CDC Reports Two Human Infections with Flu Viruses From Pigs

August 4, 2023—CDC this week reported the first two U.S. human infections with flu viruses from pigs in 2023. These human infections were caused by two different types of flu viruses that normally spread among pigs, and they occurred in two people who attended different agricultural fairs in Michigan and had exposure to pigs. CDC recommends people take precautions to prevent the spread of swine influenza viruses to people and has guidance for people exhibiting pigs at fairs, people attending fairs, and fair organizers.

Every year, there are rare sporadic human infections with flu viruses that usually spread in pigs. When found in people, these are called “variant flu virus” infections and designated with the letter “v” after the subtype. Variant flu virus infections are usually associated with contact with pigs, often at agricultural fairs. While infections with swine influenza viruses usually cause mild illness, they are concerning because they can cause severe illness, especially in people at higher risk of serious flu complications, and because of their pandemic potential. These infections are 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.

Presumptive positive influenza A(H3)v virus infection

The Michigan Department of Health and Human Services first reported a presumptive positive influenza A(H3) variant virus infection on July 26 in a person who attended a fair that took place July 7-16. A respiratory specimen from that patient was sent to CDC and received on July 27. Diagnostic testing conducted at CDC was inconclusive, likely due to the respiratory specimen not containing enough virus. However, it is likely the patient was infected with an influenza A(H3)v virus given the presumptive positive test results from Michigan, as well as the epidemiologic findings of the local investigation.

An investigation into this first infection found that:

• The patient was exposed to pigs within 10 days prior to illness onset at an agricultural fair where swine influenza A virus was detected among pigs.
• The patient was treated with flu antivirals.
• The patient was not hospitalized and has recovered from their illness.
• Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts.
• To date, no person-to-person spread of this virus has been identified.

Confirmed influenza A(H1N2)v virus infection

The Michigan Department of Health and Human Services notified CDC of a specimen that tested positive for influenza A but lacked reactivity with diagnostic tests for contemporary human influenza viruses representing either H1pdm09 or H3 subtypes on July 31. A respiratory specimen from that patient was sent to CDC and received on August 2. This was confirmed at CDC to be an influenza A(H1N2)v virus infection. This occurred in a person who attended a different fair, which took place July 23-29.  

An investigation into the second infection found that:

• The patient was exposed to pigs within 10 days prior to illness onset at another agricultural fair.
• The patient was treated with flu antivirals.
• The patient was not hospitalized and is recovering from their illness.
• Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts.
• To date, no person-to-person spread of this virus has been identified.

Agricultural fairs take place across the United States every year, primarily during the summer months and into early fall. Many fairs have swine exhibitions, where pigs from different places come into close contact with each other and with people. These venues can increase the risk that flu viruses will spread among pigs and between pigs and people.

Take Precautions While at Settings Where Pigs are Present

General precautions that everyone can take to limit the spread of flu viruses between people and pigs include:

• Not eating or drinking while in pig areas,
• Avoiding contact with pigs that appear to be sick, and
• Washing hands often with soap and running water before and after contact with pigs.

People at Higher Risk of Serious Illness

• Like seasonal flu, some groups of people are at higher risk of developing serious complications from infections with swine flu viruses.
• People at higher risk should avoid pigs and swine barns.

• If they cannot avoid exposure to pigs, they should wear a well-fitting face mask that covers the nose and mouth to reduce their risk of exposure to flu viruses.
• They should also wash their hands with soap and running water before and after exposure to pigs or a swine barn. If soap and water are not available, they should use an alcohol-based hand rub.

People should take additional protective measures if they must come into contact with pigs that are known or suspected to be sick. This includes minimizing contact time with pigs and wearing personal protective equipment like protective clothing, gloves, and well-fitted masks that cover the mouth and nose when contact is required.

Note that human seasonal flu vaccines are not designed to protect against variant flu viruses, but the same flu antiviral drugs used to treat seasonal flu can be used to treat variant flu virus infection in children and adults. More information is available at Take Action to Prevent the Spread of Flu Between Pigs and People | CDC.

How Variant Flu Virus Infections Happen

Flu viruses can spread from pigs to people and from people to pigs. Infected pigs can cough or sneeze, and droplets with virus in them can spread through the air. If these droplets land in your nose or mouth, or are inhaled, you can be infected. These infections have most commonly been reported after close proximity to infected pigs, such as in pig barns and livestock exhibits housing pigs at fairs.

Flu Viruses from Pigs Change Constantly

Like flu viruses in humans and other animals, flu viruses found in pigs change constantly. Pigs can be infected by avian flu and human flu viruses as well as their own flu viruses. When flu viruses from different species infect pigs, the viruses can reassort (i.e., swap genes) and very rarely new viruses can emerge that can infect and spread from person-to-person. This is thought to have happened in 2009 when a new swine-origin H1N1 virus with genes of avian, swine and human origin emerged to cause a flu pandemic.

Background

In 2005, human infection with a novel flu A virus flu became nationally notifiable in the United States. Novel flu A viruses are different from currently circulating human flu H1 and H3 viruses and include variant flu viruses and avian (bird) flu viruses. Since that time, a total of 512 variant flu virus infections (of different flu A virus subtypes) have been identified in the United States and reported to CDC ranging from a high of 321 variant flu virus infections during the 2011-2012 flu season to a low of one during the 2018-2019 and 2019-2020 seasons. The 321 infections reported during the 2011-2012 seasons included 315 A(H3N2)v, four A(H1N2)v, and two A(H1N1)v viruses. More than 90% of those infections were associated with pig exposure and/or attendance at agricultural fairs.