Potential Health Risks from Microbiological Contamination of
Watercress
There is no formal data available on the incidence of microbial hazards from watercress
grown in New Zealand. Therefore, it is difficult to assess the impact of watercress on enteric
infection rates in the human population. However, from the national outbreak information that
has been reviewed to date (ESR Reports, 1997-2000), there has been no known enteric
disease outbreaks linked to the consumption of watercress in this country.
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Many overseas outbreaks of human gastro-enteritis (e.g. Shigella (USA)), have been linked to
the consumption of contaminated fresh vegetables (Beuchat, 1996) and watercress is included
in a list of salad vegetables which have caused extensive outbreaks of Salmonellosis (ICMSF,
1998).
There is considerable international information available on the microbiological
contamination of salad vegetables. However, studies of general salad vegetables must be
interpreted with caution in relation to watercress, as this study indicates that due to the nature
of its growing environment it should be considered a more at ‘risk food’ than salad vegetables
generally.
Studies have shown that thorough washing and treatment of produce with chlorinated water
can reduce the populations of pathogenic and microorganisms on fresh produce but it cannot
eliminate them. Beuchat (1997) suggests that the reduction of risk for human illness
associated with raw produce can be better achieved through controlling points of potential
contamination e.g. during harvesting, processing and distribution.
The significant variability of E. coli counts in watercress found in this study would make it
difficult to assess levels of microbiological contamination to determine suitability of raw
watercress for human consumption. In regard to raw watercress, controls on the quality of the
growing waters would be the most appropriate method of reducing health risks for consumers.
The US FDA “Guide to Minimise Microbial Food Safety Hazards for Fresh Fruits and
Vegetables” (FDA/CFSAN, 1998) states that “whenever water comes into contact with fresh
produce, its source and quality dictate the potential for pathogen contamination. If
pathogens survive on the produce, they may cause foodborne illness”. The Food and Drug
Association (1999) suggested that the best means of monitoring sprout production was to test
growing water. Testing for pathogens as an indicator of watercress or water quality is not cost
effective due to the potential number of pathogens which could be present. It is more
appropriate to use a suitable faecal indicator organism e.g. E.coli, as an indicator of the
growing water quality (ICMSF, 1998; Vanderzant and Splittstoesser, 1992). However, as E.
coli concentrations may not correlate well with protozoan pathogens in growing waters, strict
quality controls on source waters/growing conditions would be required in addition to a
suitably strict E. coli standard (e.g. New Zealand Drinking Water Quality Guidelines (2000)
standard of less than 1 E. coli/100mls), to ensure minimal potential for watercress
contamination.
A report on the hygienic production of watercress (Public Health Laboratory Service, 1961)
recommended that watercress grown wild in the United Kingdom should not be sold for
human consumption. The microbiological results from this Wellington study support this
recommendation.
4.3 Fascioliasis
Liver flukes (Fasciola hepatica) are parasitic in the bile ducts of mammals. Cercariae encyst
on aquatic vegetation (e.g. watercress) and the cysts (metacercariae) are then swallowed by
the final host (humans, sheep, goats, cattle) while feeding (Dalton, 1998). In humans the
typical symptoms resulting from liver fluke infestation are anaemia, inflammation and
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haemorrhage of the intestinal tract and damage to hepatic tissue (WHO, 1998). Wild
watercress is reported as the main source of infection in Europe where there is a high rate of
endemic fascioliasis in domestic animals (Dalton, 1998).
Over the last 30 years or so, liver fluke infection of livestock has become considerably more
widespread in New Zealand, largely in association with the spread of the exotic snail host,
Lymnaea columella (Mitchell, 1995). A 1984 nation-wide sheep survey showed the
prevalence of liver fluke infection in sheep in the North Island to be 7.5% in the North Island
and 1.07% in the South Island. However, the levels varied considerably between regions e.g.
South Auckland (12.6%), Taranaki (16.9%), Nelson (18%) and Westland (29%) (Charleston
et al., 1990). A 1984 survey of cattle livers slaughtered over a six-month period in the
Moerewa works in Northland showed a prevalence of about 10% ( Kearns, 1987). Movement
of infected stock into liverfluke free areas where intermediate snail hosts live is how the
infestation spreads from area to area (Mitchell, 1995). Further spread can be expected as more
properties with suitable resident snail populations have infected stock grazed on them.
Although there have been no documented cases of fascioliasis in New Zealand related to
consumption of watercress gathered from New Zealand waterways, there is definite potential
for cases of fascioliasis in New Zealand through consumption of raw watercress. The risk of
fascioliasis would be minimised by ensuring that raw watercress for human consumption is
only grown under strictly controlled conditions.
4.4 Potential health risks due to water contact while gathering watercress
At many growing locations, gathering watercress exposes people to contact with
contaminated water. Gathering watercress exposes people to the risk of infection, for
example, through cuts and abrasions, splashes to the eyes and mucus membranes, hand to
mouth activity (e.g. eating and smoking) and exposure to aerosols. To give an indication of
potential health risks through water exposure, the E. coli levels in the growing waters at each
site were compared to the 1998 Ministry for the Environment ‘Bacteriological Water Quality
Guidelines for Fresh Water’.
As the guidelines were developed on the basis of only a few international studies relating
bacteriological indicators to illness in the general public, their suitability for use as fresh
water guidelines in New Zealand requires further evaluation (MfE, 1998). Nonetheless, the
guidelines are an appropriate reference standard to assess potential health risks. The Ministry
for the Environment and the Ministry of Health are undertaking a Fresh Water
Microbiological Research Programme that aims to develop more robust fresh water
guidelines, but conclusions and recommendations from the study are not yet available.
The measured E. coli levels were compared against the single sample exceedence limits in the
guidelines. Alert Mode II and Action/Red Modes are triggered when a single bacteriological
sample exceeds a predetermined level. Under Alert Mode II, the guidelines recommend that a
sanitary survey should be undertaken to identify the sources of contamination. Under the
Action Mode the guidelines recommend that the local authority and health authorities should
warn the public through the media that the water body is unsafe and arrange for the local
authority to erect signs warning the public of a health danger (MfE, 1998).
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All of the sites except for the golf course site exceeded the action/red mode level on one or
more occasions. Persons gathering watercress from these or similar sites could potentially be
at risk of illness from contact (e.g. hand to mouth contact, direct skin contact, aerosols) with
contaminated water. Water conforming to the guideline values may still pose a potential
health risk to high-risk user groups such as the very young, the elderly and those with
impaired immune systems (MfE, 1998). Until recently it was believed that gastro-enteritis
was the main health effect from contact with polluted water, but now it is becoming clear that
respiratory effects also occur, and may be more prevalent than gastro-enteritis (MfE, 1998).
Skin and eye/ear infections are also common health effects (MfE, 1998).
4.5 Potential Health Risks from Heavy Metal Contamination of Watercress
Measured heavy metal concentrations in watercress at all the sites, with the exception of one
zinc result, were within applicable food regulations and thus were not considered a health
risk. However, some of the site results indicate that there is the potential for contamination in
excess of food regulation guidelines in watercress growing at sites subject to current or
historical heavy metal contamination. A study of watercress collected from three sites in
Hong Kong showed that the levels of iron, zinc and magnesium were higher in watercress
collected from heavy metal contaminated sites (iron ore tailings) than cultivated and
uncultivated land areas (Wong, 1995).
There are certain waterways in New Zealand where, due to the natural presence of heavy
metals such as arsenic, it is not safe to collect watercress e.g. Waikato River System. Studies
undertaken in areas of the central North Island, have revealed high levels of arsenic in the
watercress, probably due to the geothermal activity in the area and the impact of geothermal
power stations (Robinson et al., 1995; Deely, 1997). For example, watercress collected in late
February 1994 from the Waikato River near a geothermal power station (Ohaaki near
Broadlands) averaged 412 ug/g (dry weight) arsenic (Robinson et al., 1995). The amount of
arsenic found in these samples of watercress (even when the concentration is converted to a
fresh-weight basis) is well in excess of the WHO limit for arsenic in foodstuffs (2ug/g fresh
weight). A sample taken as a control from the Tiritea stream near Massey University had less
than 0.001 ug/g d.w. arsenic. The watercress in the Waikato River appears to behave similarly
to other aquatic or semi-aquatic macrophytes in that it accumulates arsenic (Robinson et al.,
1995).
A companion 1994 Massey University study (Robinson et al., 1996) looked at the relationship
between arsenic levels in growing waters and the corresponding level in watercress. It was
found that plants grown in arsenic solutions of 0.4 ug/ml or greater, exceeded 2 ug/g, (the
WHO limit for arsenic in foodstuffs). The report recommended that humans should not
consume watercress growing in water that at any time has 0.05 ug/ml arsenic. Robinson et al
(1995) showed that arsenic concentrations in geothermal waters that flow into the Waikato
River and other lakes of the Taupo Volcanic Zone contain as much as 6 ug/ml arsenic, thus
exceeding the recommended limit.
The fact that this Wellington study did not reveal heavy metal levels of concern in the
watercress does not mean that watercress grown at other sites in New Zealand would be safe
to consume. Overseas and NZ studies have shown the ability of watercress to bio-accumulate
heavy metals to levels which could pose health risks. A specific site risk assessment would
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need to be undertaken to determine potential heavy metal contamination before the safety of
consuming watercress gathered from a site could be assessed.write only the main points
- No known enteric disease outbreaks linked to watercress consumption in New Zealand
- Overseas outbreaks linked to contaminated fresh vegetables, watercress included
- Thorough washing and treatment with chlorinated water can reduce but not eliminate pathogens on produce
- Water quality controls during harvesting, processing, and distribution are crucial in reducing health risks
- Liver flukes (Fasciola hepatica) can be transmitted through watercress consumption
- Potential for heavy metal contamination in watercress grown in certain areas of New Zealand
- Contact with contaminated water while gathering watercress can pose health risks, especially for high-risk groups