Tests found it contained faeces, E.Coli and Salmonella.
Experts have issued an urgent warning to dog owners after a toxic algae was found to be deadly to pets.
The blue green algae blooms over lakes and rivers, giving water a green colour and a musty smell.
Testing carried out around the shore of Lough Neagh in Northern Ireland found it contained bacteria primarily associated with faeces, E.Coli and Salmonella.
Researchers believe the toxins in the lough could have played a role in the deaths of some dogs which entered the water last summer.
Jan Edgginton lost her two-year-old Flat-Coated retriever Cove after coming into contact with the lethal algae.
Despite trying to move Cove away from the away, he passed away less than 45 minutes later.
Jan said: ‘Cove was unforgettable. He was so much more than just a dog.
My husband first realised all was not well when Cove started staggering as he walked back from the lake, and by the time I’d caught up with them, he was already paralysed.
‘The frightening aspect to me is that something so dangerous, that was not even in the water, can kill your dog in 45 minutes.’
Lead author, Dr Neil Reid at the Institute of Global Food Security, Queen’s School of Biological Sciences, said: ‘These results confirm Lough Neagh as “hypertrophic” which is the worst category of waterway nutrient pollution, indicative of decades of agricultural, industrial and domestic runoff.
‘Our results are consistent with claims of faecal contamination of Lough Neagh and its tributaries, most likely from farm livestock and human-effluent wastewater treatment plants.’
High levels of the toxins in humans can cause liver damage and attack the nervous system.
Outbreaks of aquatic toxic microalgae have emerged as a global problem in recent decades driven by nutrient enrichment, industrial discharge, modification of surface waters, climate change and invasive species.
The new study, published in the journal Environment International, found that more than 80% of the bacterial DNA recovered from algal mats in Lough Neagh belonged to potentially hazardous microbes, including E.coli, Salmonella and 11 others that cause human illness.
Dr Reid said: ‘No one wants our environment full of potentially nasty bacteria and harmful toxins, so we need to look forward and prioritise ecological restoration and recovery.’
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