Abstract

Ongoing, though mainly subclinical, gill infestations with the monogean trematode Pseudodactylogyrus anguillae have been observed in American Eels displayed at the Aquarium du Québec. In recent years, this parasite has been occasionally associated with mortalities, especially in conjunction with stressful events. Anguillicola crassus was recently detected in the swim bladders of asymptomatic eels from individuals that had been collected in New Brunswick in 2005. Since this nematode has not been previously reported in the St. Lawrence River, the aquarium was invited by the provincial wildlife agency to cull this cohort of eels based on the theoretical risk of translocation of this exotic parasite. The cull provided an opportunity to evaluate: the sensitivity of non-lethal gill biopsies in detecting P. anguillae infestation; the effect of anesthesia with tricaine methanesulfonate baths on the number of trematodes detected and the efficacy of treatments with emamectin benzoate (0.1 mg/kg PO once or twice q5days) to control A. crassus infection. The prevalence of P. anguillae was 70% with an infestation intensity ranging from 1 to 110 trematodes per eel, whereas the prevalence and infestation intensity of A. crassus was 37% and ranged from 1 to 4 respectively (n = 54). Gill biopsies had a low sensitivity (29%) for detecting the presence of P. anguillae in this group of subclinically infested eels, and the number of trematodes detected was not affected by the use of tricaine methanesulfonate. Two weeks following the first treatment with emamectin benzoate, approximately 40% of the A. crassus occupying the swim bladders were dead.