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College of Veterinary Medicine Nursing & Allied Health


          Level I  › Epidemiologic Systems Approach ›  


Epidemiology Systems Approach



Determinants of Disease

* Intrinsic (Genetic, Age, Physiological state)
* Extrinsic (Animal use, Level of husbandry)

Etiological agents as Determinants of Disease

The concept of "one agent - one disease" ( Koch's postulates) - do not hold up to scrutiny any longer as more complex host-parasite relationships are established.
Etiological Agents

1. Physical - mechanical injuries - trauma, thermal, radiation, noise (psychological stress), light, etc.
2. Chemical - organic/inorganic forms, gases
3. Nutritional - metabolic, primary/secondary, nutritional deficiencies, hormonal imbalances, etc.
4. Biological - all living organisms that cause disease or infection - bacteria, virus, rickettsia, protozoa, fungi, etc. Genetic defects are included here.


Epidemiologic/Ecologic Concepts
Health and Disease

Health is defined as a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.
Medical ecology and epidemiology

Concepts defining the interrelationships of an organism and its environment have been derived by that branch of biology called ecology. Application of ecologic concepts is the foundation on which an understanding of the natural history of disease is built. Understanding the many phenomena involved in the natural history of disease, however, requires the input of a wide array of disciplines. The synthesis of information from these various disciplines into a meaningful explanation of the phenomena observed is termed medical ecology.

Host-parasite Relationships

Any susceptible organism that may harbor a parasite and more specifically, is usually a parasitized or infected organism.

In the widest terms, an organism living in or on or depending on a host and subsisting on the tissues or discharges of that host. This includes the parasites that are traditionally covered in parasitology as well as a considerable array of microorganisms.

Types of hosts

Definitive host
  One in which the parasite reaches sexual maturity.

Incidental or tangential host
  One which is not necessary for the perpetuation of the parasite but may be involved in a disease process.

Intermediate or secondary host
  One in which the parasite does not reach sexual maturity.

Reservoir host
  The host species in which the parasite is usually maintained over long periods of time.

Paratenic host
  Serves purely for mechanical transporting of agents.









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