Structure of Intestinal Helminths in <i>Rhytiodus microlepis</i> (Characiformes: Anostomidae): Distribution, Histopathology and Relationships With Seasonal Dynamics of an Amazonian Floodplain Lake

The composition and structure of the helminth fauna in the intestine of the anostomid fish Rhytiodus microlepis was studied during two distinct periods in a floodplain lake in Central Amazonia. Additionally, the tissue damage in the infected intestine caused by the endoparasites was observed. Of the 38 fish analysed, a high prevalence of infection by Digenea (metacercariae) and Acanthocephala (> 90%) was observed; the metacercariae preferred the initial portions of the intestine, with a high frequency of occurrence in the lamina propria of the mucosa (75.9%), while the acanthocephalans occurred in the intermediate portions of intestine, often free in the lumen (99.5%). The histological alterations caused by the presence of the acanthocephalans were slight compression of the villi, desquamation/abrasion of the mucosal epithelium and hemorrhagic foci. Villi hypertrophy, intense leukocyte infiltration, formation of fibrous capsules and necrosis were the main responses associated with metacercariae. The composition and structure of infracommunities of intestinal helminths in R. microlepis showed seasonal variations, indicating that the hosts are susceptible to infections in the low and in the high-water seasons, with high values of intensity, abundance and pathogenicity (IAH value: 358.8 ± 99.22) of metacercariae during the low waters, suggesting that fish are more vulnerable to infections by these parasites in this period.