Grey squirrels may harbour a serious infection that can be spread to humans by ticks.

Such rodents from the US already have previous convictions for decimating Britain’s red squirrel populations and damaging woodland by stripping bark from trees.

Now scientists have discovered that they 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.

If left untreated, Lyme disease can lead to a chronic inflammatory disorder affecting the joints, nervous system and heart

Scientists who tested 679 squirrels from across Scotland and the North of England in 2012 and 2013 found that grey squirrels frequently carried ticks, and could be infected by all four bacteria species.

Around 12 per cent of the collected squirrels were infected, most commonly by a species of Borrelia usually found in birds.

Lead researcher Caroline Millins, from the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow, said: “Frequent infection of grey squirrels with bird strains of Borrelia was unexpected, and challenges our current understanding of host pathogen interactions for this zoonotic pathogen.”

Results from the research are reported in the journal Applied and Environmental Microbiology.