Parasites of zoonotic interest in selected edible freshwater fish imported to Australia

Abstract

Australia imports a significant amount of edible freshwater fish. The safety of the imported product is therefore of great importance. Previous research has shown that certain types of edible freshwater fish imported into Australia are not compliant with Australian importation guidelines and additionally are contaminated with many species of parasites, some of which may cause illness in humans if consumed. The present study, to the best of authors knowledge, is the first to publish the occurrence of zoonotic parasites in edible fish imported into Australia. Eustrongylides sp. Jägerskiöld, 1909 (P. 15.5%), family Dioctophymidae; Euclinostomum sp. Travassos, 1928 (P. 4.8%), family Clinostomidae, were recovered from imported edible and consumer ready Channidae fish and Isoparorchis sp. Southwell, 1913 (P. 11%), family Isoparorchiidae, from imported edible Bagridae fish. Euclinostomum sp. and Isoparorchis sp. were identified using morphological method. Molecular identification of Eustrongylides sp. was achieved through sequencing of the 18S ribosomal RNA gene sequence. Eustrongylides sp. and Isoparorchis sp. have been identified as the causative agent in cases of human infection and are a recognised zoonosis. Euclinostomum sp. is considered to have zoonotic potential and for this reason this species has been included in the importation risk assessments for freshwater fish from certain countries. This study confirmed the presence of zoonotic parasite species in edible imported fish. Whilst this fish product was frozen and parasites therefore inactivated, both fish species according to importation commodity codes, at the time this manuscript was written, are permitted entry into Australia chilled. Further study using a greater sample size is required to understand the human health risks.

Keywords: Imported fish; Inspection procedures; Zoonotic parasites.

Graphical abstract

Fig. 1

The global fish supply chain and risk points for zoonotic parasites in edible fish. In green are the basic steps, from farm/capture to consumption which edible imported freshwater fish may follow. In grey are the risk factors at each stage of the supply chain which may increase the risk of zoonotic parasites in edible freshwater fish. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Current inspection procedure for edible seafood imported into Australia.

Fig. 3

Eustrongylides sp., Euclinostomum sp. and Isoparorchis sp. 3A. anterior and 3B. posterior of Eustrongylides larvae. 3C. Euclinostomum ex-cysted metacercariae. 3D Isoparorchis sp. metacercariae anterior, E. mid-section with muscularised acetabulum surrounding ventral sucker and F the posterior. Characteristic undulating intestinal caecum morphology in view E and F.

Fig. 4

The lifecycles of the nematode Eustrongylides spp. (yellow arrows) and the digenean Euclinostomum spp. (green arrows). The life cycle hosts to do not vary greatly between these two species although the parasite developmental strategies between nematodes and digeneans do. The first intermediate host for Eustrongylides is a range of oligochaetes and for Euclinostomum certain species of freshwater snails. Both species utilise small fish as 2nd intermediate hosts including a number of aquarium species for Euclinostomum. Predator fish have potential to become highly infected with both species of parasite. The life cycle is completed in water birds as the definitive hosts (Original figure created from images available via open creative commons). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)