SCIENTISTS have discovered that grey squirrels host the bacteria responsible for Lyme disease, which if left untreated can lead to a chronic inflammatory disorder affecting the joints, nervous system and heart. In the UK, Lyme disease is caused by four species of Borrelia bacteria carried by the deer or sheep tick, Ixodes ricinus, and transmitted to humans through its bite. Birds and rodents are both thought to be the most important wildlife hosts of the bugs, but until now the role played by grey squirrels in their life cycle was unknown. Scientists the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow who tested 679 squirrels found around 12 per cent of the collected squirrels were infected most commonly by a type of Borrelia usually usually found in birds.