Kristen D. Arkush

Molecular and bioassay-based detection of Toxoplasma gondii oocyst uptake by mussels (Mytilus galloprovincialis)

Toxoplasma gondii is associated with morbidity and mortality in a variety of marine mammals, including fatal meningoencephalitis in the southern sea otter (Enhydra lutris nereis). The source(s) of T. gondii infection and routes of transmission in the marine environment are unknown. We hypothesise that filter-feeding marine bivalve shellfish serve as paratenic hosts by assimilation and concentration of infective T. gondii oocysts and their subsequent predation by southern sea otters is a source of infection for these animals. We developed a TaqMan PCR assay for detection of T. gondii ssrRNA and evaluated its usefulness for the detection of T. gondii in experimentally exposed mussels (Mytilus galloprovincialis) under laboratory conditions. Toxoplasma gondii-specific ssrRNA was detected in mussels as long as 21 days post-exposure to T. gondii oocysts. Parasite ssrRNA was most often detected in digestive gland homogenate (31 of 35, i.e. 89%) compared with haemolymph or gill homogenates. Parasite infectivity was confirmed using a mouse bioassay. Infections were detected in mice inoculated with any one of the mussel sample preparations (haemolymph, gill, or digestive gland), but only digestive gland samples remained bioassay-positive for at least 3 days post-exposure. For each time point, the total proportion of mice inoculated with each of the different tissues from T. gondii-exposed mussels was similar to the proportion of exposed mussels from the same treatment groups that were positive via TaqMan PCR. The TaqMan PCR assay described here is now being tested in field sampling of free-living invertebrate prey species from high-risk coastal locations where T. gondii infections are prevalent in southern sea otters.

Identification of a rosette-like agent as Sphaerothecum destruens, a multi-host fish pathogen.

A recent threat to European fish diversity was attributed to an infectious pathogen, a rosette-like intracellular parasite carried by invasive cyprinids. Here we show that the emerging rosette-like agent is Sphaerothecum destruens, originally found to be responsible for disease outbreaks in salmon in the United States. Sequencing of the ribosomal internal transcribed spacer (ITS) DNA highlights some level of geographical isolation. Unlike the situation in the United States, its occurrence in invasive fishes presents a risk of spread from wild invasive populations to sympatric populations of susceptible native fish and as such represents a risk for fisheries, as movement of fish for stocking purposes is common practice.

Introduced Pathogens and Native Freshwater Biodiversity: A Case Study of Sphaerothecum destruens

A recent threat to European fish diversity was attributed to the association between an intracellular parasite, Sphaerothecum destruens, and a healthy freshwater fish carrier, the invasive Pseudorasbora parva originating from China. The pathogen was found to be responsible for the decline and local extinction of the European endangered cyprinid Leucaspius delineatus and high mortalities in stocks of Chinook and Atlantic salmon in the USA. Here, we show that the emerging S. destruens is also a threat to a wider range of freshwater fish than originally suspected such as bream, common carp, and roach. This is a true generalist as an analysis of susceptible hosts shows that S. destruens is not limited to a phylogenetically narrow host spectrum. This disease agent is a threat to fish biodiversity as it can amplify within multiple hosts and cause high mortalities.