In America, Gray squirrels are often thought of as "cute", but in Europe they are not looked on nearly as favorably. Europeans have their reasons. The gray squirrel (or grey squirrel as they're referred to in the UK) is a non-native, invasive species in Europe. They crowd out native european squirrel populations, and they alter the way that forests there grow. Adding to those old concerns is a new and troubling one: A recent study shows that gray squirrels in the UK are carriers of the bacteria that causes Lyme disease in humans.
Lyme disease is caused by the Borrelia burgdorferi sensu lato bacteria. It is transmitted to humans through a bite from the Ixodes ricinus, commonly known as the deer tick or the sheep tick in the UK. The main carriers of these ticks are birds and rodents. The ticks pick up borrelia from biting and feeding on these carrier species and then transmit the borrelia to humans when they bite and feed on them.
Caroline Millins, a graduate student at the University of Glasgow's Institute of Biodiversity, Animal Health, and Comparative Medicine is the lead author of the new study which recently appeared in Applied and Environmental Microbiology, the journal of The American Society for Microbiology. "Grey squirrels are abundant and widely distributed in the UK," Millins said, "so we were interested in finding out how this invasive species may contribute to maintaining the bacteria which causes Lyme disease in humans."
Millins and her team tested 679 squirrels from across Scotland and the north of England in 2012 and 2013. They found that grey squirrels usually carried ticks, and that roughly 12% of the squirrels collected were infected with Borrelia. A surprising finding was the most common strain of Borrelia found in these squirrels is a strain that is usually found in birds. "Frequent infection of grey squirrels with bird strains of Borrelia was unexpected," Millins said, "and challenges our current understanding of host pathogen interactions for this zoonotic pathogen. ... We found quite different patterns and duration of infection in grey squirrels compared to native woodland rodent species. Grey squirrels become infected with whichever strains are circulating in the local area, and our models suggest that the duration of infection isn't life-long. In contrast, native rodents tend to develop chronic lifelong infections with strains of Borrelia that have adapted to these hosts. ... We can't say from this study whether grey squirrels lead to an increase or decrease in the number of ticks infected with Borrelia species in an area," Millins explained, "but we have produced comprehensive baseline data that future studies could use to investigate this."