Last updated Dec. 3, 2009:
At the end of April 2009 Mexico and the USA reported conspicuous cases of influenza in humans, in Mexico sometimes with fatal outcome. An infection with Influenza A virus of the subtype H1N1 showing similarities to influenza viruses occurring in pigs was diagnosed. Further analyses revealed that the virus is a new pathogen which up to then had never been detected in pigs. The virus genome contains part of the genetic information of influenza viruses of the type A from swine, humans, and birds. Thus, it is not a pathogen that is directly transmitted from pigs to humans, but an influenza infection that is transmitted from humans to humans. The term “Swine Flu” is misleading.
Infections of pigs with influenza viruses of various subtypes occur worldwide. Outbreaks are registered year-long with a peak during the colder seasons. Influenza viruses in pigs are mainly transmitted aerogenically by close contact between the animals and possibly also by contaminated objects which were in contact with infected pigs. The establishment of new groups and the introduction of infected animals to unvaccinated holdings promote the transmission and lead to outbreaks. Virus replication is limited to the respiratory tract and causes sudden fever, depression, coughing (“barking”), nasal and/or ocular discharge, respiratory problems, redness or inflammation of the eyes and reduced or no food intake.
Swine influenza is distributed worldwide. Influenza viruses of the subtypes H1N1, H1N2 and H3N2 occur endemically in pigs in Europe, America and Asia. Studies have shown that in pigs influenza viruses of the subtype H1N1 are the most common, followed by subtypes H3N2 and H1N2. Investigations in the USA have confirmed that antibodies against influenza viruses of the subtype H1N1 can be detected in 25 to 51 % of pigs. Currently, there is no possibility to differentiate vaccinated from naturally infected pigs, as in both cases the same antibodies are produced.
Human infections with influenza viruses occurring in pigs are very rare. The new influenza virus infections which occurred in Mexico and the USA in April 2009 are not associated with contact to pigs. The World Health Organization WHO announced that the pathogen is a new influenza virus of the subtype H1N1 which so far had neither been detected in humans nor in pigs. The virus may have been introduced to the human population some time ago and may have mutated to permit human-to-human transmission. In the meantime however, infections of pigs and turkeys transmitted by infected humans have been detected which demonstrate the zoonotic potential of the new pathogen.
Information on human infections can be found on the web pages of the Robert-Koch-Institute.
Vaccination and Treatment
For pigs, various inactivated vaccines against influenza viruses are available, however not against the Novel Influenza virus A/H1N1. Diseased pigs can be treated symptomatically with drugs, e.g. against fever.
Commercially available vaccines against influenza viruses occurring in pigs have been on the market worldwide for years. Usually, only mild or no symptoms at all are observed in immunized animals. No vaccine is available against infections of pigs with the novel influenza virus A/H1N1/09.
Information on the vaccination of humans against the Novel Influenza virus A/H1N1/09 and on the available vaccines can be found on the web pages of the Robert-Koch-Institute and of the Paul-Ehrlich-Institute.