Parasites of grey squirrels: an additional threat to red squirrels in Italy?

Parasitic infections in north Italian populations of alien grey squirrels (Sciurus carolinensis) and native red squirrels (S. vulgaris) were investigated to (i) verify whether the invader lost, introduced or acquired any parasite species following its introduction into the new range and (ii) detect any alteration to native hosts’ parasite community induced by grey squirrels. Overall, results of field studies show that introduced grey squirrels harbour an impoverished parasite community compared to their native range and this reduction in parasite pressure may have facilitated their establishment and spread, as postulated by the enemy-release hypothesis. However, the invader introduced to Italy the North American nematode Strongyloides robustus which successfully spilled over to potentially naive red squirrels, as revealed by subsequent analysis of native hosts co-inhabiting with the alien congener. In addition, where grey squirrels are present, a significant increase in prevalence of infection by a local parasite Trypanoxyuris sciuri has been observed, suggesting that competition with invaders may indirectly reduce red squirrels’ ability to cope with parasitic infections. These findings highlight the need to further investigate the role of parasites in the red-grey squirrel system, both in the context of enemy-release facilitating invasion and as an added threat to red squirrel conservation.