Environmental Engineering Reference
In-Depth Information
enteroviruses pass through sewage treatment plants, survive in surface waters,
and may pass through water treatment plants providing conventional treatment.
According to WHO, water treatment plants maintaining a free residual chlorine
in the distribution system of at least 0.5 mg/l for at least 30 minutes and low
turbidity [less than 1 nephelometric turbidity unit (NTU)] in the finished water
can achieve satisfactory virus inactivation. Other approved disinfection treatment
(e.g., ozonation) can accomplish satisfactory virus destruction.
Substance Dose to Cause Illness
The development of illness is dependent on the toxicity or virulence of a sub-
stance or pathogen, the amount of the substance or pathogen ingested (at one
time or intermittently), and the resistance or susceptibility of the individual. The
result may be an acute or long-term illness. Sometimes two or more substances
may be involved to produce a synergistic, additive, or antagonistic effect. The
microbial modes of disease transmission include ingestion of a pathogen or toxin
in contaminated water or food, contact with an infected person or animal, or
exposure to an aerosol containing the viable pathogen.
If the dose of a chemical substance administered to a series of animals is
plotted against the effect produced, such as illness, and increased doses pro-
duce no increases in illnesses, the substance is said to cause “no effect.” If
increased doses cause increasing illnesses, the substance has “no threshold.” If
increased doses cause no apparent increases in illnesses at first but then continu-
ing increased doses show increasing illnesses, the dose at which illnesses begin
to increase is referred to as the substance “threshold.” Below that dose is the
“no-observed-effect” range. Variations between and within animal species must
be considered.
Table 1.2 contains a list various microorganisms and the approximate infec-
tious dose required to cause disease. Bryan
24
has summarized the work of numer-
ous investigators giving the clinical response of adult humans to varying challenge
doses of enteric pathogens. For example, a dose of 10
9
Streptococcus faecalis
was required to cause illness in 1 to 25 percent of the volunteers, 10
8
Clostrid-
ium perfringens
type A (heat resistant) to cause illness in 26 to 50 percent of the
volunteers, and 10
9
C. perfringens
type A (heat sensitive) to cause illness in 76
to 100 percent of the volunteers.
If one were to consume 16 ounces of water containing a pathogen having a
high infectious dose value (pathogen A) and the same amount of water containing
a pathogen of low infectious dose value, it might be concluded that illness would
be less likely through infection with pathogen A than pathogen B. Such thinking
contains several fallacies, however. Pathogen infectious dose data should be used
only as a guide and must be tempered in the knowledge that many variables
influence the host-parasite relationship.
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In any specific situation, virulence of
the pathogen, physiological state of the pathogen, distribution of the infective
units (pathogen) in a unit volume (in this case water), susceptibility of the host
(infant, young, old, healthy, sick, immunocompromised), and route of infective
