Swine influenza in humans: update from Iowa State (United States)Archived

This Public Health Development is an update of an earlier Public Health Development published by ECDC on 28th October 2011 (1). It summarizes recent reports of infections of humans with swine-origin influenza viruses in the State of Iowa, in the United states, involving swine origin triple reassortant A(H3N2) influenza virus subtypes so called S-OtrA(H3N2). These American cases have been described and published on the CDC publication MMWR, in its rapid edition of November 23rd, 2011 (Vol. 60 / Dispatch). Though not a lot has changed ECDC following consultations will publish a Rapid Risk Assessment on the topic to inform those in Member States about the public health implications of these recent findings in the United States.   

Limited Human-to-Human Transmission of Novel Influenza A (H3N2) Virus — Iowa, November 2011 Download Issue 

CDC Morbidity and Mortality Weekly Report (MMWR), November 23, 2011 / Vol. 60 / Dispatch

This report describes the investigation of three cases of swine-origin triple reassortant influenza A (H3N2) (S-OtrH3N2) infections, confirmed by the Centers for Disease Control and Prevention (CDC) in Atlanta on November 20th, 2011, in children in two counties in Iowa state (US). None of the children were hospitalized, and each has recovered from a mild episode of febrile respiratory illness. All three were in contact with one another and none had a known recent exposure to swine. No additional human infections with this virus have been detected in Iowa, and no evidence of sustained human-to-human transmission of this S-OtrA(H3N2) virus has been found, although surveillance is ongoing. The report notes that eighteen human infections with swine-origin influenza A (H3N2) viruses have been identified since 2009. The most recent 10 cases, including the three Iowa cases described in this report, were infections with S-OtrH3N2 viruses containing the matrix (M) gene from the pandemic 2009 influenza A (H1N1) virus (pH1N1). These viruses are considered re-assortant viruses between a swine-origin influenza A (H3N2) virus circulating in North American swine and a pH1N1 virus. The report also notes that all cases of human infection with S-OtrH3N2 virus containing the M gene from the pH1N1 virus have occurred in 2011 and have been reported from four states: Pennsylvania (three cases), Maine (two), Indiana (two), and Iowa (three).

Epidemiologic and laboratory investigations carried out by the Iowa Department of Public Health (IDPH) revealed that the families of the three children reported no recent travel or attendance at community events where they might have been in touch with pigs. A possible common epidemiological link is attendance at a gathering of children the same day as the first children showed symptoms, and no illnesses were reported among adults or among the five other children who were present at this gathering on that day. In addition, no exposures to pigs have been identified among adults or children attending this gathering. Despite the enhanced surveillance for Influenza Like Illness (ILI) surveillance implemented by IDPH, no indications of signals of sustained community transmission have been detected so far and surveillance data from the state have shown low levels of influenza activity currently and at the time of all these patients' illnesses; thus there is only limited human-to-human transmission.

At CDC laboratories, preliminary rRT-PCR diagnostic results were inconclusive but indicated probable infection with a swine-origin influenza A (H3N2) virus. Subsequent complete genome sequencing at CDC confirmed all three specimens as S-OtrH3N2 with the M gene from the pH1N1 virus. The viruses from these three patients are resistant to amantadine and rimantadine but are expected to be susceptible to the neuraminidase inhibitor drugs oseltamivir and zanamivir based on their genetic sequence. Because these viruses carry a unique combination of genes, little information currently is available regarding the capacity of this virus to transmit efficiently in swine, humans, or between swine and humans.

ECDC Comment (24th November 2011): It is important to note here that the viruses circulating in pigs in North America though  endemic there are different from those reported in pigs in Europe. In America, A(H3N2) viruses predominate as triple re-assortant (TRA) viruses, whereas in Europe pigs are more commonly infected by Eurasian A(H1N1) swine viruses (2). Reports of swine-origin viruses in humans in North America occur most years as there is active case ascertainment for atypical viruses in humans encouraged by the United States CDC (3). In contrast, reports of swine influenza virus infections in humans in Europe are rare with only two published reports since 2008 (one in Spain in 2009 and one in Germany in 2011) (4-6). It is unclear whether human cases are that rare in humans in Europe around large pig herds. Since the discovery of both European cases in humans was almost accidental, the suggestion is that they may not be as rare as they seem. Given the experience of the emergence of the 2009 pandemic from pigs in Mexico there must be a strong public health case for more active active ascertainment for human infections with swine viruses in Europe (5). Because of this ECDC and the Community Network of Reference Laboratories for Human Influenza in Europe (CNRL) have been working together since July 2011 to ensure there is test capability to detect these viruses in Europe. Moreover the European Food Safety Authority (EFSA) are engaged in work with CDC and other stakeholders to determine the way in which these viruses might be assessed for their pandemic potential.