#publichealth crash course on Epidemiological Triad:
The causes of a disease are often considered in terms of a
triad of factors (Epidemiological Triad): the host, the agent, and the environment. For
many diseases, it is also useful to add a fourth factor, the
vector. In measles, the host is a human who is
susceptible to measles infection, the agent is a highly infectious virus that can produce serious disease in humans, and
the environment is a population of unvaccinated individuals,
which enables unvaccinated susceptible individuals to be
exposed to others who are infectious. The vector in this case
is relatively unimportant. In malaria, however, the host,
agent, and environment are all significant, but the vector, the
Anopheles mosquito, assumes paramount importance in the
spread of disease.
Host factors are responsible for the degree to which the
individual is able to adapt to the stressors produced by the
agent. Host resistance is influenced by a person’s genotype
(e.g., dark skin reduces sunburn), nutritional status and
body mass index (e.g., obesity increases susceptibility to
many diseases), immune system (e.g., compromised immunity reduces resistance to cancer as well as microbial disease),
and social behavior (e.g., physical exercise enhances resistance to many diseases, including depression). Several factors
can work synergistically, such as nutrition and immune
status. Measles is seldom fatal in well-nourished children,
even in the absence of measles immunization and modern
medical care. By contrast, 25% of children with marasmus
(starvation) or kwashiorkor (protein-calorie malnutrition
related to weaning) may die from complications of measles.
Agents of disease or illness can be divided into several
categories. Biologic agents include allergens, infectious
organisms (e.g., bacteria, viruses), biologic toxins (e.g., botulinum toxin), and foods (e.g., high-fat diet). Chemical agents
include chemical toxins (e.g., lead) and dusts, which can
cause acute or chronic illness. Physical agents include kinetic
energy (e.g., involving bullet wounds, blunt trauma, and
crash injuries), radiation, heat, cold, and noise. Epidemiologists now are studying the extent to which social and psychological stressors can be considered agents in the
development of health problems.
The environment influences the probability and circumstances of contact between the host and the agent. Poor
restaurant sanitation increases the probability that patrons
will be exposed to Salmonella infections. Poor roads and
adverse weather conditions increase the number of automobile collisions and airplane crashes. The environment also
includes social, political, and economic factors. Crowded
homes and schools make exposure to infectious diseases
more likely, and the political structure and economic health
of a society influence the nutritional and vaccine status of
its members.
Vectors of disease include insects (e.g., mosquitoes associated with spread of malaria), arachnids (e.g., ticks associated with Lyme disease), and mammals (e.g., raccoons
associated with rabies in eastern U.S.). The concept of the
vector can be applied more widely, however, to include
human groups (e.g., vendors of heroin, cocaine, and methamphetamine) and even inanimate objects that serve as
vehicles to transmit disease (e.g., contaminated needles associated with hepatitis and AIDS). A vector may be considered
part of the environment, or it may be treated separately. To be an effective transmitter of disease, the vector
must have a specific relationship to the agent, the environment, and the host.
In the case of human malaria, the vector is a mosquito of
the genus Anopheles, the agent is a parasitic organism of the
genus Plasmodium, the host is a human, and the environment includes standing water that enables the mosquito to
breed and to come into contact with the host. Specifically,
the plasmodium must complete part of its life cycle within
the mosquito; the climate must be relatively warm and
provide a wet environment in which the mosquito can breed;
the mosquito must have the opportunity to bite humans
(usually at night, in houses where sleeping people lack
screens and mosquito nets) and thereby spread the disease;
the host must be bitten by an infected mosquito; and the host
must be susceptible to the disease.
VIDEO KEYWORDS:
EPIDEMIOLOGICAL TRIAD, TRIADS, EPIDEMIOLOGICAL TRIAD EXAMPLES, HOST FACTORS IN THE CAUSATION OF DISEASE, IMPORTANCE OF EPIDEMIOLOGICAL TRIAD, DISEASE TRIAD, EPIDEMIOLOGY TRIAD, EPIDEMIOLOGY TRIANGLE, DISEASE TRIAD EXAMPLES, HOST AGENT ENVIRONMENT MODEL, HOST AGENT ENVIRONMENT, HOST AGENT, HOST VECTOR RELATIONSHIP, HOST VECTOR, HOST VECTOR SYSTEM, DISEASE CAUSATION IN EPIDEMIOLOGY, DISEASE CAUSATION, DISEASE CAUSATION THEORIES, VECTOR, HOST, AGENT, ENVIRONMENT, DISEASE TRIANGLE, MPH, EPIDEMILOGICAL TRIAD PSM, EPIDEMIOLOGICAL TRIAD IN COMMUNITY MEDICINE, EPIDEMIOLOGICAL TRIAD IN COMMUNITY HEALTH NURSING
19 мар 2023
