First identification of Echinococcus multilocularis in rodent intermediate hosts in Sweden

• Overall prevalence of Echinococcus multilocularis in rodents is low in Sweden.
• The distribution of infected rodents was focalized.
• Absence of competent intermediate hosts may limit parasite occurrence.
• Arvicola amphibius and Microtus agrestis are confirmed intermediate hosts in Sweden.

Echinococcus multilocularis is a zoonotic tapeworm with a sylvatic lifecycle and an expanding range in Europe. Monitoring efforts following its first identification in 2011 in Sweden have focused on the parasite's definitive host, the red fox (Vulpes vulpes). However, identifying rodent intermediate hosts is important to recognize opportunities for parasite transmission. During 2013–2015, livers from a total of 1566 rodents from four regions in Sweden were examined for E. multilocularis metacestode lesions. Species identity of suspect parasite lesions was confirmed by PCR and sequencing. E. multilocularis positive lesions >6 mm in diameter were also examined histologically. One Microtus agrestis out of 187 (0.5%, 95%CI: 0–2.9%), 8/439 (1.8%, 95%CI: 0.8–3.6%) Arvicola amphibius, 0/655 (0%, 95%CI: 0–0.6%) Myodes glareolus, and 0/285 (0%, 95%CI: 0–1.3%) Apodemus spp. contained E. multilocularis metacestode lesions. Presence of protoscoleces was confirmed in the infected M. agrestis and in three of eight infected A. amphibius. Six of the nine positive rodents were captured from the same field. This is the first report of E. multilocularis in intermediate hosts in Sweden. The cluster of positive rodents in one field shows that local parasite prevalence can be high in Sweden despite overall low national prevalence in foxes (<0.1%). The presence of protoscoleces in infected M. agrestis and A. amphibius indicate these species can serve as competent intermediate hosts in Sweden. However, their relative importance for E. multilocularis transmission in the Swedish environment is not yet possible to assess. In contrast, the negative findings in all M. glareolus and Apodemus spp. suggest that these species are of no importance.

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