By Dr Carl Eden BVM&S MRCVS — Virbac
There are many worming pastes and drenches available in Australia to treat the common worms in horses. Flicking through the pages of ‘Mims’ quickly tells us that despite over 20 different brands being on sale there are only five different classes or ‘families’ of drug in use. The chances of a client using the same class of drug for years despite attempts at rotation are therefore high. When rotation is employed clients are all too often rotating products with every treatment and are not aware of the need to rotate drug class as opposed to rotating the brand names or box colour.1,2
Given that resistance to benzimidazoles, and tetrahydropyrimadines is already documented in horses,3-6 we as a profession need to ensure that our clients are using wormers correctly to maximise the benefit we obtain from the macrocyclic lactones or ‘mectins’. If resistance occurs horses may carry heavy worm burdens despite being wormed on a regular basis. To date resistance has not been documented to ML’s or mectins in horses despite its emergence in other species such as cattle and sheep.
Strategies to avoid resistance
Three distinct strategies can be employed:
- Monitoring for the emergence of resistant parasites using FECRT’s.
If the FECRT indicates that the current wormer is controlling the problem it is feasible to stick with that product and monitor its success on an annual basis. If values of less than 90% reduction are being obtained from any establishment, resistance may be becoming an issue. In the absence of overstocking or pasture hygiene issues rotation away from the current wormer would be indicated, (e.g. from A ‘BZ’ wormer to a ‘ML’ wormer).
- Rotation of anthelmintic class on an annual basis.
Current advice is to rotate the class of wormer on an annual basis if monitoring practices are not being employed. This entails using one class of wormer all year round and changing to a different class for the next year. A return to the original wormer can be made in the following year.
It is important to remember that the rotational choice can not contain any active (or active from the same class or family) that is contained in the original wormer. For example changing from a paste containing ivermectin to a paste containing moxidectin does not constitute rotation. Changing from an ivermectin/ praziquantel combination paste to an oxfendazole/pyrantel combination does constitute as good rotational rational however, as neither paste contains an active that is related to those in the other paste.
- Strategic dosing.
Strategic worming or dosing involves administering anthelmintics to horses at various times of the year based on the life cycle of the parasites and the activity of the anthelmintic being used. It is a strategy employed in many countries and generally involves the use of narrower spectrum products. The availability of such products in Australia is low and as such worming tends to be focussed on the use of broad-spectrum anthelmintics all year round. Given that horses will be infested with a broad range of helminth and arthropod parasites at any given time, a broad-spectrum drench is likely to be the best means of control.
How does resistance develop?
If a worming product (or the same family of worming product) is used continuously on a property for long periods of time there is a possibility of the worms developing resistance to that wormer. In this scenario the parasites would be capable of surviving drenches administered at recommended dose rates. This would make that product and others in its family useless. If a horse with resistant worms were moved from one premise to another it would carry the problem with it to the new premises.
The speed with which resistance emerges is dependent upon the pharmacodynamics of the drug in question. For example ivermectin has been used in horses for over 20 years and resistance has not emerged to the macrocyclic lactones. The introduction of moxidectin to equine anthelmintic armoury is regarded as a significant threat to the resistance status of macrocyclic lactones in horses however. It is for this reason that leading parasitologists are recommending that its use be limited in the horse.7
Although the macrocyclic lactone class of drug is widely used in Australia today to great success, one of the concerns in the equine industry is that since the introduction of moxidectin, resistance may develop like it has in some sheep, cattle and goat parasites. Indeed shifts in the genetic diversity of the cyathastomin gene pool has already been documented and whilst these strains are not resistant as yet, these findings may represent the first steps in the development of resistance.8
How does a rotational plan help?
A number of methods can be incorporated into a rotational strategy to stop the emergence of resistance and it is important that clients are aware of these strategies and how to implement them. Clients will bond to a practice if they feel that they can talk to their vet about the everyday aspects of their horse’s wellbeing as well as the more nasty topics such as lameness, colic and laminitis. The following is a list of topics that can be discussed with clients to educate them in their horse’s care.
- Use anthelmintics.
It is important to use an effective anthelmintic that does not already have resistance to it. There are no reported cases of resistance to Equimax and it is formulated with praziquantel so it also treats tapeworms as well as roundworms.
- Rotate class of anthelmintic on an annual basis.
By using a different mixture of drugs from a different family we can slow the emergence of resistance to anthelmintics. Strategy-T paste contains oxfendazole and pyrantel. These are two different drugs, (from different families to abamectin and praziquantel), that work together synergistically killing roundworms and tapeworms. This synergy allows Strategy T to work well against benzimidazole resistant worms showing little or no cross-resistance with other members of its family. This synergy confers a similar spectrum of activity in Strategy-T to that found in drenches containing the macrocyclic lactone class of drug, (exception Gasterophilus spp).
When rotating away from macrocyclic lactone “or mectin” based wormers, Strategy-T is therefore the drench of choice.
The reason we do not rotate on a per wormer basis is because this may allow the worms to "remember" the last wormer and thus resistance to two classes of anthelmintic may develop concurrently.
- Use the minimum types of different anthelmintic a year.
Don’t rotate during the year. Change the wormer only when the year changes.
- Give the correct dose.
Modern anthelmintics are very safe and it is better to err on the side of slight overdose than to underdose. (Exception-Wormers containing moxidectin). Under dosing will accelerate selection for resistance.
- Avoid introducing resistant worms to your yard.
Worm all new horses prior to turn out and keep them confined to a box for 48 hrs after worming to prevent pasture contamination. Ideally, have faecal worm egg counts performed after worming and prior to turn out to establish if a problem exists with newly introduced horses.
- Practice good pasture hygiene and “poo pick” at least twice a week.
Worm eggs and larvae are contained in the horse’s faeces and by removing the faeces you are preventing reinfestation. Stables should be cleaned out at least once daily. Clients should be advised not to spread the manure back onto the pasture.
- Have faecal worm egg counts performed on ALL your horses to establish if a problem is emerging.
Routine visits are a good time to discuss worm control strategies. Practices such as annual FEC are not expensive and could save the client a lot of time, money and heartache in the future.
These interfere with parasitic nerve transmission by opening chloride channels in the post-synaptic membrane. They are effective against a wide range of nematodes, (round worms) but have no activity against trematodes or cestodes, (flukes and tapeworms). They are also active against some mites and lice and the larval stages of bot flies. They are lipophilic and therefore not only kill worms but also prevent re-infection for a period after treatment because they can stay in the horse’s body for a while by binding to fat. Abamectin amd ivermectin are extremely safe molecules and can be administered to all equids of all ages and physiological states.
Abarmectin (EQUIMAX, EQUIMINTH)
Ivermectin (EQUIMAX LV, EQUIMINTH IVERMECTIN LIQUID, ERAQUELL)
These act by disrupting worm energy metabolism by binding to tublin, a protein required for the uptake of nutrients. Single doses are effective against larval and adult roundworms and multiple doses may be effective against encysted roundworms by encouraging the larvae to emerge. They are not effective against tapeworms or bots. Resistance to this class of drug has been documented but oxibendazole does not seem to exhibit the same cross-resistance patterns typical of other drugs in this class. They are also safe drugs to use.
Oxibendazole (OXIMINTH, OXIMINTH PLUS)
Fenbendazole (FENCARE 100, PANACUR 100)
Praziquantel (active against tapeworms):
Acts to disrupt the parasites outer membranes causing it to become more permeable to excessive glucose loss and to be more susceptible to the enzymes found in the gut. It is currently the wormer of choice against tapeworms in horses.
Praziquantel (EQUIMAX, EQUIMAX LV)
Tetrahydropyrimadines (active against tapeworms and roundworms):
These interfere with parasitic nerve transmission as cholinergic stimulants leading to paralysis. These products are active against larval and adult roundworms and pyrantel is effective against tapeworms. (Morantel is not regarded as having significant activity against tapeworms). They again are very safe drugs.
Organophosphates (active against bots):
Inhibit the actions of acetylcholinesterase causing a spastic paralysis. The worms are then expelled from the gut by the normal peristaltic waves. Very non-specific wormer causing effects in a multitude of organisms, roundworms and bots included. Toxicity has been reported in America. Highly toxic to aquatic species.
Dichlorvos (OXYMINTH PLUS)
Trichlorfon (Farnam worma and bot paste, Telmin Plus)
The combination of two active’s from different classes of drug can result in a drench that has a greater activity than the single active alone. For example praziquantel has no inherent activity against nematodes but has excellent activity against cestodes. The combination of praziquantel at the correct dose with ivermectin or abamectin, (as found in Equimax) results in a drench capable of killing all three types of internal parasites found in horses, nematodes, cestodes and arthropods.
Similarly the problem of benzimidazole resistance can be negated and the action of the drench increased by combining oxfendazole with pyrantel, (as found in Strategy-T). This combination shows synergistic effect achieving a greater kill rate than with either one drug alone. The combination treats nematodes and cestodes and is therefore the broadest spectrum benzimidazole based drench available in Australia.
This method of parasite control is reliant on knowledge of individual parasitic life cycles, local weather patterns and the spectrum of activity of the anthelmintic in question. For example a worming strategy could be based on the use of a benzimidazole type wormer on four occasions through the year to control the nematode burden. Whilst tapeworms and bots would only need dosing against twice a year. This could be accomplished by using a wormer containing only praziquantel, (or a double dose of pyrantel) on a biannual basis and the inclusion of a ML or OP prior to and after the peak bot fly season.
In areas with cold winters or high annual rainfall a strategic five-day course of fenbendazole could be employed after the cold snap or during the peak rainy season to decrease the hypobiotic larval burden and pasture contamination of cyathastomins.
Due to the diversity of weather patterns encountered in Australia, a thorough discussion on strategic worming strategies is beyond the scope of this article.
Specific advice pertaining to mares and foals
As vets we are often asked for specific advice relating to the treatment of pregnant mares and their foals. In this respect I would recommend that mares and foals are incorporated into the treatment schedule of the rest of the herd whilst bearing in mind the following.
- Quite often the mare provides the greatest source of contamination to her foal. This is especially relevant when we give consideration to those parasites potentially transmitted to the foal through the mares’ milk, (Strongyloides westeri).
- It is important to use anthelmintics with known safety data in pregnant mares and foals. Equimax and Equimax LV have been thoroughly tested at up to five times the recommended dose in foals and at three times the recommended dose every 14 days in pregnant mares.
- A foal has no natural immunity to infection with parasites and as a result significant burdens can establish over short periods of time necessitating frequent anthelmintic administration.
- Dose limiting parasites such as Parascaris equorum can sometimes not be 100% eliminated from foals, leading to the assumption of resistance development. In these cases higher doses of a safe anthelmintic may control the problem or the foals treatments may be rotated on a fast basis (ie alternating a macrocyclic lactone with a benzimidazole every treatment until 8 months of age.)
- High environmental challenges in immunologically naive animals can also lead to the impression of resistance on a yard that has no such problems.
In essence the treatment intervals should be based on the normal dosing interval on farm and worked around a base treatment two weeks prior to the estimated foaling date. The example below is based on a treatment interval of 8 weeks.
- O’Meara B, Mulcahy G. (2002) A study of helminth control practices in equine establishments in Ireland. Vet Parasitology 109:101
- Earle CG, Kington HA, Coles GC. (2002) Helminth control used by trainers of thoroughbreds in England. Vet Rec 150(13):405
- Craven J, Bjorn et al. (1999) A comparison of in vitro tests and faecal egg count reduction test in detecting anthelmintic resistance in horse strongyles. Vet parasitology 85:49-59
- The Veterinary Formulary. (1998) Fourth Edition Pharmaceutical Press.
- Drudge JH, Lyons ET, Tolliver SC. (1984) Critical tests of morantel-trichlorfon paste formulation against internal parasites of the horse. Vet Parasitol, 14(1): 55-64
- J.F. Pook, M.L. Power, N.C. Sangster, J.L. Hodgson, D.R. Hodgson. (2002) Evaluation of tests for anthelmintic resistance in cyathastomes. Vet Parasitology 106: 331-343
- Love S. (2003) Treatment and prevention of intestinal parasite-associated disease. Vet Clin Equine, 19:791-806
- R. Tandon, E.T Lyons, S.C Tolliver, R.M. Kaplan (2005) Effect of moxidectin selection on the genetic variation within Cyliccyclus nassatus based on amplified fragment length polymorphism (AFLP) Int. J. for Parasitology 35: 813-819
Dr Carl Eden BVM&S MRCVS