Deworming Programs– One Size Does Not Fit All

By Nora Grenager VMD, DACVIM

images-2Over the past several years, there has been a huge shift in the equine veterinary community’s thoughts on parasite prevention. It is difficult to pick up a horse magazine or browse an equine website these days without reading about parasite resistance. Parasite resistance to dewormers is increasing throughout the world, which is particularly concerning given the lack of new types of dewormers available. While there are many brand names, there are really only three main types of dewormer drugs, so we need to slow emerging resistance to them as much as possible. Parasitologists say it is not a matter of when, but rather how rapidly, parasite resistance will develop in any given region. The silver lining of this impending resistance is that we are now focusing our attention on more strategic deworming programs, rather than rigid regimens that do not take into account an individual horse’s needs. Most horses do not need to be dewormed as frequently as previously thought. It makes much more sense, from both a horse and herd perspective, to tailor the deworming program to each horse.

Why do we deworm at all? Gastrointestinal parasites can cause weight loss or poor growth, rough hair coat, poor performance, colic, and diarrhea. In addition, they can cause decreased resistance to illness or treatment of illness, and decreased response to vaccinations. The main gastrointestinal parasites of concern in adult horses include: small strongyles, large strongyles, tapeworms, bots, and pinworms. This article will focus on adult horses over 18 months of age, as younger horses have deworming needs that are distinct from those of adult horses. Each type of parasite has it’s own life cycle, but there are some similarities among them. Generally speaking, grazing horses ingest parasitic larvae that then (most often after extensive migration or hibernation) develop into adult worms within the gastrointestinal tract. These adult worms reproduce and the horses pass eggs in the manure so the cycle can begin again. In the case of large and small strongyles, it is the larval stages that cause the most damage and clinical signs of parasitism, and the adult worms generally cause minimal problems.

Strongyle egg

Strongyle egg

There are three main types of large strongyles: Strongylus vulgaris, Strongylus edentatus, and Strongylus equinus. These large strongyles, also called “blood worms,” used to be the primary targets of deworming programs, but for the most part have been welleradicated by the frequent deworming protocol that has been in use since the 1960s. Large strongyle larvae penetrate the lining of the small intestine and migrate throughout the abdomen before eventually taking up residence and becoming reproducing adults in the large intestine. The damage of the migrating large strongyles can be devastating – they cause damage or aneurysms of the large arteries that supply the intestines, and can cause severe infarction colics. Alternatively the migrating larvae may damage the liver, pancreas, or other organs through which they migrate. Deworming every other month, a program that was developed in the 1960s and is still in use today, was created to eliminate large strongyles and the often fatal colics they caused. That program was very effective at eliminating large strongyles, as we rarely see problems associated with them in horses that are regularly dewormed today.

There are over forty different species of the small strongyles, or Cyathostome sp. Small strongyle larvae invade the large intestine and encyst (form a capsule in the intestinal wall just under the lining), where they can safely hibernate for months to years, a process called “arrested development,” until external environmental conditions are right for them to emerge. The larvae will often remain encysted during the hot, dry summer months (or very cold winter months) during which they would have a difficult time surviving on the pasture. When the mature larvae eventually emerge they can cause a large amount of inflammation within the intestinal wall, causing diarrhea of variable severity or colic. Diagnosis of large or small strongyle infections is dependent on finding the eggs in manure. However, during migration or arrested development there may be few to no eggs in the manure because it is larvae, not egg-producing adults, that are present in the horse and causing a problem. These small strongyles are the target of most current deworming programs, and are of particular concern because they are starting to show resistance to dewormers.



Tapeworms, or Anoplocephala perfoliata, have a more complicated indirect life cycle involving an intermediate host. Some larval stages of the tapeworm’s development take place in oribatid mites, which are prevalent on many pastures. Horses are infected by eating the tiny infected mites while grazing. Once ingested, the tapeworms attach to the region between the small and large intestine (the ileocecal valve) and complete their life cycle there. This causes inflammation at the site, with signs of colic due to spasm of the intestine, blockage of the intestine, or abnormal motility with telescoping of the intestine at the ileocecal valve (called an “intussusception”). Diagnosis is based on finding eggs on fecal examination (which is not very sensitive) or serum antibody testing (which has not yet been well validated).

There are two main types of horse botflies: Gasterophilus nasalis and Gasterophilus intestinalis. The adult botflies lay eggs in a horse’s hair coat that the horse then ingests during routine grooming. The eggs develop into larvae as they migrate to the stomach, then attach to the horse’s stomach where they can cause gastric ulceration or colic.

Pinworms, or Oxyuris equi, can live in the large intestine and small colon and cause colic or anal pruritis (an itchy hind end as evidenced by tail-rubbing). Most often these infections are not diagnosed on a fecal sample. Instead, a piece of tape is used to collect and then identify eggs from around the anus.

One of the most important concepts about parasite control is that each horse has variable susceptibility to gastrointestinal parasites. It has been shown that approximately 20% of horses shed most of the parasite eggs, and approximately 80% of horses shed very few eggs. Each horse has a genetic inherent susceptibility to parasitic infection, which generally remains the same throughout the horse’s lifetime. Therefore most horses do not need to be dewormed very frequently, and only a few horses need to be dewormed more often. This difference in an individual horse’s susceptibility to parasites underscores the fact that if we use one deworming protocol for all horses, we are deworming many horses too often and other horses not often enough.

The new approach to parasite control has several steps. First, determine each individual horse’s susceptibility to parasites by doing fecal egg counts on every horse – i.e. is the horse a low, medium, or high egg shedder. Second, determine if there is any parasite resistance to dewormers present on each farm or at each stable by looking at a representative sample of horses from each farm (if a big farm do 6-10 of the adult horses). Third, create and monitor a parasite control program for each horse based on that horse’s FEC and any known resistance, utilizing local weather and housing conditions, and considering additional management strategies.

Equine Parasitesc

Strongyle egg on left, tapeworm egg on right

How do we determine each horse’s individual susceptibility to parasites? Our methods of detecting parasite numbers are unfortunately not very sensitive. Fecal egg counts (FEC) are most often used and are relatively inexpensive, certainly noninvasive, and are easy to perform. The procedure involves floating a manure sample and counting the number of eggs seen. Horses are then categorized as non-shedders (no eggs seen), low shedders (usually less than 200 eggs per gram), moderate shedders (usually 200-500 eggs per gram), or high shedders (usually greater than 500 eggs per gram). In adult horses, this technique mostly counts large or small strongyle eggs, and sometimes tapeworm eggs. This is the most reliable method we have to noninvasively determine parasite burdens, but it has several limitations. First, as discussed above, it is often the larval stage of the parasite that causes a problem, and larvae are not sexually mature and therefore not making eggs. Second, the number of eggs does not always correlate very well with the number of adult parasites. Third, if the sample is too old or sits too long before evaluation, the egg counts can be falsely low. There are blood tests available to help diagnose tapeworm infections, but the results are difficult to interpret because the test does not distinguish well between active and previous infection. So while the fecal egg count is not very sensitive, it is the best test routinely available to us to determine a horse’s parasite burden. Parasitologists and veterinarians have done extensive work to make it as useful as possible in creating an evidence-based parasite control program.

Equine ParasitesdNext it is necessary to determine if there is any parasite resistance to dewormers on each farm. This can be determined by performing the Fecal Egg Count Reduction Test (FECRT). The FECRT involves doing a pre-treatment fecal egg count, administering a dewormer if the horse is a moderate or high shedder, and then rechecking the fecal egg count on those horses that were dewormed in 10-14 days. This provides information as to whether the horse is a low, moderate, or high shedder of eggs, and then also provides information as to whether the dewormer administered is efficacious or whether there is resistance to that dewormer at that stable.

The benefits of evidence-based deworming based on FECs and FECRTs are that only horses who need to be dewormed would be administered a dewormer. Horses that are low or non-shedders would not be treated with dewormer as frequently, thus Strongyle egg on left, tapeworm egg on right decreasing the amount of chemicals administered to horses and decreasing cost in the long run.

It is important to remember that the goal of a parasite control program is not to completely eliminate all parasites. Horses and parasites have lived together for a long time, and it is okay for horses to have exposure to parasites. No dewormer, and no deworming program, is designed or expected to eliminate 100% of parasites. The idea is to minimize the negative effects of parasites by preventing large parasitic infections. There is also an important concept called “in refugia,” which is the idea of keeping some parasites around that have not ever been exposed to dewormer. Refugia is key in the prevention of resistance, because the nonresistant parasites (that have never “seen” a dewormer) will help dilute out any resistance parasites.

Parasite control should not be all about deworming medications. Additional strategies should be considered to help minimize parasite exposure. Larvae can live for weeks in a pile of manure, and are easily disseminated when the manure gets spread out, thus increasing pasture contamination. For this reason it is also important not to drag or harrow a pasture while horses are grazing on it. It is important to not put horses on a pasture for 2-3 weeks after harrowing, and pastures should never been harrowed in the winter or wet months.

Equine ParasiteseDaily manure removal from pastures may be easier said than done, but is the most effective way to prevent pasture contamination. Also, keeping the number of horses per paddock or pasture to an appropriate level will help decrease parasite loads. Horses who live in stalls or dry paddocks are therefore at lower risk for parasite exposure. Manure should not be spread in a pasture unless it has been composted for at least two weeks, or unless the pasture will be ungrazed for at least two weeks in the hot, dry summer. Sharing pastures with livestock (cattle, sheep, goats, camellids) is a highly effective way to minimize parasite contamination, but this is not often practical.

With all this information in mind, there are many new ways to approach evidence based parasite control. In Denmark, for example, dewormers can only be purchased from veterinarians, and are only prescribed if a horse has evidence of a significant parasitic infection. This was implemented in 1996 in response to rapidly emerging parasite resistance, and is the most dramatic example of developing a dewormer program with your veterinarian’s advice. As we are slowly starting to incorporate more of these ideas into deworming practices here in the United States, each owner and veterinarian will need to work together to determine what is the best program for his/her horse.

As discussed, a deworming protocol will need to be designed for each horse and barn. A general version of Steinbeck Equine’s new plan is as follows. Determine each horse’s individual parasite susceptibility and look for any resistance on each farm as discussed above. Then, it makes sense to maintain the eradication of large strongyles, as no one wants to see the return of those devastating colics caused by large strongyles. Therefore twice yearly, in spring and fall at the time of vaccination, horses would be administered either ivermectin or moxidectin, and also use praziquantel at one of those times. Then, 2-3 months after the fall deworming, horses who have been shown (on previous fecal exams) to be medium or high shedders would have FEC done and a dewormer would be administered to them if they had FEC >200 epg. Horses that were high shedders at that time would have one more FEC performed another 2-3 months after that dewormer, prior to spring vaccines if needed. In areas where the summer is hot and dry, no additional deworming is needed during the summer months between spring and fall vaccines. Please click here to see an informational page about how and when to collect fecal samples, or contact us at 831-455-1808.

Regardless of the program chosen, it is clear that the time has come to think differently about deworming. It is not only better to have a unique plan for each horse, but in the long run will save time and money. If we do not, the resistant parasites will eventually force our hand.

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