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H1N1 (Swine Flu): Two human infections with swine origin influenza A (H3N2) in the United States (Wisconsin and Pennsylvania)

November 13, 2010

Last week CDC reported 2 human infections with a novel swine origin influenza A (H3N2) virus in the United States (Wisconsin and Pennsylvania).  The viruses are swine origin triple-reassortant H3N2 influenza viruses – these are viruses that normally infect pigs.

While human infection with swine influenza viruses is rare, it can occur. This is most likely to occur when people are in close proximity to infected pigs, such as in pig barns and livestock exhibits housing pigs at fairs.

Both of the patients were in the vicinity of live pigs. Dates of illness onset in the 2 patients are more than 6 weeks apart and the viruses from the 2 patients have some genetic differences, confirming that these 2 cases are not linked. Both patients have fully recovered from their illnesses; however, these 2 cases do underscore the importance of human and animal surveillance for influenza.

Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza viruses that regularly causes outbreaks of influenza in pigs. Swine flu viruses cause high levels of illness and low death rates in pigs. Swine influenza viruses may circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans. There are 4 main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. Most flu viruses circulating in pigs are referred to as “triple-reassortant” viruses because these flu viruses contain genes from human, swine and avian influenza viruses.

In the past, CDC received reports of approximately one human infection with a swine influenza virus every one to 2 years, but in the past few years, about 3 cases have been reported per year. Increased reporting of human infections with swine influenza could be the result of increased influenza testing capacity and capabilities in public health laboratories.

Although the vast majority of instances of human infection with animal influenza viruses do not result in human to human transmission, each case should be fully investigated to be sure that such viruses are not spreading among humans and to limit further exposure of humans to infected animals if infected animals are identified. Surveillance for both seasonal and novel influenza viruses is conducted by the CDC and its state and local health partners year round.

Flu season hasn’t yet begun in the United States as demonstrated by this weeks Flu View map…still time to get that flu shot!

2010-2011 Influenza Season Week 44 ending November 6, 2010

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