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US swine flu outbreak spikes

Today the US Centers for Disease Control and Prevention (CDC) announced that the number of reported cases in an ongoing outbreak of a strain of the H3N2 animal influenza virus (H3N2v) that transmits between pigs and humans has jumped to 145 in the past week.


On 3 August, CDC officials reported 16 total cases of H3N2v infection. In all cases, patients interacted with pigs either in their occupation or at local agricultural fairs, suggesting that the virus has not yet evolved the ability to efficiently transmit between humans. CDC first reported the variant in a 12-case outbreak from July to December 2011, with two instances of suspected weak human-to-human transmission. This week’s surge may be partially due to a change in protocol: states can now confirm positive test results before further CDC testing. But, CDC influenza division chief Joseph Bresee warned in a press teleconference today, “We’re seeing a big increase, and we think it’s a real increase.”

The outbreak covers four states, with one case in Hawaii, one in Illinois, 30 in Ohio and 113 in Indiana. Indiana’s count rose to 120 today, according to the state’s health department, and Bresee expects a continued increase in the coming weeks. The symptoms are mild and similar to those of seasonal flu, and so far the outbreak has resulted in only two hospitalizations. More than 90% of the cases have been in children — probably because many adults may have antibodies that can handle the strain.

Virologists suspect that the H3N2v strain arose from swine flu strains exchanging genetic material in a process called reassortment. What raises eyebrows is that the H3N2 virus carries a gene found in the H1N1 strain that caused a swine flu pandemic in 2009. This matrix, or M, gene may influence transmissibility. If so, the current strain offers an opportunity to study how transmission evolves over the course of an outbreak. “This is still unfolding, and we have a lot to do to understand both the biology in the natural host, pigs, and the sporadic human infections,” says Ruben Donis, a virologist with the CDC’s influenza division.

According to Bresee, the seasonal flu vaccine probably won’t be affective against the strain, but it could be a reason that adults are less susceptible. A vaccine candidate for H3N2v is slated for clinical trials this year.

Image credit: CDC


  1. Report this comment

    Henry Niman said:

    The 2012 sub-clade is novel and matches the 2011 cluster at the West Virginia day care center, which was linked to 23 ILI cases.

  2. Report this comment

    Henry Niman said:

    The CDC has released a series of sequences from July, 2012 cases from a farmer in Maui, Hawaii (A/Hawaii/03/2012), 4 cases from the LaPorte County Fair (A/Indiana/06/2012, A/Indiana/07/2012, A/Indiana/08/2012, A/Indiana/09/2012), 1 case from the Jackson County Fair (A/Indiana/12/2012), and 1 case from the Butler County Fair (A/Ohio/13/2012). All isolates were closely related and had an NA which matched NA in swine H3N2, which was distinct from the first 10 reported human cases in 2011 (which had an NA matching a swine H1N2 lineage).
    The novel sub-clade in the July cases matched the sequences from the 2011 West Virginia day care center cluster (A/West Virginia/06/2012 and A/West Virginia/07/2012) which had no swine contact or exposure and 23 contacts of teh index case had ILI. This sub-clade was also identified in the first 2012 H3N2v case, A/Utah/20/2012.
    These data highlight a novel sub-clade circulting in humans, and was only identified in 2 swine isolates prior to the July cases.

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