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

Abstract

BackgroundPersistent 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.ResultsCovert 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.ConclusionsWe 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.Electronic supplementary materialThe online version of this article (10.1186/s13071-017-2495-8) contains supplementary material, which is available to authorized users.

Iflavirus Covert Infection Increases Susceptibility to Nucleopolyhedrovirus Disease in Spodoptera exigua

“…Covert virus infections of insects persist without obvious signs of disease and are usually non-lethal [1][2][3][4][5][6][7][8]. As covertly infected hosts often survive and reproduce, the covert infection strategy is frequently associated with opportunities for vertical transmission [9,10].…”

PCR-RFLP: a targeted method to reveal host specific malacosporean infection profiles (Cnidaria: Myxozoa: Malacosporea)

“…Such screening does not identify covert infection which involves tiny isolated stages (single cells and small groups of cells) that develop in association with the body wall (Morris & Adams 2006b, Canning et al 2008. Covert infections can be detected by PCR, and extensive monitoring of UK bryozoan populations indicates that covert infections are often present in at least 50% of F. sultana colonies across time and space (Fontes et al 2017). DNA was extracted using a HotSHOT protocol (Truett et al 2000) and was used as template for PCR of malacosporean infections using the primers Malsp 1F/2R.…”

“…Our results also demonstrate that the Malsp 1F/2R primers can confirm covert infections in bryozoan samples. Covert infection dynamics appear to entail long-term persistence within bryozoan hosts (Fontes et al 2017). When conditions are permissive for the development of overt infection, tiny isolated covert stages multiply and then develop into visible sacs causing overt infection (Morris & Adams 2006b) that are relatively short-lived in the field (weeks; Tops 2004, Tops et al 2009, Fontes et al 2017.…”

“…This collective information along with amplification of parasite DNA in some 31% (118 of 382) of field-collected colonies indicates that our primers are able to identify infections even when parasite DNA concentrations are likely to have been very low in many samples. Because covert infections are persistent over space and time (Fontes et al 2017), while overt infections are transient (Tops et al 2006), the ability to detect covert infections is of particular importance. Further examination of a range of material will help to reveal how sensitive the approach may be.…”

To remain or leave: Dispersal variation and its genetic consequences in benthic freshwater invertebrates

“…We note that it is impossible to dictate exactly where bryozoan populations will be found as they can be highly patchy over space and time (Okamura, ; Okamura et al, ). Furthermore, their presence relies on availability of suitable substrata such as wood that has been submerged for a prolonged period, large stones, and tree roots (Fontes, Hartikainen, Williams, & Okamura, ; Wood & Okamura, ). Our collections therefore could not be restricted to predefined spatial scales but reflected what we managed to sample within each general region and according to hydrological connectivity of sites.…”