Sheep blamed for Manchester crypto outbreak
Sheep have been blamed for a cryptosporidiosis outbreak last year which affected over 200 people in Manchester. At the time significant numbers of Cryptosporidium oocysts could not be found in the supply network, but large numbers were found in a sample taken from the source, Thirlmere reservoir.
NHS scientist Professor Paul Hunter, who led the original investigation, has now linked the incident to sheep grazing around the reservoir, which is near Ambleside in Cumbria. Crypto is carried by many grazing animals. Last year, the UK tightened regulations concerning water companies’ crypto monitoring (see related story).
Professor Hunter said: “Steps have been taken by North West Water to remedy the situation. We must now wait and see if they have worked.”
Water from the 6km-long reservoir enters Dunmail Raise, Houghton and Chorley WTWs, from where water is pumped to 400,000 households in northern Manchester. These WTWs are now being modified to include monitoring systems for Crypto oocysts and increased turbidity.
North West Water is installing new filtration systems at several WTWs deemed ‘high-risk’ by the Drinking Water Inspectorate (DWI) but this may not occur at Dunmail Raise, the plant downstream of the intake where Dr Hunter took the contaminated sample. NWW spokeswoman Helen Lord said: “This was really just a one-off, exceptional sample and no other samples like it have been recorded since.” Nevertheless, precautions have been taken: “We have now got permission to fence off one feeder stream to keep out sheep and we have also arranged for farmers to gain access to alternative grazing areas where possible. However, the reservoir is surrounded by unfenced common land so it will be impossible to guarantee complete protection.”
It is unlikely that grazing restrictions will adequately protect raw water supplies as Crypto oocysts remain dormant and suspended in water for a long time. At the recent Water Quality 2000 conference in London, Yorkshire Water’s public health scientist Gary O’Neill told delegates: “The main problem in our region is cattle grazing by rivers, and as we cannot dictate what farmers do, we have to rely on treatment.”
The relationship between turbidity and Crypto numbers is also poorly understood. The DWI’s official advice to water companies, based on the Badenoch (1990) and Bouchier (1998) recommendations states: “It is significant that the majority of (Crypto) outbreaks identified have been associated with increased turbidity,” and asks water companies to minimise the turbidity of water entering supply.
But according to Mr O’Neill: “The relationship between turbidity and Crypto is really not that strong; if there is a relationship in the data we have collected, it is between heavy rainfall and Crypto without increased turbidity. All we know at the moment is that turbid raw water is more likely after heavy rain, and that Crypto capture rates decrease with increased turbidity in sand filters.”
Advanced membrane filters (see Water & Waster Treatment, February 2000, p.8) are capable of removing Crypto particles, but these are far too expensive for installation at most WTWs.
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