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True swine influenza in humans: recent different findings in the United States and Europe (Germany)

28 Oct 2011
This Public Health Development describes some recent episodes of proven infection of humans with swine influenza viruses, both in America – involving A(H3N2) subtypes – and in Europe (Germany) – involving A(H1N1) subtypes. The American cases have been well described and published on the CDC publication MMWR. The European cases, from Germany, have been published in German in the Epidemiological Bulletin of the Robert Koch Institute (RKI).

Swine-Origin Influenza A (H3N2) Virus Infection in Two Children --- Indiana and Pennsylvania, July--August 2011
CDC Morbidity and Mortality Weekly Report, September 9, 2011 / 60(35); 1213-1215

This report describes the investigation of two cases of febrile respiratory illness caused by swine influenza A (H3N2) viruses identified on the 19th and the 26th August 2011 in two different states in the US (Indiana and Pennsylvania). No epidemiological link between the cases were identified, and no additional confirmed human infections with this virus were detected around the cases despite extensive field investigations. These viruses are similar to eight other swine-origin influenza A (H3N2) viruses identified from previous human infections over the past 2 years, but are unique in that one of the eight gene segments is from the 2009 influenza A (H1N1) pandemic virus. Hence, these viruses are "re-assortants" i.e. they contain genes of the swine-origin influenza A (H3N2) virus circulating in North American pigs since 1998 and the 2009 influenza A (H1N1) pandemic virus that might have been transmitted to pigs from humans during the 2009 influenza A(H1N1) pandemic.  The authors note that re-assortments of the 2009 influenza A (H1N1) virus with other swine influenza A viruses have been reported previously in swine, and particularly in the US.

Surveillance data from both states showed low levels of influenza activity at the time of both patients' illnesses. Preliminary genetic characterization of these two influenza viruses identified them as swine-origin influenza A (H3N2) viruses. Full genome sequences were posted to publicly available web sites and, although the viruses were similar, they were not identical. As already mentioned, seven of the eight gene segments were similar to those of swine A(H3N2) influenza viruses circulating among U.S. pigs since 1998 and previously identified in the eight other sporadic cases of human infection with swine-origin influenza A (H3N2) viruses in the United States since 2009. The one notable difference from the viruses previously identified in human infections with swine-origin influenza A (H3N2) virus is that these two viruses have a matrix (M) gene acquired from the 2009 influenza A (H1N1) virus, replacing the classical swine M gene present in prior swine-origin influenza A (H3N2) virus infections in humans.

The article goes on mentioning that, although re-assortment between swine influenza and 2009 influenza A (H1N1) pandemic viruses has been previously reported in pigs in the United States, this particular genetic combination of swine influenza virus segments is unique and has not been reported previously in either swine or humans. Neither of the cases reported exposure to pigs through both were in localities where pigs were found and the authors closed their report with noting the need for close surveillance for atypical viruses like these causing human infections.

Germany – Human cases of infection by swine influenza virus
National Reference Centre for Influenza (NRZ), Robert Koch Institute (RKI), 2nd Sept 2011 (in German)

The Lower Saxony state health department (NLGA) and the Department of Infectious Disease Epidemiology at the RKI have recently informed and published on their website that in a swab sample of a 18-month-old boy from Lower Saxony a swine influenza virus A(H1N1) was identified. The genetic characterization of the HA, NA, NP and NS genes showed that the virus is typical of the A(H1N1) Eurasian swine lineage unrelated to the A(H1N1) 2009 pandemic virus or the earlier seasonal A(H1N1) virus. The  virus is known to be circulating among European pig herds for in an endemic fashion.

ECDC Comment (10 October 2011):

It is important to note here that the viruses circulating in pigs in North America are quite different from those reported in pigs in Europe. A(H3N2) viruses predominate in North America as triple reassortant viruses while in Europe pigs are more commonly infected by Eurasian A(H1N1) swine viruses.(1,6)  Reports of swine-origin viruses in humans in North America occur most years and there is active case ascertainment for atypical viruses in human encouraged by the United States Centers for Disease Control and Prevention.(2)    In contrast reports of swine viruses in humans in Europe are rare with only two published reports since 2008 (one in Spain) including this case in Germany.(3-5)  What is unclear is 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 case for more active American-style active ascertainment  for human infections with swine viruses in Europe.(4)  Finally it should be noted that human influenza viruses from the 2009 pandemic may be circulating in pig herds in Europe. It is important that these be referred to by their proper nomenclature of A(H1N1)pdm09 and certainly not by the confusing term of swine influenza.(6)

  1. Olsen CW, Brown I, Easterday BC, Van Reeth K. Swine influenza. In: Straw B.E., Zimmerman J.J., D’ Allaire S., Taylor D.J., eds. Diseases of Swine, 9th edition, Iowa State University Press, Ames, Iowa, 2006: 469-82.
  2. Centers for Disease Prevention and Control  Swine-Origin Influenza A (H3N2) Virus Infection in Two Children --- Indiana and Pennsylvania, July--August 2011 CDC Morbidity and Mortality Weekly Report, September 9, 2011 / 60(35); 1213-1215
  3. Adiego Sancho B, Omeñaca Terés M, Martínez Cuenca S, Rodrigo Val P, Sánchez Villanueva P, Casas I, Pozo F, Pérez Breña P. Human case of swine influenza A (H1N1), Aragon, Spain, November 2008. Euro Surveill. 2009;14(7):pii=19120. Available online:
  4. Van Reeth K, Nicoll A. A human case of swine influenza virus infection in Europe – implications for human health and research. EuroSurveill. 2009;14(7):pii=19124. Available from:
  5. Robert Koch Institute Germany – Human cases of infection by swine influenza virus National Reference Centre for Influenza (NRZ), Robert Koch Institute (RKI), 2 Sept 2011 (in German)
  6. WHO  Antigenic and genetic characteristics of zoonotic influenza viruses and development of candidate vaccine viruses for pandemic preparedness and  Standardization of terminology of the pandemic A(H1N1) 2009 virus Weekly Epidemiological Record 21 October 2011, vol. 86, 43 (pp 469–480)
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