Pet owners warned about decomposing fat balls washing up on beaches after dog had to put down when he ate one Surfer Daniel Hart, 32, was forced to put his pet dog Maisey to sleep She suffered pneumonia and a bleeding nose after eating ball of fat Substance has been washing up along Welsh coast for a fortnight A dog has died after eating a decomposing ball of fat that had washed up on a Welsh beach. Surfer Daniel Hart, 32, was forced to put his pet Maisey to sleep after she became seriously ill with pneumonia and a bleeding nose. Now, pet owners are being warned about the white lumps of fat - which look like snowballs - that have been washing up in small quantities along the Welsh coast for more than a fortnight. Warning: Pet owners are being warned about decomposing fat balls that are washing up on the Welsh coast The rancid substance has also been reported in areas along the Cornish coast. Experts are at loss to explain where the fat has come from, but believe either Atlantic storms could have washed it over to the UK or passing ships might have dumped it at sea. While posing no risk to people, the substance can cause potentially fatal problems for dogs whose digestive systems are unable to cope. Mr Hart had been surfing at the Llangennith beach on the Gower Peninsula, near Swansea, on Sunday when his beloved dog Maisey started being violently sick. The decomposing fat was first reported in Ogmore, pictured, in the Vale of Glamorgan, almost two weeks ago I was surfing and Maisey was around the high tide line. I called her to me. She ran half way to me then started being sick,' he told BBC Wales online. Maisey, who is believed to have eaten congealed vegetable oil, bled from the nose as she was carried to her owner's car. She was quickly taken to an emergency vet, where she was given antibiotics for pneumonia. Mr Hart, from Gorseinon, near Swansea, took Maisey home and slept with her on his settee - however, her condition failed to improve and blood began pouring from her nose. Less than six hours hour, Mr Hart decided it would be kinder to have his beloved pet put to sleep. 'It was horrible taking the decision but I didn't want her to suffer,' he said. She had lost the will to live. Natural Resources Wales (NRW) has warned dog walkers using the region's beaches to be 'vigilant' A spokesman for Natural Resources Wales (NRW) has warned dog walkers using the region's beaches to be 'vigilant'. He added that the first reports of rancid fat on Welsh beaches had come in from Ogmore, in the Vale of Glamorgan, almost two weeks ago. 'It appears to have started off in the east and is gradually making its way west,' he said. He said he believed that there had been one death and up to 10 cases of dogs becoming ill after consuming the fat. 'There is nothing particularly to worry about environmentally and there is no danger to humans - I'm assuming that nobody would try to eat it,' he said. The fat has been tested. It has been found to be largely vegetable matter, possibly oil, that is relatively harmless. 'We are not talking about large amounts here but it is something that dogs might eat so we have warned people to be vigilant for the sake of their pets,' he said. A spokesman for Swansea County Council said it was investigating reports of the fat, but had so far encountered nothing. He said cleaning the beach would be the responsibility of the landowner in the area affected.