Helminth, protozoan and viral infections of the European eel, Anguilla anguilla

The European eel, Anguilla anguilla, is an important catadromous fish that has been classified as critically endangered due to severe population decline. The reasons for this are not fully understood; however, infectious disease undoubtedly makes a contribution. To explore this further, a total of 509 A. anguilla were sampled between 2008 and 2010 from 27 locations across England and Wales. Following dissection, the swimbladder nematode Anguillicoloides crassus was found present in 245 specimens sampled from 20 locations (48.1%). Significant correlation was found between eel length and intensity of infection. However, no significant correlation between eel condition factor and A. crassus infection was found. To explore the A. crassus population further, microsatellite analysis of 7 loci from 72 nematodes, sampled from 10 different locations, was conducted. Although multiple alleles (between 12 and 38) existed at all analysed loci, genetic differentiation of A. crassus was low (average FST = 0.037). Further analyses (STRUCTURE, BAPS and UPGMA phylogeny) showed the existence of two distinct A. crassus populations; one originating from the river Piddle and the other represented by the remaining sites. To examine eel for infection with the protozoan haemoflagellate Trypanosoma granulosum, a nested PCR assay was developed to amplify a region of the parasite 18S rRNA gene from heart tissue. Giemsa staining of eel blood sampled from specimens obtained the river Gowy confirmed the robustness of the molecular diagnostic test. Overall, T. granulosum was found to be highly prevalent (87.2%) in UK eel. A PCR-based assay for detection of the Herpesvirus anguillae (HVA) DMA polymerase gene was also developed. Heart screening revealed that H. anguillae was present in 176 eel sampled from 26 locations (34.5%). Tissue-based screening of river Gowy eel showed that HVA detection was tissue dependent. Morphological examinations showed that some eel presented clinical signs of HVA infection, though most did not, suggesting that the virus is primarily latent. Overall, the prevalence of triple co-infections (A. crassus, T. granulosum and H. anguillae), varied significantly according to sampling location; however, there was no association between these infectious agents