Swine influenza (flu) virus

calendar icon 8 November 2018
clock icon 7 minute read

Background and history

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.

Each serotype is identified by surface proteins referred to as "H" and "N". The three common strains that affect the pig are described as H1 N1, H1 N2, and H3 N2. There are also different strains within these serotypes with differing pathogenicity (capacity to produce disease).

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.


This can often be made reliably on clinical grounds because there are no other diseases that are so dramatic in their onset and clinical effects. Blood samples taken at the time of onset of disease from affected sows and repeated 2-3 weeks later show rising levels of antibody to the specific virus. SIV can be readily grown from nasal and throat swabs and identified in the laboratory. This is often the best approach to confirm the diagnosis.

Similar diseases

In acute disease the spread is so dramatic across all ages that little else can be confused with it. In endemic disease however differentiation from other viral infections can be difficult, but PRRS, PRCV, AD and erysipelas should be considered.


It is important to prevent any secondary bacterial infections.

  • Keep sows within an environmental temperature of 20-23ºC (70-75ºF)
  • Reduce all possible stress, such as draughts.
  • Maintain dry bedding and floor surfaces.
  • Monitor boars carefully for evidence of illness
  • Identify boars that have been ill and cross-serve with another boar for the next four weeks.
  • Serve sows by AI followed by a natural mating and consider a second dose of AI (am/pm/am).
  • Medicate the water with soluble vitamins for seven days.
  • Medicate the feed as described above for 2 weeks.
  • If periods of inappetence occur in boars, blood test them twice two weeks apart to establish a diagnosis.
  • Because of the possible ways by which the virus may enter the herd it is extremely difficult to maintain populations free of infections. In some countries inactivated vaccines are available and appear to be protective. In herds in which the virus periodically circulates and causes disease this route should be explored.
  • If you believe your herd to be free (and this can be confirmed by serological tests,) purchase breeding stock from herds that have a similar disease history and that are also serologically negative. The practicality of this however is not easy.


  • Individual breeding females or boars showing acute illness, and raised temperatures, particularly with increased respiratory rate should be treated with broad spectrum antibiotics for three days.
  • Suitable medicines would include penicillin/streptomycin, long-acting OTC or synthetic penicillins such as amoxycillin. If the illness is severe then medicate the drinking water with either CTC or OTC at a level of 25g (100% pure) per 1000kg of live weight per day, for five days.

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