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Disease Concepts

by Dr. Timothy Deveau
Date Posted: 01-01-2010

No matter the species of livestock, wildlife or companion animals, the method of how disease infects and affects individuals and populations of animals follow similar concepts.  The knowledge of these concepts allows the development of programs for prevention, treatment and eradication of diseases. This article will attempt to explain in examples these concepts and how programs are developed.

The disease triad is an accepted explanation of how diseases enter individuals and populations and become
established.  The triad is formed by the host, which can be an individual or population of animals, the pathogen,
which is the agent that can cause the malady, and the environment surrounds the host and the pathogen.  When
conditions bring the host, the pathogen and the environment into alignment, then disease can be established. An example of the triad would be the relationship between Fascioloides magna, the giant liver fluke and whitetail deer. The liver fluke requires the use of a mollusk such as snail to be an intermediate host to complete its life cycle.  Therefore, an important part of the environmental portion of the triad is moist environmental conditions that are vital to the snail’s life cycle. The liver flukes cannot be established, even if introduced, in an arid climate that will not sustain the mollusk or intermediate host.

Also deer that have liver flukes present in their livers may not demonstrate clinical disease as long as they are not stressed by adverse environmental conditions. Therefore, the disease caused by fluke infestation in the liver of a whitetail deer is caused by the fluke, a moist environment and environmental stresses as well as many other factors.

If we remove these factors, whether by using flukacides to kill the immature fluke in the deer, molluscacides to
remove the snail (intermediate host) or drain the pasture to reduce snail habitat, then we can reduce or eradicate liver fluke infestation from farm raised deer. If we have a different species of host such a sheep, one liver fluke could cause the death of the individual.  Whether the pathogen is a living organism like a parasite or bacteria or a metabolic disorder like a mineral or nutrient deficiency, the alignment of the host and environment are required to create clinical disease.

Most programs for prevention of disease attempt to enable the host to fight off attack of the pathogen by its immune response. Vaccination of animals against specific diseases gives the animal the ability to recognize the pathogen at the time exposure and quickly mount an immune response. Part of this program is providing proper nutrition and reduced environmental stress so that the animal’s immune response is sufficient to prevent the establishment or colonization by the bacteria or virus.  Attempting to keep the pathogen out of the host environment is also an important program in prevention. Testing or isolation of new animals prior to their introduction onto a farm attempts to find pathogens  before they are introduced into established herds. Biosecurity practices such as foot baths, clean or dedicated clothing and cleaning and disinfecting new or used equipment are methods to reduce the introduction of pathogen into “clean” herds.

Treatments of disease attempt to fortify the host’s immune response and reduce the pathogen’s ability to multiply and establish itself in the host. Antibiotics work by either killing or reducing the bacteria’s ability to multiply and colonize so that the animal’s immune response can then mop up the infection. Antiparasitic drugs can be use as a preventative prophylaxis or as a treatment for existing parasite infections.  As in the programs for preventing diseases, treatment regimens are more successful when the host is in the best condition. Having the host on an optimum level of nutrition and stress reduced environment increase treatment success rates.

Eradication programs attack the disease at all parts of the triad. With certain diseases, the reduction of infected or susceptible high risk hosts is the only means of reducing the multiplication of the disease and stops its spread. Viruses such as those that cause foot and mouth disease  need animals to multiply the organisms and precipitates its spread. If animal concentrations are reduced by removing sick and susceptibles from the population, then the disease is halted because the ability of the pathogen to spread from infected animals to susceptible animals  is reduced. Cleaning and disinfecting after depopulation reduces pathogens from the environment.  Proper cleaning and disinfection require removal of organic materials from the environment prior to disinfection. The organic materials potential protect and nurture the pathogen and block effectiveness of the disinfectants.

As the three legs of a tripod are required for it to stand, the three elements of the disease triad are required for
illness.  If we interrupt or eliminate any portion of the triad, we can prevent, treat or eradicate disease.