What Is Swine Influenza? Causes, Symptoms, and Effective Prevention and Treatment

According to the World Organisation for Animal Health (WOAH), swine influenza is a highly contagious respiratory disease that spreads rapidly within pig herds. During an outbreak, the morbidity rate can reach 100%, while the mortality rate is generally low unless secondary bacterial infections or other respiratory complications occur.
Not all infected pigs show obvious clinical signs. Some may exhibit only mild symptoms or remain asymptomatic carriers while still shedding the virus and transmitting the infection to other pigs. The major economic impact of swine influenza results from reduced feed intake, decreased weight gain, uneven herd growth, and a prolonged time required for pigs to reach market weight.
In this article, the Fivevet Field Technical Department shares essential information to help pig producers better understand the causes, transmission routes, clinical signs, lesions, diagnostic methods, preventive measures, and effective control strategies for swine influenza. 1. Swine Influenza
Swine influenza is caused by Influenza A virus, a member of the Orthomyxoviridae family. The virus is enveloped with a glycoprotein membrane and contains a single-stranded RNA genome.
Influenza A viruses in pigs are classified into multiple subtypes based on their surface glycoproteins. The most common subtypes are H1N1, H1N2, and H3N2. Other subtypes, including H1N7, H3N1, H4N6, H3N3, and H9N2, have also been reported.
Influenza A virus strains isolated from infected pigs include H1N1, H1N2, H3N2, and H3N1. Among these, H1N1 is one of the most frequently detected subtypes in swine populations worldwide.
Virus Stability
Swine influenza virus is heat-sensitive and has limited resistance to high temperatures:
- At 56–60°C, the virus is inactivated within a few minutes.
- At 100°C, the virus is destroyed immediately.
- At 4°C, the virus can survive for approximately 2 months in embryonated chicken egg allantoic fluid.
When rapidly frozen and stored at −70°C, the virus can remain viable for an extended period.
The virus is readily inactivated by common disinfectants, including formaldehyde (formalin), Virkon, lime, and cresol.
2. Transmission of Swine Influenza
Swine influenza virus primarily infects pigs, although it has also been reported in humans, turkeys, and ducks. Infected pigs may begin shedding the virus within 24 hours after infection and typically continue to shed the virus for 7–10 days.
The virus is transmitted from infected pigs to susceptible pigs through respiratory droplets, as well as by direct and indirect contact. Pigs infected with the virus but showing no clinical signs can still shed the virus and contribute to its spread.
Swine influenza can occur throughout the year, although outbreaks are more common during the autumn and winter in temperate regions.
Although swine influenza viruses are generally host-adapted to pigs, certain strains are capable of crossing the species barrier and infecting humans. Some influenza viruses circulating in pigs, such as H3N2, are believed to have originated from human influenza viruses.
3. Pathogenesis of Swine Influenza Virus
Following infection, the swine influenza virus replicates in the respiratory epithelium, including the nasal mucosa, tonsils, trachea, lungs, and tracheobronchial lymph nodes. Viremia, if it occurs, is typically transient.
The lungs are the primary target organ of the virus. Influenza viruses can induce cellular damage through the activity of neuraminidase (NA) and the PB1-F2 protein. In addition, cytokines produced by the host during the early stage of infection play a critical role in disease progression.
Key cytokines involved include interferon-alpha (IFN-α), tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6). These cytokines contribute to pulmonary dysfunction and are responsible for many of the clinical manifestations of the disease, including inflammation, fever, lethargy, anorexia, and other systemic signs.
As the virus penetrates deeper into the respiratory tract, cytokine production increases, leading to more severe disease. However, certain cytokines also exert antiviral effects and stimulate the host immune response against influenza virus infection. Therefore, further research is needed to better understand the complex role of cytokines in the pathogenesis of swine influenza.
4. Clinical Signs of Swine Influenza
The incubation period of swine influenza is typically 1–3 days, although it may extend to 7–10 days in some cases. Affected pigs commonly develop fever, reduced feed intake or anorexia, coughing, profuse nasal discharge, dyspnea, and pneumonia.
The bronchial mucosa becomes inflamed and damaged, with excessive mucus accumulation in the bronchi. In addition, the lymph nodes, particularly the tracheobronchial lymph nodes, may become enlarged.
Piglets with swine influenza are often depressed, huddle together, and may develop reddish discoloration or erythematous patches on the skin. 4.1. Clinical Signs of the Acute Form
In the acute form, swine influenza has a sudden onset and spreads rapidly throughout the herd. Affected pigs develop a high fever (41.5–42°C), huddle together, show an unsteady gait, tremble, and eventually become recumbent.
Respiratory signs are prominent, including tachypnea, dyspnea, abdominal breathing, or a characteristic dog-sitting posture to facilitate breathing. Affected pigs exhibit reduced feed intake or anorexia, along with lacrimation and nasal discharge.
Erythematous patches may develop on the ears, limbs, snout, and areas of thin skin, such as the abdomen and groin.
4.2. Disease Severity by Age Group
- Piglets (1–5 weeks of age): The disease usually progresses rapidly and is associated with a high mortality rate.
- Pigs older than 5 weeks: Mortality is generally lower, approximately 4–5%. Deaths are primarily caused by severe bronchopneumonia. However, when swine influenza occurs in combination with other respiratory pathogens, such as Mycoplasma spp., Streptococcus spp., or Pasteurella spp., the mortality rate may increase dramatically, reaching up to 100% in severe outbreaks.
- Pregnant sows: Clinical disease is often more severe than in growing or finishing pigs. Affected sows may abort 3–5 days after the onset of clinical signs or give birth to weak piglets with poor viability and increased neonatal mortality.
5. Lesions of Swine Influenza
- In piglets 1–5 weeks of age, the bronchi and lungs contain abundant mucus, serous exudate, and foamy fluid.
- The bronchial mucosa is characterized by multifocal areas of congestion (hyperemia). The lungs, pulmonary lymph nodes, and bronchial lymph nodes are enlarged and edematous.
- The bronchioles and pulmonary lobes contain fibrinous exudate, which may obstruct the airways and contribute to respiratory distress.
- In severe cases, the pulmonary lobules become grayish-red, congested, and edematous, with lesions affecting approximately 30–50% of the lung volume.
- The nasal mucosa is swollen, edematous, congested, and covered with abundant mucoid nasal discharge.
6. How Is Swine Influenza Diagnosed?
A presumptive diagnosis of swine influenza can be made based on clinical signs, gross lesions, epidemiological information, and differential diagnosis. However, a definitive diagnosis requires laboratory confirmation using one or more of the following diagnostic methods: Reverse transcription polymerase chain reaction (RT-PCR), Virus isolation, Immunohistochemistry (IHC), Fluorescent antibody (FA) test, Enzyme-linked immunosorbent assay (ELISA), Hemagglutination (HA) assay, Hemagglutination inhibition (HI) assay.
7. Prevention of Swine Influenza
Effective prevention of swine influenza requires an integrated approach that combines isolation of infected pigs, routine cleaning and disinfection, improved herd management, strict biosecurity measures, careful control of incoming pigs, and vaccination with an appropriate swine influenza vaccine.
7.1. Hygiene and Biosecurity Measures
Because swine influenza virus is transmitted primarily through direct contact and respiratory secretions, pigs suspected of infection should be isolated immediately to minimize the spread of the disease within the herd.
In addition, all pig housing facilities and surrounding areas should be thoroughly cleaned, disinfected, and sanitized to eliminate the virus and reduce environmental contamination.
Additional preventive measures include:
- Provide optimal herd management, including proper nutrition, husbandry, and routine health monitoring.
- Implement strict biosecurity measures throughout the production system.
- Regularly clean and disinfect pig housing facilities and the surrounding environment.
- Administer all recommended vaccinations in accordance with the guidelines of the veterinary authorities.
- Strictly control the importation of live animals and animal products, particularly the movement of pigs and uncooked or unprocessed pork products.
- Strengthen disease surveillance. If pigs show clinical signs suggestive of swine influenza, promptly notify a veterinarian or the competent veterinary authority for investigation and laboratory testing.
- Purchase pigs only from reliable sources with clear traceability and valid veterinary health certification.
7.2. Vaccination
Commercial swine influenza vaccines are currently available. Clinical studies have shown that these vaccines can provide effective protection when pigs receive the complete two-dose vaccination schedule.
Most commercially available vaccines are formulated with H1N1 and H3N2 influenza A virus strains. They are primarily used in breeding sows to enhance herd immunity and are less commonly administered to growing and finishing pigs. In addition, recombinant vaccines and DNA vaccines are under development and evaluation.
Recommended vaccination schedule (reference only):
- Weaned piglets: Administer the first dose at 6 weeks of age, followed by a booster dose 4 weeks later.
- Pregnant sows: Vaccinate at 70–80 days of gestation.
8. Treatment of Swine Influenza
Swine influenza is caused by a virus, and no specific antiviral treatment is currently available. Once infected pigs are identified, they should be isolated immediately and treated individually with supportive care, while appropriate herd-level management and control measures should be implemented to limit disease spread and reduce production losses. Isolate Sick Pigs for Individual Treatment:
+ Fever reducers and anti-inflammatory agents such as: Five-Gluco.KC Namic, Five-MetaMax.50, Five-Anagin.C, Five-Clofenac, Five-Tofen@LA, Five-Medemin,… Duration: 3-5 days.
+ Supportive and strengthening supplements: Five-Butasal, Five-Cafein, Five-Acemin.B12, Five-Medemin,... Duration: 3-5 days.
+ Relieve coughing and promote mucus clearance: Administer Five-Bromhexin by injection or provide Five-Extra Oil orally at a dosage of 1ml/4 L of drinking water, preferably in the morning, for 4–5 consecutive days.
+ Antibiotics for secondary treatment such as: Five-Tulaket, Five-Tulacin 1ml/40 kg body weight, Five-FlorMax 1ml/30 kg body weight, Five-Tylan20@LA 1ml/15 kg body weight, Five-Cetifor Inj 1ml/16 kg body weight, Five-Doflo@LA 1ml/10 kg body weight, or Combo Cef-Ke-Sal,.... For the entire herd.
+ Use one of the following antibiotics: TW5-Doxy.500, Five-Tylvasin, Five-Doflo@LA, Five-Tilmo Super Oral, Five-DoxColis, Five-Tialin, Five-Tilmosin,… Duration: 5-7 days.
+ Supplements to boost immunity, reduce fever, electrolytes, and provide probiotics: Five-Anti Virus, Beta-Glucan.C, Five-No Stress Super, Five-Enzym, Five-Orgacid,... Duration: 3-5 days
+ Detoxifying the liver and kidneys: Five-Acemin.B12, Five-Orgamin, Five-Bogama,…
To prevent and control swine influenza, pig farms should promptly identify and isolate infected pigs, strengthen herd management and husbandry, maintain routine cleaning and disinfection of pig housing, control the introduction of replacement pigs, and implement vaccination programs in accordance with the recommendations of the veterinary authorities.
If pigs show clinical signs suggestive of swine influenza, a veterinarian or the competent veterinary authority should be notified immediately to conduct an investigation, confirm the diagnosis, and collect samples for laboratory testing.
This article was prepared by the Pharmaceutical Research and Development Department, Central Veterinary Medicine Joint Stock Company No. 5 (Fivevet).
References
World Organisation for Animal Health. Swine Influenza. https://www.woah.org/en/disease/swine-influenza/
Frequently Asked Questions:
1. What causes swine influenza?
Swine influenza is caused by Influenza A viruses belonging to the Orthomyxoviridae family. The most common subtypes circulating in pigs are H1N1, H1N2, and H3N2.
2. What is the incubation period of swine influenza?
The incubation period of swine influenza is typically 1–3 days after pigs become infected with the virus. Within a herd, the disease can spread rapidly, causing most pigs to develop clinical signs within just a few days.
3. What are the signs of swine influenza?
Affected pigs commonly develop high fever, reduced appetite or anorexia, coughing, sneezing, nasal discharge, ocular discharge (watery eyes), difficulty breathing, huddling together, and reduced activity. In severe cases, pigs may exhibit abdominal breathing or adopt a dog-sitting posture due to respiratory distress.
4. Can swine influenza be transmitted to humans?
Yes, but transmission of swine influenza viruses to humans is relatively rare. The greatest risk occurs in people who have close contact with infected pigs or contaminated environments, such as pig farmers, farm workers, and veterinarians.
Several swine-origin Influenza A variant viruses, including H1N1v, H1N2v, and H3N2v, have been detected in humans. Most reported cases have not resulted in sustained human-to-human transmission, and ongoing community spread remains very uncommon.
5. Can antibiotics treat swine influenza?
No. Antibiotics are not effective against influenza viruses. They should only be used to prevent or treat secondary bacterial infections when indicated by a veterinarian based on clinical diagnosis.