Persistence, impacts and environmental drivers of covert infections in invertebrate hosts

Ines Fontes, Hanna Hartikainen, Chris Williams, Beth Okamura*

*Corresponding author for this work

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9 Citations (Scopus)
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Background: Persistent covert infections of the myxozoan, Tetracapsuloides bryosalmonae, in primary invertebrate hosts (the freshwater bryozoan, Fredericella sultana) have been proposed to represent a reservoir for proliferative kidney disease in secondary fish hosts. However, we have limited understanding of how covert infections persist and vary in bryozoan populations over time and space and how they may impact these populations. In addition, previous studies have likely underestimated covert infection prevalence. To improve our understanding of the dynamics, impacts and implications of covert infections we employed a highly sensitive polymerase chain reaction (PCR) assay and undertook the first investigation of covert infections in the field over an annual period by sampling bryozoans every 45 days from three populations within each of three rivers.

Results: Covert infections persisted throughout the year and prevalence varied within and between rivers, but were often > 50%. Variation in temperature and water chemistry were linked with changes in prevalence in a manner consistent with the maintenance of covert infections during periods of low productivity and thus poor growth conditions for both bryozoans and T. bryosalmonae. The presence and increased severity of covert infections reduced host growth but only when bryozoans were also investing in the production of overwintering propagules (statoblasts). However, because statoblast production is transitory, this effect is unlikely to greatly impact the capacity of bryozoan populations to act as persistent sources of infections and hence potential disease outbreaks in farmed and wild fish populations.

Conclusions: We demonstrate that covert infections are widespread and persist over space and time in bryozoan populations. To our knowledge, this is the first long-term study of covert infections in a field setting. Review of the results of this and previous studies enables us to identify key questions related to the ecology and evolution of covert infection strategies and associated host-parasite interactions.

Original languageEnglish
Article number542
Number of pages15
JournalParasites & Vectors
Publication statusPublished - 2 Nov 2017

Bibliographical note

This project was supported by a NERC Open CASE studentship (NE/019227/1) which received financial contributions from the Centre for Environment, Fisheries & Aquaculture Science and the Environment Agency (EA). IF was also funded by SA Pescanova and a PhD research fellowship from the Fundação para a Ciência e a Tecnologia (SFRH/BD/86118/2012). None of the funding sources were involved in study design, data collection, data analysis, data interpretation or in writing the manuscript.


  • Tetracapsuloides bryosalmonae
  • Fredericella sultana
  • Proliferative kidney disease
  • Myxozoans
  • Bryozoans
  • Productivity
  • Temperature
  • Host condition-dependent effects
  • Covert infection strategies
  • Fish disease reservoirs


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