(HealthDay) -- U.S. health authorities on Thursday reported a large jump in the number of H3N2 "swine" flu cases in humans.
Last week, only 29 cases had been reported since July of 2011, when the virus first emerged. Sixteen of the cases appeared in the past month.
But this week, the tally soared to 146 cases, one each in Hawaii, Utah and Illinois, 113 in Indiana and 30 reported from Ohio.
This represents "clearly a significant increase," Dr. Joseph Bresee, of the U.S. Centers for Disease Control and Prevention's Influenza Division, said during an afternoon news conference.
Authorities have seen no human-to-human transmissions yet this year -- all people infected had had some contact with swine, usually at agricultural fairs -- but they are nevertheless concerned about the virus because it contains an element seen in the pandemic 2009 swine flu strain, H1N1, which may make it more likely for the virus to spread from person to person.
Also, the latest cases contain a variant gene that may confer increased transmissibility to and among humans, compared to other variant influenza viruses.
Some of the initial 12 cases from 2011 did not have any obvious swine exposure, Bresee added.
Numbers are likely to jump again tomorrow, when the CDC posts updated numbers on its website, Bresee noted.
In fact, a representative from CNN Medicine said during the news conference's question-and-answer period that Indiana health authorities had just this morning updated their figures to 120 cases of swine flu, bringing the total to 153.
Greater awareness of and testing for the virus along with the proliferation of agricultural fairs around the country during the summer are likely also contributing to the increase, Bresee said.
Bresee stressed that this is "not a pandemic situation."
The severity of the illness resembles that of the seasonal flu, with most cases being mild and resolving on their own. More than 90 percent of cases have occurred in children who not only have extensive exposure to swine they may have raised for exhibit but who also have less immunity to the virus.
Two people of unknown age were hospitalized but are now recovering at home, Bresee said.
To prevent contracting this flu, the CDC advises people to limit their contact with swine and avoid contact with sick swine. People who have contact with these animals should take precautions such as washing their hands, not eating or drinking in areas with swine and controlling their cough.
The virus is responsive to currently available prescription antivirals and an H3N2 candidate vaccine has been prepared and clinical trials are being planned for this year.
More information: For more on H3N2 flu, visit the U.S. Centers for Disease Control and Prevention.