National Cervid Farm-Level Biosecurity Standard
Chapter 2: Principles of disease transmission

Understanding the determinants of infectious disease is necessary for their prevention and control. Infectious diseases in cervids result from a complex interaction of three factors referred to as the disease triad:

  • An animal that is susceptible to disease (the host);
  • A pathogen such as a bacterium, virus, prion, fungus or parasite capable of causing disease (the agent); and
  • An opportunity for the host and agent to come into contact (the environment).
The disease triad

The disease triad. Description follows.

Description for The disease triad

The disease triad is represented by three circles that overlap in the centre of the image. The top circle represents Cervid (Host), the circle to the left represents Pathogen (Agent), the circle to the right represents Environment. In the centre where the three circles meet is a picture of a scale tipping towards the Pathogen (Agent) circle. The left corner of the image has text and an arrow pointing to the centre of the scale. The text reads: Disease may occur when a host, agent, and the environment combine and sufficient factors tip the balance resulting in disease. The right corner of the image has text and an arrow point to the Environment circle. This text reads: Other cervids and animals, the facility, food, water, soil, insects, equipment and humans.

Figure 1: The disease triad illustrates the relationship between a cervid (the host), a pathogen (the agent), and the environment. Disease may occur when a susceptible animal, a pathogen, and an environment favourable for disease development combine. There are many factors that influence whether disease will occur including the health of the animal, adequate nutrition, external stresses, the number of pathogens present and the ability of the pathogen to cause disease. No one element is responsible for the expression of the disease, but the 3 elements of presence of a pathogen, susceptible host and negative environmental factors combine and tip the scale to favour the expression of a disease. Understanding the elements necessary for disease to occur provides the ability to influence and manage their impact to minimize disease.

Tipping Points for Disease Expression

Tipping Points for Disease Expression. Description follows.

Description for Tipping Points for Disease Expression

The image has both a left and a right margin. Below the left margin is a title. The text reads: Impact on host by stressors (summation of agent and environment). To the left of the left margin is an arrow, that gets wider as it runs from the top to the point at the bottom. On the left margin is a scale from. From top to bottom the text reads: Negligible, Low, Medium, High.

Below the right margin is a title. The text reads: When disease is expressed. On the right margin is a scale. The text reads from top to bottom: Diseased, Healthy, Diseased is further divided, from top to bottom as Clinical Disease, Subclinical Disease. These are indicated by three vertical co-linear coloured line: red (clinical disease, top), yellow (sub-clinical disease, bottom) and green (healthy, bottom).

In the centre of the image is a triangle pointed upwards serving as a fulcrum on which is balanced a double ended red dotted arrow running from bottom left margin (High) to top right (Clinical Disease).

Figure 2: Tipping point for disease – The cumulative impact of potential stressors (such as exposure to a significant accumulation of a parasite in addition to poor nutrition, recent transport, or overcrowding) on an animal can overwhelm the animal's ability to resist infection resulting in disease. The disease may be sub-clinical (the animal is infected yet appears healthy) or clinical (the animal is infected and appears sick) depending on the degree of the impact of the stressors, characteristics of the disease agent and the health status of the animal prior to exposure).

There are many host, agent and environmental variables that influence whether an animal will become diseased. Infection prevention and control programs rely on approaches that target these environmental and host factors. Three broad approaches to the prevention and control of infectious diseases include:

  • Decreasing exposure of animals to pathogens: This is the most important approach; if contact between animals and pathogens is prevented, infection and resulting disease will not occur. Animals exposed to a pathogen may not become infected, or if infected may, or may not, show clinical signs of disease. There must be a sufficient number of viable organisms (an infectious dose) that can bypass the animal's defence systems and then multiply to cause disease. Many of the biosecurity practices focus on reducing exposure, including separating healthy animals from animals that are ill or of undetermined health (and therefore may be carrying a pathogen), minimizing contact between individuals or groups over fences and pens, and cleaning and disinfecting equipment. Maintaining stocking density at the appropriate level can reduce animal-to-animal contact, and contamination of feed, pens and pastures, thereby reducing the accumulation of pathogens. For certain diseases, this includes managing biological vectors such as insects and other pests.
  • Decreasing susceptibility of animals to disease: There are factors that can be managed to reduce susceptibility to disease including: providing proper nutrition, managing underlying disease, reducing stress, providing shelter, implementing effective parasite control, managing pain, and the appropriate use of antibiotics and other medications). There are other factors that affect an animal's susceptibility to disease that cannot be influenced to a significant degree: age, sex, genetics, and pregnancy.
  • Increasing resistance to disease: Vaccination is the primary method used to improve resistance to certain, specific infectious diseases.
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