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 PAN International Website |
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Spraying could spread food poisoning bacteria
New research from Canada shows that farmers and growers can spread dangerous food poisoning bugs in their pesticide sprays. Scientists at Manitoba University have found that some pesticide mixes promote the growth of bacteria which are then sprayed onto crops. Although UK scientists say the risk from pesticides is small compared to those from animal manure spread on farmland, the Manitoba team insists that more research should be done urgently. John Harvey reports.
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Anyone with a farm or business is on a metered water supply, and filling up a 500-gallon sprayer from the mains would be very expensive. |
‘Several investigators from the US went to Guatemala trying to find out where this parasite came from. Investigators were suggesting water as a source, and perhaps the application of pesticides had something to do with it.’
At the same time, the Centre for Disease Control in Atlanta, Georgia, was reporting that as many as 41% of all cases of food-borne illness can be traced to the consumption of fresh fruit, vegetables and their juices. ‘So the question is where is all this coming from and what else do we put on our fruit and vegetables?’ said Professor Blank.
To answer this, scientists at the university’s food science department tested a range of herbicide, fungicide and insecticide solutions and mixed them according to recommended application rates over different times and temperatures. Bacteria such as Listeria, Salmonella and Escherichia coli 0157:H7 were added to the solutions.
They found that bacteria which have caused some of the worst food poisoning outbreaks in the UK – E coli 0157: H7 and Salmonella – survived and multiplied in some pesticide formulations. Post-emergence products are more likely to cause problems than pre-emergence products, according to Professor Blank, and it is standing crops such as strawberries and lettuces which are most at risk because they are sprayed throughout the growing season.
In Canada, the catalyst for food-poisoning bacteria is contaminated water mixed with pesticide concentrate before the solution is sprayed on to crops. ‘Surveys have shown that if a farmer is near any standing water – whether it is from a lake, a stream, or a well – that is what he will use for his pesticide mix,’ said Professor Blank. E coli has been found in water on farms, and a recent incident in Ontario killed 11 people and left 2000 ill after E coli 0157:H7 contaminated municipal drinking water.
It is thought the outbreak was caused by run-off containing animal wastes escaping into watercourses from nearby farms. ‘We are saying two things,’ Professor Blank said. ‘First, those who test pesticide formulations should assess their potential to spread bacterial growth; secondly, if farmers mix a pesticide, they should use it within 24 hours. Otherwise, when they re-constitute that pesticide, they will allow these organisms to grow.’
There are three key questions about Professor Blank’s work: why some products grow bacteria while others kill them off quite quickly; whether farming practices in Canada are mirrored in the UK; and whether other factors – such as the weather – will reduce amounts of bacteria sprayed on to crops.
Chemicals studied included Round-Up (glyphosate), Gramoxone (paraquat), 2,4-D, Dithane M45 (mancozeb), Benlate (benomyl), Bravo 500 (chorothalonil), Ridomil 240EC (metalaxyl), Thiram 75WP, Sevin XLR+ (carbaryl), Lorsban 4E (chlorpyrifos), Diazinon 500 and Ambush 500EC (permethrin).
Pathogens such as E coli 0157:H7, Salmonella typhimurium, Salmonella enteritidis, Shigella sonnei, Shigella flexneri and Listeria monocytogenes were cultured and added to the solutions. Researchers tracked the growth rates of various bacteria over one to 24 hours. In some cases – such as Round-Up, the glyphosate chemical added to Monsanto’s genetically modified, herbicide-tolerant crops – the pathogens died within an hour or were gone after 24 hours. In others – such as Ambush 500, a synthetic pyrethroid – bacterial levels increased when researchers raised the temperature of the mixes from 20-22°C to about 31°C. E coli 0157:H7 even survived in Lorsban 4E and Ambush 500 when the solutions were left for as long as 96 hours.
The highest growth for all types of bacteria – including E coli 0157:H7 – was seen in Bravo 500, said Professor Blank, who added that more research would have to be done to discover why some products make bacteria grow more than others.
Two of the UK’s leading microbiologists – Hugh Pennington, professor of bacteriology at Aberdeen University and Dr Norman Simmons, a consultant – said UK farmers would tend to use clean drinking water from mains supplies for their sprayers.
‘The first question is how the bugs ended up in the spray in the first place,’ said Professor Pennington. ‘The next question is the composition of the pesticide solution, and then you have to find out what temperature that solution is held in. Obviously, if someone messes around putting the solution into old buckets which have manure in them, many pesticides are not inimical to bacterial growth.’
There is some evidence that drinking water troughs used by animals in the United States were a source of food poisoning bacteria, said Professor Pennington. ‘The jury is still out on that, though, and it is often very difficult to isolate these bugs from water supplies anyway.’
Dr Simmons said farmers using potable water to make up their sprays would have no risk of contamination. ‘If they use water from troughs or something, there is a potential risk – but why do that when it will bung up the sprayer anyway?’ A recent survey of intestinal disease covering 1992-99 showed that only 4.3% of food-borne infections were linked to contaminated salad items, fruit and vegetables. ‘That is a very low figure, and most of the incidents that were known about in that survey were due to viruses,’ said Dr Simmons.
Brian Chambers, a soil scientist with the agricultural advisory service ADAS who has studied how pathogenic bacteria move through the food chain, said he thought the risk of E coli and other bugs spreading via pesticide spray was low. ‘Most farmers make up pesticide mixes with mains water. It is not worth the risk of using potentially dirty water because you could block up the spray nozzle with sediment.’
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Rivers may provide the quickest way to fill up a sprayer if the field is a long way from the mains. |
In some areas, farmers might use water from
boreholes above aquifers for their tank mixes, he believes.
‘But it is the surface waters from rivers and reservoirs
that are more likely to be contaminated by E coli and other pathogens.
Where surface waters are used for irrigation, there is a much higher risk. On
water-hungry vegetable crops, for example, irrigators can spray up to an inch of
water every four to five days. We have found E coli in irrigation water,
although we have never looked for the 0157:H7 strain.’
Mr Chambers said contaminated pesticide mixes could be
something growers should think about in terms of the due diligence provisions of
the Food Safety Act. Taking the example of contaminated solutions sprayed on to
the surface of lettuce leaves, he said that ultra-violet light should quickly
destroy any bugs. ‘But down at the bottom of the plant where the leaves meet
the stem, there is not much UV light. If spray is going on close to harvest, it’s
something people should be aware of.’ Professor Blank recognises that UV light
could dramatically reduce bacterial growth, and says it is something his team
will have to investigate.
So professional scientists who keep an eye on the UK food
chain acknowledge that there is a risk from mucky pesticide mixes, but it is
small compared to dirty irrigation water and the millions of tonnes of animal
manures spread on farmland every year. That is the view from the bridge, but
what happens below decks – on the ground where spraying practice may vary from
farm to farm and between farmers and contractors? Does the law allow farmers to
take water from rivers and boreholes to mix their sprays?
Under the 1991 Water Resources Act, a farmer can abstract up
to 4,400 gallons of water from a river or stream to use in spraying without
obtaining an abstraction licence from the Environment Agency. But a licence
would be required if the farmer wanted to abstract water from a well or
borehole, or needed more than 4,400 gallons. ‘It should be noted that the
granting of an abstraction licence is dependent on the availability of water,
the impact of the abstraction on the environment and existing legitimate users
of water – and a demonstrable need for that water,’ said Paul Wilman, a
regional abstraction licence review officer for the agency. ‘The agency does
not issue a licence to everyone who applies.’
While UK microbiologists argue that farmers would not risk
using dirty water in expensive sprayers, one Kent farmer has told Pesticides
News that farmers will use the nearest source of water when they are in the
middle of a large arable field and keen to finish spraying before the weather
changes.
Gary Coomber farms 300 acres near Headcorn in Kent, and used
a contractor to spray his crops. ‘One of the reasons he used a pond or a river
was that it was the quickest way to fill the sprayer up if the field was a long
way from the mains,’ said Mr Coomber. ‘The other thing is that by going back
through the crop to fill up, you are damaging it even more.’ Anyone with a
farm or business would be on a metered water supply, and filling up a 500-gallon
sprayer tank from the mains would be very expensive. ‘If you have a few
hundred acres or the farm is very spread out, it is much quicker to fill up from
a natural water source. I would say that is fairly standard practice around
here.’
The National Association of Agricultural Contractors
acknowledges that smaller farms could take supplies from the troughs, streams
and rivers, which Professor Blank maintains are contaminated with bacteria.
Larger farms would use bowsers pulled by a tractor to fill the sprayer – or
they would have large water tanks strategically placed round the farm for the
sprayers to lock on to. Tanks and bowsers should be connected to mains water,
the Association said.
Any farmer filling a sprayer from a pond or river will have
at least a suction filter to prevent the nozzles clogging up with sediment –
so the technology can overcome the need for mains water. There may be two more
filters before the spray comes out of the nozzle – a finer pressure filter and
some ultra fine filters on the nozzle bodies. Ideally, farmers should have a
separate pump to take the water into the sprayer in case pesticide solution
accidentally escapes back into watercourses.
- Reference
1. Manitoba Co-operator, 27 July 2000.
John Harvey is a writer and broadcaster on farming issues.