Seven cases of a unique type of swine flu in humans  have U.S. health officials concerned whether it is linked to a strain responsible for more than 130 cases of severe respiratory illness in Mexico.

The five individuals in California and two in Texas have all recovered, and testing indicates that antiviral medications seem to work against the virus.

But federal Centers for Disease Control and Prevention (CDC) officials worry because none of the seven people were in contact with pigs, the normal route of contracting swine flu, and only a few were in contact with each other.

Anne Schuchat of the CDC says officials believe the virus can spread human-to-human, which is unusual for a swine flu virus.

It’s expected that further investigation of the seven cases, which all occurred in March-April, will turn up more cases of the virus, according to Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases.

CDC officials say the swine virus presents a unique combination of gene segments that have not been seen in people or pigs before. The virus segments are from human virus, avian virus from North America and pig viruses from North America, Europe and Asia.

The virus was first detected in two children in southern California – in a 10-year-old boy in San Diego County and in a 9-year-old girl in neighboring Imperial County.

The swine flu’s symptoms mirror those of regular flu, mostly involving fever, cough and sore throat, with some of those affected experiencing vomiting and diarrhea.

CDC officials say these cases don’t yet represent an outbreak, although it’s not known if anyone is getting sick from the swine flu virus right now.

As a precaution, CDC is preparing the virus as a vaccine seed strain that could be used to make immunizations, Schuchat says.

“We haven’t seen this strain before, but we haven’t been looking as intensively as we have these days,” she remarks. “It is very possible that this is something new that hasn’t been happening before.”

Swine flu is a respiratory pathogen normally spread among pigs, but can spread to humans, and human-to-human transmission has previously been documented, according to the CDC.

To learn more, go online to Centers for Disease Control and Prevention.