Reports of Human Infections with Swine Origin Influenza A (H3N2)

The November 12, 2010 FluView reports two human infections with swine origin influenza A (H3N2) viruses in the United States. Test samples from two patients submitted by Wisconsin and Pennsylvania have been confirmed at CDC as positive for swine origin triple-reassortant (tr) H3N2 influenza viruses—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 with confirmed trH3N2 infection reported in FluView were in the vicinity of live pigs. Dates of illness onset in the two patients are more than six weeks apart and the viruses from the two patients have some genetic differences, confirming that these two cases are not linked. Ongoing investigations in both states have not shown any evidence of community transmission of these viruses. The most likely scenario at this point is that these are two isolated cases of human infection with swine influenza viruses that, while very rare, do occur from time to time. Both patients have fully recovered from their illnesses; however, these two cases do underscore the importance of human and animal surveillance for influenza.

These two cases reported in FluView bring the total number of human infections with swine origin influenza viruses reported to CDC since 2005 to 18. Previously, three of these reports had been swine origin A (H3N2) viruses. The Pennsylvania and Wisconsin cases bring the number of reports swine origin A (H3N2) infections in humans in the United States to five. The viruses identified in Pennsylvania and Wisconsin are similar to viruses that infected a patient in Iowa in September 2009, a patient in Kansas in August 2009 and a patient in Minnesota in May 2010.

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 four 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.

Most commonly, cases of human infection with swine-origin influenza viruses occur in people with direct exposure to pigs. The patient in Pennsylvania lives in an area where live pigs are farmed and the patient in Wisconsin became sick two days after attending a state fair where pigs were exhibited. It's important to note that swine influenza viruses are not transmitted to humans by food. You can not get swine influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe.

In the past, CDC received reports of approximately one human infection with a swine influenza virus every one to two years, but in the past few years, about three 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.

These trH3N2 viruses are different from the 2009 H1N1 virus that has been circulating in the United States since late April 2009. They are also different from human seasonal influenza A (H3N2) viruses that typically circulate among people during the flu season. Swine trH3N2 viruses commonly circulate in pigs in North America, but rarely infect humans. These viruses are different from the swine classical H1N1 or swine trH1N1 influenza viruses that also circulate in pigs in North America because they have H3N2 surface antigens. Tr H3N2 viruses first emerged in North American swine herds in the late 1990s. The H3 and N2 genes which first emerged in swine flu viruses originated from human seasonal H3N2 influenza viruses that circulated globally among humans in the late 1990s.

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.

For more information about swine influenza, visit the CDC swine flu website.

Weekly U.S. surveillance updates are published in FluView.