Swine influenza frequently asked questions
What is swine influenza?
Swine influenza,or “swine flu”,is a highly contagious acute respiratory disease of pigs,caused by one
of several swine influenza A viruses. Morbidity tends to be high and mortality low (1-4%). The virus
is spread among pigs by aerosols,direct and indirect contact,and asymptomatic carrier
pigs. Outbreaks in pigs occur year round,with an increased incidence in the fall and winter in
temperate zones. Many countries routinely vaccinate swine populations against swine influenza.
Swine influenza viruses are most commonly of the H1N1 subtype,but other subtypes are also
circulating in pigs (e.g.,H1N2,H3N1,H3N2). Pigs can also be infected with avian influenza viruses
and human seasonal influenza viruses as well as swine influenza viruses. The H3N2 swine virus was
thought to have been originally introduced into pigs by humans. Sometimes pigs can be infected with
more than one virus type at a time,which can allow the genes from these viruses to mix. This can
result in an influenza virus containing genes from a number of sources,called a 'reassortant' virus.
Although swine influenza viruses are normally species specific and only infect pigs,they do
sometimes cross the species barrier to cause disease in humans.
What are the implications for human health?
Outbreaks and sporadic human infection with swine influenza have been occasionally reported.
Generally clinical symptoms are similar to seasonal influenza but reported clinical presentation ranges
broadly from asymptomatic infection to severe pneumonia resulting in death.
Since typical clinical presentation of swine influenza infection in humans resembles seasonal
influenza and other acute upper respiratory tract infections,most of the cases have been detected by
chance through seasonal influenza surveillance. Mild or asymptomatic cases may have escaped from
recognition; therefore the true extent of this disease among humans is unknown.
Where have human cases occurred?
Since the implementation of IHR(2005)1 in 2007,WHO has been notified of swine influenza cases
from the United States and Spain.
How do people become infected?
People usually get swine influenza from infected pigs,however,some human cases lack contact
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