Helminth infections of the European eel Anguilla Anguilla in the United Kingdom

The European eel, Anguilla anguilla, is designated a threatened species and parasites are considered to be one of the contributory factors associated with the species decline. To address the lack of knowledge concerning parasite infections in the UK European eel population, a total of 140 specimens were obtained from the Environment Agency at 14 river sites across England and Wales. Each specimen was dissected and infection status determined for the different classes of helminth using a combination of morphological and molecular approaches, including a PCR-based restriction digestion approach to discriminate between the gill monogeneans Pseudodactylogyrus anguillae and P. bini. Overall, 101 eels (72.1 %) were infected with a total of 1504 helminths and these included gill monogeneans (prevalence = 35.7%), gastrointestinal nematodes (prevalence = 33.6%), the swim bladder nematode Anguillicoloides crassus (prevalence = 25.0%), acanthocephalans (prevalence = 30.0%) and cestodes (prevalence = 9.3%). Seven identified helminth species; the pathogenic pseudodactylids P. anguillae and P. bini, the pathogenic swim bladder nematode A. crassus, the gastrointestinal nematodes Spinitectus inermis and Paraquimperia tenerrima, and the tapeworms Proteocephalus macrocephalus and Bothriocephalus claviceps, are known eel specialists. A further 4 non-specialist eel helminths; the acanthocephalans Acanthocephalus clavula, A. lucii and the category 2 parasite Pomphorhynchus laevis, plus the gastrointestinal nematode Raphidascaris acus, were also identified.Helminth infection appeared to have a significant association with the larger eels and this was particularly evident for infections involving acanthocephalans. This presumably reflects the age and hence prolonged exposure to infection risk, as well as a more diverse diet of the larger, compared to the smaller, eels. Interestingly, the condition factor of the eels infected with helminths was also significantly greater than that of the uninfected eels. Helminth co-infections were commonly observed (54.5% of the infected eels) and the majority of these co-infections (75%) involved one, or both, of the pathogenic A. crassus and pseudodactylids. With regard to catchment sites, interesting differences were noted in the primary helminth infection data. Eel specimens analysed from the rivers in South East England contained the most numerous and diverse range of helminths. Indeed, helminth prevalence was 100% and co-infections, including up to 5 taxa, were common; pseudodactylids and acanthocephalans were present in all the observed co-infections. Eels sampled from rivers in North West England also contained a rich diversity of helminths and although pseudodactytlids were common, the gastrointestinal nematodes were the dominant taxa observed in co-infections. A. crassus and the pseudodactylids were the dominant helminths in co-infected eels sampled from South Wales and interestingly, even though this region had the second highest helminth prevalence, tapeworm infections were not observed. Only 50% of eels sampled from North Wales were infected with helminths; specifically, acanthocephalans and A. crassus were not observed in eels from this region and gastrointestinal nematodes were present in all the co-infections. Taken together, the data in this thesis confirms that European eels in the UK are commonly infected with a plethora of helminths. Some of the eel specimens exhibited known pathologies associated with their infections. However, the precise impacts that these helminths have upon eel health, and importantly the migratory capacity of the host, remain unknown and are worthy of further investigation