Five cases of human infection with influenza viruses that usually spread only in pigs, also known as variant influenza virus infections, were reported to CDC in August 2022. These cases include three infections with influenza A(H3N2) variant (A(H3N2)v) virus and two infections with influenza A(H1N2)v virus. These cases were identified in West Virginia (3), Oregon (1), and Ohio (1). Four of the five cases reported exposure to pigs or attendance at an agricultural fair prior to illness, and one reported no contact with pigs or attendance at an agricultural fair prior to illness. Clinical characteristics of these cases have been similar to those of seasonal influenza infections and have included fever, cough, pharyngitis, myalgia, and headache. No hospitalizations or deaths have occurred among these five cases, and all patients are recovering or have recovered from their illnesses. To date, no person-to-person spread associated with the five recent variant influenza virus infections has been identified.
Early identification and investigation of variant influenza virus infections are important to determine whether the virus is spreading efficiently among people. Rapid detection and characterization of novel influenza A viruses and efforts to reduce transmission to other people remain important components of national efforts to prevent the emergence of new viruses that could have pandemic potential. To accomplish this, testing for influenza viruses and monitoring for novel influenza A virus infections, including variant influenza virus infection, should continue year-round. Individuals, especially those at increased risk of influenza complications, can take public health measures to limit their risk of infection (e.g., limiting exposure to infected animals). Clinicians are encouraged to consider variant influenza virus infection as a possible diagnosis when evaluating patients with acute respiratory illnesses and exposure to pigs or agricultural fairs prior to illness.
Since 2005, 504 variant influenza virus infections (of different influenza A virus subtypes) have been identified in the United States; most of these infections have been associated with exposure to pigs or attendance at an agricultural fair prior to illness onset. Agricultural fairs occur across the United States each year, primarily during the summer and early fall. Many fairs have swine barns, where pigs from different geographic locations come in close contact with each other and with people. These venues may allow influenza viruses to spread among pigs and between pigs and people. Infected pigs may spread influenza viruses even if they are not symptomatic (e.g., coughing or sneezing).
CDC anticipates that state health departments may identify more cases of infection with variant influenza viruses in 2022 as the agricultural fair season continues. Testing for variant influenza viruses should focus primarily on persons with exposures known to be associated with variant influenza virus infection (e.g., agricultural fair attendance or workers in the swine industry). Novel influenza A virus infections, which include those caused by variant influenza viruses, are notifiable conditions in the United States, and all confirmed cases should be reported to CDC within 24 hours.
