An outbreak of a flu variant infecting swine and humans has picked up momentum from contact between people and pigs at fairs. The U.S. Centers for Disease Control and Prevention (CDC) reported today a tally of 289 human cases since August 2011 and cautioned that the start of school could bring a fresh wave of illnesses.

“...This is an evolving situation that could change quickly,” Dr. Joseph Bresee, chief of the Epidemiology and Prevention Branch of the CDC’s Influenza Division, said in an update posted today.

Human infections of influenza A H3N2v virus were first detected last summer; by the end of the year, the case count was 12. The case total this year so far is 277. The first reported incident occurred in April, and many more followed in July and August.

The flu variant generally has been causing illness comparable to seasonal flu, with 13 cases to date requiring hospitalization, and no deaths. What makes this virus of particular concern to public health authorities is not only its zoonotic nature but the fact that it contains a gene found in the H1N1 influenza virus that was responsible for a pandemic in 2009.

The CDC explains that the virtually identical gene, known as a matrix gene or M gene, “may confer increased transmissibility to and among humans” compared with other swine flu viruses that have infected people in the past.

The agency has documented a few cases of limited human-to-human transmission. The newest such instances involve three unrelated individuals who lived in households with someone who was exposed to pigs at an agricultural fair.

Last fall, three cases occurred among Iowa youngsters who attended the same day care center and had no contact with swine.

“We’re not particularly surprised to see what looks like limited human-to-human transmission,” Bresee said in today’s update. “It’s clear though,” he added, “that this virus is much better able to spread to people. It’s reassuring that we are seeing most cases in people with prolonged contact with pigs and that we are not seeing any sustained community transmission, but this situation definitely warrants our close attention.”

The 289 cases have occurred in 13 states. Indiana leads with 140 cases, followed by Ohio with 98. The rest of the states — Maryland, Wisconsin, Pennsylvania, Michigan, West Virginia, Illinois, Iowa, Maine, Minnesota, Utah and Hawaii — have had 12 or fewer.

(Owing to reporting lag time, individual states may post higher figures than shown in the CDC tally.)

Most of those infected have been children. Ruben Donis, associate director of the CDC Influenza Division, said in an interview that adults likely have some immunity owing to past exposures to related influenza viruses.

Bresee said during an Aug. 9 news teleconference that he believes the start of fair season explains the surge in infections. “This time of the year is when you have county and state fairs ... around the country. There’s thousands of them,” he said. “So I think in that setting, there’s lots of exposure between humans and pigs, especially kids. I think that accounts for the increased transmission more than anything else.”

The start of school might also trigger an increase in cases. “As the school year gets underway and we move into fall and winter, the opportunities for spread of respiratory viruses like influenza increase,” Bresee said in today's statement. “It’s possible we could see isolated cases of H3N2v infection, and even some localized outbreaks, particularly in schools or day cares.”

In pigs, influenza tends not to be a serious illness causing high numbers of death, and this subtype is no different, said Dr. Tom Burkgren, executive director of the American Association of Swine Veterinarians (AASV).

“Typically, clinical disease will be lethargy, coughing, they might spike a fever for a day or so, go off feed and just kind of lay around,” Burkgren said. “It can be high morbidity, in that a high number of pigs can be affected in a barn, but the mortality is not very high.”

Burkgren said no commercial vaccine is available for the H3N2v flu virus in pigs. Neither is the seasonal flu vaccine for people designed to protect against this variant of the virus.

From a pork producer’s perspective, Burkgren said the greatest concern about the virus is the potential for consumers to avoid eating pork out of the misplaced fear that they will become ill. The CDC states: “Swine influenza has not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs.”

In a fact sheet called "Protect yourself against H3N2v," the CDC says:

“Influenza viruses are thought to spread from infected pigs to humans in the same way that seasonal influenza viruses spread between people. Mainly, the spread of influenza happens when droplets infected with influenza spread through the air after an infected pig coughs or sneezes (and) land in your nose or mouth, or when the droplets are inhaled. There is also some evidence that you might get infected by touching something that has virus on it and then touching your own mouth or nose. A third way to possibly get infected is to inhale dust containing influenza virus. Scientists aren’t really sure which of these ways of spread is the most common.”

The agency also has posted a fact sheet with advice on how to prevent the spread of flu between people and pigs at fairs, such as by not taking food and drink, toys or strollers into pig areas and washing hands before and after visiting pigs.

For people at high risk of serious flu complications, the CDC recommends avoiding pigs and swine barns. Those at high risk include children younger than age 5, people age 65 and older, pregnant women and people with chronic health conditions such as asthma, diabetes, heart disease, weakened immune systems and neurological or neuro-developmental conditions.

Dr. Radford Davis, a public health professor at Iowa State University College of Veterinary Medicine, said it makes sense for the CDC and other public health agencies to closely monitor the virus because there’s “always that question of, ‘Is this going to be the next pandemic virus?’ ... There’s always this concern that because influenza mutates so readily, you just don’t know whether this is a strain that will become readily transmissible to people.”

But he added: “For me personally, it’s another influenza virus. I mean, there’s going to be no end to influenza viruses in the future.”

With the greatest concentration of cases coming from nearby Indiana, Davis said Iowans are keenly aware of the latest swine flu. However, he doesn’t expect people to faithfully follow the CDC’s guidance for avoiding potential exposure to pigs at fairs.

“These are farm families. You have a lot of elderly people and young kids. They’re not going to stay away,” he said. “... Kids are there to show their pigs, and they may be there all day for a day or two. They’re sitting there, eating there, they have cots — they’re sometimes sleeping there.”

Beyond exhibitors, visitors aren’t likely to stay away, either, or to follow all precautions, Davis said. “Don’t take food or drink? Don’t use strollers? Gosh, that is so not reality,” he said. “You just wouldn’t have anybody go to the fair if people adhered to all these things.”

On the swine health side, the number of animals infected is unknown because swine influenza is not a reportable or regulated animal disease in the United States.

The U.S. Department of Agriculture (USDA), working with states and industry, conducts voluntary surveillance for swine influenza. The goal of the surveillance is not to define prevalence but “to identify viruses that may be circulating in swine and gain knowledge to contribute to improved animal health diagnostics and vaccines,” the agency states on a webpage about the program.

The USDA notes that surveillance first identified H3N2v virus isolates in late 2010. Among 12,662 samples collected between Oct. 1, 2010, and July 31, 2012, the program found 1,488 tested positive for influenza A infection — slightly fewer than 12 percent.

Among those positive for influenza A, 211 were subtype H3N2 — about 1.7 percent of all samples tested. However, not all contained the attention-getting M gene.

The USDA reports that of 138 H3N2 cases identified in the 10 months between Oct. 1, 2011, and July 31, 2012, 57 were found to contain the pandemic M gene.

The full implications of the M gene’s presence aren’t clear. “While we know the M gene plays a role in influenza virus infection, assembly and replication, the significance of this change in these swine-origin influenza A (H3N2) viruses is unknown at this time,” the CDC stated last November in a report about new cases at the time.

Donis explained that the matrix gene controls the “glue” that holds a flu virion together, binding the core to its outer shell. Whether the matrix of the 2009 pandemic H1N1 virus is what conferred that virus’s ability to spread easily from person to person hasn't been proven.

“The evidence is very, very limited,” Donis said. “It’s a correlation more than evidence. Whether there is causative association is still to be determined.”

The virus does seem to have the ability to spread readily among swine, he noted. Although hard data on the prevalence of infection isn’t available, Donis said officials have better-than-anecdotal information that “the virus is really becoming dominant in swine. ... The reason we see it more in humans is because it’s expanding and spreading very successfully and very far and widely in pigs,” he said.

Asked at a telebriefing with reporters earlier this month how this year’s outbreak differs from past instances of pig flus transmitting to people, the CDC’s Bresee said that while the occurrence is not new, the level is different.

Prior to 2007, Bresee said, the agency would hear about one case of flu transmission from pigs to people every year or two.

In 2007, novel influenza A viruses became a nationally notifiable disease. At the same time, Bresee said, a national pandemic preparedness initiative “got the right diagnostics into state departments’ and public health departments’ hands to use.” With the increased monitoring and diagnostics, the average number of pig-to-person flu cases rose to five or six a year, he said.

But closer monitoring and diagnostics do not explain this year’s surge. “Clearly, the large number of cases ... that have occurred this summer and the 13 cases that have occurred with this virus before this summer represent a big increase in the normal number of cases that we hear about from this virus,” he said. “.. We think it's a real increase in cases ...”

That is not to say that influenza viruses are evolving to become more versatile in their infective powers, Donis said. To his thinking, the potency of H3N2v has more to do with how much of the population has never been exposed to related flu viruses. “I have no reason to believe that there is a fundamental change in the biological properties of the virus (in general) with regards to its ability to infect humans,” he said.