|
In this issue we continue the review of
water quality as it pertains to usage for drinking and food preparation. The source for
this information is the Australian Drinking Water Guidelines - 1996.
Enteric Protozoa
Enteric protozoa may be found in water following direct or indirect contamination with
human or animal faeces. The protozoan cysts are transmitted by the faecal-oral cycle.
Ingestion of contaminated drinking water is one of several mechanisms for completing this
cycle. The parasites occur in water as dormant infectious cysts, which have natural
mortality rates probably determined by temperature and incident UV light. In principle,
removal or disinfection at the water source should be sufficient to prevent contamination
by enteric protozoa, provided that adequate measures are in place to prevent
re-contamination. In practice, however, this may be difficult as protozoan cysts are
generally more resistant to water disinfectants than most bacteria and viruses. Cryptosporidium
and Giardia species are likely to be the most important enteric protozoa in
Australia, although infection by Entamoeba histolytica is also endemic in some
communities. All these organisms cause moderate to severe enteritis in susceptible people
and they appear, in Australia, to be mostly transmitted by direct contact with a carrier. Cryptosporidium
can be acquired from drinking and swimming water and also from pasture animals and
possibly from pets. There is evidence that Giardia infections in Australia may
result from direct contact with septic tank waste or from faecal contamination of drinking
water or water used to wash uncooked foods.
Free-living Protozoa
Two groups of free-living protozoa, Naegleria and Acanthamoeba, have been
responsible for human infections in Australia. Infection generally results from contact
during recreational bathing, or domestic uses of water other than drinking. Public water
supplies can contaminate swimming pools. The occurrence of these organisms is not related
to faecal contamination. Naegleria fowleri, although rare, causes a cerebral
infection with is usually fatal. Acanthamoeba species cause both a cerebral and eye
infection.
Helminths
The major helminths (worm) parasites which can be transmitted by water do not occur in
Australia. While the eggs of some helminths may enter water, water-borne transmission is
generally regarded as unimportant in Australia.
Viruses
Viruses are among the smallest of all infective agents. Viruses multiply only inside
specific host cells and they are totally dependent on the host's cells to be able to
survive and multiply. The viruses of most significance to drinking water and water used
for food washing are those that multiply in the human intestine and are excreted in large
numbers in the faeces of infected individuals. Although they cannot multiply outside the
tissues of infected hosts, some enteric viruses can survive in the environment and remain
infective for long periods. Human enteric viruses occur in water largely as a result of
contamination with sewage or human excreta. Some infections, for example with the
hepatitis A virus, are difficult to trace to a source because of the long incubation
periods before symptoms develop after ingestion.
Cyanobacteria (blue-green algae)
Cyanobacteria are true bacteria, although they are often called "blue-green
algae" because of their similarity to the green algae in morphology, habitat and
photosynthetic ability. They are primarily of concern in drinking water because of the
toxins they produce, which are of two types: Hepatotoxins (which damage liver cells) and
Neurotoxins (which damage nerve cells). They also cause skin irritations, hay-fever like
symptoms, conjunctivitis and asthma. Cyanobacterial blooms sporadically in late spring to
autumn in many parts of Australia.
The subject of microbial drinking water quality will continue in the next newsletter.
|
|
