Eurasian teal ( = 46,581) were inspected during ringing operations for the presence of the hymenolepidid cloacal cestode between 1954 and 1971 while wintering in the Camargue, Southern France. These birds become infected when ingesting seed shrimps (Ostracoda) that act as intermediate hosts, largely while on migration across Western Europe. The prevalence ranged from 4% to 14% per year and increased significantly over time. This long-term trend was further supported by studying 366 teal shot in 2006-2008, for which prevalence of was 27.6%. We found no evidence to suggest that this increase in prevalence has been caused by an increase in temperature, but eutrophication and an increase in duck densities are potential contributing factors. Adult teal were more likely to be infected than first-year birds and females more so than males, probably due to differences in diet and/or habitat use. Within a given age−sex class, heavier birds were more infected than lighter ones, suggesting low pathogenicity and a causal effect of ingestion rate. Within a year, the highest prevalence was observed in mid-winter.