After a lull of several months, an influenza virus that is sporadically jumping from pigs to people in the United States has made another appearance.

U.S. public health officials have reported a new human infection with the swine-origin H3N2 virus — officially called H3N2v (for variant) virus. The case is a young girl living in Utah; she is the 13th person known to have been infected with this new virus since it was first spotted last July.

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A scanning electron microscope image depicts the H3N2 flu virus that has infected 13 people in the U.S. since last July. (Centers for Disease Control and Prevention/Canadian Press)

Twelve of the 13 cases have been children under 18. In this case, state officials are asking that the girl's precise age not be revealed.

She was taken for medical care because of a fever in late March. When she tested positive for influenza she was given the flu drug oseltamivir, or Tamiflu, and has since recovered. Members of her family and close contacts were tested for flu, but no additional cases were found.

The girl is believed to have become infected when she visited a swine processing plant in the week before she became ill, Dr. Michael Jhung of the Centers for Disease Control said in an interview.

This is the first of these cases seen in Utah, and the farthest west this virus has been spotted. Previous human cases have been reported in Indiana, Pennsylvania, Maine, Iowa and West Virginia.

About half of the cases had some exposure to pigs. But the rest did not and the CDC has acknowledged some limited person-to-person spread of the virus has likely taken place in some of these infections.

Utah is not a major pork producing state. In fact, it ranks 27th in hog production, according to the U.S. National Pork Producers Council. But the processing plant the child visited handled pigs from other states, Jhung said. He declined to specify which ones.

The new H3N2v case is the first spotted since November.

Flu experts are keeping a close eye on this virus, which contains a gene from the 2009 pandemic H1N1 virus; that gene, the M gene, is believed to enhance the ability of flu viruses to infect people.

It remains unclear what kind of a threat the swine H3N2 — a distant cousin of the human H3N2 virus — poses to people.