TRACES of blue-green algae have been spotted in Windermere prompting warnings to swimmers and pet owners.
Toxins from the algae can kill wild animals, livestock and pets.
Lake District National Park Authority guidelines say the algae can cause rashes on humans after skin contact and illness if swallowed.
Illnesses including eye irritation, vomiting, diarrhoea, fever and muscle and joint pain have occurred in people who have swallowed or swam through algal scum.
These haven’t led to long-term effects or death but, in some cases, the illnesses can be severe, said the LDNPA.
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The SLDC-operated Lake Wardens took to Twitter to say: "Please be aware before swimming in the lake there are traces of blue green algae in the water at this time."
Call the Environment Agency on 0800 80 70 60 to report a sighting.