Diversity of bird schistosomes in anseriform birds in Iceland based on egg measurements and egg morphology.

Radix balthica is the only freshwater mollusc in Iceland that has been found to be the intermediate host of ocellate furcocercariae. Infections are common. On average, 4.8% out of almost 7,000 snails examined during 2003-2006 were found to shed cercariae in late summer and autumn. Search for adult bird schistosome worms, eggs and miracidia in the intestines and in the nasal cavity of 110 water birds of the orders Gaviiformes (one species), Podicipediformes (one species) and Anseriformes (nine species) revealed 39 (35.5%) infected hosts. Exclusively, anseriform birds were infected. Based on morphological features and measurements of eggs, seven distinct bird schistosome species were detected in four of the bird species. Allobilharzia visceralis was found in 26.7% of the whooper swans Cygnus cygnus. In Anas platyrhynchos, the nasal Trichobilharzia sp. I (prevalence 73.3%) and the visceral Trichobilharzia sp. III (66.7%) were detected. In Mergus serrator, the distinct egg types of Trichobilharzia sp. IV and Trichobilharzia sp. V were found in 83.3% of examined birds. In Anser anser, also two different egg types were found: large eggs, referred to as Trichobilharzia sp. II (prevalence 54.2%), and the small eggs of Dendritobilharzia sp. (8.3%). Egg size and shape comparisons indicate that Trichobilharzia sp. III and T. franki might be the same species.