Environmental Engineering Reference
In-Depth Information
appears to be best for survival of pathogens; drier conditions generally decrease
the ability of a pathogen to survive. Nutrients increase survival. The pH is not
a major factor. For most pathogens, exposure to sunlight acts as desiccant rate.
Low temperatures favor survival.
4
,
5
The survival of pathogens in soil, on foods,
and following various wastewater unit treatment processes as reported by vari-
ous investigators, is summarized by Bryan and others.
6
,
7
Most enteroviruses pass
through sewage treatment plants, survive in surface waters, and may pass through
water treatment plants providing conventional treatment. Water treatment plants
maintaining a free residual chlorine and low turbidity less than 1 nephelometric
turbidity unit (NTU) in the finished water, as noted under Chlorine Treatment
for Operation and Microbiological Control in Chapter 3, or using other approved
disinfection treatment, can accomplish satisfactory virus destruction.
INFECTIOUS DOSE
The development of illness is dependent on three basic things: the toxicity or
virulence of a substance, the amount of the substance or microorganisms ingested
(at one time or additively over a specified period of time), and the susceptibility
of the individual. The result may be an acute or a chronic illness. Exposure to two
or more substances may produce a synergistic, additive, or antagonistic effect.
Persons may be exposed to a microorganism by direct 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 pathogen.
When the dose of a chemical substance administered to a series of animals is
plotted against the effect produced, such as illness, if increased doses produce 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 continuing 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 animal species must be considered.
Table 3.2 lists various microorganisms and the approximate number (infectious
dose) of organisms required to cause disease. Bryan
6
has summarized the work
of numerous 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 healthy volunteers,
10
8
Clostridium perfringens
type A (heat resistant) bacteria caused illness in 26
to 50 percent of the volunteers, and 10
9
C. perfringens
type A (heat sensitive)
bacteria caused illness in 76 to 100 percent of the volunteers.
For some viral infections, ingestion of as few as one viral particle can infect
a susceptible host. In that case, it would appear that viral infections should be
readily spread through drinking water, food, shellfish, and water-contact recre-
ational activities. Fortunately, the tremendous dilution that wastewater containing
viruses usually receives on discharge to a watercourse and the chemical treat-
ment of drinking water greatly reduce the probability of an individual receiving
