The three toughest deworming questions for most horse owners are:
How often should I deworm'
Must I rotate drugs'
Do brands matter'
These questions apply to every deworming schedule, whether you use pastes - or purge-dewormers or feed daily dewormers.
No one deworming drug gets every equine parasite - not even the heavy-hitters moxidectin and ivermectin - so you’ve got to rotate in other drugs. And it’s the drugs you care about, not the brand names.
To maximize your deworming dollar, decide which drug you want to base your deworming program around: ivermectin, moxidectin, pyrantel tartrate (daily dewormer) or a drug-rotation system. You then need to decide which additional drug(s) you need. Time your schedule on whether your horse is in a high-risk or low-risk situation, and use the add-in drugs as appropriate seasonally.
High vs. Low-Risk Schedules
If your horse isn’t in a high-risk situation, you can stretch out the intervals between dewormings. Horses in low-risk situations are those rarely exposed to other horses’ manure. They’re turned out in well-maintained fields and have a low density of resident horses per acre. If your horse is out with lots of horses or is frequently exposed to manure from other horses, such as at a show, he’s at high risk.
We based our schedules on how long you can expect various drugs to keep egg counts and adult populations of worms low. High-risk horses will need the more-frequent timing recommendations (such as six weeks with ivermectin), while horses in low-risk areas can go longer between dewormings (such as eight weeks with ivermectin).
Ivermectin doesn’t cause dewormer resistance - meaning worms won’t adapt to it and become immune to its killing effects - and targets more parasites than any other drug. The only major parasites it won’t get are tapeworms and encysted small strongyles, although these may not be concerns for your horse.
With ivermectin, you deworm adult horses every six to eight weeks, year-round. Foals through yearlings are dewormed every three to four weeks. If your horse is also on pasture, consider a yearly or twice-a-year deworming with double-dose pyrantel pamoate or praziquantal to target tapeworms. This tape deworming would be substituted for one or two of the ivermectin dewormings.
Time it at the end of the grazing season and, if you need it twice a year, also at the beginning of the grazing season.
Moxidectin is used a little less frequently than ivermectin, at an eight-to-12-week schedule. Foals that are four months old through yearling are dewormed every four to eight weeks. Moxidectin is not recommended for foals under four months of age. Like ivermectin, moxidectin eradicates a large spectrum of parasites and doesn’t cause resistance problems. You’ll still need to deworm for tapeworms, just as described with ivermectin, but moxidectin will kill most encysted small strongyles.
The daily-dewormer drug pyrantel tartrate isn’t as comprehensive as ivermectin and moxidectin. Therefore, any horse on a daily dewormer whose condition suggests parasitism - rough coat, inability to hold weight, general unthriftiness - should have a fecal egg count checked.
As a matter of fact, before you begin a daily dewormer schedule, we recommend treatment with an agent with good larvicidal activity, such as moxidectin, ivermectin or a five-day, double-dose fenbendazole schedule. The fenbendazole is the most effective choice of the three.
Once on daily pyrantel, deworming with moxidectin or ivermectin is still necessary at least once a year to remove bots. Horses who graze may also require a separate deworming for tapes, as with ivermectin and moxidectin. You must be careful with daily dewormers not to miss a dose. Used improperly, under-dosing or skipping a day may negate its benefits. In addition, studies show that daily dewormers may not be effective enough for foals.
Some people like to rotate in a few less-expensive drugs over the year to save a few dollars. This is fine, if you really want it, but you’ll need to consider dewormer resistance and that these drugs aren’t as broad-spectrum as ivermectin and moxidectin. Most significantly, they won’t get bots.
Also, the pyrantels, oxibendazole, oxfendazole, piperazine and fenbendazole require deworming adult horses every four weeks and foals to yearlings every two weeks.
Resistance to drugs in the benzimidazole family develops fairly rapidly. A study at North Carolina State showed oxibendazone resistance developed just as fast when dewormers were rotated as when they were not. Resistance to pyrantel in small strongyles is also increasing.
So far, no equine parasite resistance problems have been found with moxidectin or ivermectin. However, studies in cattle are showing some parasites becoming ivermectin-resistant. Furthermore, a study published in the Veterinary Record revealed that moxidectin and ivermectin have similar mechanisms of action and that parasites resistant to ivermectin were also resistant to moxidectin.
Many different schedules have been worked out for rotation systems. One approach is to alternate ivermectin or moxidectin with a pyrantel pamoate or one of the benzimidazoles.
Another choice is to use ivermectin or moxidectin immediately before, during and for one treatment after the grazing season, and to use one or more of the other dewormers at low-risk times of the year, such as winter. If tapeworms may be a problem for your horse, you’ll want to double one of the pyrantel pamoate doses for better effectiveness.
For us, ivermectin is the king of equine dewormers in terms of simplicity, safety and efficacy. We don't think rotating in cheaper dewormers over the year saves enough money to warrant the fiddling and possibly developing resistance problems.
Daily deworming remains a viable option, but you are still going to need to rotate in at least the drugs ivermectin or moxidectin.
Also With This Article
Click here to view ”Simple Deworming.”
Click here to view ”Same Drug, Different Name.”
Click here to view ”The Small Strongyles.”
Click here to view ”Horse Journal Recommended Deworming Schedule.”
Click here to view ”Comparing Brands.”
Click here to view ”Dewormer Drug Active Ingredients.”