Gábor Nagy

The large American liver fluke (Fascioloides magna) is considered an invasive trematode parasite in Europe. Its origin dates back before the Cretaceous-Paleogene Mass Extinction, after which it survived at least three population bottlenecks and successful host switches before it arrived in Europe. The authors review the evolutionary history of F. magna, the distribution by its ancient proboscidean hosts, and the probable drivers of the switch to the white-tailed deer (Odocoileus virginianus). The review collects knowledge on the biology of intermediate hosts, which helps understand the factors that influence the epidemiology of F. magna in aquatic ecosystems. The authors demonstrate the adaptation potential of the parasite using data from both North American and European endemics. Finally, the study calls attention to the epidemiological risk of human-induced global change, with a special interest in the invasive snail species Pseudosuccinea columella.

The giant liver fluke (Fascioloides magna) is an invasive parasite found permanently in three foci in Europe. The fluke has an indirect life cycle involving a final and an intermediate host. The currently accepted terminology determines three types of final hosts: definitive, dead-end, and aberrant hosts. Recently, roe deer (Capreolus capreolus) has been classified as an aberrant host, which cannot contribute to the reproduction of F. magna. This study investigated the hatchability of F. magna eggs of red deer (Cervus elaphus) and roe deer origin to compare the suitability of the two host species for the maintenance of the parasite. The study was carried out on a newly invaded area, two years after the first reported observation of F. magna. The prevalence of the parasite proved to be 68.4% (CI95% 44.6–85.3%) in red deer and 36.7% (CI95% 24.8–50.0%) in roe deer. The difference between the two species was confirmed to be significant (p = 0.02). The mean intensity proved to be 10.0 (CI95% 4.9–22.6) and 7.59 (CI95% 2.7–24.2) in the red deer and the roe deer, respectively. The difference of the mean intensities did not prove to be significant (p = 0.72). Of the 70 observed pseudocysts, 67 originated from red deer and 3 from roe deer. Most of the pseudocysts contained two flukes, while a few pseudocysts contained one or three parasites. Egg production was observed in all three types of pseudocysts. We did not find more than three flukes in any pseudocyst. The apparent proportion of self-fertilisation in flukes without mating partners was 23.5% and 100% in red deer and roe deer, respectively. The survival of single-parent eggs was not confirmed to be worse than that of gregarious parents. The viability of offspring originating from roe and red deer differed significantly. Our findings suggest that F. magna adapted to the new populations of susceptible hosts rather than vice versa.