Environmental Engineering Reference
In-Depth Information
of the waterborne outbreak, there were 73 cases of the disease. Supposing that
secondary infections occurred among the population to add another 43 cases of
the disease bringing the total to 116 cases of the disease for the year. In the
community of 10,000 people, the prevalence rate of the disease for the year of
the outbreak would be 1 percent.
The incidence rate can be determined for both the exposed and unexposed
individuals identified with the waterborne outbreak above. Looking at the data, we
find that 52 people became sick out of 84 people that drank water and 21 people
became sick out of 85 people that did not drink water. The incidence rate for the
two subgroups of individuals is 62 percent and 25 percent, respectively. From
these data, an attributable risk can be determined by subtracting the incidence
rate of nondrinkers from drinkers of the water, which would be 37 percent.
Incidence measures reflect the level of infectivity of the causative agent
of the disease. They do not establish the virulence of the causative agent
because virulence relates to the damage produced as a result of the infection.
Damage resulting from infection of an individual can range from a few mild
symptoms to life-threatening symptoms, depending on many contributing factors
(e.g., health and nutrition status, age, infectious dose of the pathogen received,
how the pathogen was received, genetic disposition and others). In the study of
an outbreak, a case is defined not by the severity of the infection but by the fact
that an infection occurred.
The subject of risk assessment has advanced considerably in the last 20 years.
Mathematical models have been constructed to estimate the probability of infec-
tion using databases of human exposure. Before models could be formulated it
was necessary to ascertain the variables of the infection process. In the case of
microbial risk assessment, such variables might include etiologic disease agent
identification, human health effects manifested through infection, dose-response
data relating dose received and probability of infection/disease in the target pop-
ulation, physiology of host-parasite relations, and epidemiological data.
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Molecular Detection of Waterborne Pathogens
Water, especially drinking water, when under suspicion of the transmission of
pathogens, requires laboratory examination for proof of contamination. Cultural
methods may prove inadequate for the isolation of pathogens, may produce uncer-
tain results, or may be too time-consuming to support ongoing epidemiological
investigations. During the past three decades, environmental laboratories have
exploited molecular-based protocols to gain insight into the presence of sundry
infectious bacteria, viruses, and protozoa in aquatic environments and water
supplies. These techniques can be useful to investigations of disease outbreak,
especially, where no cultural evidence can be obtained to show the existence
of an infectious agent. In fact, a fundamental challenge in proving the hypoth-
esis that a disease outbreak has occurred is to establish conclusively that the
suspected agent of disease existed at the suspected source of the disease. A
broad range of sophisticated laboratory techniques, such as fluorescent antibody,
