CDC Confirms Two Human Infections with Novel Influenza Viruses
December 9, 2011 — CDC has confirmed two cases of human infection with two different novel influenza A viruses in different states. Both patients have fully recovered. While the viruses infecting both patients have been found in U.S. swine and some of the prior human infections with these viruses have been associated with direct or close swine contact, there are no reports of direct or close contact with swine prior to illness onset in either of these cases. Laboratory testing at CDC has confirmed that both novel viruses are susceptible to the antiviral medications oseltamivir (Tamiflu®) and zanamivir (Relenza®).
One case of human infection with a novel influenza virus was reported by West Virginia and involves infection of a child with the novel influenza A (H3N2) virus with genes from swine, human, and avian lineages with the M gene from the 2009 H1N1 virus that was first identified in August 2011. Ten prior human infections with this virus in four other states have been confirmed. These occurred in Indiana (2), Pennsylvania (3), Maine (2), and Iowa (3).
These novel influenza A (H3N2) viruses are substantially different from currently circulating seasonal (human) influenza A (H3N2) viruses, but are distantly related to human influenza viruses that circulated among people in the 1990s. For that reason, some adults may have some residual immunity against this virus. This might help explain why 10 of the 11 cases that have been detected have occurred in children.
Most human infections with viruses that circulate in swine (but not humans) have been associated with swine exposure, but limited human-to-human transmission associated with these viruses is thought to have occurred as well, most recently in Iowa. While an investigation is ongoing in West Virginia, no direct or indirect contact with swine has been reported, implying that limited human-to-human transmission of this virus may have occurred again.
No ongoing community transmission of this virus has been detected in the United States. However, CDC is taking this situation very seriously. Surveillance surrounding reported cases is being further enhanced and, as a precaution, a vaccine virus has been developed and provided to manufacturers for them to begin vaccine production should that become necessary.
The other case of novel influenza A virus infection was reported by Minnesota, and is associated with a different influenza virus; an influenza A (H1N2) virus that circulates in swine in the United States, but does not normally infect or cause illness in humans. This case also was in a child. This is only the second case of human infection with this novel influenza A (H1N2) virus reported to CDC since novel influenza virus infections became nationally notifiable in 2007. The first such case was identified in Michigan in 2007. By some characteristics, this H1N2 virus is close to human influenza A (H1N1) viruses called “A/New Caledonia /20/99-like,” which circulated and caused illness among people as recently as 2007*. As a result, people who were exposed to A/New Caledonia/20/99-like viruses may have some existing immune protection against the virus detected in Minnesota. Again, no direct or indirect contact with swine has been reported with this case, implying that limited human-to-human transmission may have occurred in this instance as well.
Detection of Swine Influenza Infections in Humans
Human infections with novel influenza A viruses normally found in swine are rare events. Recently, however, the frequency of such detections has increased. This could be occurring for a number of reasons, including one or more of the following factors: First, laboratory methods for testing for these viruses in the United States were improved following the 2009 H1N1 pandemic. These improvements may be resulting in viruses being identified now that would have gone undetected previously. Second, this could be due to increased surveillance in the United States for influenza at this time of year. CDC has requested that states analyze, and then send, their first influenza virus specimens of the season for seasonal influenza surveillance purposes. This practice, coupled with very low levels of seasonal flu activity at this time, could mean that sporadic novel influenza infections are more likely to be tested. Third, it is possible that the increased frequency of detection of novel influenza viruses with swine origins identified by CDC represents a true increase in the number of such cases, possibly occurring from exposure to infected swine or through subsequent, limited human-to-human transmission.
The novel influenza A (H1N2) virus identified in Minnesota is known to circulate in U.S. swine herds. While the prevalence of the novel influenza A H3N2 virus with the 2009 H1N1 M gene in swine is unknown, the virus has been detected in U.S. swine through the United States Department of Agriculture’s swine influenza surveillance program.
In response to recent human infections with novel influenza viruses, CDC would like to convey the following information:
- CDC recommends an annual seasonal flu vaccine to protect against seasonal influenza viruses; however, a seasonal flu vaccine is unlikely to protect against flu viruses that normally circulate in swine.
- There are two FDA–cleared drugs that are expected to be effective in treating illness associated with these viruses. The antiviral drugs oseltamivir and zanamivir – which are used to treat infection with human seasonal influenza viruses – also have shown activity against influenza viruses from swine. (For more information about influenza antiviral medications, please see What You Should Know About Flu Antiviral Drugs.)
- 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. For more information about the proper handling and preparation of pork, visit the USDA website fact sheet Fresh Pork From Farm to Table.
At this time, CDC recommends the following:
- People who experience flu symptoms following direct or close contact with swine and who require medical attention (see below) should mention this exposure to their doctor or health care provider. (A list of flu symptoms is available at Flu Symptoms & Severity.)
- For people who have NOT had exposure to swine and develop ILI, CDC’s recommendations for seeking treatment are the same as they are for seasonal influenza.
- If you have symptoms of flu and are very sick or worried about your illness contact your health care provider.
- Certain people are at greater risk of serious flu-related complications (including young children, elderly persons, pregnant women and people with certain long-term medical conditions) and this is true both for seasonal flu and novel flu virus infections. (For a full list of people at higher risk of flu related complications, see People at High Risk of Developing Flu–Related Complications.)
- If these people develop ILI, it’s best for them to contact their doctor. (The majority of recent novel influenza A (H3N2) cases have been in children.)
- Your doctor may prescribe antiviral drugs that can treat the flu. These drugs work better for treatment the sooner they are started.
More information about swine influenza and links to all previous reports related cases of novel influenza A (H3N2) virus infections are available on the CDC swine influenza website.
* Antigenic testing at CDC (one of several laboratory tests used to monitor influenza viruses) has concluded that the H1N2 virus detected in Minnesota is actually more similar to human influenza A/Brisbane/59/2007 (H1N1)-like viruses than to the A/New Caledonia/20/99 virus mentioned above. Human influenza A/Brisbane/59/2007 (H1N1)-like viruses circulated and caused illness among people as recently as 2009 (before the 2009 H1N1 influenza viruses began circulating). This means that people who were exposed to A/Brisbane/59/2007-like influenza viruses may have some existing immune protection against the novel influenza A virus detected in Minnesota.