Vaccines are commonly used to prevent various pathogenic challenges of viral, bacterial, and protozoan origins that usually lead to diseases affecting health and performance of livestock. Some of the disease challenges in swine where vaccines play a crucial role in preventing and controlling are listed in Table 1.
Table 1. Swine diseases commonly addressed through vaccination.
3 types of vaccines
There are two major types of vaccines normally used in swine production –live and inactivated—while other types of vaccines are seldom used.
1. Live, attenuated vaccines
Live-type vaccines contain either virus or bacteria in small amounts with the objective to infect the host and multiply in its body to produce immunity, preferably with minimal reaction. This leads to the recognition of increased amounts of the same type of pathogen by the host’s immune system, thus resulting in an enhanced immune response.
2. Inactivated/killed vaccines
Inactivated/killed vaccines have inactivated and processed virus or bacteria which then stimulates the immune system for a longer period of time inside the host. Inactivated vaccines are usually combined with an adjuvant (an oil or aluminum hydroxide) to increase their stability, and to stimulate the host immune response.
These include toxoids (contain inactivated toxin of a bacterial pathogen), subunits/conjugates (contain pieces of the pathogen they protect against), and recombinant (contain virus with gene code for a vaccine protein against another virus) vaccines. Autogenous vaccines (autovaccines) are for therapeutic use, individually tailored for a host, made from cultures of pathogens isolated from the infection site.
The immune response
Two different mechanisms are involved in establishing an immune response: the inflammatory and acquired immune responses.
1. The inflammatory response
Inflammation is a non-specific response that occurs very rapidly and leads to the activation of phagocytes (macrophages and neutrophils). The activated phagocytes secrete many different molecules such as cytokines (involved in the recruitment and the activation of other cells)
2. The aquired immune response
Acquired immune responses are associated with immunogenic memory carried out by B cells (humoral) and memory T cells (cellular). These cells are generated from naïve precursor cells after exposure to the microbial antigens. Upon interaction with the antigen presenting cells, B cells start to secrete specific antibodies. Naïve T cells rapidly proliferate and differentiate into effector T cells which target the host cells infected by pathogens. This phase of proliferation is followed by a contraction phase during which about 90% of the effector T cells die, whereas the remaining cells differentiate into memory T cells. Thus the immune response is highly complex and various cells interact with one another to produce the desired effect.
Causes and consequences of vaccination failure
Factors leading to higher rates of vaccination failure result either from 1) a failure to provide potent vaccines properly to the host or 2) immune suppression in the host.
Vaccine delivery can be hampered by contamination, improper storage or procedural errors. The five factors causing immune suppression directly in the animal include stress, poor nutrition, infectious agents, maternal antibody interference, and mycotoxins.