Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S. pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. The six genes from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses.[20] While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S.[21]
In April 2009, an outbreak of influenza-like illness (ILI) occurred in Mexico and then in the United States; the CDC reported seven cases of novel A/H1N1 influenza and promptly shared the genetic sequences on the GISAID database.[22][23] With similar timely sharing of data for Mexican isolates,[24] by 24 April it became clear that the outbreak of ILI in Mexico and the confirmed cases of novel influenza A in the southwest US were related and WHO issued a health advisory on the outbreak of "influenza-like illness in the United States and Mexico".[25] The disease then spread very rapidly, with the number of confirmed cases rising to 2,099 by 7 May, despite aggressive measures taken by the Mexican government to curb the spread of the disease.[26] The outbreak had been predicted a year earlier by noticing the increasing number of replikins, a type of peptide, found in the virus.[27]
