Influenza subtypes — Clinical influenza can be caused by several different swine influenza A subtypes, although H1N1 is the most common subtype implicated in both pig and human infections . Human cases of swine H3N2 influenza virus infection have been reported rarely . Other subtypes that have circulated in pigs include H1N2, H3N1, and H3N2.

Role of pigs — Pigs may play an important role in interspecies transmission of influenza virus. Susceptible pig cells possess receptors for both avian and human influenza strains, which allow for the reassortment of influenza viruses from different species if a pig cell is infected with more than one strain . The outbreak in the spring of 2009 was caused by an H1N1 virus that had not been recognized previously in pigs or humans . This strain represents a genetic reassortment of swine, human, and avian strains of influenza . It is not clear yet how this virus arose or was initially transmitted to humans.

Person-to-person transmission — Influenza virus is present in respiratory secretions of infected persons. As a result, influenza virus can be transmitted through sneezing and coughing via large-particle aerosols, as well as by contact with surfaces that have been contaminated with respiratory droplets . (See “Clinical manifestations and diagnosis of influenza in adults”, section on Transmission).

In contrast to previous outbreaks described above, the outbreak of swine H1N1 influenza A infection in the spring of 2009 appears to involve sustained human-to-human transmission, as suggested by the large numbers of patients with respiratory illnesses identified within a short period of time at various locations around the world . Several of the isolates causing disease in the United States have been found to be nearly genetically identical to isolates in Mexico, supportive of person-to-person transmission .

Infection control and social distancing measures are discussed separately. (See “Treatment and prevention of swine H1N1 influenza A”, section on Social distancing measures and section on Infection control”).

Shedding — Since the duration of shedding of swine H1N1 influenza A virus is currently unclear, the estimated duration of shedding is based upon what is known for seasonal influenza virus .

Patients with swine H1N1 influenza A virus infection are considered to be infectious from one day prior to the development of signs and symptoms until resolution of fever . Individuals should be considered contagious until seven days after illness onset. Longer periods of shedding may occur in children (especially young infants), elderly adults, patients with chronic illnesses, and immunocompromised hosts. (See “Clinical manifestations and diagnosis of influenza in adults”, section on Transmission and see “Clinical features and diagnosis of influenza in children”, section on Transmission”).

Incubation period — Although the precise incubation period has not been established for swine H1N1 influenza A infection, it could range from one to seven days, and most likely from one to four days .


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