Yellow grub diseases on two seasonal killifish (Cyprinodontiformes, Rivulidae): a histopathological study

Yellow grub disease is a parasite infection, caused by Clinostomum spp. Metacercariae and affecting axial muscles and internal organs of freshwater fish. Killifishes live in seasonal ponds and are short-lived; in the Neotropical region they are among the most vulnerable vertebrates. The current study analyzes the possible injury and inflammatory reaction caused by parasitic infection with Clinostomum spp. metacercariae in testes, ovaries, liver, intestine and mesentery in the seasonal killifishes Trigonectes aplocheiloides and Titanolebias monstrosus. Apparently, this parasite could migrate through the circulatory system to different organs, since a cercaria was found in the lumen of a hepatic sinusoid. In all organs the inflammatory response showed dilated, congested vascular areas and infiltration of numerous inflammatory cells, mainly composed of lymphocytes, eosinophilic granular cells and macrophages. However, the most affected organs were the pancreas, ovaries and testes. In these organs, severe pancreatitis with degranulation of the pancreatic acini, scarce ovarian follicles and spermatocysts dilated with few or no seminiferous cells were observed. In seasonal killifish, yellow grub disease could cause low fertility rates or even sterility by reducing or impairing their reproductive capacity. Therefore, this type of parasitism has a considerable impact on the perpetuation of this species due to the short periods they have to breed before their habitats dry out.