Swine Influenza

Influenza viruses are the cause of outbreaks of acute respiratory disease, known as influenza or 'flu', which affects man and animals. Swine influenza is caused by a number of closely related influenza A viruses. The virus can be introduced by infected people and carrier pigs. Water birds act as reservoirs of infection.

About Swine Influenza (SI)

• Etiology
• Epizootiology
• Clinical signs
• Public health
• Treatment and prevention

Etiology
Influenza A viruses infect a wide range of avian and mammalian species, with the latter group including man, pigs, horses and aquatic mammals.Type A viruses are known for their ability to change their antigenic structure and create new strains.

Serotypes
The type A viruses are further divided into serotypes, based on the antigenic nature of their surface glycoproteins haemagglutinin (HA) and neuraminidase (NA). 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 varying in pathogenicity.

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Epizootiology
Swine influenza is endemic worldwide. It is virtually impossible to maintain a population of pigs that is influenza virus free.

SI in large herds may become endemic with intermittent bouts of disease and infertility. Different influenza strains may 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. Outbreaks are seen throughout the year but peak in the winter months.

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Clinical signs
The incubation period is very short, less than 48 hours.
Severity of the disease is dependent on:

• The strain of virus
• Age of the pigs
• Immune status of the pigs

High morbidity with low mortality is usually seen.

Piglets
Unusual in this age group as are probably protected by colostrum.

• High temperatures which cause abortions and other reproductive problems
• Widespread coughing
• Pneumonia

Weaners & Growers
Acute disease:

• The classical picture is pigs that are fine one day and then prostate and breathing heavily the following morning.
• Can often recover without any intervention within 2-6 days unless other respiratory diseases are present.
• SI causes severe pneumonia on its own but when it is combined with other infections such as App and PRRS a chronic respiratory disease syndrome can develop.

Endemic disease:

• Small groups of pigs often weaners are affected. It may be responsible for continuing respiratory diseases with symptoms as in acute disease but less dramatic.

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Public health implications
Pigs might be responsible for the transmission of influenza viruses to man in the following ways:

• Act as an intermediate host and “mixing pot” for genetic re-assortment between human and avian viruses. New strains of virus emerge that are transmissible in man.
• Directly transfer avian influenza viruses to man.
• Be responsible for the re-emergence of a virus that caused an epidemic previously. Pigs can act as reservoir for old human influenza viruses.

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Treatment and Prevention
Treatment
Antibiotic cover may be necessary to prevent or treat secondary bacterial infections.

Prevention

• General management procedures to prevent respiratory diseases are effective.
• Inactivated vaccines are available against swine influenza.

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