Recent cases prompt CDC updates on swine-related flu variants

Jan 9, 2012 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) recently updated some of its background documents on swine-origin flu viruses in humans, based on the latest information it has learned about recently detected novel flu infections.

The CDC released the updated information, aimed at both the laypeople and medical audiences, on Jan 6. The information covers key facts about swine-origin variant viruses and infections involving the strains in humans, particularly the H3N2 type that has so far infected 12 people in five different states, most of them children.

Some of the patients had contact with swine, but other cases likely reflect limited human-to-human transmission, the CDC has said.

The new virus is a triple-reassortant that has acquired the M gene of the 2009 H1N1 virus. The CDC and many of its global partners are referring to the new strain as an H3N2 variant and are using the H3N2v abbreviation.

The CDC has said it's unclear why the number of detections in humans is increasing. It said factors could include better lab capacity to detect novel viruses, changed domestic and international novel flu virus reporting requirements, or a true increase in the number of cases from exposure to swine or through limited human-to-human transmission.

Though there's no evidence of sustained transmission, the changing nature of flu viruses means that circulation may become widespread, according to the CDC.

So far the severity of H3N2v infections has been similar to seasonal flu, the CDC said. The symptoms of variant flu infections also mirrors seasonal flu, though some people infected by novel viruses have reported other symptoms, including runny nose, sore throat, eye irritation, nausea, vomiting, and diarrhea.

Limited serologic studies suggest that adults, but not children, may have some pre-existing immunity to the new virus, according to the CDC. H3N2v is susceptible to oseltamivir and zanamivir, but not the other antivirals, amantadine and rimantadine.

"CDC is closely monitoring human infections with all novel influenza viruses, including H3N2v viruses, and will provide more information as it becomes available," the agency said in its background information.

It said each case of human infection with a swine influenza virus should be fully investigated to ensure that viruses are not spreading efficiently or in a sustained manner.

On Dec 23, the CDC confirmed an infection with an H1N1 variant (H1N1v) that had never been reported in humans before. The patient is an adult from Wisconsin who had occupational contact with swine and has recovered. That virus is also a triple-reassortant that acquired the M gene of the 2009 H1N1 virus.

Infections with a swine-origin H1N2 variant virus have also been reported in the United States, most recently last month in a patient from Minnesota, only the second such case to be reported since 2007. That virus is similar to a seasonal H1N1 strain that circulated as recently as 2007.

Since 2005 the CDC has received reports of 35 human infections with variant flu viruses.

See also:

Jan 6 CDC updated information about human infections with variant viruses

Jan 6 CDC background information on variant influenza viruses in humans

Jan 6 CDC updated information on H3N2v viruses

Dec 23, 2011, CIDRAP News story "CDC reports two more novel flu infections" (includes H1N1v)

Dec 9, 2011, CIDRAP News story "CDC reports two more novel flu cases" (includes H1N2 case)

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