Influenza is a respiratory disease of high importance due to its fast transmission and zoonotic potential.

Alternative names: Porcine Influenza, influenza typeA


Porcine influenza is caused by several Influenza Type A viruses closely related, characterized to have the ability to change the antigenic structure and create new strains.

Each serotype is identified through surface proteins called "H" and "N". The three most common serotypes affecting pigs are H1 N1, H1 N2 and H3 N2. There are also different strains of these serotypes that have different pathogenicity and no crossed protection.

The incubation period is very short, 12 to 48 hours. It affects the respiratory system. The virus can infect a farm in few days. This high transmission is almost the main feature helping in the diagnosis. 

There are three important periods in which the disease can cause infertility, and all of them are caused by fever and its consequences depending on the pregnancy stage because the virus is not systemic and does not cross the placenta. Thus, fetuses are not infected.

  1. First, if sows get ill during the 21 first days of the pregnancy, the virus can prevent implantation of developing embryos, which produces and increase in returns at day 21. If gestation is already established 14 to 16 days after insemination there will be late returns.
  2. Second, if the infection occurs during the first 5 weeks of gestation, embryo death and reabsorption can occur, which lead to pseudo-pregnant and empty sows. Litter size can be affected at this time due to the embryo reabsorption. By the end of gestation, there will be abortions or late mummified piglets at farrowing.
  3. The third main effect is on the boars, in which increased body temperature can affect semen and reduce fertility during 4 to 5 weeks.

In big farms, influenza can end up being endemic with intermittent outbreaks of clinical symptoms and infertility. Different strains can consecutively affect one farm.




  • The elevated temperature may cause abortions.
  • General coughing.
  • Pneumonia.

When the virus enters a farm for the first time, 2 or 3 animals show symptoms of disease during the first 2 days, followed by:

  • A quick outbreak in which almost all animals stop eating, and appear clinically very ill at 2 to 3 days of the virus introduction.
  • The effects on the reproductive system are produced after the onset of the respiratory disease with coughing, pneumonia, fever and loss of appetite.
  • Acute respiratory distress without subjacent infections persists during 7 to 10 days (depending on how long contact between groups lasts).

In the rest of the farm:

  • Quick onset of the acute disease in sows.
  • Coughing and pneumonia which quickly disseminates.
  • Animals go back to normality in 7 to 10 days.
  • Late returns after weaning.
  • Increase of returns at 21 days.
  • Increase of returns outside the normal cycle.
  • Increase in the number of sows that arrive empty to the farrowing area.
  • Increase in the number of abortions, especially during the last 3 months of pregnancy.
  • Increase in the number of stillborns and slow farrowings.
  • Occasionally an increase in the number of mummies. 

Lactating piglets

  • Symptoms in piglets are rare, unless the disease enters the farm for the first time.
  • Colostrum can prevent the infection during the lactation period.
  • Coughing
  • Pneumonia.
  • Fever.

Weaners and growers

Acute Disease

  • Usually pigs suddenly lie down.
  • Difficulty to breath.
  • Severe coughing.
  • Most of the animals look like dying, but survive without a treatment, unless the farm already has a respiratory problem.
  • Influenza causes severe pneumonia on its own, but when combined with other infections, such as Actinobacillus pleuropneumoniae and PRRS, a chronic severe respiratory syndrome difficult to treat can occur. Severely affected pigs must be treated with antibiotics to prevent the development of secondary pneumonia.

Endemic Disease

  • In this case, the virus stays in the farm, affecting small groups of pigs, mainly weaned pigs. The virus can be responsible for persistent respiratory diseases, having symptoms similar to the acute disease, but less severe. 

Causes / Contributing Factors

Influenza can be introduced by:

  • Infected people.
  • Carrier pigs.
  • Birds, mainly aquatic birds are reservoirs of the disease.
  • Secondary bacterial infections.
  • PRRS co-infections.
  • Temperature fluctuations.
  • Stress.
  • Humid bedding and flooring.
  • Inadequate environments and bad ventilation increase the severity of the disease.


In the acute disease, a reliable diagnosis can be done based on clinical signs, due to the fact that there are no other diseases with such a dramatically quick transmission and clinical effects. Blood samples taken from sows at the beginning of the disease and 2 to 3 weeks after, show an increase in antibody titters. Influenza virus can be identified from nasal swabs analyzed by PCR. Oral fluids can also be used, with good results.

However, the endemic disease can be difficult to differentiate form other viral diseases. PRRS and Aujeszky’s disease must be considered.

Sequencing is critical to establish the vaccine strain. 


  • Breeding sows and boars showing symptoms of the acute disease such as increase of temperature and of breathing speed must be treated with wide spectrum antibiotics.
  • Control secondary bacterial pneumonia with antibiotics.
  • Inject the most affected pigs.
  • Medicate water for 3 to 5 days.
  • Use anti-inflammatory medication to minimize the respiratory system damage.
  • Several effective vaccines exist, but it is important that the vaccine contains the appropriate strain for each farm.