First record of chronic Fascioloides magna infection in roe deer (Capreolus capreolus)

Fascioloidosis is an allochthonous parasitic disease in Europe caused by the digenean trematode Fascioloides magna. The final hosts of F. magna in Europe are defined as definitive, aberrant and dead-end. Roe deer are aberrant hosts in which juvenile flukes permanently migrate through the liver parenchyma. Failure in pseudocysts formation leads to the death of both the host and the parasite. In this paper we present gross and histological findings of F. magna infection in 34 roe deer. The special emphasis is on the pseudocyst formation accompanied with new fluke's migratory channels observed in 7 animals, suggesting reinfection and prolonged survival of roe deer. No F. magna eggs were recovered from the faeces of the infected animals. These findings indicate that pseudocyst formation is essential for roe deer survival, but also point out the potential beginning of adaptation processes in roe deer, altering otherwise acute and fatal disease into a chronic one.