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Accepta Newsletter: Issue 7
Articles in issue 7:
- Cryptosporidium and Cryptosporidiosis - What is it?
- A Bug's Life.
- Nitrate - Where Does it Come From?
********************
Cryptosporidium and Cryptosporidiosis - What is it?
Cryptosporidium
is a parasite that infects man and a wide range of domestic and wild
animals. It causes cryptosporidiosis, which in healthy adults, is an
unpleasant diarrhoea lasting for up to two weeks. At present there is
no cure for cryptosporidiosis and the condition can be serious for,
and fatal to, the immuno-compromised e.g. those receiving chemotherapy
or AIDS patients.
Cryptosporidiosis
is relatively uncommon. It can be contracted through person to person
contact, from contaminated food, poorly operated swimming pools or contaminated
drinking water. The environmentally resistant form of the parasite,
the oocyst, is excreted in the faeces of infected animals and humans.
Pollution of watercourses can occur where there is poor control over
the disposal of faecal slurries from infected farm animals. There is
evidence that sewage effluents discharged to rivers used for drinking
water abstraction can play a role in recycling oocysts excreted by infected
humans. There have been a number of drinking water-related outbreaks
of cryptosporidiosis, some of which have attracted considerable media
attention. These outbreaks have involved from a few dozen to thousands
of cases of cryptosporidiosis.
Cryptosporidium
poses a challenge to water treatment processes because of its small
size and resistance to disinfection processes. However, the Inspectorate's
investigations of outbreaks of cryptosporidiosis have shown that they
are invariably related to inadequate provision or poor operation of
water treatment. There is good evidence that careful operation of the
coagulation/flocculation and filtration processes provides consumers
with a very high level of protection against exposure to the parasite.
The
Regulatory Position
In
June 1999 the UK Government introduced regulations that required water
companies to carry out risk assessments to establish whether there is
a significant risk from Cryptosporidium in water supplied from each
of their treatment works. Where there is a risk, water companies must
use a process for treating the water to ensure that the average number
of oocysts is less than 1 per 10 litres of water. Water companies must
use a regulatory method for sampling and analysis to check that they
are complying with the standard.
The
concentration of 1 oocyst in 10 litres is a treatment standard and not
a health-related standard. It is set to ensure that water companies
optimise their treatment processes and pay careful attention to operation
and maintenance. It is not feasible to set a health related standard
because of the wide variation in susceptibility of different sectors
of the population. There is also a very poor understanding of virulence
factors that are known to influence the ability of the parasite to cause
infection. Nevertheless, there is evidence that where outbreaks of cryptosporidiosis
have occurred, the concentrations of oocysts were in excess of 1 oocyst
in 10 litres. Furthermore, most outbreaks have been associated with
problems in the operation of treatment processes. The UK approach to
risk reduction may be summarised as the requirement to install and operate
effectively, physical barriers to optimise removal of Cryptosporidium
from water supplies.
Current
Research Questions
Following
the 1989 outbreak of cryptosporidiosis in Swindon and Oxfordshire (UK),
which affected some 5,000 persons, the UK Government established the
Expert Group on Cryptosporidium in Water Supplies. Under the chairmanship
of the late Sir John Badenoch and, more recently, Professor Ian Bouchier,
the Expert Group has guided the development of policy on reducing the
risk of Cryptosporidium in water supplies. The research recommendations
of the Expert Group have led to a collaborative programme of research
involving initially the Government and the UK water industry. More recently
the DETR National Cryptosporidium Research Steering Committee has broadened
its membership to include representation from the United States, Australia
and mainland Europe.
The last ten years have seen great progress in the development of catchment control strategies. There is now better understanding of issues such as: removal and inactivation during water treatment; infectious dose; and techniques for sampling and analysis. At the same time there have been significant advances in the understanding of the molecular biology of the parasite. It is now possible to investigate the characteristics of the parasite that confer resistance to attack by chlorine and susceptibility to attack by ozone or UV light.
Researchers
are also beginning to develop genetic fingerprinting techniques that
should lead to improved techniques in assessment and control of risk.
Some
Current Research Issues are Summarised in the Following List: -
Design
of disinfection studies
- If laboratory or pilot scale studies are to be used as a basis for
treatment plant design, a consistent approach is desirable. Thorough
attention to reporting of experimental details is crucial. Most importantly
the origin and preparation of the oocysts and the choice of surrogate
for human infection potential must be specified carefully if the results
from different studies are to be comparable. The Publications and Information
section of the DWI website includes the report of a recent international
workshop on this subject.
Surrogate
for human infectivity
- There is growing evidence that animal studies do not provide a good
model for human infection. There is also evidence that the precision
of animal infection studies is very poor and this may explain the very
wide variations in reported results. There appears to be a consensus
that tissue culture using human cells offers the best way forward.
Variations
in resistance to disinfectants among genotypes
- All disinfection studies have been performed on oocysts obtained from
infected calves. It is currently not possible to obtain sufficient quantities
of the human specific genotype for disinfection studies. There is some
evidence that genotype 1 and genotype 2 may show different resistance
to disinfectants. This, coupled with the comments on the selection of
surrogates for human infectivity, raises concerns about whether results
of published disinfection studies provide a reliable basis for treatment
plant design.
Use of genetic fingerprinting techniques - If it were possible to develop at the molecular level a means to identify the exact source of an infectious parasite, it would be possible to introduce improved surveillance and control techniques. DWI is currently collaborating with the Scottish Executive to support an UK-wide consortium of researchers to investigate this possibility.
©Crown
copyright.
This article
was reproduced with the kind permission of the UK Drinking Water Inspectorate
- www.dwi.gov.uk
A Bug's Life
 |
When someone swallows bacteria that cause food poisoning, there is a delay (incubation period) before symptoms begin. This is because most bacteria that cause food poisoning need time to multiply in the intestine. The length of the incubation period depends on the type of bacteria and how many are swallowed. It could be hours or days. |
The bacteria stick to the lining of the intestine and destroy those cells, either by sheer weight of numbers or by the toxins (poisons) they produce. Sometimes these toxins are absorbed and cause damage elsewhere in the body.
Some bacteria produce
toxins when they grow in food. Because the toxins themselves are harmful,
the bacteria don't need to multiply in the intestine to make someone ill,
so the symptoms come on very quickly.
Because the bacteria enter the body through the digestive system, symptoms will generally be in this part of the body - nausea, vomiting, abdominal cramps and diarrhoea. In some cases, food poisoning can cause very serious illness or even death.
How Bacteria Grow
Bacteria need warmth
and moisture to grow. They reproduce by dividing themselves, so one bacterium
becomes two and then two become four and so on. In the right conditions
one bacterium could become several million in 8 hours and thousands of
millions in 12 hours.
This means that if
a food is contaminated with a small number of bacteria and you leave it
out of the fridge overnight it could be seriously contaminated by the
next day. Then just one mouthful could make someone ill. If you put food
in the fridge it will stop bacteria from multiplying.
Since you can't see, taste or smell bacteria, the only way that you can be sure that food is safe is to follow good food hygiene at all times. See the Keeping food safe section.
The Food Hygiene Campaign is part of the UK Food Agency's strategy to reduce food poisoning. The success of this strategy is being measured by a reduction in the number of laboratory-confirmed cases of the following five bacteria: campylobacter, salmonella, listeria, E.coli O157, clostridium perfringens.
Clostridium Perfringens
Clostridium perfringens is found in low numbers in many foods, particularly meat and poultry and their products. It is also found in the soil, the intestines of humans and animals, in sewage and in animal manures.
Salmonella
Salmonella is the second most common cause of food poisoning after campylobacter. It has been found in unpasteurised milk, eggs and raw egg products, meat and poultry. It can survive if food is not cooked properly.
Listeria
|
Listeria monocytogenes is present all around in the environment. It has also been found in low numbers in many foods. In certain foods, such as soft mould-ripened cheeses and pâtés, it may be present in higher numbers. Eating foods containing high levels of listeria monocytogenes is generally the cause of illness. |
E.coli O157
 |
Most strains of E.coli are harmless, but those that produce verocytotoxin (called verocytotoxin-producing E.coli, or VTEC) can cause severe illness. In the UK, the most common type is E.coli O157. |
Campylobacter
 |
Campylobacter is the most common identified cause of foodborne disease. It has been found mainly in poultry, red meat, unpasteurised milk and untreated water. Although it doesn't grow in food it spreads easily, so only a few bacteria in a piece of undercooked chicken could cause illness. |
©Crown copyright.
This article
was reproduced with the kind permission of the Food Standards Agency -
www.foodstandards.gov.uk
Nitrate - Where Does it Come From?
Nitrate
is present in all tap and bottled waters. It is produced during the natural
decay of vegetable material in soil. Rainfall washes nitrate from sub-soil
into groundwater. Nitrogenous fertilisers used on arable farmland can
be a significant source of nitrate in groundwater and surface water.
Why
is it Necessary to Control Nitrate in Water?
High
concentrations of nitrate in water can cause methaemoglobinaemia (blue
baby syndrome) in very young children. This is a potentially fatal illness.
Nitrate is converted to nitrite in the gut and interferes with the absorption
of oxygen by the blood. This extremely unusual illness only occurs at
very high nitrate concentrations. The last recorded case in the UK occurred
in the 1950s and was associated with the use of a shallow private well.
No cases have arisen from use of public water supplies.
The current
regulatory standard of 50 mg/l nitrate is derived from the standard in
the European Union's Drinking Water Directive. The EU standard is based
on the World Health Organisation's guideline value for drinking water,
which is also 50 mg/l. That standard is intended to ensure that drinking
water will not cause methaemoglobinaemia.
What
About Nitrate and Cancer?
A number
of studies have demonstrated that extremely high doses of nitrate can
cause cancer in laboratory animals. A number of epidemiological studies
have investigated the possible association of nitrate in tap water and
incidence of cancer. None have provided any evidence for an association.
Indeed several have reported an inverse association i.e. cancer incidence
decreased as the nitrate levels in water increased. It is important to
note that nitrate is a natural component of many foods including green
vegetables and that food provides the highest proportion of our dietary
nitrate intake.
How
Much Nitrate is in Your Water?
In 1999,
99.94% of tests for nitrate on samples taken from public water supplies
in England and Wales met the nitrate standard. Remedial action is being
taken in the few locations where the standard is exceeded. This action
involves installation of treatment processes to reduce nitrate concentrations.
Alternatively, blending of high nitrate water with water that is low in
nitrate is practised to achieve compliance with the nitrate standard.
©Crown
copyright 2001.
This article was reproduced with the kind permission of the UK Drinking
Water Inspectorate - www.dwi.gov.uk
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