As County Fairs Reopen, CDC Warns of Swine to Human Influenza Cases

Five people in 4 US states in the 20-21 influenza season contracted influenza A virus variants commonly spread among pigs, but without person-to-person spread.

The CDC recently announced the fifth human infection of influenza A virus subtypes that are commonly spread among pigs, occurring in 4 US states during the 20-21 (October 2020 through September 2021) influenza season. There is no evidence of subsequent person-to-person spread.

“These types of infections occur in people rarely, and usually in the context of exposure to pigs, but are concerning because of their pandemic potential,” the CDC report indicates.

The 5 reported infections of the 20-21 season occurred in Wisconsin (one H3N2v [v designating variant] and one H1N1v), North Carolina (one H1N1v), Ohio (one H1N2v), and Iowa (one H1N2v).

As this summer county fair season gets underway, the CDC advises avoiding the close proximity to pigs that can occur in pig barns and livestock exhibits; noting that infected pigs can cough or sneeze droplets with influenza virus.

Prevention measures recommended by the CDC include not eating or drinking in proximity to areas with pigs, avoiding contact with pigs that appear to be sick, and washing hands often with soap and running water before and after exposure to pigs.

Influenza viruses from different species can infect pigs, according to the CDC report, where virus can reassort, swapping genes to produce new strains that are a mix of swine, human, and/or avian influenza viruses.

"This is thought to have happened in 2009 when a new H1N1 virus with genes of avian, swine and human origin emerged to cause a flu pandemic," the report notes.

In June of 2020, a surveillance study of swine influenza virus in China, published in the Proceedings of the National Academy of Sciences, described the emergence of a genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 virus which bears 2009 pandemic (pdm/09) and triple-reassortant (TR)-derived internal genes. The investigators find this influenza A variant to have been the predominant subtype in swine populations in China since 2016.

Similar to the pdm/09 virus, the investigators indicate, the G4 viruses bind to human-type receptors, proliferate in human airway epithelial cells, and have shown efficient infectivity and aerosol transmission in animal (ferret) studies.

"Of concern is that swine workers show elevated seroprevalence for G4 virus," the investigators observe."Controlling the prevailing G4 EA H1N1 viruses in pigs and close monitoring in human populations, especially the workers in swine industry, should be urgently implemented."

Since 2005, a total of 489 Influenza A virus variant subtype have been identified in the US, with some limited, non-sustained, person-to-person spread. Except for the 2009 H1N1 pandemic, there has not been ongoing community transmission. The CDC points out that seasonal flu vaccines are not formulated to protect against variant viruses; although the same influenza antiviral drugs used to treat seasonal flu can be effective against variant viruses.

"In general, the risk to the public from these infections is considered low, but each case of human infection with a variant influenza A virus should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in humans," the CDC report advises.