Unit 5
Principles of Control of Infectious Disease

Vaccination as a Means of Disease Control

A vaccine is composed of material from an infectious agent that induces immunologically mediated resistance to a disease. The vaccine may contain one or more adjuvants such as alum, aluminum hydroxide, oils, or other materials that enhance the immune response.

Vaccines are not used to treat disease, but rather, to prevent disease by establishing an acquired resistance to the disease causing agent.

The material from the infectious agent may be present in several forms in a vaccine.

Live (attenuated) vaccines contain living organisms that have been rendered less virulent, but are still capable of multiplication and spread within the body.

Advantages of using attenuated live vaccines are:

    • provide strong, long lasting immunity
    • fewer inoculating doses are required
    • vaccine is often effective by nonparenteral routes (oral, intranasal)
    • less chance of hypersensitivity, since the initial antigenic mass is less

Disadvantages of using attenuated live vaccines are:

    • it may cause a mild form of the disease
    • it may revert to a virulent state
    • the attenuated agent may remain in the host/environment
    • it may infect the fetus and cause abortion/birth defects
    • has a limited shelf life; defined storage conditions
    • possibility of contamination with other adventitious agents (canine vaccine contaminated with Blue Tongue virus) during production

Killed (inactivated) vaccines contain organisms that have been inactivated by treatment with formalin or other chemicals.

Advantages of using killed vaccines are:

    • the vaccine is unlikely to cause disease
    • the vaccine usually is far more stable in storage

Disadvantages of using killed vaccines are:

    • possibility of incomplete inactivation of the agent
    • immunity is short-lived
    • effective only by the parenteral route
    • multiple doses are usually required
    • hypersensitivity may occur due to greater antigenic mass
    • use of adjuvants may cause local reactions

 

Biosynthetic, genetically engineered vaccines may contain agents that have been manipulated to remove virulence factors from the agent, rendering them incapable of causing disease even though they can reproduce; or they may be composed of a noninfectious, protective subunit immunogen of the agent.

Advantages of using genetically engineered vaccines are:

    • safety (unlikely to cause disease or carry other adventitious agents)
    • subunit immunogens are usually stable and easy to store
    • ability to distinguish the genetically engineered agent from field strains of the agent serologically

Disadvantages of using genetically engineered vaccines are:

    • cost is higher due to high cost of research and development
    • short duration of immunity, multiple doses required with subunit vaccines

A number of factors determine whether vaccination to prevent a given disease is desirable or even possible.

  1. Positive identification of the causative agent(s) of the disease is necessary. In many diseases, the exact etiology (causation) is unknown and vaccination may result in immunity to an agent that may not be involved in the current disease. Proper diagnosis of the disease by a qualified veterinarian and diagnostic laboratory is essential!!
  2. If the causative agent is known, will an immune response to the agent protect against disease? Sometimes the immune response of the animal to the agent is what actually causes the disease.
  3. Is the risk of vaccinating the animal greater than the risk of disease? There are a variety of adverse reactions to vaccination, such as toxic or allergic reactions to vaccine components, establishment of a latent, attenuated infection in the animal, and infection of the developing fetus resulting in malformation or death.

An "Ideal Vaccine", if there were such a thing, would have the following properties:

  1. provide effective resistance to disease (to the host animal and/or to it's fetus or offspring)
  2. provide long-lasting protection
  3. be free of adverse side effects
  4. be inexpensive
  5. be stable, and suitable for use in mass vaccination
  6. establish an immune response which is distinguishable from natural infection. This is especially important if vaccination is used in a disease eradication program, such as pseudorabies eradication in the swine population.

In order for a vaccine to promote "effective resistance" to disease in an animal, it must:

  • induce the right type of immune response in the animal.

Vaccines that stimulate a B cell immune response are effective against agents which are best neutralized and cleared from the animal by antibodies.

Vaccines that stimulate a T cell immune response are effective against agents that are cleared from the animal by mechanisms of cell-mediated immunity.

  • induce an immune response in the right place.

Production of circulating IgG, and IgM may be effective against agents that cause systemic infections.

Production of IgA may be most effective in preventing infection with agents that enter through mucus membranes.

  • induce an immune response to the right antigens.
    • outer surface protein
    • receptor/attachment factor
    • toxin ("toxoid" vaccines)

There are a number of ways in which vaccines are administered to animals. Administration by the parenteral (injection) route, either subcutaneously (SC-under the skin) or intramuscularly (IM-into the muscle) usually results in development of a systemic immune response. Administration through intranasal instillation, aerosol, or in the feed or drinking water will stimulate a local immune response with secretion of IgA by the mucous membranes. Vaccines should only be given by the route recommended by the manufacturer!

No vaccine can be expected to be 100% effective in all cases. There are a number of reasons for vaccination failures, some of which are listed below:

  • the animal may have been exposed to the agent prior to vaccination
  • the infectious agent may have mutated beyond recognition by the immune system
  • the infectious agent may be present at such a high concentration that it overwhelms the host immune system
  • the host immune system may be suppressed due to stress, poor nutrition or other concurrent infections (especially viral infections)
  • there may be interference with the host immune response due to high levels of maternal antibody, presence of too many antigens within the vaccine or mixing and administration of multiple vaccines
  • the vaccine may not have been stored, handled or administered properly
  • the vaccine may never have been properly tested for efficacy (until recently, the ability of a vaccine to produce reasonable levels of serum antibodies was accepted as proof of efficacy, rather than the ability to prevent disease in challenged animals)
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