Mucous and a multi-factorial approach: The key to disease control post-malachite

The Fish Site
by The Fish Site
30 April 2007, at 1:00am

By Dave Cox, Fish Vet Group - Malachite green was something of a cure all for farmed fish. Used successfully for preventing and treating fungal infections and ectoparasites, such as those causing white spot disease, it was also effective in controlling internal parasites such as those causing Proliferative Kidney disease (PKD).

However, since its prohibition, and the subsequent appearance, and may be not coincidentally, of diseases such as PKD in locations that have never shown evidence of this parasite before, are we beginning to realise its preventative role in other diseases. These observations have been readily acknowledged.

I agree whole heartedly with the sentiments expressed by Professor Ron Roberts in International Fish Farmer earlier this year. The nature and circumstances of the demise of malachite green are indeed worthy of closer examination. It is difficult to see the scientific grounds for its removal because, as far as I am aware, the only work to demonstrate any potential problems, for rabbit embryos and trout eggs, involved feeding doses that bore no resemblance to those experienced during treatments.

Many chemicals are toxic at high concentrations - even life-saving oxygen is lethal when administered at high levels - so what was the relevance of this work and why is it always quoted by pressure groups and ‘antis’?

Perhaps, if we look at the political reasons for losing malachite green then we may draw different conclusions. Cynics could say the campaign to remove it appealed to the vote catching aspirations (and spin doctors) of certain politicians?

But grudgingly I must accept that malachite green was an unlicensed product. I just wish that someone had had the foresight to officially take it through to licensing. Perhaps then we would have managed to retain what was a very valuable medicine.

So what are the alternatives to malachite green? I believe that there is no single solution and producers and health professionals may have to rely on more multi-facetted disease control strategies.

Novartis’ newly launched product Pyceze will never fully replace malachite green, but it has to be included in any overall strategy because it is currently the only product licensed for anti-fungal treatment. It has a Marketing Authorisation for use in eggs and the Scottish Environmental Protection agency (SEPA) has given interim permission to some farms to use it in treating fish until it obtains a full Marketing Authorisation.

Work is clearly needed on determining how we use this product, but as an industry we do need to concentrate on a much broader approach to dealing with and preventing fungal disease.

Control of PKD and other external parasitic infections will have to be addressed separately. So what should we be doing?

Carry out risk assessments for fungal infections for each site?

Fish are more likely to be at risk during egg incubation, following handling such as grading, and definitely post-vaccination. Also, certain locations are clearly more severely effected than others. For example loch-based farms are more likely to suffer a higher degree of fungal infections than those supplied by rivers.

Good management is also vital and means reviewing all current practises and reducing the potential avenues where fungal infections can develop. In essence it producers cannot be complacent about health. Successful control will involve all the old strategies that tend to be periodically dropped when that ‘Auld Enemy’ appears.

To be fair, losing malachite green has hit trout farmers throughout the world as hard as it has hit fish farmers in Scotland. It is something that we are just going to accept and deal with head on as soon as possible.

Well-proven strategies must ensure hygienic environmental conditions at all stages of the farming procedure and particularly at key periods such as during grading or vaccination. Maintaining low stress levels when fish are handled and using clean tanks and nets with good water exchange and oxygen levels plus a reduction in stocking densities is very good practice.

Understand the dynamics of how the fish and fungus interact

I am not convinced that we fully appreciate all the reasons why the balance tips in favour of the fungus and fish then become infected.

Fish continually prevent fungal zoospores from settling and developing into infection. It is only under certain conditions that the fungus proliferates and invades skin and underlying tissues.

Under normal conditions, fish continuously produce mucus and the zoospores are shed with the mucus. The mucal layer also contains a range of chemicals comprising immunoglobulins (commonly known as antibodies), anti-inflammatory products such as C-reactive protein, a chemical called lyzozyme and a series of complementing compounds that are involved in the destruction of bacterial cell walls. These naturally produced compounds have a damaging effect on a range of micro-organisms, including fungi, so mucus is not as simple as you think.

May be we should be looking at ways of ensuring optimal mucus production. Nucleotides are reported to increase mucous cell numbers, so why not feed these products at times when a loss of mucus is expected.

And we must consider an immune approach to control. There appears to be specific immune mechanisms involved in preventing fungal infections. For example the winter kill of catfish in the USA has been shown to involve a loss of function of immature lymphocytes exposed to a sudden drop in temperature.

More research into vaccines against Saprolegnia, and the development of immunostimulants, or so-called nutrichemicals, would also be very welcome. Conversely maybe we should be looking at which vaccines, if any, are more likely to predispose fish to developing fungal infections.

As an industry, it’s time we adopted an effective multi-factorial approach to controlling fungal infections. Treatment and prevention strategies do not rely on malachite green, other methods could be equally effective. So, the grass could well be greener on the other side, for fish, producers, welfare groups and environmentalists.

April 2007