WATER QUALITY - Water Quality Engineering in Natural Systems - page 20

those of similar natural habitats within a region. Exceed-
ance of biological standards in either of these categories
can cause the water body to be impaired.

TABLE 2.3. Pathogenic Microorganisms Commonly Found
in Surface Waters

Pathogen Group and Name Associated Disease

Virus
Adenoviruses

2.4.1 Human Pathogenic Microorganisms

respiratory disease, eye
infections Microorganisms are too small to be seen clearly without

the aid of a miscoscope, and although there is no precise
definition, the size that separates microorganisms from
other organisms is approximately 100 µ m. Pathogenic
microorganisms are disease-causing organisms that typ-
ically originate in the intestines of infected people or
animals. Pathogenic microorganisms are mostly trans-
ported by urban stormwater runoff, domestic and
municipal wastewater discharges, combined sewer over-
flows, septic systems, and runoff from pastures and
animal feedlots.
Water uses that are affected by the presence of
pathogenic microorganisms include bathing, fishing, and
shellfish harvesting. In most cases, simply bathing in a
stream or swimming in an ocean will not cause humans
to become ill, since a person must first come into contact
with the pathogens, the pathogens must gain entry to
the body, and the dose of pathogens must be sufficient
to overcome the natural defenses of the body. under
rare circumstances, an infection can develop from a
single pathogenic microorganism, but the minimum
infective dose for pathogenic bacteria is typically in the
range of 10 2 -10 6 , depending on the species (Majeti and
Clark, 1981; NAS 1996). This indicates that recreational
waters need not be absolutely free of pathogens to be
relatively safe, but the higher the concentration of
pathogens, the greater the probability that illness will
result from contact with the water body.
A list of pathogens typically found in natural waters
is given in Table 2.3, and all of the microorganisms
shown in Table 2.3 originate primarily from human
feces. Microorganisms of most concern from a water-
quality viewpoint are viruses, bacteria, protozoans, hel-
minths, and algae. These are described in more detail
below.

Enteroviruses

Polioviruses Aseptic meningitis,

poliomyelitis

Echoviruses Aseptic meningitis,

respiratory infections,
diarrhea

Coxsackie viruses Aseptic meningitis,

herpangina, myocarditis

Other enteroviruses Encephalitis

Hepatitis A virus Infectious hepatitis

Norwalk and related GI
viruses

Gastroenteritis

reoviruses Mild upper respiratory and

gastrointestinal illness

rotaviruses Gastroenteritis, a diarrhea

Bacterium
Salmonella typhi

Typhoid fever

Salmonella paratyphi Paratyphoid fever

Other salmonella Salmonellosis,

gastroenteritis

Shigella spp. Bacterial dysentery †

Vibrio cholerae Cholera

Gastroenteritis

Escherichia coli

Yersinia enterocolitica Gastroenteritis

Campylobacter jejuni Gastroenteritis

Cyanobacteria
Legionella spp.
Leptospira spp.
Plesiomonas shigelloides
Pseudomonas aeruginosa
Protozoan
Acanthamoeba castellani Amoebic

meningoencephalitis

Balantidium coli Balantidosis (dysentery)

Amoebic dysentery

Entamoeba histolytica

Giardia lamblia Giardiasis (gastoenteritis),

diarrhea

Cryptosporidium spp. Cryptosporidiosis, diarrhea

Viruses are complex molecules that typically contain
a protein coat surrounding a deoxyribonucleic acid
(DNA) or ribonucleic acid (rNA) core of genetic
material. Viruses have no independent metabo-
lism and depend on host living cells for reproduc-
tion. They range in size from 0.01 to 0.4 µ m in
diameter. Viruses do not exist for long periods of
time outside a human or animal body, but while
they exist, they can survive heat, drying, and chem-
ical agents. More than 130 types of enteric viruses
are excreted in human feces and urine. Viral
pathogens are responsible for diseases such as
poliomyelitis,

Naegleria spp.
Helminth
Ancylostoma duodenale Hookworm

Ascaris lumbricoides Ascariasis

Hymenolepis nana Hymenolepiasis

Necator americanus Hookworm

Schistosoma spp. Schistosomiasis

Strongloidiasis

Strongyloides stercoralis

Taenia saginata Tapeworm

Trichuris trichiura Trichuriasis

a Gastroenteritis is an inflammation of the lining membrane of the
stomach and intestines.

aseptic meningitis, infectious