Prevalence, incidence, and morphology of Eustrongylides (Nematoda: Dictophymatoidea) in freshwater Galaxias maculatus in Victoria, Australia.

Eustrongylides is a genus parasitic nematode, comprising of three species, E. tubifex (Nitzsch in Rudolphi 1819), E. ignotus (Jägerskiöld 1909) and E. excisus (Jägerskiöld 1909), with larvae commonly encysted in the body cavity and musculature in fish. Eustrongylides have a complex lifecycle involving first intermediate hosts (oligochaetes), second intermediate hosts (benthic fish), paratenic hosts (predatory fish, frogs, reptiles) and definitive host (piscivorous birds). There is a lack of general knowledge regarding Eustrongylides in Australia and their influence on host growth, condition and reproductivity. Our objectives were to 1) quantify the prevalence of infection by Eustrongylides in two populations of Galaxias maculatus, 2) determine the site of infection within the host, 3) examine the association between infection, fish condition (including length-weight relationships) & gonad development, 4) determine parasite size and sex distribution, 5) determine the survival of parasite outside of a host, 6) to examine Eustrongylides sp. morphology. Galaxias maculatus were collected from two locations (Deans Creek and Yarram Creek), measured and weighed, and all Eustrongylides were removed and worm morphometrics were recorded. A number of removed worms were also kept alive in the laboratory. There was a similar level of infection between Deans Creek and Yarram Creek with a prevalence of 38% and 35% respectively. These prevalence’s in this study are much higher than those reported in other small-bodied fish in Australia and overseas. For the site of host infection, 80% occurred in the abdominal cavity (muscle 20%), and there didn’t appear any significant effect of Eustrongylides on host condition or reproductive development. However, Deans Creek specimens were in significantly poorer condition than Yarram Creek. There was sexual dimorphism between the Eustrongylides sp. including external morphology and size (females bigger than males). This is the first study to have recorded the number of days these nematodes can survive outside of the host, and this ranged from 3 – 21 days. SEM was used to identify key morphological 3 features, and several new morphological structures were identified, which have not been described in this genus before. Therefore, it is possible that the Eustrongylides sp. in this study, could be a new species, but further morphological and molecular analysis is required to confirm this.