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Swine Influenza Virus


Sabrina L. Swenson, DVM, PhD1
Gene A. Erickson, DVM, PhD2

Background: Swine influenza virus (SIV) infections in swine are common causes of respiratory disease in pigs. Infection of pigs occurs throughout the world. Prior to 1998, swine influenza in pigs in the United States was caused by H1N1 SIV. In August of 1998, a North Carolina herd exhibiting clinical signs of influenza was identified with H3N2 SIV. Since that time, isolates from pigs in Texas, Iowa, Minnesota, and Illinois have been typed as H3N2 SIV.

Pathogenesis: Viral replication is generally restricted to the respiratory tract of the pig. The primary area of infection is the respiratory epithelium lining the airways. Immunity to infection develops very rapidly and is detectable around 6-7 days post-infection, which provides a very narrow window of time for diagnosis by nasal swab specimens. Only pigs with acute disease should be swabbed for virus detection. Typically, those pigs are febrile with high temperatures of 105 F or higher, and are off feed (sows or boars) or are reluctant to move at all (nursery pigs).

Epidemiology: Two forms of disease occur in swine, endemic or epidemic (epizootic). Sow herds
with endemic H1N1 or H3N2 influenza will ordinarily have sporadic abortions (due to the high fever associated with acute infection) and may also be manifested as a decreased conception rate due to first and second trimester abortions that are recorded as “not in pig” events. Aborting sows will usually be off feed for 2 to 3 days with a fever of 105 F.

Epidemic influenza is readily apparent in all age groups. Onset of disease is acute and dramatic, particularly in the sow herd. For naive sow herds, abortion may be widespread, ranging from 5 to 10% of the sow herd. Unlike PRRS-induced abortion, influenza-induced abortion storms resolve within 2 to 3 weeks. Spread is rapid, typically sweeping a breeding/gestation facility within a week. Similarly, the respiratory disease associated with epidemic influenza in growing pigs is of rapid onset, and the initial viral pneumonia-induced coughing within the herd is of short duration due to rapid spread among susceptible pigs. Transmission is by direct contact and highly infectious viral aerosols within facilities. Under appropriate environmental conditions, airborne spread may result in explosive spread among farms within very dense swine production areas. Unless appropriate management decisions and interventions are made, herds with acute or epidemic disease very commonly have endemic disease thereafter. Epidemic disease is seasonal in nature, and peaks in periods of greatest environmental stress, such as the late fall and winter months of the Midwest. Disease onset often occurs  during extreme temperature fluctuations. Commonly, it is associated with chilling that can occur in the Midwest due to dramatic drops in outdoor temperature or loss of heating.

Clinical Signs: Pigs infected with SIV show a variety of clinical signs including coughing, nasal and/or ocular discharge, sneezing, depression, anorexia, dyspnea, and hyperthermia. It is not uncommon for pigs to develop temperatures of >105 F. Reproductive performance in both males and females can be affected by SIV infections. Elevated rectal temperatures in boars can result in adverse effects on the testicles and sperm, resulting in reduced fertility. Depending on the stage of gestation, affected females can show a variety of reproductive problems including delayed return to estrus, abortions, and reduced viability of piglets at birth. Milk production in lactating females may be reduced, resulting in adverse effects on the nursing piglets.

Diagnosis: Swine influenza virus infections can be diagnosed based on a combination of clinical signs, typical gross and histopathologic lesions, diagnostic tests to detect live virus, nucleic acid, or antigen and serologic assays.  Tissues, particularly lung, can be evaluated for the presence of live or inactivated virus. The presence of SIV can also be evaluated from nasal or lung airway swabs. Virus isolates can be typed to determine the H and N components, which is important for determining the type(s) of SIV vaccine to use in a herd.

Treatment/Prevention/Control: Treatment for SIV infections is based on supportive therapy and can include reducing stress, medication to lower body temperatures, and ensuring the pig is made as comfortable as possible. Prevention is based on good management practices to reduce the risk of SIV entering the herd and include good biosecurity measures such as limiting people and vehicle traffic on the premise, properly cleaning and disinfecting trucks that take pigs to market, and quarantining new pigs before placing them in the general pig population of the farm. Due to the antigenic differences between SIV strains vaccination can prove to be challenging.  Options for vaccination include autogenous or commercially available vaccines.

Easterday BC and Van Reeth K. 1999. Swine Influenza. Pages 277-290 in Diseases of Swine. Iowa State University Press. Ames, Iowa.
Fenner F, Bachmann PA, Gibbs EPJ, Murphy FA, Studdert MJ, and White DO, eds. 1987. Orthomyxoviridae. Pages 473-484 in Veterinary Virology.

Academic Press, Inc. San Diego, California.

1National Veterinary Services Laboratories, Ames, IA
2Rollins Animal Disease Diagnostic Laboratory, Raleigh, NC

© 1999 National Pork Board as implemented by the National Pork Producers Council • Des Moines, Iowa