Proliferative kidney disease (PKD) is an economically important parasitic condition - costing the trout industry over £2.5 million per annum in the UK alone - primarily affecting first season freshwater salmonid fish in areas of Western Europe and North America. The causative agent involved was originally known as PKX, denoting its uncertain taxonomic position in the phylum Myxozoa. Subsequent studies discovered that bryozoans – known colloquially as “moss animals” - acted as alternate hosts, and the organism was named Tetracapsuloides bryosalmonae and placed in the new myxozoan class Malacosporea.
The focus of this PhD project was to try to achieve fundamental progress towards the development of effective prevention and control measures against PKD. Preliminary work has involved the laboratory culture of the invertebrate bryozoan hosts. As few previous findings have been published regarding suitable nutritional sources for these animals, over 50 species of algae and protozoa were screened to gauge their value as food to bryozoans. Several species were ascertained to be readily digested and these formed the basic diet for bryozoan colonies collected from trout farms in the south of England endemic with PKD.
Following collection from the sites, the bryozoan colonies were routinely monitored using dissecting and inverted microscopes. The development of two myxozoan parasites was observed within the colonies. Within the bryozoans Fredericella sultana and Plumatella repens, the myxozoan parasites T. bryosalmonae and Buddenbrockia plumatellae respectively were studied by means of light and electron microscopy. Successful transmission of PKD to rainbow trout exposed to material released from F. sultana demonstrated the value of the culture method in the maintenance and further investigation of PKD in the research scenario.
Investigations have also been carried out investigating the use of chemotherapeutic agents delivered in feed as a method of limiting clinical disease. This work carries on from previous studies at the Institute of Aquaculture. Two further drugs found to be well tolerated in salmonids were examined as in-feed medications for PKD.
Further studies explored the relationship between host and parasite with particular emphasis on molecular and immunological techniques. Studies have been conducted to demonstrate the minimum number of spores which can lead to infection in rainbow trout. Attempts have been made to demonstrate specific immune protection using crude vaccine preparations in rainbow trout. Work has been carried out concentrating on T. bryosalmonae using confocal laser scanning microscopy techniques, allowing the development of 3D models of the structure of the spore released from the bryozoan host. The overall aim of the project was to study the development of malacosporean parasites and relate this to possible control methods for this highly damaging disease of salmonid fish.
Following the PhD project, I have been employed as a postdoctoral researcher on a project aiming to develop a vaccine against PKD. I have also been involved in teaching duties for the MSc courses during this time which has been most enjoyable and satisfying.
Professor Alexandra Adams
Doctor David J Morris
The Fishmongers'
Company
