Pests and diseases: A tale of two squirrels

THE two species of squirrel resident in the UK are Sciurus vulgaris (native European red squirrel) and Sciurus carolinensis (North American eastern grey squirrel).

Introduced to estates in the 1870s, the grey squirrel now numbers 2.5 million, its population spread over most of the UK, dwarfing the 140,000 native red squirrels pegged back into parts of Scotland and northern England, plus a few island redoubts like Anglesey and the Isle of Wight.

The native red squirrel is now one of the nation’s most highly prized biodiversity icons, with the grey an alien invasive classed as vermin.

However, this tale of two squirrels has a ‘sting in the tail’ which came to light during archaeological research carried out by scientists at Cambridge University. It appears that red squirrels infected with the bacterial pathogen (Mycobacterium leprae) which causes leprosy may have brought the disease to England along Viking trading routes for fresh meat and fur.

The North American eastern grey squirrel (Sciurus carolinensis) is now more at home in England than the native European red squirrel.

A study published in the Journal of Medical Microbiology (Inskip et al. 2017) found medieval leprosy cases in human remains in Eastern England including a pre-Norman female skull unearthed in a garden at Hoxne in Suffolk, dated 885–1015 AD. Analysis of the bacteria revealed she had been infected with the same strain of leprosy previously found on the skeleton of a man from Great Chesterford in Essex who had lived as early as 415–545 AD. This suggests the disease had persisted for centuries in the south-eastern part of Britain, probably arriving on the east coast. The same strain of the bacterium has been found in skeletal remains dating from the medieval period in Denmark and Sweden.

The last case of human leprosy in Britain was over 200 years ago, but a recent study showed leprosy infection in red squirrels on Brownsea Island in Dorset. Genetic sequencing of the bacterial strain showed it was closely related to that detected in the skull unearthed at Hoxne in Suffolk.

Lead author Sarah Inskip, research associate at St John’s College Cambridge, said: “It is possible that this strain of leprosy was proliferated in the south-east of England by contact with highly prized squirrel pelt and meat and traded by the Vikings at the time this woman was alive.

“Strong trade connections with Denmark and Sweden were in full flow in the medieval period, with King’s Lynn and Yarmouth becoming significant points of entry for fur imports. It is questionable how long the bacteria could have survived on fur or meat, but it’s notable that squirrels were also sometimes kept as pets.”

That pieces of red squirrel are not being found on Brownsea Island may suggest the native red is tolerant but perhaps the alien grey is not. However, anyone thinking of introducing leprosy-causing bacterium into the grey population to wipe it out might like to imagine their hands with fewer digits.

Like beech trees, the similarly thin-barked hornbeam suffers considerable gnawing damage from grey squirrels. But characteristic fluting of the stems to create distinct folds means hornbeam, unlike beech, rarely suffers complete ring barking and is therefore more likely to survive.

Reasons for the successful invasion of the British Isles by S. carolinensis in such a short space of time are many. However, its ability to carry and transmit the squirrel pox virus to which S. vulgaris has no immunity is considered a clincher in its success. In areas where the virus has been introduced by S. carolinensis, the decline in S. vulgaris numbers is 17 to 25 times more rapid than to be expected by competition alone.

There is yet another sting (and a whiff of hypocrisy) in this tale. Today, S. carolinensis is blamed by UK foresters for widespread bark stripping of trees, to such an extent that some native broadleaf deciduous hardwood species like common beech (Fagus sylvatica) are no longer planted commercially where greys proliferate.

However, when S. vulgaris ruled supreme, this native squirrel was blamed for the same manner of damage and persecuted for its activity. In 1903, the Highland Squirrel Club in Scotland was established to control red squirrels, which were causing severe bark stripping, and over 82,000 were killed in the 20 years up to 1933.

Therefore, it’s possible that South-East England may have proved such fertile territory for the invading grey simply because the native red had already been hunted to near extinction.

REFERENCE

Inskip, S., Taylor, G. A., Stewart, G. (2017) ‘Leprosy in pre-Norman Suffolk, UK: Bimolecular and Geochemical analysis of a Woman from Hoxne’. Journal of Medical Microbiology Volume 66, Issue 11.