An anthelmintic is a drug that expels parasitic worms (helminths) from an animal’s body. Unfortunately, more and more worms are showing anthelmintic resistance (AR). The drug kills worms susceptible to the ‘poison’. Those that survive pass on their resistance genes. Resistant worms accumulate and, finally, the treatment fails.
In a previous issue, we discussed how careful husbandry can help in the fight against worm parasites and AR. In this article, we look at how to get the best out of anthelmintics.There are two ways of doing this. One involves optimising anthelmintic use – that is, creating the best possible situation or environment for the drugs to work. The second involves improving the drug’s efficacy, or how well it works.
- Establish the parasite species present and its economic importance: Without this knowledge, worm management becomes a matter of costly guesswork.
- Use the most suitable drug: If the parasites are ranked in order of economic importance, and their susceptibility to groups of anthelmintics is known, it is then possible to decide which drug/s will be the most suitable in each situation. This should include a cost/benefit analysis. Neither the cheapest nor the most expensive drug is necessarily the best one to use. Beware of a generic drug sold by an unknown company.
- Avoid administering treatment too frequently: Forget the old approach of ‘dosing clean’. Instead, administer the drug just often enough, and to a sufficient number of animals, to maintain an equilibrium between parasite, host and environment.
- Treat all and stay: This goes against the advice given to farmers for nearly a century. If all the sheep are to be treated, they should remain in the camp where they were grazing before treatment for at least two to three weeks after treatment. This will prevent them from contaminating a new pasture with resistant parasites. If a long-acting anthelmintic is used, this period will have to be longer (two to three weeks after the effective residual action ends).
- Treat selectively: Treat only those sheep or goats unable to cope with the current infection challenge, provided the percentage of non-copers remains below 20%. This can be ascertained with the FAMACHA system for haemonchosis and, possibly, with body condition scoring for other parasites. In the absence of such selective treatment, leaving even a small percentage (10% to 25%) of the flock intentionally untreated can retard AR development.
- Move, then treat: Another way to achieve the same result as ‘treat all and stay’ is to move the flock to a new, safe camp and delay treatment for two to three weeks in order to allow ‘seeding’ of the new pasture with unselected worms.
- Dose over the tongue: Guiding the tip of the dip gun over the tongue towards the back of the mouth prevents the oesophageal groove from closing and the full dose therefore lands in the rumen, where it is absorbed more slowly. This is particularly important for anthelmintic groups such as the benzimidazoles and macrocyclic lactones that rely on prolonged presence in the blood for their effect. This means that a drug against which worms have developed a moderate degree of resistance can be made more effective. Be warned: if carried out carelessly, dosing (drenching) over the tongue can see the dose ending up in the lungs, causing pneumonia; or the nozzle of the dosing gun can penetrate the pharynx and cause a fatal infection. If the sheep jumps forward, the operator must let the gun ‘ride’ with the sheep and not oppose it. Deliver the dose by a measured, steady pressure rather than a single squeeze.
- Reduce feed intake: In the case of benzimidazoles and closantel, it has been found that reducing feed intake (starvation) for 24 hours prior to treatment will improve the absorption of the remedy because of the slower flow of ingested matter. This results in a more effective exposure of the parasite to the drug.
- Repeat the dose: This only applies to benzimidazoles and macrocyclic lactones. Two doses given 12 hours apart will increase the efficacy of these drugs, allowing more time for a cumulative killing effect. Thus, resistant worms can still be killed, although this is achieved at a cost since two normal doses instead of one are needed. A double dose, given at one time, will have no beneficial effect with these two groups of anthelmintics.
- Increase the dose:This applies only to drugs that rely mainly on peak concentration for their effect. In this case, a double dose of a drug given at one time can overcome drug resistance in worms. This is useful for the imidasothiazoles (Levamisole). But check with your vet, as two or three times the suggested dosage may cause toxicity problems.
- Correct dosage: Many problems are caused by not weighing sheep, not checking the dosing gun for accuracy, and not reconciling the amount of drug used with the number of sheep treated.
- Sustained delivery: Medicated blocks or controlled release capsules will increase the efficacy of drugs that rely on prolonged action for their effectiveness. Bear in mind, though, that prolonged exposure to a drug at low levels will increase selection for resistance. This approach should therefore be used only for very specific, limited purposes (weaners on green pasture, for example).
Based on a paper by the University of Pretoria’s Gareth Bath (Department of Production Animal Studies) and Jan van Wyk (Department of Veterinary Tropical Diseases) distributed by the Livestock Health and Production Group of the SA Veterinary Association as part of its monthly disease report.