Health – Aberdeen University
Progress in PKD research
Development of parasite vaccines for fi sh no longer seems an intractable goal, says Aberdeen team
P
roliferative Kidney Disease (PKD), caused by the myxozoan parasite Tetracapsuloides bryosalmonae, remains a considerable threat to the sustainability of the rainbow trout industry in western Europe and North America.
With the proven seasonality of parasite development in the defi nitive
bryozoan host (colonial fi lter feeding invertebrates) and links to elevated water temperatures, PKD is moving northwards and occurring at higher altitudes, with outbreaks reported in recent years in Norway, Switzerland, Austria and even in Iceland, where it is impacting on Atlantic salmon and brown trout populations. Losses to the UK aquaculture industry emphasise the need to develop
effective methods to control this disease. This parasite primarily infects and proliferates in the kidney and spleen of
the fi sh (Fig. 1), where it causes an abnormal immune response resulting in severe kidney swelling. This, in turn, causes renal dysfunction that leads to infected fi sh becoming highly susceptible to other stressors that ultimately can lead to fi sh losses, depending on seasonal water temperature profi les. Chemical treatments (fumigillin, malachite green) are effective in reducing parasite numbers but can be damaging to the fi sh and are not approved for use.
Importantly, fi sh recovering from PKD are immune to re-infection,
allowing fi sh farmers to employ carefully timed exposure programmes. This approach has had a degree of success but is limited by production bottlenecks at both hatcheries and table farms, compounded further by the unpredictable nature of the timing and quantity of parasites released from upstream bryozoan populations. A great deal of progress has been made in recent years in understanding parasite growth and persistence in bryozoan populations, the infection route taken by the parasite following attachment to the fi sh host and its life cycle.
With respect to the European strain of T. bryosalmonae, native salmonids such as brown trout harbour parasite stages that are able to infect native bryozoans to complete the life cycle. However, parasites in rainbow trout
fail to develop infective spores and are thus considered as dead-end hosts. Work at the Scottish Fish Immunology Re-
Left: Figure 1. Parasite stages (highlighted with arrows) observed in infected spleen ti ssue (Gomori’s trichrome, x400 magnifi cati on) Above: Atlanti c salmon Next page: Brown Trout
search Centre (SFIRC) at Aberdeen and Stirling Universities has been at the forefront of research to understand the nature of the fi sh immune mechanisms that drive PKD disease pathology. In particular, BBSRC-funded research at Aber- deen has highlighted the dysregulated nature of the host response to the parasite (see article below by Dr Bartolomeo Gorgoglione et al, a former SFIRC PhD student). This feature is often seen in pathogen-mediated
chronic diseases, believed to favour the patho- gen infection, in contrast to a tightly controlled immune response more likely to bring about pathogen clearance. Gene expression studies at Aberdeen also suggest that PKD pathology could be due to abnormal development and activity of immune house-keeping functions in the kidney and not due to a more classical infl ammatory response. These studies have been instrumental in identi- fying immune inhibitory molecules that could be
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