Dec 9, 2011 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) today confirmed two more infections with novel flu viruses, in children in Minnesota and West Virginia, only one of which involves the novel H3N2 strain found this year in four other states.
Though both of the viruses have been detected in US pig populations, investigations so far haven't revealed any connections between the two kids or their close contacts to pigs, which could signal limited human-to-human transmission, the CDC said in a statement today. Both of the children have recovered, the agency said.
The infection in the West Virginia child involved a swine-origin H3N2 reassortant strain that includes the M gene from the 2009 H1N1 pandemic virus, raising the number of such cases detected in the United States so far to 11. At the end of November, Iowa reported that the virus infected three children who had contact with each other but no known swine exposure, making theirs the first known cases to involve likely human-to-human spread.
The CDC said the virus is distantly related to human H3N2 viruses that circulated in the 1990s, which suggests that adults may have some protection against it. These factors might explain why 10 of the 11 cases so far have been reported in children.
Toby Wagoner, spokesperson for the West Virginia Department of Health and Human Resources, told CIDRAP News that state health officials have increased their surveillance for flu-like illnesses and are reminding the public about the importance of basic flu prevention methods, such as covering coughs and sneezes, observing proper hand hygiene, and staying home when sick.
Minnesota's novel flu case involves an H1N2 virus that circulates in swine but doesn't usually infect humans, the CDC said, adding that it is only the second novel H1N2 case reported since 2007, when novel flu infections became nationally notifiable. The first case was identified in Michigan in 2007.
The agency said some of the virus's characteristics are similar to a seasonal human H1N1 virus (A/New Caledonia/20/99-like) that circulated as recently as 2007, and that people who were exposed to the latter may therefore have some protection against the novel Minnesota strain.
Since 2005, 33 human infections with swine-origin flu viruses have been reported in the United States, the CDC said. Among those cases, 12 were triple-reassortant H1N1, 19 were triple-reassortant H3N2, and two involved triple-reassortant H1N2. All of the patients, who included 25 children, recovered from their illnesses.
In 24 cases, people had direct or indirect exposure to pigs before they got sick. Investigations of other cases have suggested likely transmission from close contacts, but none have led to sustained human-to-human transmission, according to CDC background information on human infections with swine-origin flu viruses.
Though human infections with novel viruses typically found in swine are rare, the CDC said detections have become more frequent, for which there are three possible reasons: Improvements in lab testing for flu viruses since the 2009 H1N1 pandemic may be causing identification of viruses that wouldn't have been detected before, flu surveillance has increased as the nation enters its winter flu season, or the findings could signal a true increase in the number of cases from infected swine or limited human-to-human exposure.
The CDC continues to recommend the flu vaccine to prevent seasonal flu but said it isn't likely to protect against viruses that circulate in pigs. It said tests on the two novel viruses show that both are susceptible to the antiviral medications oseltamivir (Tamiflu) and zanamivir (Relenza). The agency reiterated that flu isn't transmissible through eating properly handled and prepared pork products.
It advised people who seek medical attention for flu symptoms after direct or close contact with pigs to mention the exposure to healthcare providers.
In other flu developments, the CDC said today that seasonal flu activity in the US remained at low levels last week. The percentage of respiratory samples that tested positive for flu dipped slightly to 1.7%, and the percentage of doctor's visits for flu-like illnesses was 1.2%, below the national baseline.
The number of deaths from flu and pneumonia increased slightly, but was still below the epidemic threshold. The CDC received reports of two pediatric flu deaths, but they occurred during the 2010-11 season, raising the total to 122. So far no pediatric flu deaths have been reported for the current season.
Lab analysis of 36 flu viruses showed that the H3N2 and 2009 H1N1 isolates closely matched the seasonal flu vaccine components. Of 10 influenza B strains that were characterized, 6 belonged to the Victoria lineage vaccine component, while 4 belonged to the Yamagata lineage. The CDC said it is too soon in the season to determine how well the seasonal flu vaccine and circulating strains match.
Only one state, Virginia, reported local flu activity. The number of states reporting sporadic flu activity rose to 30, an increase of 2 from the previous week.
Dec 9 CDC "Have you heard?" statement
Dec 9 CDC flu surveillance update
CDC background on human swine-origin flu virus infections