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Human cases of influenza A (H3N2) variant appear on the rise
CDC. MMWR. 2011;60(early release):1-4.
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In the last 4 months of 2011, the CDC received reports of 12 cases of human infections with influenza A (H3N2) variant viruses that have the matrix gene from swine-origin influenza A and pandemic influenza A 2009 viruses, according to a recent report from the CDC.
Only 35 cases have been reported in the United States since 2005, but the frequency with which they have been detected increased in 2011, according to an early-release Morbidity and Mortality Weekly Report.
“When different influenza viruses simultaneously infect a single host (eg, a human or swine), exchange of genetic material can occur, resulting in a new influenza virus,” the MMWR report said. “Depending on the antigenic distance between the new virus and recently circulating seasonal viruses, little or no immunity might exist in the human population. Influenza A (H3N2)v viruses resulted from reassortment of influenza A (H1N1)pdm09 viruses with swine influenza A (H3N2) viruses.”
The 12 cases occurred in Indiana, Iowa, Maine, Pennsylvania and West Virginia. Eleven of the patients were children, and six of the 12 patients reported no recent exposure to swine.
The most recently reported cases occurred in an adult man from Indiana with occupational exposure to swine and two children in West Virginia who regularly attended the same day care. Three of the 12 patients were hospitalized, and all have recovered fully.
The adult patient reported direct contact with swine during his work in the week before illness onset and said he did not wear any personal protective equipment because the swine did not exhibit signs of illness.
The cases in the two children from West Virginia who attended the same day care are likely unrelated because the symptom onset dates are more than 10 days apart.
“This represents a scenario of limited human-to-human transmission occurring in a day care setting,” according to the report in MMWR. “Therefore, clinicians also should consider the possibility of influenza A (H3N2)v infections in patients who have not had exposure to swine, particularly young children in those states where influenza A (H3N2)v cases have been reported.”
Nonhuman influenza virus infections rarely result in human-to-human transmission, but the implications of sustained ongoing transmission between humans is potentially severe. For this reason, according to the CDC, prompt and thorough identification and investigation of sporadic human infections with novel influenza viruses are needed to reduce the risk for sustained transmission.
According to the MMWR report, swine-origin influenza viruses identified in humans are now referred to as “variant” viruses and denoted with a “v.” Influenza viruses identified in swine populations will continue to be referred to as “swine influenza” viruses.
This change in nomenclature follows an announcement by WHO of a decision to standardize nomenclature for the pandemic influenza A (H1N1) 2009 virus, which has had diverse names, as influenza A (H1N1)pdm09.
Per the new naming convention, these H3N2 viruses will now be referred to as “influenza A (H3N2) variant viruses with genes from avian, swine and human viruses” and will be abbreviated as “A(H3N2)v” for scientific use and “H3N2v” for general public use. The 12 patients mentioned in the MMWR report with A(H3N2)v infections also have the M gene from the A(H1N1)pdm09 virus.
This change is a result of discussions among WHO, the World Organization for Animal Health, the Food and Agriculture Organization, CDC and other US agencies.
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