Swine Influenza


Description, causes and risk factors:

An acute respiratory disease of swine caused by strains of influenza virus type A; it is believed to have become adapted to swine in the United States during the great human pandemic in 1918; fatal cases, as in such cases of pandemic influenza in humans, are commonly associated with secondary bacterial pneumonia.

Abbreviation: SI.

ICD-9-CM code: 488.1

Swine influenza (SI) is a virus disease that can cause epidemics of acute respiratory disease in pigs. The disease is due to viruses from the type A of the Orthomyxoviridae family, (there are three types of Orthomyxoviridae, A, B and C). Etiology of SI is complex according to the high genetic variation of the causative viruses, mainly on two glycoprotein: hemagglutin (H) and neuramidase (N). A nomenclature for virus designation has been established by WHO in 1980

Pigs are the main host. However, strains of swine influenza virus can also be directly transmissible to humans, and reciprocally. SI was responsible for the human outbreak in 1918-20 that killed more than 20 million people over the world (Spanish flu). A recent strain has also been responsible for an outbreak in 1976.

Like all influenza viruses, the swine flu viruses are also in a constant state of change. This change helps viruses to form other genetic mutations. Pigs, apart from the swine influenza viruses, can be infected by both avian influenza and human influenza viruses. In pigs, these various viruses reassort (i.e. swap genes) and new mutated viruses emerge.

At this time, there are four main influenza A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. However, most of the recently isolated influenza viruses have been the H1N1 viruses.

There is increasing evidence of interchange of influenza viruses between pigs, other mammalian (including marine mammals) and avian hosts, either directly or after a process of genetic reassortment or mutation.

Swine influenza is transmitted by direct contact between pigs. In the acute stages of the disease, high concentrations of virus are found in nasal secretions. Virus is transmitted by aerosols over a short distance. The virus can be shed for 30 days after infection and has been recovered from clinically normal animals.

The disease is spread to new areas and farms by the movement of infected pigs or carrier people. The virus is easily carried and spread by avian species, particularly waterfowl and turkeys. Outbreak of Asian subtypes of influenza A may involve duck to pig transmission since these animals are traditionally kept together. Care should be taken to prevent spread from and between birds and humans to swine.

In endemic areas, while virus is present throughout the year, outbreaks are seasonal, tending to occur in late autumn and early winter. Epidemics are often explosive, with outbreaks occurring on most pig farms in a locality over a short period.


Symptoms include:

  • Fever.

  • Anorexia leading to loss of weight.

  • Inactivity, prostration and huddling leading to weakness.

  • Paroxysmal coughing.

  • Sneezing.

  • Runny nose.

  • Irregular abdominal breathing.

  • Muscle ache and fatigue.

  • Ocular and nasal discharges.

  • Diarrhea and vomiting.


Identification of the virus can be done by by many techniques including immunohistochemical detection, hemagglutination-inhibition coupled to neuramidase inhibition, ELISA and PCR, sometimes several techniques must be combined to identfy (H) and (N) subtypes. Identification tests can be performed from nasal (preferably) or pharyngeal swabs from live animals. Samples must be suspended in glycerol saline and kept at 4°C if they are tested within 48 hours and at 70°C, shipped with dry ice if they are tested after a longer delay. Alternatively lungs from spontaneously dead or sacrificed animals can be sent under similar conditions.

For serology, blood samples should be collected from pigs that are in the acute and convalescent stages of the disease (2 to 3 weeks later). Tests are made by hemagglutination inhibition and demonstrate a raise in the antibody level. The interpretation of serological results may be complicated in young pigs because of the persistence of maternal antibodies.

Some tests include:

    Rapid influenza antigen test.

  • Real-time reverse transcriptase (rRT)-PCR.

  • Viral culture.

  • Immunofluorescent antibody staining.

  • CBC.

  • CXR.


The best way to deal with swine influenza is to prevent the occurrence and spread of the disease. Where the disease does occur, the primary treatment is supportive therapy. Infected pigs require a dry, clean, dust free environment. Antibiotics are also essential to treat and control any secondary bacterial infections that usually develop. Expectorants are commonly used as a herd treatment and are administered in the drinking water.

Commercial vaccines are available in Europe and North America. Results from various studies show that vaccinated animal exposed to swine influenza virus have markedly reduced nasal shedding, virus infection in lung tissue, and lung pathology compared to non-vaccinated animals. Studies have also shown than maternally derived antibody in vaccinated sows protected 5 week old pigs from clinical disease, virus infection in the lung, and lung pathology but did not prevent nasal viral shedding.

As the passage of swine influenza to humans is a serious threat if suspected, the disease should be diagnosed and notified to Public Health Department.

Oseltamivir or zanamivir are the two drugs recommended for the treatment or prevention of infection with the H1N1, or swine, influenza virus.

As of today, four influenza antiviral drugs are approved for use in the United States. They are:


  • Zanamivir.

  • Rimantadine.

  • Amantadine.

Preventive steps:

1. Avoid close contact.

2. Covering nose and mouth with a tissue upon coughing and sneezing followed by proper disposal of the tissue.

3. Staying home form work and/or school upon onset and for the duration of symptoms.

4. Clean your hands.

5. Avoid touching your eyes, nose or mouth.

6. Encouraging pursuit of medical evaluation at earliest onset of symptoms.

7. Use of masks to those who are exhibiting symptoms or who are ill.

8. Practice other good health habits.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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