Toxic algae is believed to have caused the deaths of at least four dogs in Co Fermanagh.
The dogs are suspected to have come in contact with the blue-green algae that formed on Lough Melvin on the border with Co Leitrim.
Toxic algae forms in slow moving or stagnant water in warm weather and is linked to pollution.
Aoife Ferris, of Lakeland Veterinary Services, told BBC NI that four dogs had died at her surgery since Tuesday.
All had been in the water or near the shoreline of the lake. Ms Ferris said the deaths may have been caused by toxic blue-green algae.
"Dogs are obviously very warm in this weather but I would urge any dog owner not to let their pet near any stagnant, lake water," she said.
Blue-green algae is a term for cyanobacteria, a type of bacteria that produces toxic chemicals harmful to humans and animals.
The algae can bloom in lakes and ponds during warmer weather, producing toxins that can stop the liver from functioning. Symptoms following ingestion include vomiting, seizures and sudden collapse.
The algae can look like green or turquoise wispy paint, green scum or clumps of green particles, according to the UK Centre for Ecology and Hydrology.
"Often these surface scums accumulate along the shore where children or dogs play, or where livestock come to drink," the centre says.
Ms Ferris said the dogs became unwell "quite quickly and displayed symptoms consistent with having ingested the algae". Algae are not always visible to the naked eye.
"Sometimes you will see a sludge around the shore line but the algae can also be very subtle. We have made the authorities aware of the deaths so hopefully we will see testing done on the water," she said.
The Northern Ireland Environment Agency (NIEA) received a report on Thursday of the death of a dog on May 31 at Lough Melvin near Garrison.
"A subsequent report of another dead dog was also reported on June 2," a spokesperson said.
"A Water Quality Officer was tasked to investigate for evidence of water pollution.
"No visual evidence of an algal bloom or any other signs of water pollution were detected. As a precautionary measure samples have been collected for algal analyses.
"Fermanagh and Omagh District Council have been notified of the reports, and as Lough Melvin is a cross border lake, authorities in Republic of Ireland have also been informed.
"Dog owners should be aware of the dangers posed by poisonous plants and algae growing in the environment, but especially along riverbanks and in wet grassland or edges of lakes."
