Histopathological Examination of Wild American Eels Infected with Anguillicola crassus.

We describe for the first time the histopathology caused in wild American eels Anguilla rostrata by the introduced nematode Anguillicola crassus. All of the American eels examined showed signs of previous or ongoing infections with A. crassus. Gross observations included opacity of the normally translucent swim bladder and dilation of blood vessels. The swim bladders of infected American eels showed focal, multifocal, and diffuse histological changes. Consistently observed pathologies included abnormal papillose appearance of the mucosa; hyperplasia of the lamina propria, muscularis mucosa, and submucosa; edema of the mucosa and muscularis mucosa; dilation of the blood vessels; and damage in the submucosa caused by migrating A. crassus L3 and L4 larvae. Less-common pathologies included fibrosis and lymphocytic aggregates around L3 and L4 larvae in the submucosa; destruction of the mucosa, which in some cases completely exposed the mucosal blood vessels; L2 larval penetration of the tissues of the swim bladder; bacterial infections in the submucosa and muscularis mucosa; and migration of an L4 larva through the rete mirabile.