H3N2v Case Numbers Rising, But No Pandemic on the Horizon, Says CDC

Influenza Variant Is Mild, Shows Little Human-To-Human Spread

August 10, 2012 05:30 pm Matt Brown

The CDC is reporting a fivefold increase in the number of influenza A variant (H3N2v) virus infections confirmed since it reported case numbers on Aug. 3, but experts are far from jumping on the pandemic bandwagon.

The influenza A (H3N2) virus, shown here, caused illness in Indiana and Pennsylvania in 2011. As of Aug. 9 of this year, a variant of H3N2 was responsible for 145 confirmed infections in four states.

According to Joseph Bresee, M.D., a medical epidemiologist in the CDC's Influenza Division who spoke during an Aug. 9 press briefing(www.cdc.gov), case confirmations poured in during the week -- 145 H3N2v cases had been confirmed as of that date, up from 29 on Aug. 3. All confirmed cases have been associated with exposure to swine; most occurred in people who exhibit swine at agricultural fairs, he said.

However, Bresee noted, because the strain is a mild one, resulting in only two hospitalizations and no deaths to date, and the virus has spread mostly from pig to human, he and his colleagues are not unduly worried.

"At this point, there is no evidence that there is sustained human-to-human spread," said Bresee. "This is not a pandemic situation, but the CDC will continue to monitor what's going on."

Doug Campos-Outcalt, M.D., M.P.A., of Phoenix, chairman of the Department of Family, Community and Preventive Medicine at the University of Arizona College of Medicine, Phoenix, told AFFP News Now that although it is hard to predict what will happen, the CDC is ready for contingencies.

story highlights

  • The CDC reports a fivefold increase in the number of influenza A variant (H3N2v) virus infections confirmed since it reported case numbers on Aug. 3.
  • The cases all have been associated with exposure to swine, most often among people who exhibit swine at agricultural fairs.
  • Because the resulting symptoms are mild and there has been only limited human-to-human spread, the CDC is not forecasting a pandemic.

"This is the way pandemics start," Campos-Outcalt acknowledged, "but, on the other hand, most of these outbreaks don't lead to a pandemic. I think you have to be cautious not to be seen as continuously saying that the sky is falling, or people will stop paying attention.

"This is what flu people watch, and they do that as an attempt to get an early start in seeing antigens. So while it bears watching, there's no reason to panic."

Bresee said the CDC changed the reporting protocol for H3N2v on Aug. 6, allowing states to confirm their own flu cases before submitting specimens to the CDC for lab confirmation. Most of the new cases are coming from Indiana, which reported 113 confirmed infections as of Aug. 9. Ohio reported 30, while Illinois and Hawaii each have reported one confirmed case.

"We anticipate that the change in reporting requirements will allow for more real-time estimates," Bresee said. "For now, the CDC will update its counts every Friday, which means the states will have the most up-to-date case counts. Because of that, the numbers we report tomorrow may be different."

Substantially Different From Seasonal Flu

In a previous press briefing(www.cdc.gov), Bresee noted that although the seasonal influenza vaccine contains an H3N2 component, it is substantially different from the H3N2v virus. "We definitely want people to get vaccinated with this season's virus, because flu's right around the corner," he said. "But it's not a way to prevent infection with this virus."

An H3N2v vaccine candidate has been prepared and clinical trials are being planned for this year, Bresse noted, but there is no sense of urgency at this point from the CDC's perspective.

"The trigger for going forward with vaccine production clearly is more evidence that an A virus like this virus represents a pandemic threat," he said. "We're clearly not there yet."

Because all of the confirmed cases to date occurred in people -- mostly children -- who had direct or indirect contact with swine and most were associated with attendance at agricultural fairs where swine were present, the CDC continues to recommend preventive actions to cut down on exposures between pigs and humans.

Diagnosis, Management and Prevention

The CDC recommends that physicians and other health care professionals who suspect influenza in people with recent exposure to swine should

  • obtain a nasopharyngeal swab or aspirate, place the swab or aspirate in viral transport medium, and contact the state or local health department to arrange transport and request a timely diagnosis at a state public health laboratory;
  • consider reverse-transcription polymerase chain reaction (RT-PCR) testing for patients with influenza-like illness prior to the start of the traditional influenza season in October;
  • consider RT-PCR testing for influenza throughout the year for patients with influenza-like illness who report recent swine exposure and for those who can be epidemiologically linked to confirmed cases of H3N2v; and
  • consider antiviral treatment with oral oseltamivir or inhaled zanamivir in patients with suspected or confirmed H3N2v virus infection. Antiviral treatment is most effective when started as soon as possible after influenza illness onset.

Commercially available rapid influenza diagnostic tests may not detect H3N2v virus in respiratory specimens, the agency notes, so a negative rapid influenza diagnostic test result does not exclude infection with H3N2v or any influenza virus. In addition, a positive test result for influenza A cannot confirm H3N2v virus infection because these tests cannot distinguish between influenza A virus subtypes. Therefore, respiratory specimens should be collected and sent for RT-PCR testing at a state public health laboratory.

The CDC is encouraging people exposed to pigs to

  • wash hands with soap and water before and after exposure to animals,
  • avoid drinking, eating, or putting things in their mouths while in animal areas,
  • not take food or drink into animal areas, and
  • avoid close contact with animals that look or act ill.

Bresee said high-risk patients with conditions such as lung disease or diabetes should consider avoiding exposure to pigs in swine barns altogether, especially if sick pigs have been identified.

"If you develop flu symptoms following direct or close contact with swine and you see your medical provider, tell the doctor about this exposure," he said. "(He or she) can advise if testing or treatment is appropriate. Medications that have been effective in treating and preventing seasonal influenza infections are also likely to be effective in treating these variant virus infections."

Fortunately, H3N2v has not been shown to be transmissible to people through eating or proper handling of pork or pig meat or other products derived from pigs. "It's not a foodborne disease," Bresee said.

So how does a person at an agricultural fair identify a sick pig?

According to Lisa Ferguson, D.V.M., a veterinarian in the National Animal Health Policy Program office at the U.S. Department of Agriculture, pigs respond to flu viruses in much the same way humans do, often presenting with respiratory-type signs.

"So, if you see a pig that has a runny nose, that is coughing, they've got goop in their eyes, they're standing away from the rest of the pigs in the group, that would be a pig that looks sick," Ferguson said.