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Emerging diseases: The aquatic health perspective
Dr Brian Jones,
Principal Fish Pathologist, Department of Fisheries, Western Australia; Adjunct Professor, Muresk Institute, Curtin University; Adjunct Associate Professor, Murdoch School of Veterinary and Biomedical Sciences

Something like 47 per cent of the animal protein that is eaten in the world comes from fish, and it is an activity that employs something like 35 million people, 70 per cent of them in the developing world.

Status summary
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Aquaculture can be a small-scale activity, as shown in the top illustration (Slide 1), but in combination those small-scale activities produce an awful lot of fish. You may be surprised to know that Indonesia produces over a million tonnes of fish each year from aquaculture, and any of you who have been to Indonesia and eaten the fish will almost certainly have eaten fish that were grown in a pond like that.

Aquaculture may be a small industry in Australia, but it is big internationally. In Europe, salmonid culture competes directly with the chicken industry for space on the supermarket shelves, and the cost of production is down to fractions of a euro per fish.

This industry has been severely affected by emerging and old animal diseases, and that is due to several factors. There are new and emerging diseases, there are diseases which have become a problem through translocation from one area to another, and, increasingly, there is a problem with changes to the environment, usually generated by man.

Progress
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So where are we going?

Worldwide there is an improvement in biosecurity. There have been moves for international reporting, and great strides forward in national surveillance and monitoring, including in Australia.

Improving health management: the aquaculture industry is really young. Although the Chinese have been growing carp for thousands of years, aquaculture as a major food producer really only started in the 1970s and 1980s.

And so we have gone right through from having no antibiotics at all, through the antibiotic problem and, now that the industry is just coming of age, we find that antibiotics are no longer acceptable.

I mention malachite green there (Slide 2). That is a treatment that has been around since the 1930s and is no longer acceptable because malachite green turns out to be a carcinogen and a teratogen, and traces in food are unacceptable.

I also mention vaccines. Although the industry is relatively young and has grown very quickly, it is rapidly becoming a major attractive item for international drug companies. I learned last week that Novartis, one of the major vaccine producers, now make more vaccine shots for fish in Europe than they do for the chicken industry. And the salmonid industry is their major market.

Attention Area: new diseases
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So what about new diseases?

Diseases are becoming a problem, as I said. The classic example for Australia is the pilchard mortalities in 1995 and 1998. This was a disease where pilchards started to die in large numbers – and you can see (Slide 3) a picture of pilchards on a beach in Western Australia. The mortalities were first reported in South Australia and moved east and west like a bushfire, at about 40 kilometres a day. It was quite predictable, and it moved right through the entire biological range of the pilchards.

There was considerable concern about the mortalities, and a number of ideas were put forward as to how the mortalities were being caused. It was an effort by States and by CSIRO that came up with the diagnosis. It involved not only traditional histopathology but also electron microscopy and epidemiology. It was a multi-jurisdictional issue, and it was only solved by coordination at a national level.

There were critical delays in reaching a diagnosis, mainly due to a lack of trained staff, and that has not changed. There is no veterinary school in Australia that teaches aquatic animal health for anything more than a couple of hours in total.

It is not just a case of saying, ‘Well, fish are basically aquatic chooks.' There are 24,000 species of teleosts alone, not including crustaceans and molluscs. It is the most speciose and diverse group of vertebrates.

Just to give you an example: of the 1600 species that have been subject to cytogenic study, the genome size varies greatly, if you just take haploid chromosome number, from 20 to 40 and up to 80 in salmonids. The great tunas and some of the sharks can maintain their body temperature at greater than ambient; ice fish can live quite happily in -5°C. Some fish species have a true liver, others have had a pancreas, some have a stomach, some do not. Elasmobranchs have a spiral valve. There is a huge diversity, not only in morphology but also in biochemistry.

A lot of the old skills like histopathology are not being taught. People are going for the high-tech things like PCR and molecular diagnostics, but they only work if you know what you are looking for. A classic example is taurus syndrome or white spot syndrome virus or IHHNV. All of those viruses have strain differences, and some of the PCRs that are available will not pick up all of the strains. If you use the wrong primers you will get a false negative.

Prawn diseases
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Slide 4 is a graph I did some years ago but it is still relevant today, just showing the published papers on, in this case, prawn diseases but it is the same for molluscs. There is a steady increase in the number of new diseases being reported. In this case, the rate is about one every two years.

Attention Area: movements
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The translocation of animals is a big issue. There are known diseases of concern like spring viraemia of carp, which broke out in the US in 2002. Carp farming in America is worth US$22 million. It was controlled by slaughter of affected ponds. Again in 2002, white spot syndrome virus, a well-known disease, affected prawn farms in Iran and cost 5000 jobs – which in a country like Iran is devastating. We are having outbreaks in Australia of Streptococcus iniae, which has been around for a long time but it would appear, from the epidemiology, that we are dealing with a ‘hot' strain, a strain that is killing fish much more efficiently than it used to.

There are unknown diseases which were assumed to be insignificant – an example is Bonamia, which I will go on to shortly – and changes of host. The introduction of live eels for the food market from the Pacific, particularly New Zealand and Australia, into Europe took with it an insignificant nematode that lived in the gas bladder called Anguillicola. Now, Anguillicola has a two-stage life cycle: it lives in the eels and also in a copepod. When released into the environment in Europe, it found a copepod which it could use as intermediate host, attacked the local species of eel and caused 80 per cent mortality. It is a major problem in Europe; it is an incidental and insignificant finding in Australia.

Spread of Bonamia ostreae
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Bonamia is an interesting example of a mollusc disease. These (Slide 6) are the microcells that live inside the haemocytes. The microcell disease had been reported in the USA as an insignificant finding. The French, in an attempt to improve the genetics of their oyster industry, imported some brood stock from the US, and Bonamia broke out in the oyster beds in France in 1979 and reduced the production from 1000 tonnes to nothing. The disease spread into Holland in 1980. In 1982, despite rigorous quarantine restrictions, it was smuggled into the UK. The Irish managed to keep it out till 1985, when it was smuggled into Ireland. And it reached Spain in 1981.

We talk about bio-terrorism, but really it is bio-stupidity. People will always assume that the grass on the other side of the paddock is always greener, and in this case, although the farmers knew that Bonamia was out there and that it was killing oysters, there were deals done which were too good to ignore. And the idea, ‘Well, it won't happen to me. These are good oysters and I'll escape' always seems to win out in the end. No matter how strict your quarantine requirements are, it would seem that sooner or later these diseases are going to introduce themselves – usually because someone has been stupid.

Environmental 'stress'
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I mention environmental stress. Just quickly: pollution is a big problem, particularly for fish. Those of you who watched the Four Corners program last night on water resources will know how valuable water is, but just think: when they mentioned that water in London goes through seven sets of kidneys before you drink it, they forgot about all the fish.

Water is really important to fish. It is what they live in. And pollution is absolutely critical. A big issue that I think is going to rival TBTs in the future is that of endocrine disrupters in the water supply. They are already having major effects on some species of fish. (Heaven knows what they do to us when we drink them!) Pharmaceuticals are also a problem, and in Germany there are moves under way to develop toilets that will actually strip pharmaceuticals out of the water, or out of the sewage, at source.

Global warming is an issue. It is changing the way that fish live, it is changing their access to food and their access to spawning grounds.

Habitat alteration, loss of estuaries: that is going to be a problem if the sea levels rise, because the heads of the estuaries where many fish spawn are all going to be fenced off in order to protect the high-value properties on the waterfront.

And loss of rivers: it is all very well Sydney pumping out rivers to feed water to the population, but the fish have got to live somewhere too.

All of those things cause stress, and stress in turn results in disease.

Biosecurity measures
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Biosecurity measures I have mentioned; changes in international quarantine. You can see a lot about what is being done in Australia and in our region from the web sites.

There is a need for harmonisation, both internally and for imports, and while we are very good through AQIS and Biosecurity Australia in saying what comes into the country and under what conditions, there is very little control over what we do within the country. That will have a big impact on how we manage the border.

The Indonesians, for example, could quite rightly say, ‘Why should Perth take prawns without question from the eastern states, 3000 kilometres away and across one biogeographic boundary, but not take prawns from Bali, which is 450 miles off the WA coast? Why should we take prawns from Sydney and not take prawns, for example, from New Zealand, when New Zealand is closer to Sydney than Perth is?' So we need to make sure that our internal controls are equivalent to those that we impose on our international partners.

Because of all this, biosecurity is shifting to the farm gate. It is becoming harder and harder to keep things out, and the only way that farms are going to ensure that they are biosecure is to actually do it themselves. A good example of that is the protocols that are under way in the pearling industry in WA. There are zones around farms and there are strict controls on movements into and out of the zones within the fishery, and into and off farms.

Health management
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I have mentioned that food standards are changing and aquaculture is having to keep up – things like chloramphenicol testing. Chloramphenicol is often used as an antibiotic in Asia, but in fact the ability to test for chloramphenicol has now reached the stage where natural levels of chloramphenicol produced by natural soil bacteria are being picked up as contaminants.

There is almost a complete absence of therapeutics for fish. We are using things that were developed in the 1930s. There hasn't been the money invested in therapeutics in fish, and anyway we know almost nothing about their immune reactions, particularly those of molluscs and crustaceans.

Malachite green works exceptionally well as a fungicide: we know that it blocks the respiration of the fungus. There have been hundreds of thousands of US dollars spent trying to find alternatives to malachite green, without any success at all. We can't use it but there must be, with our modern molecular methods, ways of finding out why it is so good and manufacturing an alternative.

I mentioned immunology. We know almost nothing about the immunology of crustaceans and molluscs. They don't have acquired immunity as vertebrates do, yet they cope very well.

Also I mentioned vaccines. There is a massive investment being made overseas in vaccines, not only traditional killed vaccines but also modern plasmin vaccines. At least one plasmin vaccine is being registered for fish at the moment.

Goals — Beyond dead fish!
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Just to give you a bit of an idea of where these sorts of things can go to: it is not just a matter of trying to find what is killing fish, but there are a lot of things that we can learn from fish and apply to other things.

For example, injuries to fish skin are life-threatening, and fish have evolved a system of closing over wounds that involves epithelial cells sliding. They can close a wound up in a matter of minutes. If you could find out how they do that and put it in a bottle, you would make a fortune.

I mention that crustaceans and molluscs can sequester viruses in huge titres and parcel them away so that they don't do any damage. Again if we understood how they did that, bearing in mind that they don't have the acquired immune systems of teleosts, we might be able to learn something about how our own bodies cope with viruses, because many of those ancient systems have been conserved.

There are antimicrobial peptides. Hundreds have been found in crustaceans and molluscs, again with application to humans.

Crustaceans have a brilliant anti-fouling mechanism on their shells. If we understood how that was done, perhaps our dependence on modern anti-foulings would change.

And I mention also crustacean nerve regeneration. Crustaceans are adept at regenerating their nerves, and again we know very little about how that is done.

Aquaculture & Genomics
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But aquaculture is also leaping into the future, and I just put that (Slide 11) up. It is a job from one of the major drug companies, looking for a fish ‘ecotoxicogenomist'. I did a search on ‘aquaculture' and ‘genomics', and there are 3000 web sites and over 100 papers have been produced, just on that topic alone.

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