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Swine Influenza (SI)

(457) Swine influenza is similar in most respects to human flu. In the pig there are at least four different serotypes each stimulating immunity to itself but not to the other serotypes. It is thus possible for the pigs to be infected by one virus and develop disease and then some two to three months later be infected by a different serotype and develop disease yet again. In large herds of over 300 sows the virus may circulate in young growing pigs, disease becoming more active in the winter time associated with reduced ventilation rates. In the growing and finishing pig disease is not seen until maternal antibody has disappeared at sometime between 7 and 12 weeks of age. SI viruses can interchange between man, pig and birds and the carrier state can exist in the pig for some two to four weeks. They are also spread by birds, particularly ducks. It is also thought that the viruses may spread on wind up to 3km (2miles) on wind but this has never been demonstrated. Thus it is virtually impossible to guarantee or maintain a population of pigs that is free of this disease.

Clinical signs

Acute disease
The incubation period is short, less than 48 hours. The onset can be extremely rapid and dramatic. The classical picture is a house full of pigs that are normal on one day and most of them are prostrate and breathing heavily by the following morning. Severe coughing and laboured breathing will be observed. You may think most of the pigs are going to die but rest assured most of them survive, and provided the herd does not have a history of ongoing pneumonia the pigs will recover on their own, but it is always difficult to predict the outcome. Severely affected individuals or groups of pigs are therefore best given antibiotic cover.

Endemic Disease
This is where the virus continually circulates through the herd infecting individual pigs within groups. SI causes severe pneumonia on its own but when it is combined with other infections such as App, EP and PRRS a chronic respiratory disease syndrome can develop.


In acute disease the rapidity of development and spread, together with typical clinical signs, are diagnostic. No other disease will affect so many pigs so quickly.

In the chronic respiratory disease syndrome it is necessary to carry out serological tests and virus isolation to determine the presence and serotype of the virus.


  • There is no treatment specifically for flu viruses. However secondary bacterial pneumonia may be involved and in such cases antibiotic treatments by injection or in the drinking water, would be advised.
  • In-feed medication in acute disease is a waste of time because pigs do not eat. Although labour intensive, it is far more efficient to treat individual pigs that have secondary pneumonia with long-acting antibiotics, such as oxytetracycline or amoxycillin.
Management control and prevention
  • Use the management procedures already outlined for the other respiratory diseases.
  • Reduce the weights of exposure to other organisms.
  • Vaccination is used in some countries with mixed results.
  • Avoid buying pigs from sources where SI is active.

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