Inflammatory response to parasitic helminths in the digestive tract of Anguilla anguilla (L.)

The European eel, Anguilla anguilla, is a major warm water fish species cultured in North and South Europe. A total of 140 eels measuring 28–92 cm in total length (70.9 ± 14.7, mean ± SD cm), collected on 10 separate occasions during October 2005 to May 2006 from the Comacchio lagoons, were examined. Ninety-six (69%) harbored parasitic helminths. Of infected eels, 55% contained 3 digenean species, 2% a single cestode species, and 5% 2 nematode species. Intestinal pathology associated with digenean and cestode infection was minimal. The main damage caused by digeneans was destruction of the mucosal epithelium of the villi. Necrosis and degeneration of epithelial cells were also evident. At the site of digenean infection, a high number of rodlet cells (RCs) and mucous cells were observed in the epithelium, with both types of cells exhibiting discharge activity. The number of RCs per area (30,000 µm2) in parasitized A. anguilla (10.83 ± 7.08, mean ± SD, n = 40) was significantly greater than in uninfected (2.18 ± 2.15, mean ± SD, n = 40, t-test, P < 0.01). The majority of RCs in both infected and uninfected intestine were mature cells and presented the typical cell cortex. The number of mucous cells per area (30,000 µm2) was significantly higher in intestine of parasitized eels (70.58 ± 17.95, mean ± SD, n = 40) than in uninfected (27.18 ± 5.58, mean ± SD, n = 40, t-test, P < 0.01). Severe intestinal damage was caused by Contracaecum rudolphii A larvae encysted within the tunica propria and over the external surface of the stomach and intestine. At these sites, conspicuous granulomas showing chronic inflammatory responses characterized by infiltration of mast cells and fibroblasts were observed.