From Nature News Blog

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.

This article is reproduced with permission from the magazine Nature. The article was first published on Aug 10, 2012.