Gastrointestinal Parasites in Adult Horses
By Nora Grenager, VMD, DACVIM
Over the past ten to fifteen 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. 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 or four main types of dewormer drugs. Unfortunately diatomaceous earth is ineffective in killing gastrointestinal parasites in horses. 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? Horses have evolved with parasites over millions of years and it is okay for a horse to have some parasites; actually it probably keeps their immune system active. Our goal of deworming programs is not to completely eliminate parasites. While often small worm burdens (i.e., just a few worms) cause no clinical signs, when there is a large burden these parasites steal nutrition from the horse. Gastrointestinal parasites can cause weight loss or poor growth, rough hair coat, poor performance, tail rubbing, colic, diarrhea, and decreased ability to fight other infections or respond to treatment for illness.
The main gastrointestinal parasites of concern in adult horses (over 18 months of age) include: small strongyles, large strongyles, tapeworms, bots, and pinworms. This article will focus on adult horses, as younger horses have deworming needs that are distinct from those of adult horses. Each type of parasite has its own life cycle, but there are some similarities among them. Generally speaking, grazing horses ingest parasitic larvae that then (sometimes after extensive migration or hibernation that can cause significant damage) 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. Pasture conditions, environmental temperature and humidity all affect how long this takes and parasites in our part of California typically prefer late winter/early spring conditions when it is wet and getting warmer.
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 well-eradicated by the frequent deworming protocols used for the last several decades. Large strongyle larvae penetrate the lining of the small intestine and migrate throughout the body along blood vessels, causing hemorrhage and inflammation in the liver, pancreas, or abdomen before eventually taking up residence and reproducing in the large intestine. The migration of these large strongyles can cause organ damage or blood vessel damage with subsequent intestinal infarction and necrosis, peritonitis, and severe colic signs; the only way to diagnose this condition is at surgery or after death.
There are over forty different species of the small strongyles or cyathostomes. Small strongyle larvae invade the large intestine and encyst (form a capsule in the intestinal wall), where they can safely hibernate for up to 2.5 years, a process called “arrested development,” until it is time to mature and then emerge. The larvae will often remain encysted during the hot, dry summer months or cold winter months during which they would have a difficult time surviving on the pasture. They are able to detect when environmental conditions will be favorable for their propagation and emerge at that time. When they 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.
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 intestinal lining, preferentially at the ileocecal valve between the small and large intestine, and complete their life cycle there. This can cause colic due to spasm of the intestine, blockage of the intestine, or telescoping of the intestine at the ileocecal valve (called “intussusception”). Diagnosis is based on finding eggs on fecal examination (which is not very sensitive) or serum antibody testing (which is difficult to interpret).
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. While these are very common, they rarely cause notable clinical signs.
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 inherent (likely at least partially genetic) variable susceptibility to gastrointestinal parasites. It is said that 20% of horses shed most of the parasite eggs, and 80% of horses shed very few eggs. Therefore most horses do not need to be dewormed very frequently, while 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 some horses too often and other horses not often enough.
Therefore, it would be ideal to only deworm horses that have too many parasites. So how do we determine which horses have too many parasites? Currently, our best option is the fecal egg count (FEC), which is relatively inexpensive and easy to perform but unfortunately this method of detecting parasite numbers is not very sensitive. 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 150 eggs per gram), moderate shedders (usually 150-500 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 AND there can be variation in the amount of eggs passed between piles of manure. 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 it is not particularly helpful in diagnosing tapeworms in a single horse (better at telling if there is an issue within a herd). So while the FEC is not very sensitive, it is the best test routinely available to us to determine a horse’s parasite burden. There’s a cool new smartphone app that should be available soon to do FECs stall-side for less cost and faster results.
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.
The good news is that research has shown that if a FEC is performed long enough after the last dewormer was administered so that no more dewormer is present in the horse’s system (i.e., 4–5 weeks for pyrantel/fenbendazole/oxibendazole, 6–8 weeks for ivermectin, and 10–12 weeks for moxidectin) then the results stay consistent over time. Once a horse has shown itself to be a non- or low-shedder, this will remain true unless something change’s significantly in the horse’s health (such as the horse developing equine Cushing’s syndrome or recovering from serious systemic disease). Therefore, after one or two negative or low-shedder FEC, horses in these categories no longer need to have FEC performed unless there are suspicious clinical signs or something changes in their condition. This ideal single sample would also be taken in the late winter/early spring here in CA, when pastures are wet, because parasites here do not enjoy the hot, dry summers.
The ideal deworming program then would focus on each individual horse’s parasite susceptibility and would determine if there is any parasite resistance to dewormers on a farm/facility. Resistance to dewormers 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. Once resistance to a dewormer has been documented at a facility, that resistance will remain.
To slow the spread of resistance to dewormers, some parasites need to be kept “in refugia.” These parasite are those that are not exposed to a dewormer at the time of treatment, either because they are encysted, on the pasture, or in horses not dewormed. So, if non- or low-shedder horses are dewormed less frequently, they would then likely shed some eggs (of parasites that have not been exposed to dewormer) into the environment, thus diluting out any possibly resistant parasites. Parasites in refugia are not under selection pressure to develop resistance, and therefore are maintained as a relatively “resistance-free” population.This concept of keeping some parasites around that have not ever been exposed to dewormer is key in the prevention of resistance.
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. Manure should also only be spread on a pasture if it’s been appropriately composted first to kill parasites.
Daily manure removal from pastures may be easier said than done, but is the most effective way to prevent pasture contamination. 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, camelids) is another highly effective way to minimize parasite contamination, but this is not often practical. Lastly, using feeders in pastures/paddocks so horses are not eating off the ground and ingesting parasites (also good for sand control!).
In Denmark 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 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. Some may opt to only deworm those horses with higher egg counts, some may opt to use a particular dewormer once or twice a year to kill any migrating large strongyles, then the rest of the year only deworm those horses with higher egg counts. The FECRT should be performed as needed to evaluate for any emerging resistance. Regardless of the program chosen, it is clear that the time has come to think differently about deworming. If we do not, the resistant parasites will eventually force our hand as resistance is being documented with increasing frequency around the country and world.
In general, at Steinbeck Peninsula Equine Clinics we still recommend twice yearly deworming at the time of spring/fall vaccines. In the fall we usually recommend ivermectin or pyrantel or fenbendazole, while in the spring we recommend using a product that will kill tapeworms such as Quest Plus, Zimectrin Gold, or Equimax Plus. Each horse should have a FEC done at least once, ideally in the spring and long enough after the last dewormer. Those horses that are non- or low-shedders do not need to have FEC done again, and can be dewormed just twice a year at spring/fall. Horses who are moderate- or high-shedders should have FEC done summer and winter (so 3 months after the spring and fall deworming times) to see if they have developed worm burdens that would necessitate deworming. Additional FEC can be done at spring/fall to see “where they are” but these horses should be dewormed at those times regardless of the results. Lastly, facilities need to consider doing FECRT on a handful of horses that have parasite burdens in order to start to look for resistance to dewormers in our area.
Incorporating these newer strategies into your deworming program will help create a bespoke deworming protocol for each horse leading to decreased administration of dewormer in most horses (80%) and therefore financial savings in the long run. Most horses will require less deworming treatments each year while those that would benefit from additional treatments (approximately 20% of horses) will be identified, leading to their improved health.