H3N2v Update: New Cases Reported, Limited Person-to-Person Transmission Detected

August 24, 2012 -- Today, 52 additional cases of H3N2v are being reported, bringing the total number of such infections since July 2012 in the United States to 276 across 10 states. Investigations into H3N2v cases indicate that the main risk factor for infection is exposure to pigs, mostly in fair settings; however, CDC also is reporting three instances of likely human-to-human spread of this virus during the current outbreaks. Found in pigs in 2010 and first detected in humans in July 2011, this H3N2 variant virus appears to be more transmissible from pigs to people than other variant viruses. The Centers for Disease Control and Prevention (CDC) is working with states to respond to this evolving situation and continues to monitor the situation closely.

According to CDC’s Dr. Joseph Bresee “limited human-to-human spread of this virus has been seen in the past, but the H3N2v virus has not previously -- and is not now -- spreading easily from person-to-person.” According to Bresee, “Most cases are occurring in children who are exhibiting pigs, or helping to exhibit pigs and have occurred after a lot of very close contact with pigs over a relatively long period of time.” Dr. Bresee is Chief of the Epidemiology and Prevention Branch in CDC’s Influenza Division.

The 52 new cases reported this week are from the states of Illinois (1), Maryland (12), Michigan (4), Minnesota (1), Ohio (26), Pennsylvania (2), and Wisconsin (6). These are the first reports of H3N2v with the pandemic M gene from Maryland and Minnesota. The cases in Maryland are reported to have had exposure to pigs prior to their illness. The one case in Minnesota was confirmed following a visit to a live animal market where pigs were present. Cumulative totals for 2011 and 2012 by state are available in the H3N2v case count table. The remaining cases being reported this week are associated with exposure to pigs at fairs.

The three instances of likely person-to-person spread of H3N2v were recently identified during investigations of cases and their household contacts and are not epidemiologically linked to one another. In all three cases, transmission is thought to have occurred from one person to another person without further spread to additional people. Each of these three instances of likely person-to-person spread happened between 2 people living in the same household, with the initial infection in each household being associated with pig exposure at an agricultural fair.

“We’re not particularly surprised to see what looks like limited human-to-human transmission,” says Bresee. Limited human-to-human spread has been seen in the past, both with this variant virus as well as other variant viruses. “It’s clear though,” Bresee adds, “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,” says Bresee.

CDC is monitoring for changes in the virus and potential person-to-person transmission of H3N2v. This week CDC updated its guidance to states for enhanced surveillance for influenza-like-illness (ILI) in all people, including people reporting pig exposure. So far, genetic analysis of the viruses submitted to CDC show that all viruses are nearly identical, and very similar to the H3N2v viruses found in 2011.

Illness associated with this virus so far continues to be mostly mild with symptoms similar to seasonal flu. Like seasonal flu, however, serious illness with H3N2v infection is possible. To date, 13 hospitalizations have occurred, but all patients have recovered. Last week CDC issued information for clinicians on H3N2v; guidance which underscores the importance of rapid antiviral treatment of H3N2v virus infections in high risk patients.

High risk factors include: being younger than 5 years, or 65 and older, pregnancy, and certain chronic medical conditions like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions.

CDC has specific recommendations this summer for people attending fairs. Anyone with an age or medical factor that puts them at high risk for serious flu complications is being urged to avoid pigs and swine barns at fairs this season. CDC and USDA also have issued a fact sheet entitled Issues for Fair Organizers to Consider When Planning Fairs.

“Something else we are looking at is schools,” says Bresee “as the school year gets underway and we move into fall and winter, the opportunities for spread of respiratory viruses like influenza increase. It’s possible we could see isolated cases of H3N2v infection, and even some localized outbreaks, particularly in schools or day cares,” he cautions. CDC has issued supplemental guidance for schools on H3N2v. “The guidance document is a heads up for schools to be aware of, and on the look-out for, illness with this virus,” Bresee explains. In 2011, there was at least one documented outbreak of H3N2v in a day care setting.

“It’s important to remember that this is an evolving situation that could change quickly,” Bresee notes. “We’re constantly looking at our data and re-evaluating.”

Symptoms of H3N2v have been consistent with seasonal influenza and include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and fatigue. While influenza viruses can spread from people to pigs and pigs to people, however, they have not been shown to be transmissible to people through eating pork (pig meat).

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