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Swine Influenza Virus

20 November 2013

Michigan Pork Quarterly

Madonna Gemus, extension swine vet with Michigan State University, explains that the best means to control influenza disease are reducing the incidence of flu transmission, vaccination and best management biosecurity practices in 'MSU Pork Quarterly'.

Key Points:

  • Influenza A virus causes a highly contagious respiratory disease in a variety of hosts, transferring from humans to pigs and pigs to humans.
  • The primary means for controlling influenza disease are reducing the incidence of flu transmission, vaccination and best management biosecurity practices.
  • Effective and beneficial vaccines provide both excellent adjuvants (carriers) and updated strains of influenza virus.
  • Timing of vaccination is fundamental to efficacy. In most instances, booster vaccination programmes which are beyond three weeks from the primary dose, will be efficacious.
  • Vaccination failure is Nature’s way of telling us that our protocols are not optimal for the best immune response for the pig.

Influenza A virus causes a highly contagious respiratory disease in a variety of hosts, including humans and pigs. There is evidence that the pig becomes a “mixing vessel”. Case in point is the circulation of the H1N1 pandemic viruses which were introduced back into the swine population from humans. This strain co-circulated with swine influenza strains common only to swine and exchanged genome (Thacker and Janke, 2008).

There are over 70 human origin influenza viruses transmitted to swine documented in 12 countries during 2009–2011 (Nelson, et al, 2012). Each time influenza is transmitted from a pig to a person or person to a pig, it increases the variation of the strain of influenza. The immune system is efficient in protecting against which is recognises. However, the more variation in the virus genome, the less recognition by the immune system. Pigs as an intermediate host facilitate the genetic reassortment between avian and human populations (Thacker and Janke, 2008).

Therefore, the implications to human health cannot be ignored. A swine producers’ responsibility to both human and swine health includes the control of influenza virus infection in swine. This is critical to reduce the cross-species adaptation and minimise the risk of animals being the source of the next influenza pandemic (Thacker and Janke, 2008).

Methods of control of influenza infection in swine are three-fold:

  • reduce exposure of influenza infected humans to the swine population
  • consider vaccination of susceptible pigs and
  • retain best biosecurity management practices.

Youth Swine Exhibitors

The average cost for Swine Influenza Virus (SIV) vaccine is $0.90 cents for a one-dose product; $0.51 per dose or $1.02 per pig for a two-dose product – preferred if your pigs have not been previously vaccinated for SIV. Both the single-dose and two-dose products have six different antigens included.

If your pigs have not been previously vaccinated for SIV, it is strongly recommended that you use the two-dose product. The primary dose should be given at six weeks, followed by the booster at two to three weeks prior to the fair season. Most vaccines have six-month duration of immunity. (*Note this cost analysis is provided by Dr Jim Kober.)

Commercial Producers

Visit with your veterinarian on the best timing, vaccine product and utilisation of best management biosecurity practices for your system.

Reduced Exposure

As a shared virus, there continues the risk of people infecting pigs with influenza virus.

Research from Norway documented the sero-negative status of their pigs for many years and only when the 2009 pandemic virus was introduced, the pigs became flu-positive (Grontvedt et al, 2011). In the author's practice, a “stay at home” policy was invoked. Producers, stockpersons or visitors exhibiting signs of flu such as a fever, runny nose or sore joints, were asked to stay away from pigs.

The best indicator is a thermometer to determine fever and they had one present at the entry of each farm site. In addition, they encouraged workforce vaccination, personal hygiene and wearing masks during human influenza season. The human influenza vaccines offered are updated annually to include strains based on predictions of circulating strains.

Vaccination of Pigs

Both inactivated licensed commercial vaccines and autogenous licensed inactivated vaccines are commonly used in pigs. Studies have shown that pigs that have received the SIV vaccination have decreased severity, clinical signs and viral secretions when exposed to the virus (Van Reeth et al., 2002; Kitikoon et al., 2006).

Another study (Romagosa et al., 2011) demonstrated that transmission to pigs was significantly reduced by vaccination but it could not be completely prevented when a commercial vaccine with more than two strains was used. Of more concern to this research group is that a silent spread of active transmission took place in the absence of clinical indications. However, vaccination paired with proper herd management strategies, such as timing, herd closure or blanket (whole herd) has the potential to stop virus transmission at the population level (Torremorell, 2011).

Vaccination timing is dependent on animal age and pathogen exposure. The principle of vaccination is to provide at least two doses of vaccine in the animal’s life. The pig receives antibodies from sucking the sow. Typically, these antibodies levels drop after three weeks of age. The first dose of vaccine, preferably administered after three weeks of age, is the primary dose that will induce the production of antibodies and expansion in the populations of responding immune response cells to the influenza strain provided in the vaccine. The second dose or secondary response is more rapid and larger than the primary response – anamnestic response.

If allowed the appropriate amount of time between the primary response and secondary response, a “waning” period allows for maturation of immunity. This “waning process” allows “culling” of cells that may be poor responders. To have a complete and mature immune response, this clonal expansion must also include an active process of cell death.

This whole process from primary vaccination to achieving mature immune response homeostasis takes at least three weeks (Figure 1). Therefore, in most instances, if primary vaccination occurs after three weeks of age, and the booster vaccination is administered beyond three weeks from the primary vaccine the process is most likely to be efficacious (Chase, 2012).

The dogma that revaccination must occur within two weeks of the primary vaccination is not true and the anamnestic response will be better if we wait longer.

Days Post Vaccination

Immune Response over Time

The primary response to the vaccination requires eight to 16 days post vaccination. The “waning” period, necessary for maturation requires four to seven days. The secondary response is much better 21 days post first vaccination, according to Dr Chris Chase of South Dakota State University.

The 4-Star Group has developed a commercially available, multi-strain vaccine which presently includes: Pandemic H1N1, Classic H1N1, 2 different H1N2 strains (these strains are prevalent in IN, OH and southern MI) and two different H3N2 strains.

Comments on Proper Vaccination

There are many reasons why animals may develop disease even though they have been vaccinated. These reasons include:

  • Vaccine administration in the face of maternal immunity, such as piglets less than three weeks of age
  • Vaccine administered after infection
  • Improper handling of vaccines or administrative equipment, and
  • Immunosuppression such as poor nutrition, use of glucocorticoids and/or stress at the time of vaccination.

It is very important to maintain vaccine viability through proper storage conditions and administration. The use of chemical disinfectants on syringes and needles can inactivate the vaccines. One recommendation is to use a new disposable syringe per day and change the needle for each litter or every 10 pigs or sows.

Best Management Biosecurity Practices

Understanding where the virus is derived is key in your biosecurity protocol and implementation. One aspect of the national PRRSv strategy is that it provides concurrent training for flu. One caveat is that waterfowl and domestic fowls such as ducks are a natural reservoir of this virus. Avoid contact with bird feces and water systems. Keeping swine facilities confined and separated is imperative. Using a system that encourages a change into clean coveralls, boots; and washing of hands prior to entry to each swine facility will dramatically reduce the spread of influenza.

Unfortunately, the author says, the scope of this article does not allow for all areas of biosecurity relevant to this discussion.


Thacker E. and B. Janke. 2008. The Journal of Infectious Diseases. 197 (Suppl 1). S19.

Nelson M.M.R. Gramer, A.L. Vincent and E.C. Holmes. 2012. Journal of General Virology, 93, 2195–2203.

Grøntvedt C.A., B. Gjerset et al, 2011. Proc International symposium on emerging and re-emerging pig diseases. p74, June 2011, Barcelona, Spain.

K. Van Reeth, S. Van Gucht, and M. Pensaert. 2002. Viral Immunology. 15(4):583-594. doi:10.1089/088282402320914520.

Kitikoon, P.D. Nilubol, B. Erickson, B. Janke, et al. 2006. Veterinary Immunology and Immunopathology. 112 (3-4): 117-128.

Romagosa, A., M. Allerson, M. Gramer, H.S. Joo, J. Deen, S. Detmer and M. Torremorell. 2011. Veterinary Research. December, 2011.

Chase C. 2012. Proceedings 2012 AASV Annual Meeting: Integrating Science, Welfare, and Economics in Practice, March 2012, 411-416.

Torremorell, M. 2010. Proceedings 2011 Allen D. Leman Conference, September, 2011, 27-31.

Further Reading

Find out more information on swine influenza by clicking here.

November 2013

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