Amplification and transport of an endemic fish disease by an introduced species

Abstract

The introduction of American shad from the Atlantic to the Pacific coast of North America in the late 1800's and the subsequent population expansion in the 1980's resulted in the amplification of Ichthyophonus sp., a Mesomycetozoean parasite of wild marine fishes. Sequence analysis of the ribosomal DNA gene complex (small subunit and internal transcribed spacer regions) and Ichthyophonus epidemiological characteristics indicate a low probability that Ichthyophonus was co-introduced with American shad from the Atlantic; rather, Ichthyophonus was likely endemic to marine areas of the Pacific region and amplified by the expanding population of a highly susceptible host species. The migratory life history of shad resulted in the transport of amplified Ichthyophonus from its endemic region in the NE Pacific to the Columbia River watershed. An Ichthyophonus epizootic occurred among American shad in the Columbia River during 2007, when infection prevalence was 72%, and 57% of the infections were scored as moderate or heavy intensities. The epizootic occurred near the record peak of shad biomass in the Columbia River, and corresponded to an influx of 1,595 mt of infected shad tissues into the Columbia River. A high potential for parasite spillback and the establishment of a freshwater Ichthyophonus life cycle in the Columbia River results from currently elevated infection pressures, broad host range, plasticity in Ichthyophonus life history stages, and precedents for establishment of the parasite in other freshwater systems. The results raise questions regarding the risk for sympatric salmonids and the role of Ichthyophonus as a population-limiting factor affecting American shad in the Columbia River.

Parasitism by water mites in native and exotic Corixidae: Are mites limiting the invasion of the water boatman Trichocorixa verticalis (Fieber, 1851)?

“…When an introduced species is a suitable host for a native parasite, this can seriously impact the exotic hosts, but can also amplify the infection (''spillback'' from exotic to native species) with effects for native species at both the host individual and population level (Daszak et al 2000;Tompkins and Poulin 2006). At the current stage of T. v. verticalis invasion our results provide no evidence of parasite spillback, but it remains a potential risk given the density and reproductive potential of the exotic host, high susceptibility for parasites and the high reproductive potential of parasites, all factors affecting the probability of spillback (Hershberger et al 2010;Paterson et al 2013). We can expect the opposite to the dilution effect hypothesis, which predicts that the introduction of a less competent host species may reduce infection prevalence in the native host (Telfer et al 2005).…”

Phylogenetic interpretations and ecological potentials of the Mesomycetozoea (Ichthyosporea)

“…The capacity to switch hosts allows for a departure from a density dependent relationship with a specific host population, potentially allowing a disease to represent a greater risk to other susceptible hosts. In North America, the translocation of the American shad appears to have amplified the prevalence and range of an endemic strain of Ichthyophonus along several river systems along the Pacific coast (Hershberger et al 2010). In Europe S. destruens has spread with invasive cyprinid fish acting as carriers (Gozlan et al 2005) and has recently been implicated as the cause of local extinctions of an endangered cyprinid, Leucaspius delineates (Andreou et al 2012).…”

Spillover but no spillback of two invasive parasitic copepods from invasive Pacific oysters (Crassostrea gigas) to native bivalve hosts

“…While there is ample evidence for the acquisition of native parasites by invasive host species (step 1 of parasite spillback; e.g. Miller and Vincent 2006;Georgiev et al 2014;Sheath et al 2015;Lewicki et al 2015), there is little conclusive evidence for an actual amplification and/or spillback to native species (but see Hershberger et al 2010;Goedknegt et al 2016). Finally, as an alternative mechanism to parasite spillback, incompetent invasive host species can act as a sink by diluting the native parasite population and thereby reducing the disease risk for native species (transmission interference or parasite dilution; Thieltges et al 2009;Paterson et al 2011;Poulin et al 2011;Goedknegt et al 2016).…”