Resistance to dewormers is extensive and officially includes almost all deworming drugs except ivermectin and moxidectin. However, with isolated reports of possible ivermectin resistance beginning to appear in horses, as they have in cattle for many years, we’re skeptical about no ivermectin resistance.
For resistance to develop, the parasite must be exposed to the drug one or more times. For this reason, many experts now advise that healthy adults should not be dewormed on a regular schedule of every X number of weeks.
Simply rotating deworming drugs on schedule doesn’t stop resistance from building. You have to break away from the calendar-based rotation deworming schedules. By doing so, you’ll avoid unnecessarily exposing the parasites to the dewormers, which would increase the likelihood of resistance.
Instead of using a calendar, the best method is to use fecal egg counts at regular intervals, with only the horses showing moderate or heavy burdens being treated. We realize this is a bit of an annoyance, but it will pay off if your horse truly has a parasite burden. You’ll get the right drug administered at the correct time.
However, you may also determine that your healthy adult horse doesn’t need frequent dewormings. That’s the most likely reason why many natural dewormers get away with making claims that they work the horse simply didn’t need to be dewormed. A horse in a low-risk situation (on generous pasture, without coming into contact with new horses) may be as well off with minimal deworming treatments. Some parasite experts advise that horses with only light parasite burdens and no symptoms suggestive of a parasite problem should not be treated at all.
Because resistance is so widespread, it can cause problems for the moderately or heavily wormy horses that need to be dewormed. If your program includes drugs other than ivermectin or moxidectin, or if results after ivermectin or moxidectin aren’t as expected, check a fecal egg count two weeks after deworming. If the horse is still positive, change drugs.
When we talk about horses that need to be dewormed, we’re emphasizing high-risk horses. These are the horses where routine deworming schedules, according to dewormer package instructions, can’t get the job done.
The usual calendar-based recommended intervals for dewormings are:
Ivermectin products ??? every 8 weeks.
Moxidectin products ??? every 12 weeks.
Most others - every 4 weeks.
These intervals are designed to keep numbers of egg laying adults to a minimum in mature, adult horses. The method does work, but it’s often the immature stages of the parasites that are doing as much, or more, damage and the drugs are targeting adults. When dealing with heavily parasitized horses, or horses with poor immunity to parasites that rapidly become reinfected, this schedule may not be enough.
Foals have a poor immunity to parasites for about their first year of life. Immunity to parasites is mucosally based, meaning that the immune system cells in the intestine must learn to recognize the parasite antigens and mount a local immune response.
Unfortunately, the magnitude of exposure to parasites often gets ahead of their attempts to build immunity, and the general unthriftiness of parasitized foals further weakens their immune system. Studies have also sometimes found deworming drugs aren’t as effective even in eliminating adult egg-laying stages in foals. This is interpreted as meaning that a good immune response is also needed, and the drugs alone can’t give 100% control of parasites.
Senior horses are another high-risk group. Waning of the immune response commonly occurs with age. Borderline nutritional intakes related to chewing problems or poor absorption from the intestinal tract can also weaken the immune system.
It’s not uncommon for these seniors to pick up burdens of parasites not normally seen in adult horses, like roundworms. For this reason, seniors may do better on a deworming routine more geared to foals than that of younger adults.
Also at higher risk are horses on immunosuppressive drugs (e.g., corticosteroids for heaves or allergies) or those battling a serious illness, infection or injury. Heavy exercise, shipping the horse long distances, moves, even the loss of a companion are all periods of high stress that can negatively impact the immune system.
Even healthy adults with a robust immune response can be overwhelmed if their exposure to parasite larvae is high. If you board at a facility that doesn’t isolate new horses until their fecal egg counts are confirmed to be safe, and new horses use the same paddocks as others, or are turned out with them, your horse is at high risk.
Other high-risk situations are horses turned out on small fields crowded with other horses and horses that travel away from home frequently. Show grounds, camp sites, popular trails, even ship-in stalls at racetracks, have heavy traffic and can easily become heavily contaminated with parasite larvae. Healthy adults in these situations don’t necessarily need a modification of deworming, but they should at least be checked more frequently by fecals.
Foals to Yearling
•Deworm mares within 12 hours of foaling, if possible. This will prevent the foal being infected by Strongyloides larvae that have been lying dormant in the dam’s tissues but mobilize at foaling to turn up in the milk after the colostrum is gone.
• Never rely on daily dewormer products in horses under one year of age. They are excellent for tapeworms, but they don’t always kill a sufficient percentage of other types of parasites to provide good protection for this age group. Resistance is also a problem, and the daily dewormers themselves likely eventually induce resistance to the pyrantel in these products.
• Moxidectin should not be used in foals under six months. For horses six months and older, do not use it unless you have an accurate body weight.
• Timing of the first deworming depends to some extent on the drugs being used and how heavy the exposure risk is. Discuss this with your vet. The first deworming can be as early as four to six weeks of age in high-exposure situations and should never be past the eight-week mark.
• Always deworm the mare and the foal at the same time.
• Start by deworming foals at intervals no longer than the recommendations on the last dewormer used.
• Be alert to warning signs of parasitism (see sidebar ). At the first sign of a problem, either institute fecal checks two weeks after deworming with each of the drugs used, to see if you have a resistance problem, or switch over to deworming only with ivermectin and talk to your vet about using shorter intervals between deworming until the problem is controlled. Because it is often the immature forms that are causing as much, or more, trouble as the adults, deworming at four to six weeks with ivermectin, to get these forms as they become susceptible, may be advisable until you are on top of the problem.
• Deworm for tapes at the end of their first season on pasture.
Deworming is basically the same for older and ill horses as it is for foals (see section above).
• Begin with regularly scheduled dewormings, according to the interval recommended for the last used deworming product.
• Combining a daily dewormer with regularly scheduled paste dewormings may benefit this group, but before trying it do fecals before and after a deworming with Strongid paste, and at yearly intervals to make sure you are not getting resistance problems.
• Again, be alert to the signs of a parasite problem and don’t delay in acting on them.
Healthy Young and Middle-Aged Adults at High Exposure risk
The simplest approach is to use only ivermectin or moxidectin, at manufacturer’s suggested intervals, with tape dewormings at the beginning and end of the grazing season.
• If using drugs other than ivermectin or moxidectin, check a fecal egg count two weeks after deworming with those products at least once a year.
• If high exposure is only part of the year, use regularly scheduled dewormings during high risk, fecals to determine need at other times of the year.
Healthy Low-Risk Adults
This is the group where routine dewormings on a schedule are least likely to be needed and most likely to cause resistance problems.
• If the horse is on turnout, consider routine timed dewormings only during the grazing season, checking fecal egg counts once during the winter to see if additional deworming is required. The last deworming of the season should target bots and tapes.
• Alternatively, the horse may be dewormed with a comprehensive product, including bots (not detectable on fecal) and tapeworms twice a year, with fecal checks in between to see if any additional treatment is needed.
The ready availability of dewormers directly to owners is convenient and economical, but your vet is still your best source of advice on the deworming program that is right for your own situation.
If your horse is low-risk (not under stress, has a healthy immune system, doesn’t travel and isn’t on a farm with lots of new horses in and out), you’re probably better off relying on fecal counts to determine your deworming schedule. However, if this is just too much of a nuisance for you, back off a bit and deworm less frequently. Note: Bots and tapeworms aren’t easily detected on fecal exams and require specific dewormers.
Until research tells us otherwise, stick with ivermectin or moxidectin, as they have the lowest likelihood of resistance developing, and both of these dewormers also target bots. For tapes, you’ll need to use double-dose pyrantel pamoate or a drug containing praziquantel. We recommend twice-yearly tape deworming schedules. In fact, for some horses, twice-a-year with a broad spectrum dewormer that targets bots and tapes may be sufficient.
If your horse is high-risk, by all means, get the veterinarian involved until you have the parasite problem under control. It’s foolish to keep throwing dewormers at this horse without knowing what types of pests you need to target.
If the horse is heavily parasitized, and hasn’t been dewormed in a while or you don’t know its deworming history, you may want your vet to pre- and post-medicate the horse with Banamine and/or an antihistamine. This will help reduce the risk of adverse reactions to the large parasite die-offs.
In choosing a brand of deworming drug, check the package labels carefully to ensure that the actual active ingredients of the drugs and amounts of those drugs in the specific products are the same. If they are, let price be your guide in choosing among brands.
Try to get an accurate measurement of your horse’s weight. Underdosing your horse is a waste of time and money, as it won’t work effectively. The option of "more is better" is not always wise, especially if you’re using moxidectin, which has a lower safety zone than other dewormers. Follow label instructions on all drugs, and get a weight-tape if you’re not good at estimating weight.