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Swine influenza is caused by a number of closely related influenza A viruses that are noted for their ability to change their antigenic structure and create new strains. SI can be introduced by infected people, carrier pigs and probably on the wind although this has not been proved. Birds particularly water fowl, are reservoirs of infection.

Sample

Clinical signs

The incubation period of the disease is very short, as little as 12-48 hours. When the virus first enters the herd two or three animals may be observed sick for the first two days, followed by a rapid explosive outbreak of inappetence and clinically very ill pigs. The effects on the reproductive system follow the sudden onset of a rapid spreading respiratory disease with coughing, pneumonia, fevers and inappetence. Acute respiratory distress persists over a period of 7-10 days (depending on the amount of contact between groups of sows). There are three important periods when infection causes infertility. First, if sows are ill in the first 21 days post-service pregnancy their developing embryos may not get established and an increase in 21 day returns results. If pregnancy has been established 14-16 days after mating, and it then fails returns will be delayed. Second if infection occurs in the first five weeks of pregnancy, there could be total embryo mortality and absorption with sows becoming pseudo-pregnant and not in-pig. Litter size may also be affected at this stage due to absorption of embryos. Towards the end of the pregnancy period abortions or late mummified pigs at farrowing may also be experienced. The third major effect is on the boar, where high body temperatures affect semen and depress fertility for a 4 to 5 week period.

SI in large herds may become endemic with intermittent bouts of disease and infertility and different strains may also sequentially infect the herd. Immunity to influenza viruses is often short lived (6 months) and the immunity profile in the breeding herd varies considerably with time.

At a herd level the following may also be seen:

• A sudden and rapid onset of acute illness in sows.
• Coughing and pneumonia spreading rapidly.
• A return to clinical normality over 7-10 days.
• Delayed returns to heat after post-weaning.
• Increased repeats at 21 days.
• Increased repeats outside the normal cycle.
• Increased numbers of sows coming through not in-pig.
• Increased numbers of abortions, particularly late term.
• Increased numbers of stillbirth rates and slow farrowings.
• Occasionally an increase in mummified pigs.

During the phases of high temperatures other diseases present in the herd may be triggered off. A typical example would be an increase in abortions associated with leptospira infection.

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