investigators defined possible human-to-human cases as those that occurred 6-7 days after swine exposure, that is, beyond the normal incubation period, Finelli said. “Because human-to-human transmission was so important to us, we defined it very strictly,” and investigated such cases aggressively, she added.

“We have a handful of those [with long incubation periods], but are they truly human-to-human? We don't know,” she said. “They may be just cases that occurred at the tail end of the [incubation] period.”

The incubation periods in all of those dozen or so cases were within about seven days, which is “not out of the ordinary for a long incubation period for flu,” she commented.

In any case, she said, “Do we think there was a lot of human-to-human transmission? Absolutely not.”

Some experts have argued that stronger measures are needed to prevent swine-to-human spread of flu at fairs, especially in light of evidence that many pigs can carry flu viruses without showing any obvious signs of illness. Such experts have advocated a number of measures including shortening swine exhibition periods, vaccinating pigs, and even the sure-to-be-unpopular step of canceling swine exhibitions.

Finelli said the CDC will be alert for any return of swine-origin cases next summer. Complete plans have not yet been made, but the agency will be ready with warnings for the public if the problem resurfaces and will work with animal health agencies to take steps to reduce transmission among pigs.

“We can't anticipate what will be needed until we see how big the outbreak is and if there's a lot of pig-to-human or human-to-human transmission,” she said. “We're keeping our fingers crossed, but whatever we need to do next year, we'll do.”