Background Triple-reassortant swine influenza A (H1) viruses— containing genes from avian, human, and swine influenzaviruses — emerged and became enzootic among pig herdsin North America during the late 1990s.
Methods We report the clinical features of the first 11 sporadiccases of infection of humans with triple-reassortant swine influenzaA (H1) viruses reported to the Centers for Disease Control andPrevention, occurring from December 2005 through February 2009,until just before the current epidemic of swine-origin influenzaA (H1N1) among humans. These data were obtained from routinenational influenza surveillance reports and from joint caseinvestigations by public and animal health agencies.
Results The median age of the 11 patients was 10 years (range,16 months to 48 years), and 4 had underlying health conditions.Nine of the patients had had exposure to pigs, five throughdirect contact and four through visits to a location where pigswere present but without contact. In another patient, human-to-humantransmission was suspected. The range of the incubation period,from the last known exposure to the onset of symptoms, was 3to 9 days. Among the 10 patients with known clinical symptoms,symptoms included fever (in 90%), cough (in 100%), headache(in 60%), and diarrhea (in 30%). Complete blood counts wereavailable for four patients, revealing leukopenia in two, lymphopeniain one, and thrombocytopenia in another. Four patients werehospitalized, two of whom underwent invasive mechanical ventilation.Four patients received oseltamivir, and all 11 recovered fromtheir illness.
Conclusions From December 2005 until just before the currenthuman epidemic of swine-origin influenza viruses, there wassporadic infection with triple-reassortant swine influenza A(H1) viruses in persons with exposure to pigs in the UnitedStates. Although all the patients recovered, severe illnessof the lower respiratory tract and unusual influenza signs suchas diarrhea were observed in some patients, including thosewho had been previously healthy.
Source Information
From the Influenza Division (V.S., C.B.B., T.M.U., B.S., A.B., X.X., S.L., L.V.G., V.D., R.J.G., A.K., J.S.B., L.F.), the Epidemic Intelligence Service Program (V.S., D.D.), and the Preventive Medicine Residency Program (V.S.), Centers for Disease Control and Prevention, Atlanta; the Iowa Department of Public Health, Des Moines (M.H., P.Q.); the Chicago Department of Public Health, Chicago (S.G., K.R.); the Michigan Department of Community Health, Lansing (S.V.); the Ohio Department of Health, Columbus (F.S.); the Texas Department of State Health Services, Austin (N.P.); the Minnesota Department of Health, St. Paul (K.M.); and the Illinois Department of Public Health, Springfield (C.C.). This article (10.1056/NEJMoa0903812) was published on May 7, 2009, and was last updated on May 22, 2009, at NEJM.org.
Address reprint requests to Dr. Finelli at the Centers for Disease Control and Prevention, 1600 Clifton Rd., NE Mailstop A-32, Atlanta, GA 30333, or at lfinelli{at}cdc.gov.
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