2.1 Objectives:

• 1) To describe the use of the "systems approach" in epidemiology
• 2) To define concepts and terms of epidemiologic/ecologic significance and their interrelationships.
• 3) To describe the concept of the "epidemiologic triad" and its application
• 4) To examine the concept of multiple determinants with a focus on environmental determinants of diseases ion populations. 

2.2 Introduction

The usage of the concept of a "system" here refers to the assembly of factors connected with each other in some form of coherent relationship; all working towards the same or common goal.

For example,

An epidemiologic study of a specific disease or health entity must begin with the task of identifying and defining the various states of health (or disease), the determinants that affect the system under study, the relationships amongst the variables as these interact to affect the outcome of the specific condition under study etc. 

2.3 Epidemiologic/Ecologic Concepts

2.3.1 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.

2.3.2 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.

2.3.3 Host-parasite Relationships:

• 1) Host = any susceptible organism that may harbor a parasite and more specifically, is usually a parasitized or infected organism.

  Types of hosts:

  • a) Definitive host = one in which the parasite reaches sexual maturity.
  • b) Incidental or tangential host = one which is not necessary for the perpetuation of the parasite but may be involved in a disease process.
  • c) Intermediate or secondary host = one in which the parasite does not reach sexual maturity.
  • d) Reservoir host = the host species in which the parasite is usually maintained over long periods of time.
  • e) Paratenic host = serves purely for mechanical transporting of agents.

   

• 2) Parasite = 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. 

 

2.3.4 The Epidemiologic Triad:

2.4  MULTIPLE DETERMINANTS (MULTIPLE CAUSALITY)

2.4.1 Host Determinants of Disease

• a) Intrinsic (Genetic, Age, Physiological state)
• b) Extrinsic (Animal use, Level of husbandry)

2.4.2 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. 

2.4.2.1 Etiological Agents:

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

2.4.3 Environmental Determinants of Disease 

A triad of Environmental factors is shown below.