Identifying targets for control of Ichthyophthirius multifiliis - a major cause of disease in aquaculture

Lead Research Organisation: University of Exeter
Department Name: Biosciences

Abstract

The provision of enough food to the growing world population is extremely important. An important role in the production of healthy and safe food comes from the aquaculture sector. Over the last 10 years or so, increases in fish production have not come from catching wild fish in the seas but from fish farms who play an increasingly more important role in the fishing industry.

However, disease limits future growth and sustainability. Pathogens such as the ciliate Ichthyophthirius multifiliis are very significant factors that limit food security in this sector. Ichthyophthirius multifiliis, or Ich for short, causes white spot disease in fish, a disease that manifests itself in the form of tiny, visible, white specks on the outside of fish. White spot disease is extremely contagious and has caused large-scale mortalities in wild fish populations. In the UK aquaculture sector, it is one of the top-10 diseases that affect salmon and trout aquaculture. Due to its high virulence, it can cause 100% mortalities in disease outbreaks.

Unfortunately, there is no suitable medicine to prevent or treat white spot disease in fish. The aquaculture industry previously used malachite green but that chemical is now banned because of its toxic nature to humans. Other chemicals used to treat diseased fish such as formalin and copper sulfate are also very toxic to humans. So, there is a clear need for safer and efficient new substances to prevent and/or control the outbreak of white spot disease.

Although malachite green was effective in the prevention and treatment of white spot disease, it is not known how it works. However, we can test how it works in the laboratory and that is what this project will do. By comparing how the parasite responds to malachite green and the other dangerous chemicals, we can get a clue to which mechanisms are involved in its mode of action. We will share this information with an animal health company (Benchmark Animal Health in Sheffield) who will then use this information to develop new ways to trigger the same pathways that make malachite green so effective but using less toxic compounds.

In addition, very little is known about Ich and we are not really sure how it spreads and from where. By studying the genomes of several isolates from different parts of the world, we plan to develop diagnostic tools to follow the spread of the parasite. This will allow our project partner Cefas (the Centre for Environment, Fisheries and Aquaculture Science, a governmental institute) to predict future outbreaks and inform the government on policies to protect UK fisheries and also to warn the aquaculture sector to prevent or limit new outbreaks.

All in all, this project aims to limit or prevent the devastating outbreaks of a serious disease affecting the UK aquaculture sector and therefore protecting food security for the UK consumers.

Technical Summary

The protistan parasite Ichthyophthirius multifiliis, otherwise known as 'Ich', causes significant disease globally in freshwater finfish aquaculture. In the UK, Ich is one of the top-10 pathogens affecting Salmonid aquaculture, an industry producing ~40 million kg of high quality animal protein and contributing >£1bn to the economy annually. Ich causes significant mortality and treatment is labour intensive and expensive. The ban on the only known effective agent, malachite green, has increased losses as no equally effective alternatives are available. Interestingly, a phenomenon of senescence-associated avirulence (SAA) has been observed in the laboratory, whereby the infectivity of Ich declined after a number of passages through fish. Some genes associated with senescence have previously been identified, but causality is yet to be established, and the metabolic pathways they may control are not known. How SAA is prevented in the wild is unknown, but it has been postulated that sexual reproduction between parasites may occur on the host.

This project focuses on understanding the mode of action of malachite green, and determine whether identified genes associated with SAA are potential targets for the development of new pharmaceuticals. Together with Cefas and industry, who are both end-users of the project's output, this will be achieved via comparative transcriptomics, genomics and tanks experiments.

We will:
1) Determine the mechanisms underlying the mode-of-action responsible for the efficacy of malachite green on the parasite to help identify metabolic pathways that could be appropriate targets for future controls.
2) Determine if sexual reproduction takes place and if it is a source of genetic diversity.
3) Determine whether previously identified genes associated with SAA could be potential targets for the development future pharmaceuticals.

Planned Impact

Communication through a variety of mechanisms provides information to, and therefore impacts upon, the scientific community. This includes presentations at conferences, peer-reviewed publications and data sharing. The major conferences at which the data generated will be presented are:
1. European Association of Fish Pathologists meeting,
2. International Society of Protistologists meeting.

We will work with the media to communicate our findings and the importance of our science and science in general to help educate the public, and in doing so inspire the next generation of scientists. Both Cefas and the University of Exeter websites promote key findings of their researchers. Pages for particular communities (schools, farmers, the general public) contain interesting, important and relevant information arising from research. We will actively engage with our Press Offices to communicate our outputs. Communication with non-academic based and non-research stakeholders will include public engagement through presentations at science events, farming shows, talks to school teaches and pupils (e.g. through the Science and Engineering Ambassadors Scheme) and liaising with government policy makers. In specific we will engage in:
1. The annual Britain Needs Bioscientists workshops organised by Biosciences at the University of Exeter,
2. The secondary school 'The Year 9 - Infectious Diseases' workshop organised by Biosciences at the University of Exeter,
3. Two yearly Science career's days in both primary and secondary schools in our local area.

In addition to the general public this project has particular relevance to several more specific groups:

BBSRC - this project addresses key scientific priorities for BBSRC, food security and basic bioscience underpinning health, and they will be kept informed of any significant, important or interesting findings that are worthy of further dissemination through their auspices. BBSRC Business will be used to describe our research work to a wider audience.

NC3Rs - Replacement, Refinement and Reduction (3Rs) - are a widely accepted ethical framework for conducting scientific experiments using animals humanely. The use of in vitro cell cultures to assess host gene expression to the parasite will result in the reduction of the number of animals used and the number of animal procedures performed. Both institutes have experience in communicating with the public and the industry sectors and we will work in public engagement activities to inform on our work in 3Rs when opportunities arise.

Fish farmers - Findings from the study will be reported in trade journals, the Cefas' website, at aquaculture events and other publications. If the findings are of particular importance or relevance to fish farmers then we will seek the opportunity to speak to aquaculture associations etc. Cefas has extensive experience of working and communicating with the farming community and has excellent contacts through whom specific events can be organised.

Policy Makers - at an Institutional level Cefas hase regular meetings with key policy makers (Defra/OIE/FAO) and will make them aware of the project and any results that are of value in the development of UK or international policy through that route.

Publications

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