8.1 Disease Transmission Through Drinking Water 195
it completes its life history in a single host, unlike other
Sporozoans which complete their life history in two
hosts, such as the malaria parasite, Plasmodium spp.
Members of this group produce different kinds of
spores (hence their name, Sporozoa). Cryptosporidium
isolated from humans is now referred to as C. parvum.
Cryptosporidium infections have been reported from a
variety of wild and domesticated animals, and in the
last 6 or 7 years, literally hundreds of human infections
have been reported, including epidemics in several
major urban areas in the United States. Cryptosporidiosis
is now recognized as an important opportunistic infec-
tion, especially in immunocompromised hosts. The
disease is transmitted when feces of infected persons
or animals enter drinking water. The thick walled
oocytes of Cryptosporidium parvum, are known to be
resistant to chlorine at concentrations typically applied
for water treatment (Fig. 8.5).
Shigellosis
Shigellosis, also known as bacillary dysentery is a
water borne illness caused by infection by bacteria of
the genus Shigella. The causative organism is fre-
quently found in water polluted with human feces, and
is transmitted via the fecal-oral route. An estimated
18,000 cases of shigellosis occur annually in the United
States, mainly affecting infants, the elderly, and espe-
cially those suffering from AIDS. It is characterized by
diarrhea, often with blood or mucus in the watery stool,
cramps, fever, and sometimes vomiting. It sets in a day
or 2 after contact.
Discovered over a 100 years ago, the organism is
named Shiga, the Japanese discover of the organism.
There are four species of Shigella: Boydii, dysenteriae,
flexneri, and sonnei. The genus Shigella is divided into
four groups, A, B, C, and D, with the common nomen-
clature of Shigella dysenteriae, S. flexneri, S. boydii,
and S. sonnei, respectively. There are 49 recognized
serological types or serovars, representing subtypes
from three of the four groups. Infection and outbreaks
associated with this organism are prominent in devel-
oping countries and are strongly associated with over-
crowding and poor hygienic conditions. Shigellosis is
also travel associated, and several outbreaks have been
reported in developed countries such as Canada and
the United States which are travel-related. Shigella
sonnei (“Group D” Shigella) accounts for over two-
thirds of the shigellosis in the United States. ShigellaFig. 8.5 Life history of Cryptosporidium parvum (Note the oocysts
whose thick walls shield them from disinfectants) (From http://
http://www.dpd.cdc.gov/dpdx/html/frames/A-/Cryptosporidiosis/
body_Cryptosporidiosis_life_cycle_lrg.htm; Credit: Center for
Disease Control [CDC])