A warning has been issued after a swimmer contracted a rare disease in the River Stour.
Dedham Parish Council has revealed that a local school student has suffered a "severe infection" while swimming in the river, which straddles the Suffolk and Essex border through Constable Country.
In a letter, which was sent to pupils and parents at East Bergholt High School, the authority said the infection is a case of Weil's disease, also known as leptospirosis.
"We are writing to make you aware that unfortunately a local student has recently suffered a severe infection after swimming in the River Stour at Dedham, and has been very poorly," said a spokesman for the parish council.
"The Environment Agency is currently undertaking monitoring in the river at Dedham and, once received, we will share their findings on the Dedham Parish Council website."
According to the NHS, Weil's disease is a rare condition in the UK.
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It is spread through infected animal urine and is most common among rats, cows, pigs, dogs and mice.
The serious disease can be contracted if freshwater containing the infected urine gets into someone's mouth. This is usually through activities such as outdoor swimming.
Dedham Parish Council stated it was working with several other agencies to monitor the issue and improve river safety awareness.
It comes amid concerns surrounding "regular" accidents at an access point to the river in Mill Lane.
One child was left with a gash on his foot after litter was left behind, while children have also been seen jumping into the river.
A representative of the River Stour Trust said: "Weil’s disease is an infection introduced to water through the urine of rats and is something all river users are aware of. And frankly, short of eradicating rats there is not a lot that can be done about it.
"We generally recommend that you do not go into the river if you have a cut or graze. And if you are a swimmer, endeavour to keep it out of your mouth."
Councillor Laura Smith, Green party councillor for Sudbury South West on Babergh District Council, said: "I’m very sorry to hear that someone has contracted Weil’s disease in the Stour; unfortunately it is always a risk of swimming in rivers.
"I hope the person makes a speedy recovery, and I hope for others this is a reminder that the water might not be clean even if it looks clean, and to make sure any open cuts are covered before going in the river."
The Environment Agency said that health risks from locations not designated as bathing water sites may be higher than those which are.
Areas that are not designated as bathing waters are not monitored for bacteria, intestinal enterococci and E. coli, which are used to assess bathing water quality.
A spokesman for the Environment Agency said: "They can contain levels of sewage, faeces from livestock and pollution from farming or industry which are harmless to wildlife but would not be acceptable in designated bathing waters."
It comes after part of the River Stour near Sudbury was awarded bathing status.
The Environment Agency spokesman added that it does undertake routine sampling, although latest tests do not suggest any water quality issues at the site.
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