192 8 Disease Transmission in Water
The type and incidence of water borne diseases
varies according to the economic circumstance of the
country. This is reflected in the ability of a society to
provide clean potable water for its members. Thus,
while cholera is endemic in Asian countries, it does
not exist at all in the USA. Figures for many develop-
ing countries are not available, but if they were they
would most probably be unreliable as many of these
countries do not have laws which compel the reporting
of certain diseases. That water-borne diseases are
important is not in doubt. Even in a country of high
economic development such as the USA, where treated
water is easily available, data show (Fig. 8.3) a good
number of cases annually (Anonymous 2003 ).
As seen in Fig. 8.3, the cause of water-associated
diseases recorded in the US between 1991 and 2000
could not be determined in 39% of the cases. Where
the cause was known, protozoa topped the list fol-
lowed by bacteria, chemicals, and viruses (Fig. 8.3,
Table 8.1). Indeed the number of cases appears to be
rising, although this could be a reflection of greater
reporting and more investigation than of absolute
increase in cases.
In addition to fecally borne pathogens, other micro-
bial hazards (e.g., guinea worm Dracunculus medinen-
sis, toxic cyanobacteria, and Legionella) may be of
public health importance under specific circumstances.
The infective stages of many helminths, such as para-
sitic roundworms and flatworms, can be transmitted to
humans through drinking water. As a single mature
larva or fertilized egg can cause infection, these should
be absent from drinking water. However, the water
route is relatively unimportant for helminth infection,
except in the case of the guinea worm.
Public health concern regarding cyanobacteria
relates to their potential to produce a variety of toxins,
known as “cyanotoxins.” In contrast to pathogenic
bacteria, cyanobacteria do not proliferate within the
human body after uptake; they proliferate only in theFig. 8.2 Routes of transmission
of water-associated diseases
(From Anonymous 2006a.
With permission)
Fig. 8.3 Summary of the causes of outbreaks of diseases linked
to drinking water in the US, 1991–2000 (From the American
Chemistry Council; Anonymous 2003. With permission)