Anthelmintic resistance in horses has been reported since the 1970’s, mainly to the benzimidazole group of drugs. This has led to less use of that drug group which naturally has meant more use of ivermectin based products and more recently Moxedectin based products. The other main group (Pyrantel) is traditionally used for both routine worming but more so for tapeworm treatment and then at twice the double dose. The resistance we have seen in recent years is when treating for Roundworms (nematodes). It is thought that genes for pyrantel/ivermectin resistance are naturally rare in nematodes of equines and therefore resistance should be slow to develop. However, the advent of drugs that do kill the encysted stages (such as moxidectin) is decreasing the worms in “refugia” and thus potentially increasing the likelihood of resistance and how soon that resistance may arrive in the UK horse population. As there are no current plans for introducing new classes of anthelmintic (worming) products we have to be vigilant in the correct use of the existing products on the market and to adopt animal health plans that take all circumstances into account including the use of faecal egg counts and strategic worming.
At the agricultural level work is being done to limit the resistance issue in sheep and cattle and it is from these species that we can learn how resistance develops and how to avoid the situation becoming so significant in the equine population. A new approach is required to ensure that with regard to treatment of our horses, products remain efficacious to thereby avoiding compromising equine welfare.
So what can we do to avoid resistance developing?
Appropriately timed treatments: Use the advised dosage intervals and note that these intervals differ between active ingredients.
Pasture Management: Minimize pasture contamination by picking up droppings, graze with other species such as sheep or cattle, treat new horses on arrival and quarantine them. Harrow pastures when conditions allow such as during spells of hot and dry weather.
Correct Dosing: It is vital to dose at the correct rate for the weight of your horse. The best method is on a weigh bridge but failing that, the use of industry leading weigh tape or weight estimation formulas are of value. Under dosing is one of the main reasons for the development of resistance and remember we all lie about our weight so if you do estimate the weight think about adding an extra 10% on top of your estimation. If you administer a sub therapeutic level of the drug and thereby expose the worms to the drug, but perhaps not at a sufficient dosage to kill them, worms that survive treatment may pass on their “immunity” to subsequent generations and those generations will become more adept at surviving chemical treatments, with the potential for resistance to develop to that drug.
Use faecal worm egg counts: Monitor the parasite burden through the warmer months by testing and when you get the results consult an expert on how to read the results and how to decide on a strategy. Most of the worms live within a few susceptible animals so try to identify those animals by a risk assessment of the burden in each horse. This approach can then lead to targeted worming and treating only those animals that actually have a parasite burden. Also consider testing for tapeworm with a saliva antibody test.
Consider testing for tapeworm by using saliva samples: This test identifies horses with a low burden, a borderline result or a moderate/high burden. Treatment is recommended for any borderline or moderate/high results. In scientific terms the Tapeworm Test has both high sensitivity and specificity, which is important for correctly identifying horses with tapeworm burdens.
Rotate the active ingredients: The jury is out on this one but there is a consensus that rotation has a role to play on basis of rotating the selected products in the grazing season only, or perhaps a 1 or 2 year scheme. However, changing active ingredient each time you worm could well have the opposite effect and increase resistance as each different generation of parasite get exposed to different drug classes.
Selecting ingredients that actually work: When you do decide to use a wormer use one that is known to be effective against the particular parasite you are targeting. If you are using products where resistance is suspected check on the efficacy by using a Faecal egg count reduction (FECR).
Select the right ingredient for the target parasite: Although many different ingredients treat a broad spectrum of worms and parasites some treat for more specific burdens, some only treat for a single issue such as tapeworm and some treat parasites at different times in the life cycle. Try to understand the main threats and choose a product accordingly.
Understand the life cycle of the parasite: Different worms have differing life cycles which is why we have altered approaches with regard to when we actually treat for them, rather than a blanket plan of treating at certain times of year. A seasonal approach is fine if these life cycles and previous treatments are factored into the equation. The worms are not in the host for very long as for most of the cycle they are free living infective larvae on the pasture which is the reason for pasture management as part of the animal health plan.
Understand Refugia: Refugia could be described as those worms which are not exposed to a drug when treatment is carried out - either because they are on the pasture, or because they are in horses that have not been treated. The refugia provide a resource of drug susceptible parasites which in a way will dilute those parasites that have built up resistance to certain chemicals. These provide a dilution effect slowing the development of resistance in the worm population. By maintaining a group of parasites that are not exposed or have reduced exposure to an anthelmintic (worming chemical), horse pasture management will be improved.
Develop an Animal Health Plan: By recording all wormer treatments and egg counts as well as basic individual horse details such as age, weight etc. develop a plan to suit each individual horse.
When in doubt ask: Worming horses does not have to be complicated but the more we understand, the better equipped we are to treat correctly. If your horse has any health problems then the first point of call is your vet. Additionally in the UK we are lucky to have over 5000 SQPs who are specially trained and qualified to advise and prescribe anthelmintics (wormers). Many of those are specialists in the equine world and as such undergo continuous professional development in the field of parasitology. They are on hand instore and online to help with advice and hopefully explain why we need a new approach in the field.