Chapter 6

PROPHYLAXIS AND TREATMENT

A. Prophylaxis

Prophylaxis, or action taken to prevent the development of disease is one of the most critical factors in determining the success or otherwise of a clarias farming enterprise. Good husbandry of fish stocks is all-important and this involves a mixture of specific knowledge of some treatments and an application of a lot of common sense.

Stocking density is probably the most important figure and problems usually arise from overstocking. As indicated in the previous chapter this will lead to a decreased oxygen level in the water (of less significance to clarias because of their accessory breathing organs) and also to build-up of toxic waste products. At the same time, if growth is to be obtained, the amount of food added to a particular volume of water will also be dramatically increased and any excess uneaten food will further complicate matters through its decomposition, a particular problem with the feeding of trash fish. pH of the water is likely to decrease under such circumstances and liming of the pond and even changing of the water may be necessary (see chapter 5).

The above adverse conditions have a direct effect on the epithelium of gills and skin and cause increased mucus production, phyperplastic responses in the gill and often, erosions and ulceractions of the skin. Such lesions are readily invaded by normally saprophytic bacteria and parasites, leading to major losses, as described in chapter 3.

Correct nutrition is another important factor in preventing the occurrence of disease. A major problem with feeding trash fish is that such food is often in an advanced state of decomposition before it is fed by being stored for long periods in the sun in the open. Such food is hardly a complete diet in terms of vitamins, minerals etc. and although some farmers supplement this with vitamin/mineral mixes designed for chicken production, this still falls far short of a balanced diet (see chapter 4). The present work being carried out on the nutritional requirements of clarias is enabling the production of pelleted formulations which will give a balanced diet without wastes.

Sterilisation of pond-bottoms in between batches of fish is important in cutting down numbers of parasites. This should be carried out with calcium hydroxide (CaOH) (see Appendix 5) at a rate of approx. 200 g/sq meter. The quantity used will vary with soil type and advice should be sought from NIFI as to actual amounts of lime and length of time for exposure. For other alternatives see Appendix 5.

Removal of intermediate hosts is another way in which parasitic diseases may be prevented. This is especially important for the worm parasites whose life-cycle may involve snails. Appendix 5 gives advice on removal of predators and pests.

B. Methods of Treatment

Most comments in this section apply to the treatment of infectious disease. Nutritional or non-infectious disease should obviously be treated by identifying and rectifying the particular husbandry or dietary defect. It must be stressed at this point that the majority to infectious diseases arise directly as a result of some husbandry problem and there is not point in carrying out specific parasitic or bacterial treatment without also improving the background husbandry quality.

The types of treatment method available are somewhat limited and will depend on the size of fish to be treated but consist essentially of:

1. Immersion treatment

2. Systemic treatment by the oral route; and

3. Injection

Details of actual drugs and dosages used are give in section C and some worked examples in Appendix 7.

(a) Immersion treatment. This involves the application of chemical treatments to the water to control external disease agents affecting skin and gills. The type of immersion will vary from “dip” treatment ie. fish are placed in a high concentration of the chemical for a short time, to bath treatment which involves much lower concentrations of chemical for extended periods of time. This may in many cases in pond systems be for an indefinite period or may involve a gradual dilution of chemical if a slow flow of fresh water enters the pond. The dip method may be more useful for fry in small numbers of fish may be treated at one time and stress or skin damage may occur during netting.

The following points should be considered when using bath treatments in clarias culture:

1. High levels of suspended solids will inactivate many treatment compounds and make effective dosage computation difficult.

2. If possible, fish should be starved for 24 hours prior to treatment. This will decrease the amount of toxic waste products in the water reduce oxygen consumption by the fish. Suspended solid levels are also likely to be decreased.

3. Many treatment compounds are themselves damaging to gill, particularly if overdosed, and so assessment of the health of gill tissues should be carried out prior to treatment and, wherever possible, increase aeration during treatment.

4. Accurate calculations of the pond volume and amount of chemical needed are essential. Mathematical calculations must always be checked, preferably by a second person (see Appendix 6).

5. Treatment trials are very helpful. A small sample of fish is taken and treated in a separate container at the suggested dose-rate. This enables monitoring of toxic effects and also efficacy of treatment. Once a whole pond has been treated, little can be done to improve matters if an unsuitable treatment has been used.

6. Water hardness and pH are very important factors which need measuring before treatment with a number of compounds such as CuSO⁴ and quaternary ammonium compounds. Soft, acid water makes these treatment compounds more efficient but also more toxic than hard, alkaline water. An increase in pH will also increase the toxicity of any ammonia present in the water further increase the likelihood of fish mortalities.

7. Fish should be carefully observed during treatment. If any drastic errors are made, signs of distress will be seen in the fish and it may be possible to flush out the chemical with freshwater or remove the fish to another pond before permanent damage is done.

8. The results of treatment should be checked immediately following the treatment by wet smear examination for parasites etc. in order to determine whether repeat treatment will be necessary.

(b) Systemic treatment. This involves the incorporation of chemicals into the food to treat internal infections. Its principal use is in the treatment of systemic bacterial disease with anti-biotics although helminth infections may also be treated by this route.

A particular problem with oral antibiotic therapy is that sick fish do not feed well and it is important to diagnose and treat as quickly as possible. There is seldom time for specific diagnosis of the species of bacterium involved or even for sensitivity tests to be carried out and so a broad spectrum antibiotic such as oxytetracycline must be used in the first instance. If subsequently, the bacterium proves resistant to this treatment, then sensitivity tests carried out at a specialist laboratory will provide the best alternative treatment. Availability and cost of such antibiotics are also likely to be limiting factors.

A further problem is subsequent marketing of treated fish. A period of three to four weeks is generally recommended between the end of treatment and marketing for human consumption, and it is essential that the full course of treatment is undertaken to avoid the build up of drug resistance. Antibiotics may be incorporated into trash fish diets by simple mixing but addition to pelleted diets is facilitated by first mixing the antibiotic into a small quantity of vegetable oil and then adding the mixture to pellets at a ratio of 1 : 20 by volume.

(c) Injection. This usually involves antibiotic therapy and is seldom financially worthwhile except in the case of valuable broodstock fish. Fish may be injected either intramuscularly at a point mid-way between the dorsal fin and the lateral fin at a point half way along the fish, or by the intraperitoneal route at a point mid-way between the lateral line and the ventral body surface between the pelvic and anal fins.

Problems with this technique, in addition to the cost of antibiotic, are that it is labour-intensive and causes stress through netting and handling of the fish. In addition clarias catfish are very difficult to handle and can damage themselves or the handler in their struggles so anaesthesia is weill worthwhile if they are to be injected.

C. Treatment Compounds

A variety of factors need to be considered when choosing a method and/or chemical for treatment. First of all, it may not be worth attempting treatment if mortalities are low and cost of treatment is high. The alternatives are (i) to either let the disease continue and accept a certain percentage loss removing heavily infected fish to prevent build-up of pathogens, or (ii) if fish are near market size, it may be possible to harvest the fish and avoid losses, or (iii) treatment may be necessary.

The range or compounds at present in use in clarias farming is limited and is summarised in Table 3. Many other treatments are as yet experimental but many drugs in common usage in veterinary medicine show great promise in fish use eg. the wide range of anthelminthics. The permanent bath method, treating a whole pond by spraying the compound over the surface as evenly as possible and the use of oral therapy are really all that are easily available to the clarias ongrower. With earlier stages, dip teatments are more readily used and expense becomes less of a problem.

Repeat treatments are also occasionally necessary but care should be taken to avoid accumulated toxicity. Repeated application of many of the chemicals used eg. Formalin, Malachite green, causes irritation to the gills and may reduce the respiratory efficiency.

Finally, it can never be stressed too much that attention to background husbandry improvements as well as treatment is necessary if repeated problems are not to occur.

Table 3

COMPOUNDS AND TREATMENT REGIMES