GUIDELINES ON TREATMENT OF BACTERIAL INFECTION IN FISH

The control of bacterial diseases in farms is not limited to the use of antibiotics. In cases where the disease originated from ingestion of feeds with high microbial load or use of water from highly contaminated source, the control must start from here to get good results.

Use of antibiotics in control of bacterial infection in fish could be via bath medication, feed medication or by injection.

Bath medication has advantages and disadvantages. For example, when there is anorexia in fish, it is the easiest means of mass treatment of sick fishes, though might be more expensive. To reduce cost of medication, volume of water must be reduced, while maintaining the right concentration of drug in water for effective therapy. The element of time is very important in bath medication, and it is often related to the concentration of drug and size of fishes.

Feed medication is by far cheaper than bath medication and has advantage of mass medication. The disadvantage of this method lies in a situation where anorexia is prominent in the sick fishes or where the drug used in medicating via feed is bitter (e.g., is Enrofloxacin). The enrofloxacin should be sweetened before using it to coat the surface of feed pellets.

Treatment of sick fish by injection is not usually done but may be done on a few and expensive broodstock.

It is important to re-emphasize the need for bacterial culture and antibiotic susceptibility test before administering antibiotics to sick fishes. It is also important for the veterinarian to understand the peculiarity of medicating aquatic animals like fish with respect to withdrawal periods to be observed. Here withdrawal period is temperature dependent. For example, if the MRL (maximum residue limit) for a drug like enrofloxacin is 100µg/kg and withdrawal period is 500° days (500 degree-days), it means that at pond water daily average temperature of 25°C, it will take 20 days from last day of medication to achieve significant elimination of drug from edible tissues of fish. This also implies that at a lower pond water temperature (e.g., 20°C) as may be seen in harmattan season, the withdrawal period will be 25 days. This is because of the poikilothermic nature of fish. Generally, the withdrawal period of antibiotics in fishes could be 21 days from the last day of antibiotic treatment and changing the water every 2-3 days.

Common antimicrobials and their recommended dosages used in fish farming

Drug

Posology

Oxytetracycline

Effective in treating fish against bacterial infection. Feed medication at 50 mg/kg body weight for 5 consecutive days. Bath medication at 1gm of 100% Oxytetracycline per 25 litres of water for 6 hours in adult fish, 3-4 hours for fingerlings and juveniles and 2 hours exposure for fish larvae or fry x 3/7.

Note that feed medication is cheaper than bath medication.

Amoxicillin

Effective in treating bacterial infection in fish. Feed medication at 40mg/kg body weight for 5 consecutive days. or by bath at 50mg/L of water for up to 6 hours in adult fish and 3 hours for fry and fingerlings x 3/7

Ciprofloxacin/ Enrofloxacin	These are fluoroquinolones. They are effective against bacterial infection. Can be administered via feed medication at 10mg/kg body weight for 5 consecutive days. Bath medication at 1ml of 20% Enrofloxacin or Ciprofloxacin per 6 litres of water for 6 hours in adult fish, 3-4 hours for fingerlings and juveniles and 1½ to 2 hours exposure for fish larvae or fry. x 3/7 If water is hard (with high calcium and magnesium ions), it reduces the bioavailability and efficacy of fluoroquinolones when administered by bath. Oladele et al., 2010c
Streptomycin	Feed medication at 20 to 40mg/kg body weight for 5 consecutive days. Or by bath at 50mg/L of water for up to 8 hours in adult fish and 3 hours for fry and fingerlings. x3/7
Gentamicin	This can be administered via feed at dosage of 20mg/kg body weight of fish for 5 to 7 days.
Florfenicol	It is effective in controlling bacterial infection via feed medication at 20mg/kg body weight for 5 to 7 days.
OTHER IMPORTANT DRUGS USED IN FISH FARMING

NaCl (Common salt)	Useful in control of monogenean trematodes of freshwater fish and some protozoa at 10g/litre for 1 hour. Lower concentration of 2gm/litre has been used continuously in hatchery to protect fry and fingerlings from effect of significant level of nitrite from water source.
Copper sulphate	Safe to use as anti-fungal through feed medication at 500 gm/ton of fish feed x 3/7. Avoid bath medication especially if alkalinity of water is unknown. If total alkalinity is determined e.g. 200 ppm alkalinity in pond water sample, this should be divided by factor of 100 to get 2, hence the concentration of CuSO4 that will be safe for use in the pond is 2ppm or 2mg/L or 2g/1000 L of pond water.
Potassium permanganate (KMnO4)	Safe to use by bath as topical anti-fungal and anti-protozoal drug at 2 ppm, i.e., 2gm/ 1000 litres of water for prolonged immersion or 20 ppm for only 1 hour treatment.
Mebendazole	This is an anthelmintic effective against worms, including trematodes (Gyrodactylus and Dactylogyrus spp.) that attach to skin and gill surface. It also destroys eggs of worms. Feed medication at 100gm of 10% Mebendazole in 100 kg of feed x 3/7. Bath medication at hatchery at 1gm of 10% Mebendazole per 4 litres of water for up to 8 hours/day x2/7


Formalin(38%)	It is a parasiticide used against external parasites. Should be used with caution. At 25 ppm (1ml/40 litres of water), it is very safe at the hatchery for up to 5 hours to clear parasites (e.g., Icthyobodo spp. and trematodes like Dactylogyrus spp.) from fry x2/7. At 250 ppm (1ml/4litres of water), use for short term bath at 30 mins in well aerated ponds Should not be used if it forms a white precipitate at the bottom (paraformaldehyde).
Ivermectin	Commercially available as a 1% oral poultry medication. Useful against internal and external worms in fish. Should be administered only via feed at 0.3 mg/kg body weight x 2/7. Also effective against fish lice. Note: Ivermectin is highly contraindicated for use by bath

Medication of fish by bath in concrete or tarpaulin or plastic tanks.

Since bath medication is often more expensive, it makes economic sense to use a reduced volume of water medication. Administration of antibiotics by bath is usually done when there is anorexia.

A fish tank of 6m x 2m x 1.2m has volume of 14.4m³ and this can take 14,400liters of water(maximum). If this dimension of tank takes about 1000 adult catfish, the height of water for medication should not be up to 1m, but about ¼ of the 1.2m total height, hence 0.3m or 30cm will be the level of the water for medication.

For a drug like enrofloxacin (20%) that is used by bath at 1ml/6litres of water, using it at 1meter water level=12,000 litres of water that will need 2000mls or 2 litres of enrofloxacin. If the water level is reduced to 0.3m or 30cm before medication, the water volume =3,600 litres and quantity of enrofloxacin needed will be 600mls hence reducing the cost of medication.

If it is to be done in an earthen pond, the water should be drained towards the outlet to reduce water volume. The remaining water volume could be estimated before medication.

Also note that as the water is drained, the fishes move to aggregate towards the outlet

Example

1a). A fish farmer complained of high mortality of fish in his earthen pond (stocked with 10,000 catfishes). Cumulative mortality was 750 fishes in the last 5 days. Laboratory tests conducted revealed that fishes had severe Acinetobacteriosis and Gyrodactilosis. The bacterial isolate was susceptible to enrofloxacin, while the drug of choice for control of Gyrodactylus spp. was 1% ivermectin. Given that the average body weight of fish is 1.2 kg, dosage of ivermectin as 0.3 mg/kg and enrofloxacin as 10 mg/kg, with a 20% Enrofloxacin preparation available, how will you bring the situation under control?

Show with a clear calculation, the quantity of drug needed for daily treatment and for 5 days.
If the ivermectin is to be administered for 2 days within the 5 days, explain how this should be done via feed if at their average body weight (1.2kg), feed consumed is about 1% of their total body weight.

1b. Given a similar situation (as seen above) occurring in fingerlings at the hatchery, where recirculating aquaculture system is used and anorexia, being a prominent feature, what will you do as a Vet Doctor ?

Answer

Current population of fish in the pond= 10,000 -750 = 9,250. Total weight of fish = 1.2 kg x 9,250 = 11,100 kg.

Daily feed consumed= 1% of total body weight= 1/100 x 11,100= 111 kg of fish feed/day. Quantity of enrofloxacin needed per day:

Use the formula: Dose = B x D/C, where B= body weight of fish in kg, D= drug dosage in mg/kg body weight and C= Concentration of drug in mg/ml. Here B= 11,100 kg, D= 10 mg/kg and C= 200mg/ml (i.e., 20%).

= 11,100 x 10 = 555 mis of 20% Enrofloxacin needed per day to treat 9,250 adult catfish.

200

Quantity of Ivermectin needed:

Here B= 11,100 Kg, D= 0.3 mg/kg and C= 10 mg/ml (1%)

= 11,100 x 0.3 = 333 mls of 1% Ivermectin needed/day and for 2 days.

If the farmer makes his own fish feed, the daily quantity of drug will be expanded eg with a sweetener solution (because of bitter drugs like Enrofloxacin) before evenly mixing with ground feed which is later pelletized. If farmer uses pelletized floating feed, the surface of feed pellets will be evenly coated with expanded volume of drug. For example, the expanded drug could be divided into 10 equal parts and sprayed on the pellets divided into 10 equal parts. This is allowed to dry and fed to the fishes.

In the above case, for treatments of days 1 and 2, the 2 drugs are combined, but for days 3 to 5 of treatment, Enrofloxacin alone will be used.

If these conditions had happened to fry in the hatchery and they are not feeding as expected, the following should be done:

Subject fishes to bath treatment with formalin at concentration of 25ppm (1ml of 38% formalin per 40 litres of water) for 5 hours When it gets to 3 hours into treatment with formalin, add 20% Enrofloxacin and leave for 2 hours. After this, gradually drain the water while adding fresh water into the fish tank. This should be repeated the following day. On day 3, only enrofloxacin can be administered for bath treatment.
Note that it is easy to monitor the efficacy of the formalin on the trematodes (Gyrodactylus ) when water + sediments from the hatchery fish tank base are siphoned and about 10 to 20 mls dispensed in clear petri dish to view under the microscope. This can be done before and after treatment. Usually after treatment, a lot of dead trematodes will be seen.