Swine influenza – backgrounder

A backgrounder on swine influenza put together by the Science Media Centre in London.

What is it?

· Swine flu is a highly contagious acute respiratory disease affecting pigs, caused by one of several influenza A viruses

· Four subtypes of the virus have been identified: H1N1, H1N2, H3N2, and H3N1, though most recently identified cases have been of the H1N1 type

· H1N1 has been shown to be common throughout pig populations worldwide – studies have shown that between around 30-50% of pigs in the north central US show evidence of infection by H1N1

· Outbreaks occur in pig herds all year round, with increased incidence in autumn and winter and after the introduction of new pigs to herds

· The virus causes high rates of illness and low rates of mortality (1-4%)

Does it affect humans?

· Swine flu viruses do not normally infect humans, and H1N1 infections in humans are rare.

· However, some infections have occurred, usually in people who work in close proximity to pigs

· The extent of swine flu infection among humans is unknown, as many cases resemble normal human flu or are asymptomatic and thus go undetected

How does it spread?

· The virus spreads among pigs through both direct contact (animal-to-animal) and indirect contact (via contaminated objects and surfaces)

· It can be spread between humans and pigs, most likely to people who have been in close physical proximity to pigs (such as livestock workers)

· It can also spread between humans, in a similar manner to human seasonal influenza (via direct contact and contaminated surfaces – doorknobs, etc.)

· Viruses can survive for up to 2 hours on contaminated surfaces – frequent handwashing can reduce risk of contamination from these sources

· Swine flu viruses are not transmitted by food – you cannot get swine flu from eating pork or pork products

What are the symptoms?

· In pigs: sudden onset of fever, depression, barking cough, discharge from nose or eyes, sneezing, breathing difficulties, eye redness or inflammation, going of feed

· In humans: similar to regular flu – fever, cough, sore throat, aches and pains, chills, fatigue. Diarrhoea and vomiting have also been reported

· Severity in humans can vary from asymptomatic infection to severe, sometimes fatal pneumonia

What treatments are available?

· Two antiviral treatments are available in the US and are recommended by CDC: oseltamivir and zanamivir

· Antivirals work by preventing the virus from reproducing in the body. They have greatest benefit when given within two days of onset of symptoms

Is there a vaccine?

· There are no swine flu vaccines currently available for humans, as the swine flu virus is genetically very different from that which affects humans

· Vaccines for human seasonal flu do not protect against swine flu for the same reason

Sources / further information

New Zealand Ministry of Health


World Health Organisation:


US Centers for Disease Control and Prevention:




American Veterinary Medical Association


Center for Infectious Disease Research & Policy has current updates: