In Australia, nematodes belonging to the genus Eustrongylides were believed to be endemic species until the late 20th century when they were all considered to be E. excisus, invalid or inquirendae. Although these nematodes have frequently been reported in Australian fish, reptiles, and birds and cause disease or mortality among them, there has been no attempt to date to characterise them genetically. Globally, also, no one has validated or defined suitable genetic markers to distinguish between species of Eustrongylides. In this study, adult Eustrongylides from little black cormorant (Phalacrocorax sulcirostris; n = 3) and larvae from mountain galaxias (Galaxias olidus, n = 2) and a Murray cod (Maccullochella peelii, n = 1), and a Murray cod-trout cod hybrids (Maccullochella peelii x Maccullochella macquariensis, n = 1) were available for morphological examination and molecular characterisation. The adult nematodes from cormorants were identified as E. excisus. Sequences of the 18S and ITS regions were then obtained for all nematodes, which were identical among all specimens (larvae and adults) and also identical to those of E. excisus available in the GenBank. However, only one base pair difference exists between the 18S sequences of E. excisus and E. ignotus, with limited sequences available in GenBank accompanied with proper morphological data for the nematodes. With that limitation in mind, identifying our specimens as E. excisus suggests spill-over – that it is an introduced parasite species that has successfully established its life cycle among Australian native species – may have occurred. Our study is the first report of E. excisus in the little black cormorant, P. sulcirostris. Our results do not exclude the possibility of the occurrence of other species of Eustrongylides, either native or exotic, in Australia. This parasite is zoonotic and with increasing demand for fish and changing dietary preferences, such as the consumption of raw or undercooked fish, its occurrence in the flesh of the fish is concerning. This parasite is also associated with anthropogenic habitat alteration affecting the reproductive success of the infected hosts. Therefore, awareness among the relevant authorities of the presence of the parasite in Australia and its adverse impact on native animals is crucial for the success of conservation plans such as fish recovery and relocation efforts.
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