Insights into the aetiology and epidemiology of an emerging protistan disease of amphibians
Date: 10 May 2021
University of Exeter
PhD in Biological Sciences
Severe Perkinsea infection (SPI) is an emerging disease that has been linked to mass die-offs of tapdoles across a broad geographic range in the USA. The exact relationship between Perkinsea infection and SPI disease remains undefined and untested. It is not known if certain co-factors (e.g. co-infection with other pathogens, exposure ...
Severe Perkinsea infection (SPI) is an emerging disease that has been linked to mass die-offs of tapdoles across a broad geographic range in the USA. The exact relationship between Perkinsea infection and SPI disease remains undefined and untested. It is not known if certain co-factors (e.g. co-infection with other pathogens, exposure to pollution, host microbiome composition) contribute to disease progression, or if differences in host susceptibility affect the distribution of Perkinsea infection and SPI disease. We have yet to determine if the two known lineages of amphibian Perkinsea (i.e. a putatively pathogenic lineage and a putatively non-pathogenic lineage) are distinct strains, or if they are genetic variants of the same strain. Finally, we must continue to explore the extent of the parasite’s geographic range, particularly in regions with high amphibian diversity and/or a large number of threatened species. This thesis aims to elucidate some of the unknown aetiological and epidemiological aspects of Perkinsea infection and SPI disease in amphibians. These aims were addressed with a variety of experimental approaches. We investigated the aetiology and transmission of SPI disease by experimentally infecting healthy tadpoles with Perkinsea parasites, and by feeding tissue from putatively infected tadpoles to healthy tadpoles. We developed a field-compatible duplex qPCR assay for the rapid detection of the two lineages of amphibian Perkinsea, to be used as an epidemiological tool for monitoring amphibian health. We used phylogenetic analysis to investigate Perkinsea infection in captive tadpoles from the UK and wild tadpoles from Panama, a global centre of amphibian diversity and a biosphere greatly affected by amphibian decline. We used amplicon-based metagenomics to compare the intestinal eukaryotic microbiomes of healthy and Perkinsea-infected tadpoles. Our experimental infections did not result in a pathogenic interaction in our sample, and we did not detect ribosomal activity in the parasites. The results for the cannibalism feeding experiment were obscured by a pre-existing cryptic infection in our sample. Our results suggest that Perkisnea pathogenicity is limited, or is only manifest under specific ecological circumstances. Our optimization efforts confirmed that our novel duplex qPCR assay is specific, efficient, and sensitive in most conditions, thereby supporting its application as a tool for amphibian health surveys, and means to improve understanding of the dynamics of Perkinsea infection and SPI disease. We found molecular evidence of Perkinsea infection in a wide host range in Panama, and in the UK aquaculture samples, thereby expanding amphibian Perkinsea’s known biogeographical range and providing a broader base for risk assessment with a view to conservation. The results of our microbiome comparison did not reveal any major compositional differences in the intestinal eukaryotic microbiomes between healthy and Perkinsea-infected tadpoles; however, more specific taxonomic ranks need to be investigated before conclusions can be made. The outcomes of these experiments offer novel insights into under-studied aetiological and epidemiological aspects of this emerging protistan disease of amphibians. These insights can potentially improve amphibian management strageies to help mitigate the impact of Perkinsea infection and SPI disease, and possibly prevent disease outbreaks.
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