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A strain of swine influenza A (H1N1) virus is thought to have been responsible for the 1918 “Spanish flu” pandemic, which caused 20–40 million deaths over a 3-year period. Reemergence of related strains that cause human illness engenders fear among epidemiologists and infectious diseases specialists. In 1976, identification of four human cases of swine flu led to the vaccination of >40 million U.S. residents.
In a dispatch dated April 21, 2009, the CDC reported two cases of nonfatal respiratory infection caused by a swine influenza A (H1N1) virus. The patients — a 9-year-old girl and a 10-year-old boy — had no exposure to swine; no epidemiologic link between them has been identified except for residence in adjacent counties in Southern California. Neither child had received influenza vaccine during the 2008–2009 influenza season, and both recovered uneventfully, without specific antiviral therapy.
The viruses isolated from these children were genetically similar to each other. However, the combination of genes coding for neuraminidase and matrix proteins in these viruses had not been recognized previously in swine or human virus isolates in the U.S. Both isolates were resistant to the adamantanes; testing for susceptibility to neuraminidase inhibitors is under way.
Centers for Disease Control and Prevention (CDC). Swine influenza A (H1N1) infection in two children — Southern California, March–April 2009. MMWR Morb Mortal Wkly Rep 2009 Apr 21; 58d:1. (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0421a1.htm)
Comment
As of April 23, five additional cases of swine influenza in humans — three more in California and two in Texas — had been confirmed. None of these patients had had contact with swine.
Transmission of influenza virus from animal reservoirs to humans had long been postulated, because many human epidemics have originated in swine-raising areas of Asia. Now, such transmission is known to occur; evidence for it has recently been augmented by the finding that avian influenza virus can pass directly from birds to humans. The biggest concern, suggested by these cases, is that an animal influenza virus will adapt to human-to-human transmission, resulting in a major pandemic.