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Swine Influenza Virus (SIV)

Constantly evolving, this disease continues to pose challenges

Swine Influenza Virus (SIV)

As one of the top three swine respiratory health problems in the U.S., swine influenza virus (SIV) is a key concern for the industry. SIV – referred to as “swine flu” or “flu” – can be caused by a number of different swine influenza strains.

Influenza viruses are single-strand RNA viruses, giving them the ability to change their antigenic structure and create new, different strains. SIV has evolved from a seasonal disease caused by a stable genotype of SIV to a year-round, endemic respiratory disease caused by multiple SIV genotypes undergoing continuous change.1

This, coupled with the fact that the disease is characterized by nearly 100% morbidity2, makes this one of the most challenging diseases for today’s swine operations.

1.  Gramer MR. Defining swine influenza virus. J Swine Health Prod. 2005;13(3):157-160.
2.  Marie Gramer, DVM. Swine influenza virus: the only constant is change. Allen D. Leman Swine Conference, 2006; 61-63.

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  • Influenza virus has plagued humans for centuries. Flu-like human disease syndromes were first described in 412 B.C. In 1930, H1N1 was first isolated in pigs. This classical strain was thought to be the only strain of influenza in U.S. pig populations for many years.

     A new swine flu virus subtype, H3N2, appeared in 1998. It contained genes from human, swine and avian influenza viruses and swept through the swine population because pigs were only immune to the H1N1 subtype. Shortly thereafter, the H1N2 subtype appeared due to reassortment of H3N2 and H1N1 viruses.

    The influenza virus changes over time because of the imperfect replication by RNA polymerase and the propensity of antigenic shift and reassortment of viral RNA segments. These genetic and antigenic changes are some of the most important factors affecting the epidemiology of swine influenza throughout the world.1 Frequent updates to SIV vaccines are needed to keep up with this ever-changing virus.

    Although somewhat rare, transmission of influenza virus can occur both from humans to pigs and from pigs to humans. This zoonotic potential has increased the attention given to the virus in the media. The U.S. Centers for Disease Control and Prevention (CDC) has emphasized that “swine influenza viruses are not spread by food. You cannot get swine influenza from eating pork or pork products.” In addition, the CDC states that “vaccination of pigs with a swine flu vaccine that is effective against circulating strains might reduce the risk of flu in pigs and possibly reduce the risk of people getting infected with swine influenza viruses.” FluSure XP® and FluSure® Pandemic fit the description of vaccines that are effective against circulating swine influenza strains.

    Learn more about swine influenza and the history of flu.


    1. Gramer MR, Rossow K. Epidemiology of swine influenza and implications of reassortment. 2004; 69-73. Allen D. Leman Swine Conference. 

  • Unlike some other respiratory diseases, this illness is known for its rapid onset, and pigs usually show signs within the first 12 to 48 hours of being infected. A noticeable sign of SIV-infected pigs is a harsh, barking cough followed by flu-like symptoms. Pigs lose their appetite, become lethargic, huddle and pile on one another. Affected pigs run high temperatures (105° to 107°F) and show labored, open-mouth, abdominal breathing (thumping). 

    Sows may experience spontaneous abortions, increased number of stillborns or premature farrowings due to high fever.

    The signs of the disease remain similar from year to year with occasional variations in severity of morbidity and mortality.1 However, the viruses themselves continue to change, making this a difficult disease to control.

    1. Marie Gramer, DVM. Swine influenza virus: the only constant is change. Allen D. Leman Swine Conference, 2006; 61-63.

  • STOMP Plus®

    Although the disease is quite common, diagnosis can be challenging. Zoetis  offers STOMP Plus – a diagnostic program to help pinpoint what diseases are occurring, as well as the level and timing of infection, so control programs can be implemented when they will do the most good. The goal is to keep the overall disease load in herds below thresholds levels where clinical respiratory disease appears.

    The program involves a combination of serology, ante-mortem and post-mortem testing. Results are used to compare on-farm swine influenza strains to vaccine strains and to strategically place vaccines and/or medications. Contact your veterinarian or Zoetis representative if you are interested in testing your herd.

    SIV Surveillance Team

    The Zoetis Animal Health utilizes a dedicated group of professionals, the SIV Surveillance Team, to monitor trends in SIV pathogenesis and prevalence. This team works closely with universities, veterinary diagnostic labs and customers to evaluate which SIV strains are impacting the industry now and monitor those that may become problematic in the future.

    The SIV Surveillance Team plays a crucial role in Zoetis  ability to update FluSure XP® in a timely fashion. These updates help provide a broadly protective commercial vaccine allowing veterinarians and producers the simplicity of a ready-to-use vaccine, while still providing protection against the SIV strains impacting their herds. As a result, Zoetis  is the company veterinarians and producers can turn to first for accurate information about SIV issues.

  • Vaccination programs for controlling SIV are commonplace. In response to the continued antigenic drift and shift of SIV, Zoetis  has formulated a flu vaccine, FluSure XP® to help further protect against current circulating flu strains. The vaccine contains two strains of H1N1 and one H3N2, and is now the only commercial vaccine on the market to contain H1N2. 

    FluSure® Pandemic helps protect herds from the pandemic H1N1 (pH1N1) strain of SIV. Zoetis  is the only manufacturer to gain full licensure from the U.S. Department of Agriculture (USDA). This vaccine provides the safety and quality that pork producers need against this now common swine pathogen.

    FluSure XP and FluSure Pandemic vaccines, as part of the FluSure XP Defense System, help guard against 9 out of 10 of the most current flu strains that threat swine herds today.1

    This line of valuable SIV management tools, along with the FluSure XP Defense System, demonstrates Zoetis commitment to monitoring SIV and bringing to the market vaccines that help protect against SIV strains circulating in pork production systems today.

    Success of vaccination programs should be monitored by :
    ·        Reduction of clinical signs
    ·        Incidence of SIV in routine diagnostics
    ·        Incidence of respiratory treatments related to SIV

    If frequency or severity of documented SIV outbreaks is increasing, an evaluation of strains may be needed to see if any changes have occurred. As stated above, Zoetis  is fully engaged in the diagnosis of SIV through the STOMP Plus diagnostics program and is ready to work with herds that are experiencing problems.

    1. Genetic comparison of viruses submitted to the University of Minnesota Veterinary Diagnostic Laboratory in the last three years demonstrated that 9 out of 10 currently circulating U.S. strains were at least 90% similar to FluSure XP and/or FluSure Pandemic strains. 


  • It is difficult to estimate the total economic impact of SIV. Since it typically occurs with other respiratory diseases, it makes it difficult to attribute losses to just one pathogen.

    One hog production company conducted a study to assess a dollar value associated with SIV losses. They concluded that SIV can cost $10.31 per market pig.1 The comparison was based on closeout data from over 100,000 pigs. Costs were attributed to losses in production and veterinary-related expenses. Costs in your system may be more or less depending on the strain of SIV in your herd and concurrent infections with other pathogens.

    1. Donovan TS. Influenza isolate selection methodology for timely autogenous vaccine use, in Proceedings. Amer Assoc Swine Vet Conf 2008;557-561.



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