WATER QUALITY - Water Quality Engineering in Natural Systems - page 22

cryptosporidiosis. Severely immunocompro-
mised persons are encouraged to avoid any
contact with water in lakes and streams and
should not drink water from these water bodies.
C. parvum produces eggs called oocysts that are
not readily killed by chlorination and can be
removed effectively only by filtration. A person
infected with C. parvum may excrete as many
as 10 8 oocysts per day.

Helminths (worms) are found in domestic sewage.
Most helminths enter the human body through
the mouth, although a few gain entry through the
skin. Contamination of drinking water with these
worms can be prevented with modern water treat-
ment methods that include disinfection, however,
swimming or wading is sewage-polluted waters
may lead to outbreaks of intestinal worm infec-
tions. Such incidents are of little public health sig-
nificance in the united States, but are much more
common in other parts of the world. The best-
known helminth is the beef tapeworm Taenia sagi-
nata , which lives in the intestinal tract of humans,
and infected persons may discharge as many as 10 6
eggs per day in their feces. Symptoms of tapeworm
infection include abdominal pain, digestive distur-
bance, and weight loss. Certain species of the
blood fluke Schistosoma spend part of their life
cycle in humans, where they may cause a debilitat-
ing illness known as schistosomiasis. An electron
micrograph of a pair of Schistosoma mansoni is
shown in Figure 2.7. Penetration of the skin by
larval schistosomae may cause a skin rash known
as “swimmers itch.” The more serious symptoms
of the disease, which generally do not develop
until 4-8 weeks later, include fever, chills, sweat-
ing, and headaches.

Figure 2.7. Pair of Schistosoma mansoni . Source : uniformed
Services university (2005).

rash, flulike symptoms, diarrhea, fatigue, and
severe cramps. The protozoan attaches itself to
the upper intestinal tract and produces cysts,
which are shed in the feces. One of the major
reasons why giardiasis continues to be a
problem as a waterborne disease is that the
cysts survive well under adverse conditions.
Giardia cysts are highly resistant to chlorine
and can live in cold water for months. Virtually
all of the outbreaks of giardiasis in the united
States have been reported in mountainous
areas, where the frequent use of unfiltered,
easily contaminated surface waters by hikers
and campers has undoubtedly contributed to
the prevalence of the disease. Three of the
major hosts for Giardia are humans, beaver,
and muskrat; and beavers are believed to be a
particularly significant source of G. lamblia in
surface waters. Although water is a major means
of transmitting giardiasis, the largest percent-
age of recorded cases is caused by person-to-
person contact, primarily in child-care centers.
Cryptosporidium parvum. Cryptosporidium par-
vum is a protozoan of effective diameter 3-7 µ m
that causes the disease cryptosporidiosis and
poses severe health risks. At least 10 species of
Cryptosporidium are currently recognized, with
C. parvum (the species found in the domestic
mouse) being the major species responsible for
illness in humans. In healthy persons, crypto-
sporidiosis causes 7-14 days of diarrhea, with
possibly a low-grade fever, nausea, and abdomi-
nal cramps. The effects on immunocompro-
mised persons can be life threatening, and
no antibiotic treatment currently exists for

Algae are a diverse group of simple organisms that
are members of the Protista kingdom. Algae are
not members of the Plantae kingdom, but like
plants, most algae use the energy of sunlight to
make their own food via photosynthesis. Algae
lack the roots, leaves, and other structures typical
of true plants. Microscopic algae that float or swim,
called phytoplankton , are typically in the size
range of 1-100 µ m. Algae found in many natural
waters are usually not a health concern; however,
certain species may produce endo- or exotoxins,
which if ingested at high enough concentrations,
may be harmful. Three species of blue-green algae,
Anabaena flos-aquae , Microcystis aeruginosa , and
Aphanizomenon flos-aquae, , produce exotoxins.
The toxin-producing algae Anabaena flos-aquae is