Unit 5
Principles of Control of Infectious Disease

General concepts of prevention, control and eradication
of disease in an animal population

There are 3 concepts of control of infectious disease that will be examined in this unit. These are Prevention of disease within a group on animals, Control of pre-existing diseases within a group of animals, and Eradication of disease from an animal population.

 

I. Prevention of Disease within a group of animals

A. Quarantine and testing of animals before introducing into the group or, maintaining a completely closed herd, with no live animal additions (must use artificial insemination to introduce new genetic material)

B. Vaccination

1. doesn't prevent exposure to disease agent, but will hopefully prevent disease when exposed

 

II. Control of Disease within a group of animals -- reduction of disease to biologically +/or economically feasible levels

A number of sub-populations of animals exists within a diseased herd. They consist of :

1. infected animals

a. sick
b. "apparently" normal

2. uninfected animals

a. susceptible
b. non-susceptible

The goals of disease control within the herd or flock are threefold:

1. decrease prevalence of existing disease
2. decrease incidence of new infection
3. decrease morbidity and mortality rates

A number of management practices can be used to carry out these goals.

1. To decrease the number of currently infected animals, one must:

    • identify the infected animals
    • cull them from the herd or flock or
    • treat infected animals promptly and effectively

2. To decrease the incidence of new infection, one must reduce the infection pressure on uninfected, susceptible animals within the herd or flock. This can be accomplished through a combination of the following environmental management techniques:

    • Sanitation -- maintain a clean environment, feed and water supply. Many agents are spread from animal to animal by fecal contamination of feed, water and the environment.
    • Provide "controlled" infection pressure
      • lower the level of exposure in environment
      • expose new additions to limited doses of environmental agents (add small amounts of fecal material to isolated animals)
    • Quarantine new additions; isolate sick animals
    • Don't mix animals from different sources
    • Separate neonates from dams early in life (assure adequate colostrum intake)

Another way of decreasing the incidence of new infections in a herd is to decrease host risk factors. This can be done by:

    • providing adequate colostral immunity
    • vaccinating to enhance immunity within herd
    • decreasing stress on the animals
    • providing proper nutrition

Morbidity and mortality can be decreased by adhering to the above described environment and host management techniques.

 

III. Eradication of Disease from Herd/Geographic Area can theoretically be accomplished by several techniques outlined below.

1. Complete Depopulation -- destruction & disposal of all clinically affected and exposed animals

This can be either a compulsory program dictated and supervised by governmental agencies, or it can be voluntarily carried out by a producer who is no longer able to realize a profit due to the deleterious effects of disease in his animals.

2. Limited depopulation ­ accomplished by testing and removal of animals identified as carrying the disease "Test-and-Removal"

This method relies on diagnostic tests to detect exposed/carrier animals. No test is 100% sensitive and specific. That means that either some positive animals will go undetected or some negative animals will be removed from the herd, or both will happen.

3. Elimination of disease agent by treatment

An example of this is the elimination of psittacosis (Chlamydiosis) from the pet bird population. In order for this method to be effective, the drug must be very effective in killing the agent. The drugs are usually given for long periods of time.

The advantages of completely eradicating a disease from a group of animals are threefold:

1. There is no disease to decrease productivity
2. There is no need for vaccination (cost)
3. There is no need for treatment (cost)

There are, however, disadvantages that must be weighed against the advantages listed above. Some of the disadvantages are:

1. The cost of eradication, both in actual value of animals removed, and the value of their genetic potential, loss of income if depopulation is performed and cost of treatment if that is the alternative chosen.

2. The need for specific, sensitive tests to detect the agent.

Any successful disease eradication Program Requires Continued Surveillance for Disease to assure that it does not return .

This surveillance includes within herd surveillance for the presence of the agent, as well as surveillance within the given geographic region (an example is surveillance within the United States for foreign animal diseases)

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