Overview of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 6

CHAPTER TWO
Overview of Waterborne
Pathogens
Helen Bridle
Heriot-Watt University, Institute of Biological Chemistry, Biophysics and Bioengineering, Riccarton,
Edinburgh, Scotland
Waterborne pathogens can be divided into three main categories: viruses,
bacteria and parasites, the latter of which are comprised of protozoa and
helminths. Such pathogens reach water sources when infected people or
animals shed microbes in feces. For example, untreated, undertreated or
accidental release of sewage allows pathogens to enter water sources. An
alternative mechanism is through runoff to source water or permeation
into groundwater from animal feces or sewage utilized as fertilizer. Many
waterborne pathogens are zoonotic, i.e. they are capable of infecting both
humans and animals ( Fig. 2.1 ).
The persistence of pathogens in the environment depends upon many fac-
tors. For example, pathogens may be removed from the water supply due to set-
tling in lakes, possibly augmented by interactions of pathogens with sediment.
Furthermore, inactivation by light, temperature or chemical conditions, such as
salinity or ammonia, can occur ( Fig. 2.2 ). 1 Understanding the fate and transport
of pathogens in the environment is essential for risk management. However, the
survival and transport of pathogens is beyond the scope of this topic and the
reader is referred to other articles 2 and topics for more on this topic. It is suffi-
cient to note that some of the pathogens discussed in this topic remain infective
in the environment for long periods of time (from months to years).
As waterborne pathogens are transported through the environment, they
are often diluted to low concentrations. However, while these low concen-
trations may prove challenging for detection purposes, they may still present
a considerable public health risk, as several of the pathogens discussed here
have extremely low infectious doses.
One final factor, in addition to environmental persistence and infectious
dose, which renders some waterborne pathogens particularly problematic is
their resistance to disinfection methods. Most viruses, the spores or a veg-
etative phase of a bacterium, and the cysts, oocysts or ova of parasites are
capable of some degree of resistance to chlorination. 3