By Dyneah Classen, DVM, director of Health for the Carthage System
Influenza A virus causes respiratory disease in multiple species. Influenza A viruses in swine can circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans. Clinical signs in swine can start similar to other diseases — animals off feed, lethargy, high fevers (rectal temperatures of 104 degrees F and above), increased mortality and in pregnant sows abortions. As the disease progresses clinical signs included nasal discharge and a deep, barking cough. The author has been known to imitate this cough for owners and herdsmen, alike.
On post mortem necropsy the lungs will have pneumonia with a viral pattern. Influenza A virus outbreaks, like most diseases, can range from mild to severe clinical signs. In some cases there are no clinical signs at all and the influenza A virus is found on routine testing. The utilization of oral fluilds has increased passive surveillance of finishing herds and found non-clinical herds positive for influenza.
What factors contribute to an influenza A virus outbreak severity?
There are multiple strains of influenza A virus. Influenza virus is an RNA virus and changes constantly. Pigs can be infected by avian influenza and human influenza viruses as well as IAV-S. When influenza viruses from different species infect pigs, the viruses can reassort (i.e. swap genes) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge.
Over the years, different variations of swine flu viruses have emerged. At this time, there are three main influenza A virus subtypes that have been isolated in pigs in the United States: H1N1, H1N2 and H3N2. Within these subtypes are clusters and clades that are used to identify the virus isolates. These different strains can influence the severity of disease. The human-like influenza virus in pigs are often less virulent and produce less clinical signs, coughing and pneumonia.
The immune status of the herd is important. Sows or pigs that have previous exposure to vaccination or a previous outbreak of influenza A will have antibodies. When the new flu virus enters the host, the antibodies team up to protect the host, in this case the sow or pig. This protection results in less lung damage and pneumonia. This protection will also help shorten the course of disease and the clinical signs throughout the herd.
Vaccinations don’t prevent an animal from acquiring the influenza A virus. The goal is to reduce the clinical signs and decrease the negative impact on production parameters like; average daily feed intake, mortality and abortions in sows.
Selection and strategy
When choosing a vaccine and vaccination strategy, work with your veterinarian to determine which option is best for your herd.
There are several commercial influenza A killed vaccinations available. There is also a newer modified live vaccine available. These vaccinations are available on short notice and have gone through rigorous testing to prove efficacy and safety. The disadvantage of commercial vaccination is the influenza A strains (antigens) included in the vaccine are stagnant and difficult to change, given the regulations the manufactures must follow.
There are also several autogenous influenza A virus vaccine manufactures. Autogenous vaccines are custom vaccines that consist of herd specific same (homologous) antigens (flu strains). Autogenous vaccines are approved for use by the direction of a veterinarian. Therefore, good basic diagnostic work up, knowledge of the influenza A virus in the area/region and isolation of the influenza A virus are critical. Autogenous vaccines fit well with herds that have a close relationship with veterinarians.
Use of autogenous products is considered when no commercially licensed product is available, or when commercially licensed products have not provided adequate protection. The autogenous manufactures have rigorous quality control and safety standards. However, the efficacy of autogenous vaccinations are not tested like a commercial vaccination. The autogenous vaccines can update flu strains regularly.
Autogenous vaccines used in your herd must first identify the flu virus, obtain a sample and have adequate virus isolation and growth for vaccine production. The vaccine production takes 8-12 weeks. When it comes to updating influenza A virus strains for use in vaccination autogenous is hands-down the best option. The disadvantage of autogenous vaccines are it’s still a retrospective look and the time to manufacture the vaccine.
Once you and your herd veterinarian have decided on the vaccine product to use. The next step is to determine timing. Since influenza A virus can circulate all year round, but is more prevalent in the late fall and winter months, vaccination should be timed to increase antibody production before an anticipated outbreak. It takes 10-14 days for the animal to have an adequate antibody titer response following vaccination. If the animal hasn’t been previously vaccinated with influenza A a booster dose is recommended for best results.
Traditionally approaches have been to vaccinate replacement gilts prior to breeding, quarterly mass vaccinations to the sow herd, prefarrowing vaccinations to sows, and intranasal vaccination of the suckling piglets with the modified live vaccines, and on a limited basis, vaccination of wean-to-market pigs post weaning.
This year for the sow herds the industry is abuzz with an approach referred to as prime-boost. The strategy involves choosing two vaccines that are genetically different or heterologous and giving a priming dose and following up 2- 4 weeks later with a booster dose. This can be done by choosing two commercially available vaccines with different strains, a commercial and autogenous vaccine or two autogenous vaccines with different strains. The concept is that the two different vaccination will boost the immune response to the conserved epitopes between genetically different influenza A strains. An epitope is the part of an antigen (influenza virus) molecule that is recognized by the immune system, to which an antibody attaches itself.
For example using two autogenous vaccines:
- Prime vaccine included: H1 γ N1, H1 δ N2, H3 hl N2, and pH1 N1.
- Boost vaccine included: H1 α N2, H1 δ2 N2, H3 IV N2, and H3 prov N2
A scientific paper by Van Reeth et al. concluded that heterologous (different) prime boost gave equivalent protection to homologous (same) challenge when giving either a monovalent (single-flu antigen vaccine) or bivalent (two-flu antigen) vaccine. The use of genetically different influenza A strains heterologous prime boost showed broader antibody cross reactivity.
The author has had several clients try this strategy. At this time the herds have yet to be challenged with a new influenza A strain.
In closing, there are multiple options in approaching IAV-S. The first step is working with your veterinarian in choose the right type of vaccine for your herd, commercial versus autogenous. The next step is timing of administration of the vaccine to the herd.