15. FISH DISEASE PREVENTION AND TREATMENT

15.0   Introduction

1. Fish diseases may cause severe losses on fish farms through:

• reduced fish growth and production;
• increased feeding cost caused by lack of appetite and waste of uneaten feed;
• increased vulnerability to predation;
• increased susceptibility to low water quality;
• death of fish.

2. While it may be difficult to avoid fish diseases completely, it is better to try to prevent their occurrence rather than to allow them to develop and then attempting to cure them once they start to cause problems (see Section 15.2). To cure a fish disease is much more difficult and generally calls for the services of a specialist. By the time proper treatment can be organized, the disease may have become more serious. In some cases surviving fish are so weakened that effective treatment becomes difficult.

3. However several simple and effective treatments can be used, either for prevention or early control of disease before it becomes too serious. These methods are described in Section 15.3. In all cases, you will have to handle chemicals and know about their dangers and their use (see Section 15.1).

Main causes of disease in farm fish

4. There are several causes of disease that may affect the fish directly or may continue to cause disease problems. Basically, any factor which causes stress or difficulty to the fish decreases its resistance to disease and increases the chance of disease problems occurring.

5. The three main causes of disease are:

• improper feeding;
• stress through extreme or toxic condition;
• attack by disease organisms.

(a) Fish are not fed properly: nutritional diseases become more frequent as the culture system becomes more intensive and the fish obtain less of their nutrients from natural food organisms (see Chapter 10).

(b) Fish are stressed by being exposed to an extreme or a toxic condition: in the previous chapters, you have already learned about such factors as:  

• rough and/or excessive handling, for example when harvesting (see Chapter 11) or sorting/grading (Chapter 12);
• overcrowding and/or behavioural stresses, for example in storage (see Chapter 13) or transport (Chapter 14);
• unsuitable water temperature (see Section 2.4);
• lack of dissolved oxygen (see Section 2.5);
• changes in pH towards extreme values (see Section 2.2);
• presence of toxic gases such as ammonia or hydrogen sulphide (see Sections 2.4 and 6.2);
• toxic factors in artificial food such as particular chemicals in certain plant foodstuffs (saponin, gossypol, etc.), fungal toxins in stored foods (see Section 10.5), and pesticide residues;
• pollution of the water by agricultural or industrial chemicals, sewage effluents, heavy silt loads.

(c) Fish are attacked by successful disease organisms, either externally on the skin, gills or fins, or internally in the blood, digestive tract, nervous system, etc. You will learn more about these organisms in Section 15.3.

6. As the characteristics and management of the first two causes of disease have already been fully discussed earlier, the next sections will concentrate on the prevention and simple treatment of fish diseases caused directly by living organisms.

Life cycle of lchthyophthirius multifilis responsible for the white-spot disease

This disease may spread rapidly from one fish to another through water and pond bottom infections which makes disease control very difficult

How does disease caused by living organisms develop in fish ponds

7. The development of disease caused by living organisms is encouraged by any condition which places the fish under stress.

8. Disease risks become even greater when fish undergo combined stresses, for example handling when the water temperature is below normal or overcrowding in low dissolved oxygen conditions.

9. Other factors on the fish farm may also be responsible for the survival and propagation of disease organisms, making disease control much more difficult such as:

• the presence of diseased wild fish;
• the presence of intermediate hosts such as snails and fish-eating birds, necessary for completing the life cycle of the disease organism;
• the introduction of disease organisms through contaminated inputs such as food, trash fish or processing wastes, for example imported eggs, juveniles, or broodstock, and water from an upstream pond or farm.

Life cycle of Diplostomum spathaceum

Preventing diseases through good management

10. As already explained, disease prevention on the fish farm is better than cure. All efforts should thus be directed to applying good management practices.

11. Be particularly vigilant about the following points.

(a) Ensure good water quality: sufficient supply, with adequate dissolved oxygen concentration and free of pollution (see Chapter 2).

(b) Keep the pond environment healthy: control silt (see Section 11.6, Construction, 20),control plants (see Section 4.9),keep a healthy balance of phytoplankton and zooplankton (see Section 10.1), and exchange water if needed. If necessary, use mechanical aeration (see Section 2.8).Disinfect the pond regularly (see Section 15.2).

(c) Keep the fish in good condition:control stocking density. Keep different sizes or sexes separate if necessary to control fighting. Ensure good food supply (see Chapter 10). Handle the fish properly, especially during harvesting (see Chapter 11) and sorting/grading (see Chapter 12). Care for your fish during storage (see Chapter 13) and transport (see Chapter 14).

(d) Prevent the entry of disease organisms from outside your farm:

• control wild fish by using filters and screens (see Section 2.9) and regularly eradicate them from canals and ponds (see Sections 4.6 and 4.7);
• disinfect all fish stocks imported from outside as eggs, juveniles or adults (see Section 15.2);
• be careful when using trash fish or processing wastes as supplementary feed; if possible boil the raw material for at least 30 minutes or use it for compost or silage feeds; if natural food supplies are limited, add vitamin* supplements to the cooked food to ensure its quality;
• increase vigilance: if you have to use water downstream from a neighbouring fish farm, use screens to control escaped fish;
• for a hatchery it is safest to use spring or well water, free of disease organisms; it is also useful for rearing small fry; alternatively, consider a sand filter to help remove smaller disease organisms (see Section 2.9);
• enclose hatchery and nursery areas with a fence to control access; use footbaths and protective clothing if necessary to limit contamination.

(e) Prevent the spread of disease organisms within your farm: 

• control fish-eating predators, particularly birds and mammals (see Section 4.8);
• disinfect ponds regularly to kill both the disease organisms and their intermediate hosts (see Section 15.2); keep different age groups of fish separate; disinfect breeding ponds well and, if possible, remove broodstock from them as soon as spawning has taken place;
• use diversion ponds with parallel flow if possible; if your ponds are arranged in series, it is best to have the water flow from the ponds with the less infected and more sensitive, youngest fish into the ponds with the oldest fish (more infected and less sensitive);
• disinfect juveniles before stocking them in clean fattening ponds; treat broodstock before using them for propagation in breeding ponds (see Section 15.2);
• if a disease breaks out on your farm, remove dead or dying fish from the ponds as quickly as possible, at least daily, and do not disturb and stress remaining fish excessively;
• bury diseased fish with quicklime away from the ponds; carefully treat infected ponds and disinfect all equipment that has come in contact with them (see Section 15.2);
• in a hatchery have separate equipment for handling small and large fish, if possible keeping one set of hand nets, buckets, etc. for each tank or pond;
• use disinfectant bins for routine disinfection of equipment, and clearly mark the equipment accordingly.

15.1   Common chemicals and their use on the fish farm

1. Most chemicals used for controlling disease organisms are toxic and/or irritant for the skin and respiratory tract. Many chemicals can cause serious health problems if swallowed or absorbed through the skin. You should therefore handle chemicals with care, mark them clearly and store them safely, away from children in particular.

2. When handling chemicals take at least the following precautions:

• protect your hands by wearing rubber gloves; if possible wear overalls and boots for general protection;
• work in a well-ventilated area;
• handle chemicals carefully, avoiding splashing or spillage and, if accidentally splashed, wash off immediately;
• avoid inhaling dangerous vapours;
• clearly mark all containers, equipment and protective clothing used for storing and handling chemicals; unless they can be thoroughly and safely washed out, do not use them for other purposes; store them safely when not in use;
• thoroughly wash your hands after use, and particularly before touching any food.

3. Further precautions for handling pond disinfectants in the open are given in Section 4.6.

Buying and storing chemicals

4. Most chemicals degrade with time. When buying chemicals ensure that they are still of good quality. Check their expiration or "sell by" date, if any. Check also whether they have been stored properly. Clearly indicate on each container the date of purchase.

5. For storing chemicals, use a cool, dark, dry and lockable room. Some chemicals may require refrigerator storage; check labels or handling instructions. Generally, chemicals deteriorate more quickly in warm conditions. Keep good records and ensure a good rotation of the stocks.

Determining the strength of chemicals and their solutions

6. Treatment chemicals are costly and can, in the wrong dosage, be toxic to fish. It is therefore essential to know and understand how the strength of these chemicals is expressed and how dosages can be calculated. You should become familiar with concentrations of chemical solutions, treatment units and their conversion values. You will then be able to avoid wasting chemicals and losing fish.

7. Treatment chemicals contain one or more toxic ingredients to disable or kill disease organisms. These are called the active ingredients (AI). The amount of active ingredient contained depends on the chemical. The AI concentration is expressed in percent of the total weight or volume of the chemical.

Example

• common salt contains 100 percent active ingredient;
• chlorine bleach powder contains 33 percent Al (chlorine);
• Wescodyne contains 1.6 percent AI (iodine);
• Roccal contains from 10 to 50 percent Al (benzalkonium chlorides).

8. The usual practice is to dilute solid or liquid chemicals in water and prepare:

• a stock solution, which can be kept for some time, and be diluted as required to a working solution or a treatment dosage (also used when you need to dose very small amounts of chemical);
• a working solution, which is used directly once prepared;
• a treatment dosage, which is the concentration to which fish are exposed for treatment.

9. These solutions are prepared to a recommended strength or concentration, expressed either as:

• the concentration of the chemical required (e.g. diluted in water, or mixed with feed); or
• the concentration of active ingredient required.

10. Be very careful to make sure whether it is the chemical or the active ingredient which is being referred to. Examples are given later for making up these solutions. The concentration of material (chemical or active ingredient) in a solution may be expressed in various ways:

(a) As the weight present in the solution volume, such as milligrams per litre solution (mg/l), grams per cubic metre (1000 l) solution (g/m³), grams per 25 l solution (g/25 l);

(b) As a volume present in the solution volume such as millilitres per litre solution (ml/l), millilitres per cubic metre solution (mI/m³), millilitres per 10 l solution (ml/10 l).

In practice, you may consider that:

(c) As a percentage, the number of parts (normally by weight) of material in 100 parts of solution.

Example

 A 1.5 percent solution of salt contains 1.5 g salt per 100 g = 100 ml solution or 15 g/I; a 5 percent solution of chlorine bleach powder contains 5 g chlorine per 100 g = 100 ml solution or 50 g/I.

(d) In parts per million (ppm), the parts of material present in one million parts of solution; by weight this is equivalent to mg/l, g/m³, or by volume to ml/m³ of solution.

Example

A 1000 ppm solution of chlorine bleach powder is in fact a 1000 mg/l solution; this amounts to 1 g chlorine per litre; a 160 ppm solution of formalin contains 160 mg/m³ of formalin; this is equivalent to 0.16 ml formalin per litre solution.

(e) As a ratio, the parts of solution per each part of Al, such as a 1: 4 000 solution where for example each ml Al is present in 4 000 ml solution (1 ml/4 l).

Note: to obtain a ppm value from a ratio, multiply the ratio by 1 000 000 as shown in the chart below.

Calculating the amount of chemical to be used

11. As explained earlier, you may wish to make a stock solution, a working solution or a treatment dosage of the chemical concerned. The concentration required will be determined by the type of chemical and treatment used (see Sections 15.2 and 15.3), and the amounts used will depend on the concentration and the overall volume of the container, tank or pond involved.

12. Calculations of the amount to be used depend on the way the dosage is expressed (see paragraphs 9 and 10) and on the percentage of Al present in the chemicals.

13. If you are using a chemical with 100 percent Al, proceed in one of the following ways.

(a) When the concentration is given as a weight or volume of Al per volume of solution, multiply this concentration by the total volume of water to be treated.

Example

• A concentration of 3 mg/l malachite green is required as a treatment dosage. Your tank contains 2 m³ = 2000 l. You will need 3 mg/l x 2000 l = 6000 mg = 6 g malachite green.
• A stock solution of 1 g/I copper sulphate is to be made, to use in a 5-l drum. You will need 1 g/I x 5 l = 5 g of copper sulphate.

(b) When the concentration is given as a percentage, multiply this concentration (expressed as a decimal number) by 1000 times the water volume (in l) to obtain the amount of chemical in ml or g.

Example

Recommended treatment dosage is 2 percent common salt. Your plastic barrel contains 30 l water; you will need 0.02 x 1000 x 30 l = 600 g salt.

(c) When the concentration is given in parts per million (ppm), multiply this concentration by the water volume (in l). Divide the result by 1000 to obtain the amount of chemical in ml or g.

Example

Recommended dosage is 100 ppm copper sulphate. Your trough contains 500