Squirrel invasions and their parasites: macroparasites of Pallas’s squirrel in Italy

Biological invasions can affect native species through several ecological processes such as competition, predation and hybridization. The outcome of the invasion may also be affected by parasites, as has been reported for several introduced species, and infection transmission can become a threat to native species and humans. The aim of the present study was to investigate the macroparasite fauna of Pallas’s squirrel, Callosciurus erythraeus, a tree squirrel native to South-East Asia, co-occurring with the native Eurasian red squirrel Sciurus vulgaris in Lombardy. Pallas’s squirrel is known to be invasive and has been introduced to other countries like Japan, Argentina, Belgium, France, the Netherlands and, in the last decade, introduced to Italy. A threat to native wildlife, and in particular to the native Eurasian squirrel, may be processes of parasite spillover, spillback or parasite release. We examined a total of 74 squirrels (females = 34, males = 40) for gastrointestinal parasites, and identified five nematode species all with prevalence below 10%. In particular, the helminth with the highest prevalence was the Oxyurid nematode Rodentoxyuris (Trypanoxyuris) sciuri, species specific to the native Eurasian red squirrel. All parasites, except S. callosciureus, were native to Europe and were acquired in the invaded range. We also found two main species of ectoparasites collected from the fur of 135 Pallas’s squirrels (females= 61, males = 74): the flea Ceratophyllus (M.) sciurorum sciurorum (prevalence 50%, mean intensity 1.0 ± 0.1) and the tick Ixodes ricinus (prevalence 47%, mean intensity 3.0 ± 0.7). While the hard tick I. (I.) ricinus infests a wide variety of terrestrial vertebrate hosts in many Italian habitats, C. sciurorum is acquired from red squirrels. The poor parasite community of the Pallas’s squirrel in Italy and the loss of native parasites might have contributed to their successful invasion. The acquisition of local parasites, and the introduction of the Asiatic nematode S. callosciureus may potentially lead to spill-back and spill-over processes towards red squirrels. The low prevalence of T. (R.) sciuri seems however to preclude, at least at present, any premises for parasite spill-back. On the other hand, further studies are needed to better understand the potential consequences of C. (M.) s. sciurorum spill-back and S. callosciureus spill-over to red squirrels and the role of the ongoing parasite colonisation process on the invasion success of C. erythraeus. Moreover, special attention should be paid to vector-borne diseases, especially pathogens transmitted by I. (I.) ricinus in Europe, since the high nymph-to-larvae ratio might indicate that Pallas’s squirrels actually play a significant role in the circulation and transmission of such pathogens in the vertebrate community, humans included.