Influenza A (H3N2) Variant Virus Outbreak in Maryland, 2017

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Enhanced surveillance should extend to all regional fairs when swine influenza is detected at 1 fair.

Enhanced surveillance should extend to all regional fairs when swine influenza is detected at 1 fair, according to a recent publication in Morbidity and Mortality Weekly Report by the Centers for Disease Control and Prevention (CDC).

The Maryland Department of Agriculture was notified on September 17, 2017 by fair and 4-H officials of sick swine at an agricultural fair; an investigation revealed 5 swine with fever and signs of upper respiratory illness that subsequently tested positive for the influenza A (H3N2) virus. The next day, the Maryland Department of Agriculture was notified by the fair and 4-H officials that swine exhibitors were also ill. These events also occurred at 2 other agricultural fairs in Maryland.

A total of 76 people underwent testing for variant influenza via reverse-transcriptase polymerase chain reaction testing of nasopharyngeal swab specimens. A suspected case of variant influenza was defined as influenza-like illness, which included fever and cough or sore throat, in people with swine exposure £7 days before the onset of symptoms. The influenza A (H3N2) variant virus infection was identified in 40 patients with exposure to swine at the fairs. In these 40 patients, 75% of the infections occurred in individuals at high risk for serious influenza complications.

Further, 35% reported only indirect swine contact. This outbreak highlighted the risk for contracting variant influenza virus at agricultural fairs after direct or indirect swine contact.

The median incubation period from swine exposure to symptom onset was 2.5 days, with a range of 1 to 6 days. Fever (92.5%), cough (92.5%), and sore throat (40%) were the most commonly reported signs and symptoms. Of the infected patients, 8 reported receiving an influenza vaccination within the last year. Although there were no deaths, 2 children were hospitalized: 1 child with direct and the other with indirect exposure to swine.

In 2 of the fairs, viruses from 2 swine were isolated and whole-genome sequencing was performed. Analyses demonstrated that both human and swine viruses from the fairs were highly similar to each other in each gene segment (>99% identity). Results also showed that the Maryland influenza A (H3N2) variant and swine influenza viruses were similar to other swine exhibit-associated variant cases detected in other states in 2017.

In accordance with Maryland law and Maryland Department of Agriculture standard operating procedures for swine influenza, the correct steps were taken to successfully minimize transmission among swine, as well as from swine to humans. These steps included measures such as sending market swine to slaughter, quarantining non-market swine until 7 days after the last swine showed symptoms of influenza illness, and closing swine exhibits to the public. In addition, press releases, interviews, and a website were created to educate the public about risk, prevention, and treatment. Further, letters were sent to schools and childcare providers advising preventative measures.

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Overall, the study investigators concluded that, “Finally, this outbreak highlights the need for a One Health approach to investigating and responding to variant influenza virus outbreaks, including the application of both swine and human infection control measures, as well as collaboration between agricultural, environmental, and public health agencies on surveillance and communications strategies.”


Duwell MM, Blythe D, Radebaugh MW, et al. Influenza A(H3N2) variant virus outbreak at three fairs- Maryland, 2017. MMWR Morb Mortal Weekly Rep. 2018;67:1169-1173.