Natural History of Disease in Man, Interactions of Agent, Host and Environment

Natural History of Disease in Man

Natural history of a disease includes:

• Pathological onset
• The presymptomatic stage from onset of pathological changes to the first appearance of symptoms and signs.
• The stage when the disease is clinically obvious and may be subjected to remissions and relapses, regress spontaneously, or progress to death.

Detection and the treatment at any stage cam alter the natural history of a disease. Every disease has its own natural history.

A. Prepathogenesis period

Every condition of health and disease in man has its own origin in other processes before aman himself is involved. The precipitating and predisposing forces or causes may be continuouslyoperative in the living environment of man. Heredity, social and economic factors and physical environment may be creating a disease stimulus long before the time that man and stimulus begin to interact and disease is produced.

The preliminary interactionof potential agent, host, and environmental factors in disease production is termed as the period of prepathogenesis.

B. Pathogenesis period

The course of a disorder in man from the first interaction with disease provoking stimuli to change in form and function which result in disease until equilibrium is again reached may be termed the natural course of the disorder or the period of pathogenesis. At post-interval equilibrium, the statsus of person might be recovery, disability, defect, or carrier. If the swings is so far from equilibrium that the lever does not spring back, the terminal outcome would be chronic state and death.

C. Late pathogenesis period

During this period, most of the disease individuals develop some kind of disabilities and definitions. It may result either to recovery or in some instances death of the patients.

Interactions of Agent, Host and Environment

Agent, host and environmental factors provide an approach to elucidating causes of incident outbreaks of infectious disease. They are less helpful in predicting when and why outbreaks arise. As public health focus moves to non-infectious health problems such as obesity or mental health, limitations of this traditional approach become clearer. The agents associated with non-infectious diseases are invariably non-specific, often there are no necessary causal factors; host susceptibility cannot be measured, and the environment has complex, interacting layers of influence. In such circumstances agent, host and environment explanations have significant limitations.

The interactions between agent, host and environment can be bidirectional or triangular. There can be interactions between agent and environment, host and environment, agent and host or agent, host and environment. In either case, disease occurs when the equilibrium relationship between these factors is disturbed.

I. Agent- environment interactions

In agent-environment interactions, the agent is directly affected by the environment regardless of the characteristics of the host. This interaction usually occurs during the period of prepathogenesis but may continue during pathogenesis. For example, killing of bacteria exposed to direct sunlight, vaporizing of toxic chemicals by heat etc.

II. Host-environment interactions

In host-environment interactions, the host is directly affected by the environment regardless of the characteristics of the agent. It may occurduring the period of prepathogenesis or pathogenesis. For example, availability of health services, acid rain, behavior of the people etc.

III. Host-agent interactions

In host-agent interactions, the infectious agent after finding the portal of entry, lives and multiplies in specific organ or site of the body, and stimulate the host to respond with signs and symptoms of adisease such as fever, tissue changes and the population of immunity or other defense mechanism. This interaction results complete recovery, defect, disability or death. Otherwise, it may lead to clinical recovery without elimination of the agent.

IV. Agent-host-environment interactions

In this traingular relationship, agent, hot and environment interact and initiate or prevent a disease process. This results during both the prepathogenesis and the pathogenesis process.

Dr. John Gordon has employed the analogy of a lever balance over a fulcrum (environment- E) by weights at either end, representing respectively the agent (A) and host (H) to show the interactions between agent. host and environment and occurrence of disease.

1. Prepathogenesis period- During this period, the system is in balance or in equilibrium state. Up tp this stage, health prevails.

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Fig. At equilibrium

2. Pathogenesis period- This is the disease state in which the balance between agent, host and environment is disturbed. It causes the disease or increases the severity of disease.

a. Changes of the agent: The changes in agent such as appearance of new agent, increase in number of infectious agent, and mutation of agent are important that make disturbance in equilibrium of the fulcrum. It increases the virulence of the infectious agent, and resistance to the adverse environment.

b. Changes of the host: Changes in the host either increases host susceptibility or resistance or immunity to disease. For example, development of risk behavior such as intravenous drug use increase susceptibility to HIV/AIDS. If the measles occurs in childhood, it provides life long immunity.

c. Changes of the environment: The environment changes facilitates agent spread or alter host susceptibility. The leverage of the agent or the host would be increased. For example, faciparum malaria in post rainy season in Nepal, or increase in susceptibility to respiratory infection due to air pollution.

Biological Laws of Diseases

The well known biologic laws of disease are applicable to other conditions affecting man's health. These laws are:

1. Disease results from an imbalance between disease agents and the human host.
2. The nature and extent of the imbalance depends on the nature and characteristics of the host and the agents.
3. The characteristics of the agent and the host and their interaction are largely governed by the environmental factors.

The interaction occurs between micro-organisms and human in communicable diseases. In case of non-communicable diseases, the interaction occurs between human and non-living things.

References

Gordis, L. Epidemiology. third edition. 2004.

Joshi, Banjara. Fundamentals of Epidemiology. Kathmandu: Quality Printing Press, 2007.

Park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.