Department of Life Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK.
Scottish Fish Immunology Research Centre, Aberdeen University, Aberdeen, AB24 2TZ, UK.
Parasit Vectors. 2017 Nov 2;10(1):542. doi: 10.1186/s13071-017-2495-8.
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.
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.
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.
已提出，多房棘球蚴（Tetracapsuloides bryosalmonae）在初级无脊椎动物宿主（淡水苔藓虫，Fredericella sultana）中的持续隐匿性感染代表了增殖性肾病在次级鱼类宿主中的潜在传染源。然而，我们对于隐匿性感染在苔藓虫种群中的持续存在和变化、以及它们可能对这些种群产生的影响知之甚少。此外，之前的研究可能低估了隐匿性感染的流行率。为了更好地了解隐匿性感染的动态、影响和意义，我们采用了高度敏感的聚合酶链反应（PCR）检测方法，并在三年内，每个季度对三条河流中的三个种群进行了每年一次的现场隐匿性感染调查。
隐匿性感染全年持续存在，且在河流内部和河流之间的流行率存在差异，但通常>50%。温度和水质的变化与流行率的变化有关，这与在生产力较低、苔藓虫和多房棘球蚴生长条件较差的时期维持隐匿性感染的方式一致。隐匿性感染的存在和严重程度的增加会降低宿主的生长，但只有当苔藓虫也在投入生产越冬繁殖体（休眠芽）时才会如此。然而，由于休眠芽的产生是短暂的，因此这种影响不太可能对苔藓虫种群作为持续感染源的能力产生重大影响，从而也不太可能对养殖和野生鱼类种群中的潜在疾病爆发产生重大影响。
我们证明了隐匿性感染在苔藓虫种群中广泛存在且持续存在于空间和时间上。据我们所知，这是首次在野外环境中对隐匿性感染进行的长期研究。对这项研究和之前研究结果的回顾使我们能够确定与隐匿性感染策略的生态学和进化以及相关的宿主-寄生虫相互作用有关的关键问题。
