About Human Infections with Variant Influenza Viruses

About variant flu in people

Swine flu is a respiratory disease of pigs caused by type A influenza viruses that regularly cause outbreaks of flu in pigs (not people). When an influenza virus that normally spreads in pigs but not people is found in a person, it is called a "variant influenza virus." For example, if a swine influenza A(H3N2) virus is found in a person, that virus will be called an "H3N2 variant" virus (abbreviated as "H3N2v" virus).

These viruses are genetically different from the seasonal influenza A (H1N1) and A(H3N2) viruses that commonly spread in people. People may have little to no immune protection against those influenza A viruses that spread in pigs, and human flu vaccines do not protect against the influenza A viruses that spread in pigs.

How it spreads

Pigs to people

Influenza viruses from pigs do not normally infect people. However, sporadic variant influenza virus infections occur among people each year. Human infections with variant influenza viruses most commonly happen in people with exposure to infected pigs (e.g., children near pigs at an agricultural fair, people who raise pigs, or workers in the swine industry). This is thought to happen mainly when an infected pig coughs or sneezes and droplets with influenza virus in them spread through the air. If these droplets land in the nose or mouth, or are inhaled, a person can be infected. There also is some evidence that people can get infected by touching something that has virus on it and then touching their own mouth or nose. Scientists are not sure which of these ways of spreading is most common.

Person to person

Person to person spread of variant influenza viruses is rare. The vast majority of human infections with variant influenza viruses do not result in person-to-person spread. However, each case of variant influenza virus infection should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in people and to limit further exposure of people to infected animals, if infected animals are identified.

People to pigs

Human influenza A viruses can infect pigs and can introduce new influenza A viruses into the pig population.

Signs and symptoms

Illness from variant virus infections has been mostly mild with symptoms similar to human seasonal flu. These include:

• fever
• lethargy (tiredness)
• lack of appetite
• coughing
• runny nose
• sore throat
• eye irritation
• nausea
• vomiting
• diarrhea

Like with seasonal flu; however, serious illness resulting in hospitalization and death has occurred.

People at higher risk

The groups of people at higher risk of developing serious variant flu complications are thought to be the same groups at higher risk for serious seasonal flu complications. CDC has guidance for fair exhibitors and organizers, including people at higher risk of serious flu complications. These considerations include actions to help prevent the spread of flu between animals (poultry, pigs, cows) and people at fairs.

Testing and diagnosis

There is a test to diagnose variant influenza A virus infection. Since the 2009 H1N1 pandemic, state health departments have the ability to test for novel (non-human) flu viruses. However, if a variant influenza virus is suspected, a respiratory specimen is also sent to CDC for confirmation and further testing.

Treatment

There are four different antiviral drugs that are recommended for use in the United States for the treatment of flu: oseltamivir, peramivir, zanamivir, and baloxavir.

Vaccination

Seasonal flu vaccines are not designed to protect against variant influenza viruses. However, CDC recommends that people who have contact with pigs get a seasonal flu vaccine every year to help prevent being infected with seasonal and variant influenza viruses at the same time, as well as spreading human influenza viruses to pigs.

Safe handling and preparation of pork

Variant flu has not been shown to spread through eating properly handled and prepared pork (pig meat) or other pig products. For more information about the proper handling and preparation of pork, visit the USDA website fact sheet Fresh Pork from Farm to Table.

Current case counts

CDC data on the most current case counts for variant flu virus infections in humans reported in the United States.

Pandemic potential from variant viruses

Pigs can be infected with avian (bird), human, and swine influenza viruses, they have the potential to be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. When this happens, it is possible for the genes of these viruses to mix through a process called reassortment. This can result in the creation of a new influenza virus. This type of major change in influenza A viruses is known as antigenic shift. If this new virus causes illness in people and can spread easily from person-to-person, a flu pandemic can occur. This is what happened in 2009 when an influenza A (H1N1) virus with genes from swine, avian and human influenza viruses emerged in the spring of 2009 and caused a pandemic.

Influenza A (H3N2) variant viruses, also known as "H3N2v" viruses, with the matrix (M) gene from the 2009 H1N1 pandemic virus were first detected in people in July 2011. The viruses were first identified in U.S. pigs in 2010. In 2011, 12 cases of H3N2v infection were detected in the United States (Indiana, Iowa, Maine, Pennsylvania, and West Virginia). The largest outbreaks of H3N2v occurred in 2012, when 309 cases of H3N2v infection across 12 states were detected. That year 16 people were hospitalized, and one died. Most cases of H3N2v infection have occurred in children.

Background on the 2009 H1N1 virus

The 2009 H1N1 influenza A virus was first detected in people in the United States in April 2009. The virus was new to people and was able to spread easily from person-to-person, causing the first flu pandemic in more than 40 years. This virus had two genes from influenza A viruses that normally spread in pigs in Europe and Asia, three genes from influenza A viruses that normally spread in North American pigs, as well as genes from human and avian influenza A viruses. This particular virus had not been detected in North American pigs before April 2009. The 2009 H1N1 influenza A virus is now considered a human seasonal influenza A virus.

In October 2009, the first 2009 H1N1 influenza A virus infection in a pig in the United States was confirmed. Pigs in other countries also have been infected with the 2009 H1N1 influenza A virus, including in Canada, Australia and Argentina. USDA and other researchers conducted studies in pigs that showed that the 2009 H1N1 virus caused illness in swine similar to those of other well-known, circulating swine influenza A viruses.

Reports of variant flu infections

Reports of variant influenza virus infections in the US have increased since 2012

Prior to 2012, CDC received reports of approximately one variant influenza virus infection every one to two years, but beginning in 2012, there was a jump in the number of variant influenza cases. Human infections with variant influenza are reported in FluView, and case counts are available at Novel Influenza A Virus Infections (cdc.gov) in FluView Interactive.

The increased detection and reporting of these cases could have occurred for a number of reasons, including:

• pandemic preparedness efforts have improved state level surveillance and laboratory capacity to detect novel influenza viruses in the United States;
• in 2005, novel influenza virus infections were made internationally reportable; and in 2007, they were made domestically reportable; and,
• it's also possible that there has been a true increase in the number of these cases, following exposure to infected swine.

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