Occurrence of Parasites of the Genus Eustrongylides spp. (Nematoda: Dioctophymatidae) in Fish Caught in Trasimeno Lake, Italy

Abstract

Eustrongylides spp. is considered a freshwater fish zoonotic nematode. In the present study, the prevalence of Eustrongylides spp. in six edible fish (European perch - Perca fluviatilis, goldfish - Carassius auratus, largemouth black bass - Micropterus salmoides, tench- Tinca tinca, carp - Cyprinus carpio and sand smelt - Atherina boyeri) of Trasimeno lake was surveyed. The investigations were conducted from October 2014 to September 2015 and 384 specimens per species for each season were caught in Trasimeno lake and examined for the presence of larvae in the abdominal cavity and muscle. The presence of nematodes in the abdominal cavity and musculature was revealed in three fish species. The prevalence of Eustrongylides spp. infection was 6.84, 1.89 and 0.13% in perch, largemouth black bass and sand smelt, respectively. The number of parasites per fish was only one in largemouth black bass and sand smelt and ranged from one up to three in perch. This study states that the European perch, largemouth black bass and sand smelt of Trasimeno lake are infected with zoonotic parasites; therefore, food business operators have to take appropriate measures to guarantee the health of consumers.

Keywords: Eustrongylides spp; Freshwater fish; Nematode larvae; Parasitic hazards.

Figure 1.

Photograph of the fish species parasitized: A) European perch fillet infected by the larvae of Eustrongylides spp. and capsula (red arrow); B) largemouth black bass fillet infected by the larvae of Eustrongylides spp. (black arrow); C) sand smelt infected by the larvae of Eustrongylides spp. showing the larva protruding from the anus.

Figure 2.

Scanning electron micrographs of Eustrongylides spp. fourth-stage larva. A) Cephalic end: oral orifice, surrounded by cephalic papillae, scale bar = 50 μm; B) cephalic papilla of external circle, high magnification, scale bar = 5 μm; C) middle part of the worm showing transverse striations of the cuticle, scale bar = 200 μm; D) rounded posterior end of the larva, scale bar = 200 μm.

Figure 3.

Larva encapsulated in host muscle. A-B) Transverse section of the parasite showing intact cuticular layers (arrows). A) Haematoxylin-eosin (HE), scale bar = 200 μm; B) masson trichrome stain (MT), scale bar 100= pm. C) The arrows point out degenerating and necrotic larvae comprised inside the lumen of the capsule. HE, scale bar = 100 μm. DF) Magnification of the connective capsule surrounding the larva. Arrows point out lymphocytes (D, HE, scale bar = 10 μm), eosinophils (E, HE, scale bar = 10 μm) and macrophages (F, PAS, scale bar = 20 μm) infiltrating the capsule. G) Neoformed microvessels are evident in the wall of the capsule. MT, scale bar = 20 μm. H-I) Degenerating and necrotic muscle fibers surrounding the capsule. H) MT, scale bar=30 μm. I) HE, scale bar = 30 μm.