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Triple-Reassortant Swine Influenza A Viruses in the U.S.

From December 2005 through February 2009, viruses containing gene segments from swine, human, and avian influenza viruses infected at least 11 humans in the U.S.

Triple-reassortant swine influenza A (H1) viruses, which contain gene segments from avian, human, and swine influenza viruses, have been found in North American swine since the late 1990s. In a new report, investigators characterize the clinical and epidemiologic features of 11 sporadic cases of human infection with triple-reassortant viruses, detected through routine surveillance systems in the U.S., from December 2005 through February 2009. Human infections caused by novel influenza A viruses (including those that are nonhuman in origin) became nationally notifiable diseases in June 2007.

The patients were aged 16 months to 48 years (median, 10 years); 7 of the 11 were male. All lived in the Midwest or the South, and nine reported proximity or direct exposure to pigs. Person-to-person exposure was suspected in one case. Onset of illness occurred in August for four cases and from October through February for the others. For the seven patients whose time of proximity or exposure to pigs was known to within 1 day, the incubation period was 3 to 9 days (median, 3.5 days). Four patients had preexisting medical conditions. For the 10 patients with clinical information available, symptoms included cough (in 10), fever (9), headache (6), sore throat (6), and diarrhea (3). Four patients were hospitalized; two required mechanical ventilation. All 11 survived. Four were known to have received oseltamivir, often late in the clinical course. Influenza viruses were detected initially by rapid influenza testing, viral culture, or reverse-transcriptase PCR.

The CDC confirmed the viruses to be triple-reassortant swine influenza A (H1) viruses; 10 were H1N1, and 1 was H1N2. The influenza hemagglutinin genes originated from two phylogenetic lineages found in North American swine and differed from human seasonal influenza (H1) viruses by >100 amino acids. All 11 isolates were susceptible to both adamantanes and neuraminidase inhibitors.

Comment: This report documents that swine-origin triple-reassortant influenza viruses have been present in the U.S. for several years and have caused sporadic human infections. The swine-origin influenza A H1N1 virus now spreading throughout the U.S. and to numerous other countries can pass easily from person to person, a characteristic not observed with these earlier swine-origin viruses.

The cases detailed here were detected through passive surveillance systems and thus likely reflect only a fraction of infections. Although severe infections may be overrepresented in this series, the report shows that swine-origin viruses can cause serious illness in healthy individuals. In addition, the characteristics of such infections seem to differ somewhat from those of seasonal influenza (e.g., more diarrhea and a longer incubation period); further study is needed to assess this observation. Editorialists note the need to monitor the current situation and direct readers to an H1N1 influenza center from the New England Journal of Medicine and Journal Watch.

— Mary E. Wilson, MD

Published in Journal Watch Infectious Diseases May 7, 2009

Citation(s):

Shinde V et al. Triple-reassortant swine influenza A (H1) viruses in humans in the United States, 2005–2009. N Engl J Med 2009 May 7; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0903812)

Baden LR et al. H1N1 influenza A disease — Information for health professionals. N Engl J Med 2009 May 7; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMe0903992)