Environmental Engineering Reference
In-Depth Information
Host
Environment
Agent
Host
Reservoir
VECTOR
Environment
Agent
Environment
FIGURE 2.1
Epidemiologic triangle framework for infectious disease transmission. Top
panel is for non - vectorborne diseases. Bottom panel shows interaction between insect and
pathogen. The two “environment” vertices represent the separate milieu in which the host
and reservoir reside. The vector participates in both triangles and serves as the bridge
between them. Control measures should be targeted at all three/six legs for maximum
effectiveness.
agent (Figure 2.1). For arthropodborne diseases, a second triangle is involved,
representing the biological interactions between the vector (insect) and the envi-
ronment. Host factors include genetic and behavioral susceptibility. The agent
is the infectious organism. The environment is the setting in which the interac-
tion occurs, and includes the social as well as biological environment. For most
effective interruption of disease transmission, interventions must be targeted at
all three legs of the triangle. More nuanced versions of this triangle or concept
exist, but for our purposes, the three-point model will suffice.
Phelps called the interruption of disease at each of these three points the
principle of multiple barriers.
17
It recognizes as axiomatic the fact that “all human
efforts, no matter how well conceived or conscientiously applied, are imperfect
and fallible” (ref. 16, p. 347). Sometimes it is only practical to control or break
one link in the chain. Therefore, the number and type of barriers or interventions
should be determined by the practicality and cost of providing the protection, the
benefits to be derived, and the probable cost if the barriers are not provided. Cost
is used not only in the sense of dollars but also in terms of human misery, loss
of productiveness, ability to enjoy life, and loss of life. Here is a real opportunity
to apply professional judgment to the problems at hand to obtain the maximum
return for the effort expended.
