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Challenge of Surveillance of Swine H3N2 In Humans
Recombinomics Commentary 04:20
November 11, 2010

According to Onishchenko, the first of a new influenza detected in the seven-month baby in the U.S. in Illinois, the second - a 46-year-old man in Pennsylvania.

The above translation is from one of the many reports in Russia and Ukraine on the H3N2 triple reassortants in the United States.  The Pennsylvania case is noted in the upcoming MMWR report by the CDC.  The above reports suggest the same virus was detected in both patients and indirectly suggests both cases are recent (and the information was sent over the WHO paging system). 

If both cases are current, a new pandemic has almost certainly started, assuming the two cases are not closely linked.  Swine H3N2 jumps to humans are rare, so detection of two cases at distinct locations indicates then number of actual cases is orders of magnitude greater than the two reported cases.

Detection of H3N2 swine reassortants is difficult, because unlike pH1N1, the H and N in swine triple reassortants are human.  Consequently, assays that test for influenza A or subtype for human H3 would not distinguish swine H3N2 from seasonal H3N2.  Seasonal H3N2 has been widely reported, but sequence data has been limited, and sequences from the above two cases have not been released.  However, it is likely that the virus has three human flu genes (PB1, HA, NA), two avian gene segments (PB2 and PA), and three swine gene segments (PA, MP, NS).
Distinguishing swine H3N2 triple reassortants from seasonal H3N2 will be a challenge.  Many reporting sites still use a rapid test to indentify influenza A, which is why many isolates in the CDC weekly report are “influenza A, sub-typing not performed”.  The more advanced sites do sero-typing to distinguish between seasonal H1 and seasonal H3.  However, the seasonal H3 test will be positive for swine H3 as well as seasonal H3. 

To distinguish between swine H3N2 and seasonal H3N2, a more specific test, like sequencing is required, which is why many of the seasonal H3N2 samples already reported worldwide may include swine triple reassortants.

Thus, the extent of swine H3N2 spread in humans in the United States and worldwide is unclear.  Comprehensive screening and surveillance will be a challenge.  Moreover swine H3N2 in humans will accelerate the change in swine H3N2 as well as swine H1N1 in humans and swine and the combination of serotypes will target the young and old.

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