Overview of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 12

2.1. VIRUSES
Viruses are the smallest of the waterborne pathogens, with typical
sizes ranging between 20 and 300 nm. A virus consists of a small amount
of nucleic acid (DNA or RNA) coated with and protected by a layer of
protein ( Fig. 2.3 ). Viruses replicate only within host cells, where the viral
nucleic acid directs the infected cell to produce progeny viruses. Viral infec-
tions are initiated when the outer layer of the virus protein specifically
interacts with cell surface receptors.
Gastroenteritic viruses replicate in the upper third of the intestine,
destroying the mature enterocyte covering the villi and therefore disrupt-
ing the reabsorption of water from the gut, resulting in diarrhea. Eventu-
ally, the villi are repopulated with immature, undifferentiated cells that are
resistant to the virus infection. However, it takes time for these undiffer-
entiated cells to mature sufficiently to develop the necessary ion uptake
capabilities.
Viruses are in general quite species-specific and also tissue-specific
(known as tropism) with regard to which cells they can infect. Several
of the waterborne viruses have secondary trophisms beyond the intestine,
e.g. enteroviruses, which can cause poliomyelitis or meningitis, or hepati-
tis, which can infect the liver. Additionally, there is speculation that some
waterborne viruses may be zoonotic, with potential reservoirs being cattle,
pigs, poultry or rodents. However, this has not been confirmed, possibly
due to limited diagnostics in identifying viruses in human or animal feces
and in water. Many viruses are long-lived in aqueous environments, and
they have been shown to contaminate deep aquifers, a water source often
considered as being free from microbial contamination. 6
A selection of waterborne viruses, as identified by the WHO, is sum-
marized in Table 2.1 . We will concentrate our more detailed descriptions
of each pathogen on those transmitted by ingestion of water as opposed to
inhalation or skin contact. 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 num-
ber of different species of each pathogen, their size and appearance, with
the reader being referred to original scientific articles for more in-depth
information on aspects such as cell biology, pathogen life cycle, etc. Brief
details of recent outbreaks of disease attributed to waterborne viruses are
summarized in Ref. 7 .