07/26/2012 12:36 EDT | Updated 09/25/2012 05:12 EDT

New swine flu virus re-emerges in U.S.; four people infected at county fair

A new flu virus that is occasionally jumping from pigs to people has done it again.

The U.S. Centers for Disease Control is reporting that the new swine influenza H3N2 virus has infected four people who attended a county fair in Indiana.

Indiana is one of six U.S. states in which human infections have been found since this variant H3N2 virus was first spotted at the end of last July.

The four latest cases were all people who had contact with pigs; they became infected in early-to-mid July after exhibiting pigs at a county fair in the northwestern part of the state.

The CDC says 12 pigs from the fair were tested for the virus and all were positive.

Dr. Michael Jhung of the CDC says while state health officials are still looking for additional cases, to date there doesn't seem to have been human-to-human spread in this cluster of infections.

Including these cases, there have been 17 human infections with this virus, all in the United States.

The infections have mainly been seen in children. Many, though not all, had exposure to pigs. In some of the earlier cases, the CDC acknowledged limited person-to-person spread had likely occurred.

The new Indiana cases came to light when two of the people went to a hospital emergency department because of their illness. None of the four was admitted to hospital and all have recovered.

With the start of the agricultural fair season, health officials in the U.S. are on the lookout for more cases, says Jhung, an epidemiologist with the CDC's Influenza Division.

"We're trying to reach out through animal health partners and public health partners to people who organize fairs, people who attend fairs and send them the typical (flu) prevention messages," Jhung said in an interview from Atlanta.

"We're asking them to pay special attention to handwashing and not eating or drinking in areas where there are animals and to avoid contact with sick animals."

The virus is a distant cousin of the human H3N2 viruses that circulate every flu season. In fact, the swine virus originates from the human one; it was passed from people to pigs years ago and has circulated among swine populations ever since.

At some point after the 2009 H1N1 pandemic, the swine virus picked up a gene from the pandemic virus. Scientists believe this gene, the M gene, enhances the virus's ability to move from pigs to people.

It remains unclear what kind of a threat the swine H3N2 poses to people.

Research done by CDC scientists suggests children under 10 are probably the most vulnerable to the virus. Older people probably have some antibodies that would protect against it, that work suggests.

But Canadian research points to a sharp drop in protective antibodies in people over the age of 40.