Vaccination is an important tool for disease control in swine herds. The goal of herd vaccination is to decrease the number of susceptible animals, reducing clinical disease and pathogen spread.
Passive and active immunity are the two methods used for disease protection.
Passive immunization produces immediate and temporary resistance to disease through transfer of antibodies from resistant animals to susceptible animals. Maternal antibodies are the primary source of passive immunity.
Active immunity involves exposure of the animal to an organism by either vaccination or natural exposure, resulting in a protective immune response.
Active immunization by vaccination is the most common type of immunity used to protect pigs from disease. Sow vaccination results in both active immunity and disease protection for the sow and passive immunity through antibodies in the colostrum for the pigs.
In contrast to passive immunity, active immunization has a number of advantages, including prolonged protection against disease.
An active immune response occurs when cells present an antigen from either infection or vaccination to lymphocytes, resulting in a primary immune response. It takes about a week for the primary immune response to develop, and longer for the stronger, secondary immune response (Figure 1).
The vaccine and the adjuvant administered determine the nature, length and effectiveness of the immune response. The advantage of vaccination over “controlled” infection with the organism is the identity and amount of each pathogen has been established. Therefore, vaccination is a safer, more reproducible method for exposure to pathogens.
Ideally, the protective immune response induced by a vaccine will be identical to the response from infection, with none of the adverse side effects associated with disease.
Types of Vaccines
Vaccines consist of either live or killed pathogens. Each type of vaccine has advantages and disadvantages summarized in Table 1.
The goal of live vaccines is to closely mimic natural infection with minimal disease. In contrast, killed vaccines consist of inactivated organisms similar to the living organisms.
Site of administration is based on vaccine type, the adjuvant and what works best to produce immunity. Most vaccines are given systemically, either in the subcutaneous tissues under the skin or in the muscle.
Several vaccines are effectively administered orally. Animals must be healthy enough to eat or drink so all animals ingest an adequate amount of the vaccine.
Needleless injection systems are also increasing in popularity and can be very effective. They can enhance the uptake of vaccine antigens by antigen-presenting cells, improve immune response and safety.
Killed vaccines are made of inactivated bacteria or viruses. They can consist of the whole organisms, or select proteins from the organism, such as the cell membranes or smaller pieces called subunit vaccines. The organisms are chemically inactivated, leaving them as similar as possible to the live organisms.
Inactivated, purified antigens alone usually don't induce an adequate immune response. Therefore, adjuvants are used to increase the immune response by trapping the antigen at sites that enhance their uptake by antigen-presenting cells and increase exposure to lymphocytes.
Adjuvants also induce inflammation, further activating the various cells of the immune system of the pig.
Compounds used as adjuvants include aluminum salts, water-in-oil or oil-in-water emulsions, natural bacterial fractions, surface-active agents and their combinations.
Two doses of an inactivated vaccine are normally needed for a strong immune response. Occasionally, one vaccination is enough to induce protection, as with some mycoplasma vaccines. With one-dose vaccines, infection serves as the booster.
|More rapid protection||Potential to revert to virulence|
|Longer lasting immunity||May be immunosuppressive|
|One dose usually sufficient||May cause abortion|
|No adjuvant needed||May be contaminated with other viruses|
|Improved cell-mediated immunity||Must be handled carefully to keep alive|
|Safety||Not as immunogenic (likely to cause disease)|
|No reversion to virulence||Requires adjuvants|
|Less likely to suppress immunity||Usually requires two vaccinations|
|Increased vaccine stability||Immunity of shorter duration|
One-dose vaccines may not be effective, however, when factors that decrease vaccine efficacy are present. Examples include infections occurring too quickly following vaccination or when the infection overwhelms the initial immune response, when infection occurs too quickly following vaccination or the infection overwhelms the initial immune response.
Inactivated vaccines tend to produce strong antibody responses; however, cell-mediated immune responses can also be generated.
Live vaccines, too, can consist of either viruses or bacteria. Virulent live organisms can't be used safely in vaccines, however.
A number of methods are used to attenuate or reduce the ability of the organism to cause disease.
One of the most common is to grow an organism a long time in a laboratory. Continuous culturing often causes the organism to lose the mechanism to cause disease.
While this technique has effectively produced live vaccines, there remains a danger that when the pathogen is put back into the host, reversion to virulence will cause disease.
Another technique to decrease organism virulence is to alter or remove specific genes associated with disease. This technique only works if the deleted gene is not required to produce a protective immune response.
Genetic alteration of the organism can also differentiate vaccine from wild-type responses. Pseudorabies vaccines are an example where genetic modification produced a virus incapable of causing disease and allowed differentiation of vaccine from natural infection.
Similar gene deletion technology has been used in other vaccines. Other genetic alterations of pathogens are being developed to produce safer and more efficacious vaccines.
Live vaccine use in animals with a suppressed immune system increases disease risks, so their use is not recommended.
Typically, a single dose of live vaccine is enough to induce a strong immune response, therefore, adjuvants are not required.
In recent years, vaccines have been studied that contain specific genes known to induce protection against either viruses or bacteria. This technology is based on the DNA or genetic material from the organism being injected into the animal. The cells in the animal produce the specific proteins coded by the DNA, which then induce an immune response. An advantage is DNA vaccines use specific genes, rather than the whole organism, preventing any possibility of disease.
In addition, genes from multiple pathogens can be combined and specific cytokine genes added to provide the immune response needed for disease protection. The primary drawbacks of DNA vaccines are their costs and the lack of knowledge of the genes needed for protection against many pathogens.
Herd Immunity, Vaccine Failures
In a perfect world, all vaccines would always protect all animals. However, there are many reasons why vaccines fail. In some cases, the vaccine may be ineffective. Always use vaccines from reputable manufacturers.
Of greater significance is the failure of known, effective vaccines to induce protective immunity. This type of failure may be due to improper vaccine administration, improper syringe use, or inactivation of a live vaccine through poor storage or handling. A syringe used for antibiotic therapy or chemical sterilization of a syringe will destroy a vaccine.
Carefully following manufacturer's directions for vaccine storage, handling and administration with new syringes and needles will prevent these problems.
Some animals in a herd will always fail to mount an effective immune response. Genetic or environmental factors, improper vaccination technique, or vaccine handling may cause this failure. The range of immune responses within a herd tends to follow a normal distribution pattern (Figure 2).
Response to vaccination will vary between vaccines, the herd conditions and other factors that may impact vaccine efficacy. Any factor that decreases the number of animals protected by vaccination increases the economic loss to the producer.
Since the development of an immune response is a biological activity, any factor that affects the normal function of the pig also potentially impacts its ability to induce immunity. Inadequate environment or improper nutrition will have a profound impact on this ability.
The presence of maternal antibodies or the circulation of immunosuppressive pathogens, such as porcine reproductive and respiratory syndrome (PRRS) virus, can increase the number of animals that fail to develop adequate protective immunity following vaccination.
Maternal antibodies can be an important factor affecting vaccine efficacy. Their importance varies with the vaccine and disease. For example, low levels of maternal antibodies against swine influenza virus (SIV) have been shown to block vaccine efficacy.
In contrast, very high levels of maternal antibody levels are required to block mycoplasma vaccines.
The presence of PRRS virus circulating in pigs at the time of vaccination appears to diminish the protective capabilities of mycoplasma and SIV vaccines. How this mechanism works remains unknown. PRRS virus infection is an important factor to consider in any vaccine program.
Organisms use a number of different techniques to survive in the pig. Some affect vaccine efficacy.
One evasive mechanism is the location of the organism. For example, Mycoplasmal pneumonia, which colonizes the cilia in the respiratory tract, is physically separated from the immune system. This makes a good immune response by vaccination or infection challenging.
Other organisms regularly change the proteins on their surfaces or their genetic makeup. Some do this intentionally to escape the immune system, while other changes are unintentional mistakes.
For example, as PRRS virus replicates within cells, mistakes are made. This results in a constant changing of the genetics of the virus. If there are enough changes over time, a new strain of the virus is formed. Then, the immune system must begin the process of developing a new immune response again.
Another evasive technique used by viruses is to combine genetic materials from different strains to produce a genetically different virus, as occurs with SIV. The new H1N2 subtype appears to have developed from the H1N1 subtype combining with the H3N2 subtype. The ability of the earlier immune response to each subtype to protect against the new subtype will depend on how closely the new subtype matches the old.
By constant and frequent change, pathogens are able to evade the immune system and survive. Vaccines developed against one strain or subtype of bacteria or virus may not be effective against new organisms.
The amount of change differs between pathogens. The toughest pathogens exhibit the most variation. The success of the pathogen is reflected in a vaccine's failure. Addressing this problem may require changes in vaccine development and licensing procedures.
Vaccines are important tools for herd immunity. Limited knowledge has sometimes led to production of less than optimal vaccines for some of our more problematic swine pathogens.
Vaccine failure can have many genetic, environmental or management causes.
Overall animal health, the presence or absence of maternal antibodies and immunosuppressive pathogens must also be considered for herd vaccination programs.
As our knowledge of the swine immune system and the pathogens increases, technology will provide vaccines with increased efficacy and ease of use for pork producers. rActive immunization by vaccination is the most common type of immunity used to protect pigs from disease.
Table 2. Glossary or Terms of Immunity
Adjuvant: Any substance that enhances the immune system when administered with an antigen.
Antigen: Any foreign substance that can induce an immune response.
Antigen-presenting cell: Cells that can ingest, process and present antigen on their cell surface for T cell activation. The main antigen-presenting cells are macrophages, dendritic cells and B lymphocytes.
Cell-mediated immunity: An immune response mediated by T lymphocytes and macrophages.
Cytokines: Proteins made by cells that affect the behavior of other cells. Used by cells to communicate with each other.
Humoral Immune Response: Antibody-mediated immunity. Lymphocytes active in producing antibodies are B lymphocytes.
Lymphocytes: A small white blood cell that comprises the primary effector cells of the immune system. B lymphocytes produce antibodies and T lymphocytes are responsible for mediating cell-mediated immunity.
Macrophages: Large cells in the tissues that engulf and process foreign substances.
Pathogen: An organism that causes disease.
Virulence: The ability of an organism to cause disease.