Environmental Engineering Reference
In-Depth Information
immunized, nor does the immunization need to be effective in creating protec-
tive antibodies in every person; however, the higher the rates of immunity in
the population, the lower the chances that a newly introduced infectious person
will infect enough people for the pathogen to be able to become endemic. This
concept is called
herd immunity
.
The United States has enjoyed high levels of vaccine coverage for most
childhood vaccine preventable diseases. However, there are and will always be
subpopulations who are opposed to vaccination for moral and/or religious rea-
sons. In 2005, an outbreak of polio was reported among unvaccinated Amish
residents in remote Minnesota.
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This group had eschewed vaccination on reli-
gious grounds, and U.S. law allows for such exemptions. When one member of
the group traveled abroad (it is hypothesized), he was infected with polio (and
became a carrier) and brought the disease back to the other susceptible members
of his community. Since few people had been vaccinated, the virus was able to
pass from human to human. Sanitary measures and emergency vaccination were
used to control the outbreak, but not before considerable suffering in the small
community.
In summary, vaccines play an integral role in protecting the public health.
The use of vaccines is problematic due to ethical, biological, and social reasons.
However, in the instances that they are available, they can be a crucial tool in
breaking the cycle of transmission.
Control of Infectious Agents and Vectors
Arthropods involved in the transmission of human (and animal) disease are called
vectors
. Common vectorborne (or arthropodborne) include malaria, yellow fever,
plague, dengue, West Nile virus, Japanese encephalitis, Lyme disease, Chagas
disease, sleeping sickness, and leishmaniasis. These diseases combined are the
most common infectious diseases in the world. The infectious agent is actually a
bacteria or virus, and the insect serves as a vehicle for transmitting the agent to the
human host. They are transmitted through bites (and other forms of inoculation)
from mosquitoes, ticks, sand fleas, and other insects. The bacteria or virus have
a complex biological relationship with the insect, as well as the host. In some
cases, the bacterium will cause pathology in the insect. Arthropods serve as a
site of maturation for the parasite, and passage through the insect is a crucial
step in the natural life cycle of the infectious agent. Referring back to Figure 2.1,
controlling arthropod populations can break the cycle of transmission between
human and insect. Broadly speaking, this can be achieved through interruption
of the parasite life cycle within the insect, or by limiting the reproduction of the
insect itself. Vector control is an aspect of environmental engineering that has
tremendous impact on infectious disease transmission. The most work has been
done in controlling mosquito populations, and some of these interventions will
be discussed in this chapter.
The
reservoir
is source of infection, often a nonhuman animal. Reservoirs
are not adversely affected by the presence of the infectious agent, and interaction
