Overview of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 23

2.2.5. Cholerae
The Vibrio bacteria incorporate a number of species, of which V. cholerae is the
only pathogenic species associated with fresh water. These bacteria are small,
curved (comma-shaped) and Gram negative, possessing a single polar flagellum.
Many of the serotypes have the ability to cause diarrhea, although it is
certain strains of the O1 and O39 serotypes that cause the classical cholera
symptoms. These strains produce the cholera toxin, which acts to change
ionic fluxes across intestinal mucosa, resulting in severe fulminating and
watery diarrhea. These toxigenic strains are far less common than the others,
which are widely distributed in water sources. The infectious dose is high
at around 10 8 organisms, and many infections are asymptomatic. Those who
suffer from symptomatic cholera vary in the extent and severity of their
symptoms, with some losing up to 10-15 L of water a day. Vibrio cholerae is
highly sensitive to chlorine disinfection.
2.3. PROTOZOA
Protozoa are a diverse group of unicellular eukaryotic organisms. This
group includes sporozoa (intracellular parasites), flagellates (which possess
tail-like structures for movement), amoeba (which move using temporary
cell body projections called pseudopods), and ciliates (which move by beat-
ing multiple hair-like structures called cilia).
A selection of waterborne protozoan pathogens, as identified by the
WHO, is summarized in Table 2.3 . We will concentrate our more detailed
descriptions of each pathogen on those transmitted by ingestion of water as
opposed to inhalation or skin contact. Of these pathogens, Cryptosporidium
is the most widely studied. Where available, information is presented regard-
ing the disease caused by each of these pathogens in terms of symptoms,
prevalence and treatment. Additionally, information is given on the number
of different species of each pathogen, their size and appearance, with the
reader being referred to original scientific articles for more in-depth infor-
mation on aspects such as cell biology, pathogen life cycle, etc. For an over-
view of recorded waterborne protozoa outbreaks, we recommend Ref. 21 .
2.3.1. Cryptosporidium
Cryptosporidium was recently placed on the WHO's Neglected Diseases Ini-
tiative. It is responsible for a significant proportion of childhood mortal-
ity in developing countries 22 and several outbreaks of disease, associated