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Anthelmintics

An anthelmintic is a compound which destroys or removes helminths from the gastro-intestinal tract and other tissues and organs they may occupy in their hosts.

Currently a good selection of safe anthelmintics is available, some with broad spectrum activity and others with activity against specific helminth infections. Many modern anthelmintics are effective against both adults and larval stages and an increasing number are efficacious against arrested or dormant larvae.

Due to their cost and their tendency to delay or interfere with natural host immunity mechanisms, anthelmintics may not be the most desirable method of managing helminth problems. However, in many circumstances the sensible use of anthelmintic drugs is likely to be the only available method of controlling helminth parasites. They should not be used indiscriminately.

1. Characteristics and selection of anthelmintics

The ideal anthelmintic has the following properties:

(a) A broad spectrum activity against adult and larval helminth parasites.

A number of factors influence the efficacy of an anthelmintic drug. Animals often harbour several different species of helminths, which may not have the same sensitivity to a given anthelmintic. In addition, there is usually a difference in sensitivity between adults and larval stages, with immature stages being less sensitive than the adult parasites.

Very few if any of the anthelmintics are completely effective at the recommended doses under field conditions. Some anthelmintics may be very effective in sheep but not in cattle, or?vice versa.

(b) A rapid metabolism in the body and short-lived presence at low levels in the milk and/or tissues.

Animals should not be slaughtered for human consumption and milk not distributed to consumers until the drug residues have reached acceptably low levels. The withdrawal period of the drug should be considered before its use.

(c) A low toxicity in the target species. The ratio of the therapeutic dose to the maximum tolerated dose should be as large as possible.

It is desirable that an anthelmintic has a safety margin of at least six-fold.

(d) No unpleasant side-effects to the animal or to the operator.

Drugs may cause vomiting, or pain at the injection site. Some drugs irritate the skin of humans.

(e) Suitable for practical and economical integration into various management systems.

The selected drug(s) should be competitively priced and ready to use in a simple way. They should be stable and not decompose on exposure to normal ranges of temperature, light and humidity, and have a long shelf life.

2. Administration of anthelmintics

It is important to first identify the nature of the parasitic problem in order to select the appropriate drug to treat the infection. The optimal time and mode of administration of the drug should then be considered.

WARNING: Many formulations of anthelmintics are easily adulterated and it is strongly recommended that only registered drugs from authorized sources be purchased.

A wide variety of formulations and preparations have been developed to provide methods of dosing animals, which are convenient for a wide range of species and circumstances.

2.1 Dosing by mouth

The majority of anthelmintics are given by mouth as liquid preparations, pastes, boluses and tablets.

Liquid preparations are usually sold ready to use. Several devices such as syringes, bottles and drenching guns can be used for delivering the dose. Drenching guns are generally preferable and a wide variety, including single dose, multi-dose and automatic types, are available. It is important to keep the drenching equipment clean after use. The dose to be delivered should be checked before-and several times during-dosing to ensure that the correct dose is given to all animals. A graduated cylinder should be included in the field equipment for calibration purposes. It may be necessary to fit a short piece of rubber tubing on the end of the dosing nozzle to protect the mouth and pharynx of dosed animals.

Pastes are relatively easy to administer if a proper dispenser is available. If that is not the case, care should be taken to ensure the animal receives a full dose.

Boluses and tablets can be placed deep in the mouth of the animal by using a dosing gun or a pair of long-handled forceps, both of which can be manufactured locally. Bolus and tablet formulations have the advantage that if the dose is rejected, it is usually the total dose and a replacement can then be administered.

Prolonged protection of grazing livestock can be achieved by incorporating anthelmintics into medicated salt-molasses blocks and prepared mineral mixes, but animals do not always consume the amount required for an efficient treatment. Controlled-release preparations, such as slow release boluses allow the effective delivery of anthelmintics over several months.

2.2 Dosing by injection

A number of anthelmintics are available for injection. The size of needles should be appropriate for the formulation and the site of injection. In order to avoid local reactions (such as abscess formation at the injection site) the highest possible hygienic standards should be maintained.

2.3 Dosing by external application

Several dewormers are now available in a formulation for external application, termed "pour-on" preparations. The active ingredient of the drug is absorbed through the skin reaching its target via the circulatory system. This application form, which is particularly convenient for animals kept under range conditions, has the advantage that only minimum restraint of animals is needed, as the dose is applied to their back while passing through a crush or standing at a feeding trough.

3. Testing of anthelmintics

Anthelmintics marketed by international pharmaceutical companies are usually well tested for efficiency and toxicity and can be safely applied according to the manufacturer's instructions. In cases where the origin of the drug is unknown or when it has been obtained from a source without an established reputation, it may be advisable to test the efficiency of the drug before using it in large scale control programmes. A rapid and cheap method of assessing the efficacy of an anthelmintic is to determine the effect on worm egg counts before and after treatment.

4. Summary of anthelmintics for the treatment of gastro-intestinal

Nematodes, Lungworms, Tapeworms and Flukes

Table 1. ?ANTHELMINTICS AND THEIR APPLICATION
Generic name Route of administration* Dose rate (mg/kg) Spectrum of activity**
Benzimidazoles
Albendazole O 5-7.5 GI, L, T
Cambendazole O 20-25 GI, L, T
Febantel O 5-10 GI, L
Fenbendazole O 5-7.5 GI, L, T
Mebendazole O 12.5 GI, L, T
Oxfendazole O/IR 4.5-5 GI, L, T
Oxibendazole O 10-15 GI
Parbendazole O 20-30 GI
Thiabendazole O 44-110 GI
Thiophanate O 50-80 GI, L
Immidazothiazoles
Tetramisole O 15 GI, L
Levamisole hydrochloride O/S0/SC 7.5 GI, L
Levamisole phosphate O/SC 8-9 GI, L
Organophosphates ? ? ?
Coumaphos O/F 8-15 GI
Haloxon O 40-50 GI
Naphtalophos O 30 GI, T
Trichlorfon IM/SC 10-15 GI
Tetrahydropyrimidines
Morantel O 10 GI
Pyrantel tartrate O 25 GI
Miscellaneous ? ? ?
Ivermectin D/SC/SO 200 mcg/kg
500 mcg/kg
GI, L
*O: Oral
SC. Subcutaneous
SO: Spot-on
IM: Intramuscular
IR: Intraruminal
F: Feed
**GI: Gastro-intestinal nematodes
L: Lungworms
T: Tapeworms Table 2. ?ADDITIONAL INFORMATION ON ANTHELMINTICS FORMULATIONS
Generic name Preparation
Albendazole Liquid suspension
Cambendazole Ready-to-use suspension, paste
Fenbendazole Ready-to-use suspension, granules, tablets, mineral pre-mix, licking blocks
Mebendazole Ready-to-use suspension, granules, paste, mineral pre-mix
Oxfendazole Ready-to-use suspension, tablets
Thiabendazole Ready-to-use suspension, powder, granules, tablets, paste
Tetramisole Suspensions, granules, tablets, injectable
Levamisole Pour-on preparation
Morantel Ready-to-use suspension, tablets, granules, paste
Pyrantel ?

Table 3. ?ANTHELMINTICS FOR THE TREATMENT OF INHIBITED LARVAE
Cattle Sheep
Albendazole Albendazole
Febantel Febantel
Fenbendazole Fenbendazole
Oxfendazole Oxfendazole
Thiophanate Levamisole
Ivermectin Ivermectin

Table 4. ?ANTHELMINTICS AND FORMULATIONS FOR STRATEGIC PROPHYLACTIC PROGRAMMES
Generic name Formulation Animal
Albendazole Slow release bolus, active 90–120 days Cattle
Sheep
Albendazole Pulse release bolus Cattle
Oxfendazole Pulse release bolus Cattle
Morantel tartrate Slow release bolus active 60 + days Cattle

Table 5. ?ANTHELMINTICS FOR THE TREATMENT OF LIVER FLUKES
Generic name ? Route of Administration ? Dose rate (mg/kg) Minimum age of fluke in weeks efficiency 3 90%
Sheep Cattle Sheep Cattle
Hexachlorophene* O 15 20 12 >20
Hexachloroethane O 250–300 300 12 12
Tribromsalan O 20 20 12 >12
Bithionol* O 75 30 >12 >12
Hexachloroparaxylene O 150 130 12 12
Bromophenophos O 16 12 12 >12
Clioxanide* O 20 NR 12 NR
Oxyclozanide* O 15 13–16 12 >14
Niclofolan* ? O 4 3 12 >12
SC NR 0.8 NR <12
Nitoxynil SC 10 10 8 10
Brotianide* O 5.6 NR 12 NR
Rafoxanide* O 7,5 7.5 6 12
? SC NR 3 NR 12
Closantel O 7.5-10 NR 8-6 NR
? SC NR 3 NR >12
Diamphenetide O 80–120 100 1 day–6 weeks 1 day–7 weeks
Albendazole O 4.75 10 >12 >12
Triclabendazole O 10 12 1 1
Clorsulon ? O - 7 - 8
SC - 2 - >12
O = Oral
SC = Subcutaneous
NR = Not Recommended
* = Also effective against paramphistomes (see Table 7.6) Table 6. ?ANTHELMINTICS FOR THE TREATMENT OF PARAMPHISTOMES
Generic name ? ? Route of Administration ? ? Dose rate (mg/kg) Efficiency (%)
Sheep ? Cattle ? Immature in small intestine + abom. Mature in rumen
Sheep Cattle Sheep/Cattle
Clioxanide* O 20–40 NR ? ? ?
Niclosamide ? ? O 50–100 50–100 95 0-96 0
? NR 160
(2 doses)
NR 92 ?
? ? ? ? ? ?
Niclofolan* O 6 6 76-95 42
Oxyclozanide* O 15 15 85–100 61–96 73–100
? NR 18
(2 doses)
NR 99–100 100
Rafoxanide* O 75–100 25–50 99–100 9–100 63–98
Bithionol S03* O 40 40 97–100
Resorantel O 65 65 80–90 62-99 85–100
O = Oral
NR = Not Recommended
* = Also effective against liver flukes (see Table 7.5) ?Note: Source of this article: from www.ilri.org
 
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