Backgrounder - Swine influenza

New strain of H1N1 swine flu virus
Recent reports of a new "swine flu" virus among humans are emerging. Influenza viruses from different species can mix and reassort (i.e. swap DNA) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. While called "swine flu", this new virus appears to be a hybrid of North American swine influenza viruses, North American avian influenza viruses, human influenza viruses, and swine influenza viruses found in both Asia and Europe. According to the CDC there is no connection to this new virus and contact with swine - it appears to be solely a result of human-to-human transmission. While the investigation is ongoing, the CDC recommends certain precautions humans should take to stay healthy, which are located on their website at:

Causative agent
Swine influenza (SI) is caused by type A orthomyxoviruses. These viruses are 80-120 nanometers (nm) in diameter, and consist of a core of eight separate pieces of single-strand ribonucleic acid (RNA) surrounded by a spiked arrangement of glycoproteins. Influenza viruses are classified based on the relative numbers of hemagglutinin (H) and neuraminidase (N) glycoproteins in the lipid outer layer. The most common subtypes of influenza virus in swine are H1N1, H1N2, and H3N2. Despite the same subtype classification, swine influenza viruses in Europe and the United States are genetically distinct.

Natural distribution
Swine influenza is commonly found in North and South America, Asia, and Europe. Its presence in Africa has also been reported. The H1N1 subtype was initially identified in the United States in 1918. Two avian-like variants of the H1N1 subtype exist—one in European and American pigs, and the other in Asian pigs—that are believed to be avian influenza viruses that were transmitted to pigs. The H3N2 and H1N2 subtypes are hybrids of swine, avian, and human influenza viruses; the H1N2 virus is a hybrid of two distinct human influenza strains.

Human infection with swine influenza virus has been reported infrequently in the United States, Europe, and New Zealand. A notable outbreak of human cases occurred in 1976 at Fort Dix, New Jersey—five recruits with clinical signs of respiratory disease were found to be infected with swine influenza. Five hundred people on the base were infected based on serologic results, but only one fatality occurred.

Humans who have contact with pigs may become infected with the swine influenza virus; however, because the clinical signs mimic those of human influenza, a distinction is not often made. Turkeys have also been infected with swine influenza virus.

Swine influenza virus is spread via aerosolized respiratory secretions and fomites, including contaminated inanimate objects and people moving between infected and uninfected pigs. Outbreaks of swine influenza are most commonly associated with the introduction of new animals into a herd. The incubation period is usually one to three days. Pigs begin excreting the virus within 24 hours of infection, and may shed the virus for seven to ten days. A carrier state can exist for up to 3 months.

Transmission of viruses across species is rare, but can create an epidemic in the new host species. Alternatively, a new virus can mix, or re-assort, with an already-adapted virus and result in the generation of a new strain or variant. Pigs are distinct from other species in that their cells possess receptors that can bind to swine, human, and avian influenza viruses; therefore, two or more types of viruses may co-infect swine cells and combine to produce a novel virus. For this reason, swine have been considered "mixing vessels" that facilitate the formation of new viruses.

Clinical signs
Infection with swine influenza virus results in acute upper respiratory disease, and signs include:

• fever • sneezing • eye inflammation
• anorexia • discharge from nose • abortion
• coughing • labored breathing  

Most affected pigs will recover within five to seven days in the absence of complications. Severe bronchopneumonia may develop as a secondary complication, and is a high risk factor for mortality. Other complications may include secondary bacterial or viral infections.

Turkeys infected with the swine influenza virus may exhibit decreased egg production, production of abnormal eggs, or respiratory disease.

Clinical signs in humans mimic those of human influenza infection, and the incidence of swine influenza virus infection in humans may be underestimated. Self-limiting flu-like symptoms and diarrhea have been reported in infected humans.

Swine influenza virus infection is confirmed by virus isolation, viral antigen or nucleic acid detection (with reverse transcriptase polymerase chain reaction), immunofluorescence, immunohistochemistry, ELISA, and hemagglutination serologic tests. Lung tissue or nasopharyngeal swabs from acutely infected pigs yield optimal results. The virus is best detected in febrile animals 24 to 48 hours after onset of clinical signs.

As for all viral disease, treatment is largely supportive. Good husbandry and nutrition may assist pigs in mounting an effective immune response. Secondary bacterial pneumonia in more severely affected pigs responds best to a combination of broad-spectrum bactericidal antimicrobials and maintenance of hydration via intravenous administration of fluids.

Morbidity and mortality
Morbidity associated with swine influenza can reach 100%; mortality in confirmed cases to date has ranged from 1 to 3% in the absence of complications. Secondary infection with the bacteria Hemophilus suis increases the severity and mortality of swine influenza virus infection.

Prevention and control
Once introduced, the swine influenza virus may become endemic in herds. Annual outbreaks may be observed, primarily during the colder months of the year. Young, naïve pigs are at increased risk of infection. Recovery from infection usually confers limited immunity.

The swine influenza virus is an enveloped virus that appears to be easily killed by disinfectants in common use in veterinary and swine facilities, such as quaternary ammonium compounds and 10% bleach solutions. Isolation protocols should be rigorously applied for pigs showing signs of respiratory disease. Clothing, equipment, surfaces and hands should be cleaned and disinfected after exposure to pigs suspected to be infected.

Links to More Information on Swine Influenza

Talking points

Centers for Disease Control and Prevention (CDC)
CDC Swine Flu

World Health Organization (WHO)
WHO Swine Influenza

American Association of Swine Veterinarians (AASV)
AASV Human Cases of Swine Influenza



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