Infections due to the Influenza A (H3N2) variant virus have soared from 29 cases last week to 145 this week, according to the Centers for Disease Control and Prevention.
"This increase is partly based on the change in reporting requirements ... but in fact, the increase reflects accurately what is going on in these outbreaks," Dr. Joseph Bresee said during a telephone press conference held by CDC.*
Dr. Joseph Bresee
There have been 145 confirmed cases of Influenza A (H3N2) variant infection, as of 1 p.m. EDST on Aug. 9. So far there have been 113 cases in Indiana, 30 cases in Ohio, 1 in Illinois, and 1 in Hawaii.
On Aug. 6, the CDC provided guidance to state laboratories and is now allowing states to confirm their own H3N2v cases, prior to laboratory confirmation at CDC.
"We’ve been finding that cases that were positive at the state level were overwhelmingly being confirmed also at CDC," said Dr. Bresee, who is a medical epidemiologist with the CDC’s influenza division.
"Given this, and in the context of an outbreak situation, with very little seasonal influenza circulating, we felt that it was appropriate for states to begin reporting their positives as confirmed cases rather than waiting for CDC confirmation," he said. "We anticipate that the change in reporting requirements will provide for a more real-time indication of how these outbreaks are evolving in the states."
Positive samples will still be forwarded to the CDC, where these will be confirmed using genetic sequencing.
The CDC will begin updating case counts every Friday, based on information provided by the states. States will have the most up-to-date numbers on other days.
Future issues of Morbidity and Mortality Weekly Report (MMWR) will provide an update on the numbers of cases and will provide some information about the effectiveness of rapid influenza diagnostic tests in detecting these viruses.
"The severity of human illness associated with this virus continues to resemble that of seasonal flu. Most of the cases are mild, self-limited, and resolve on their own," Dr. Bresee said.
CDC has not received any reports of deaths associated with the virus and only two hospitalizations.
Importantly, there is no evidence of sustained human-to-human spread in the community. "This is not a pandemic situation," Dr. Bresee said. However, "these viruses are all the same. They’re not completely genetically identical, but they’re very close to being so. All of the viruses that we’re seeing so far, in this latest increase in cases, are the viruses with the M gene."
The M gene comes from the human influenza (H1N1)pdm09 (2009 H1N1) virus and may confer increased transmissibility to and among humans, compared with other variant influenza viruses.
Most of the cases have involved contact or exposure to swine prior to illness onset, and many have been associated with state agricultural fairs, where swine were present.
Signs and symptoms of H3N2v virus infection are similar to those caused by other respiratory infections, including seasonal influenza virus infection. If H3N2v virus infection is suspected because of recent exposure to pigs, testing of respiratory specimens should be performed at a state health department; rapid influenza diagnostic tests may not detect H3N2v virus in human respiratory specimens, resulting in false negative results.
Two antivirals – oseltamivir (Tamiflu and zanamivir (Relenza) – are expected to be effective for treating H3N2v illness. Antiviral treatment is most effective when started as soon as possible after illness onset, according to the CDC.
A vaccine is under development and clinical trials are planned for later this year.
Influenza viruses have not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs.
For information on the prevention of infection with the H3N2v virus, see the CDC’s factsheet.
*CORRECTION:10/8/2012 Dr. Joseph Bresee's name was misspelled in the original version of this story.