Abstract
The freshwater bryozoan, Fredericella sultana, is the main primary host of the myxozoan endoparasite, Tetracapsuloides bryosalmonae which causes proliferative kidney disease (PKD) of salmonid fish. Because spores that develop in bryozoan colonies are infectious to fish, bryozoans represent the ultimate source of PKD. Bryozoans produce numerous seed-like dormant stages called statoblasts that enable persistence during unfavourable conditions and achieve long-distance dispersal. The possibility that T. bryosalmonae may undergo vertical transmission via infection of statoblasts has been the subject of much speculation since this is observed in close relatives. This study provides the first evidence that such vertical transmission of T. bryosalmonae is extensive by examining the proportions of infected statoblasts in populations of F. sultana on two different rivers systems and confirms its effectiveness by demonstrating transmission from material derived from infected statoblasts to fish hosts. Vertical transmission in statoblasts is likely to play an important role in the infection dynamics of both bryozoan and fish hosts and may substantially contribute to the widespread distribution of PKD.
Highlights
The myxozoan parasite Tetracapsuloides bryosalmonae, is the causative agent of Proliferative Kidney Disease (PKD) in salmonid fish (Anderson et al 1999; Canning et al 1999; Feist et al. 2001)
Tree roots with attached colonies from the River Lohr were glued to Petri dishes, placed in an 124 aerated 10 L bucket filled with Bryozoan Medium C (BMC) (Kumar et al 2013) and 125 maintained at 18 0C (+2 0C) by feeding with cultured algae as described in Grabner and El126 Matbouli (2008)
Higher proportions of infected statoblasts were 236 collected from the River Dun material (35% and 45% of statoblasts collected from covertly237 infected and uninfected maternal colonies, respectively) than from the River Avon material (29% and 19% of statoblasts collected from covertly-infected and uninfected maternal colonies, respectively) (Fig. 3)
Summary
The myxozoan parasite Tetracapsuloides bryosalmonae, is the causative agent of Proliferative Kidney Disease (PKD) in salmonid fish (Anderson et al 1999; Canning et al 1999; Feist et al. 2001). When conditions are favourable for host growth (e.g. high temperatures and food levels for hosts; Tops et al 2006; Hartikainen and Okamura, 2012) these cells detach and develop within the bryozoan body cavity to form sacs filled with spores infectious to fish. Because these sacs can be observed with a stereomicroscope we refer to such infections as overt (see Okamura et al 2011, for review)
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