Deworming has always been a vital part of your horse’s health care plan. Recently, the methodology by which we have carried out this plan has been questioned. Questions like, are we deworming our horses too often? Are we creating resistant populations of internal parasites? Is that dewormer you just gave your horse doing any good? What if I told you that the majority of horses may only need to be dewormed 1-2 times a year and only a few horses’ need to be dewormed 4-6 times a year? These questions are being asked as researchers are finding more and more parasite resistance to the dewormers we use and the way we use them. At the latest convention for the American Association of Equine Practioners held in Las Vegas last December, researchers shared their findings regarding this topic. The researchers reviewed the problem of increased resistance to dewormers by parasites and recommended new ideas for internal parasite control in the horse. This discussion will hinge on their findings and on how they can be incorporated into a sensible deworming strategy to control equine internal parasites, reduce the amount of resistant strains, and prevent further resistance problems.

The major factor that has led to an increase in internal parasites becoming resistance to the dewormers has been the use of the rotational deworming schedule. The current recommendation of a rotational deworming schedule where a different class of dewormer is administered at intervals has not slowed the progression of the equine internal parasites’ resistance but in fact may have sped up the development of resistance in these parasites. This method was first devised in 1966 when dewormers had a very narrow spectrum of effectiveness and there was a large amount of parasite load in horses. It was concluded that rotation of those dewormers would mean that once through the rotation most of the parasites would have been killed by the end of the rotation. Since then great advances have been made toward the efficacy, ease of administration and the availability of dewormers for horses. Without monitoring for the effectiveness the dewormers used, we have selected for resistant internal parasites while killing off the parasites that were sensitive, thereby, increasing the amount of resistant parasites. Resistance in other livestock species is well known and recently there has been suspicion that equine small strongyles are becoming resistant to the class of dewormers benzimidazole (BZ) (brand names: Anthelcide, Panacur, Safeguard) and pyrantel salts (PS) (brand names: Strongid, Rotectin 2, and many generics). While that may not seem so dire of consequences, one must realize that there are only three classes of broad spectrum dewormers approved for use in the horse and there are not any new drugs being developed in the near future. The last class of deworming drugs to be developed for use in the horse and the last to have resistance problems is the macrocylic lactones (ML). These were developed and marketed starting back in the early 1980’s with the introduction of ivermectin. Another drug within the class of ML is moxidectin (Quest), which became available in the 1990’s. Due to the apparent resistance problems currently seen to dewormers by equine small strongyles, the focus of deworming practices must be directed toward the controlling the 40+ species within this group of parasites, while keeping in mind the other less significant internal parasites.

To defeat an enemy, as in this case the equine small strongyle, we need to know something of the enemy and its life cycle. Small strongyles are also known as cyathostomins have a direct life cycle: 1) eggs hatch in manure piles and become larvae, 2) Larvae mature into third stage larvae and migrate onto grass leaves, 3) Horses consume the parasite with a mouthful of grass, 4) Larva migrate in the horse becoming adult worms 5) Adult worms start laying eggs within the intestinal tract of the horse, 6) The parasite eggs are passed out the horses in the feces and deposited on the ground to start the cycle over. This description is a simplified parasite life cycle that could describe most any internal parasite for many livestock species. What makes the small strongyle a unique parasite for the horse is that after ingestion, the third stage larvae penetrate the wall of the colon and cecum and become encysted. Hence, the term many of us have heard, “encysted small strongyles”. These encysted larvae can remain encysted for many months up to a couple of years, and will emerge under various conditions. The conditions under which they emerge can vary from the time of year, to the de-population of adults in the lumen of the intestine either from natural die offs or from the use of a dewormer. It is thought that the larvae will encyst in the fall of the year and emerge in the spring when the growing season starts again so as to continue their life cycle. The emergence of the encysted larvae is also seen when a dewormer is used that has activity against the adult forms only. This will cause a major parasite die-off that signals the encysted larvae to emerge to repopulate in the horse’s intestine. The damage to the horses’ intestine occurs when the encysted strongyle emerges from the lining of the intestine. The severity can be minor when small numbers emerge, with greater severity in those horses with larger amount of a parasite load. All three classes of dewormers have had some degree of effect on the adult small strongyles, while some resistant has been noted, as mentioned above. There are only two specific dewormers that are actually effective against the encysted stage. One dewormer is fenbendazole of the BZ class. These are most commonly known as Panacur and Safeguard. Horses are treated at double the normal dose for five consecutive days with this drug and it has been shown to cause a significant reduction in encysted small strongyles. Although with current resistance problems in this class of dewormers there is concern that this treatment may have reduced effectiveness in those areas that have benzimidazole resistance. The other dewormer that has shown effectiveness against the encysted small strongyle is moxidectin of the ML class of dewormers. Most commonly known as Quest, this dewormer will kill up to 90% of encysted small strongyles in a single dose.

Since the rotational method of deworming is no longer an effective way to control the parasites, what method is? That method is called a targeted or strategic deworming plan. For the sake of this discussion we call it a targeted plan. This plan has four components: it targets specific horses, at specific times of year, in specific management schemes, with a specific dewormer. How do we do this? The answer is with fecal parasite testing to determine what is called fecal egg counts (FEC). FEC is the foundation of a targeted deworming plan. This is a test to determine the amount of strongyle eggs (both large and small) in a particular amount of fecal material. This result of this test is quantified as the number of eggs per gram of feces (EPG).

There are particular egg levels to indicate the severity of infection and when the tested horse needs to be dewormed. Parasitologists have found that about 20% of the horses in a particular horse population carry the majority of the parasite load and they are considered the major contaminators of their environment. Since these horses carry a significantly higher FEC that the rest of the population, their identification is just one way to reduce the spread of eggs over pastures by targeting those horses with more frequent dewormings and testing. This is an example of targeting the specific horse for testing and treatment.

The FEC test will also give us information on which horses that may be infected with the majority of small strongyles prior to putting those horses out on pasture, as in springtime. If this is done at the proper time, encysted small strongyles that over wintered, will have emerged, matured into adults, and will be shedding eggs. Those horses tested with higher FEC will be treated prior to turn out on the pastures to prevent contamination of the pasture. In this regard we are using the FEC to target specific horses at specific times.

We can also use the FEC test in particular management schemes. There are two management schemes that we commonly encounter that differ in the requirements for parasite control. One is when horses are out on pasture and the other is a stall or dry lot situation. Horses that are on pasture for several hours a day to continuously grazing are at more risk of parasite infection than those on well maintained dry lots or those kept in stalls continuously. FEC need to be done more frequently for those horses with access to pasture so as to monitor (1) the duration of the previous dewormer class given; (2) the horses increase in parasite load; and (3) the need to deworm a specific horse to reduce further contamination of the pasture. On the other hand FEC can be done with less frequency in horses that are stalled and placed in runs and/or are in dry lot situations, because their chance of becoming reinfected with parasites are greatly reduced. In this situation we are using the FEC test to target specific horses, in specific management situations.

Another use of the FEC test can be to determine the relative effectiveness of a particular dewormer in a group of horses and to determine if resistant strains of parasites are present or not. This is done by doing a FEC on a group of horses that are to be dewormed. Then after deworming the group, another FEC is done 14 days after deworming to determine the effectiveness of that particular class of dewormer. The percent difference between the two tests will give the effectiveness of the dewormer and will tell us if there is resistance built up against that dewormer.

Along with FEC to reduce the risk of reinfection of horses and determine when to treat, certain management practices can also lower the risk of parasite infestation. These practices include feces removal form the horses’ environment, composting manure instead of spreading fresh manure on pastures, rotating pastures with other livestock species, resting pastures during the summer heat, reducing the stocking density to prevent over grazing, and not to feed hay off the ground. (This last practice will help in the prevention of ingesting sand and gravel which can cause other digestive problems.)

Another management practice to be used is to allow grazing on pastures only when the parasite is least infective. The ambient temperature at which the small strongyle eggs need to hatch and molt into the third stage larvae is between about 45°F to 85°F. Below 45°F the larvae are inhibited from hatching and can stay dormant for quite some time. Above 85°F the infective third stage larvae have limited nutrient stores and can run out of energy quite rapidly in the heat. Knowledge of these facts can help us to manage our pastures better. Harrowing or dragging pastures have been touted to help disperse fecal pile and to help eliminate parasite loads on pastures. That is true as long as it is done within the parameters of the life cycle of the small strongyle. The pastures need to be worked at ambient temperatures above 85°F and left vacant for at least two to four weeks with temperatures above 85°F to kill of any third stage larvae and to allow eggs to hatch and for the subsequent larvae to die off as well. Otherwise working the pasture at any other time will only spread more of the parasite around the entire pasture and make it more infective.

In conclusion, equine internal parasites that have developed resistance to certain classes of dewormers have been caused by an outdated deworming program and indiscriminate use of dewormers without testing for efficacy. Due to these reasons, equine small strongyles have become resistant to the effects of multiple dewormers. To prevent further resistant strains of parasites and to combat the resistant strains found in horse populations, radical changes need to be taken for the control of internal parasites of the horse. Combining a targeted deworming plan, FEC, and management changes, internal parasites can be controlled and multiple drug resistant strains of parasites kept to a minimum.