Internal parasites are a major threat to the health of your horse. They can cause external damage and you may not be aware that your horse is infected. Parasites can lower your horse's resistance, deplete valuable nutrients, cause gastrointestinal upset and may lead to colic, intestinal ruptures and death.
There are more than 150 internal parasites that affect horses, but the most damaging are strongyles and ascarids (roundworms), that sometimes cause permanent damage that can profoundly affect your horse's performance.
What To Watch For
Many horses that have dangerous parasite levels appear to be healthy, while inside the parasites are doing their damage. Signs are occasionally present, however, especially in young horses. These include:
Fortunately, horse owners have the power to prevent parasite damage. The prevention of parasitism in the horse broadly falls into 3 categories:
Many horse owners and veterinarians continue to adhere to old-fashioned and out-dated practices when it comes to minimizing parasite pathology in their horses. Deworming of horses should not be reserved for times when a horse appears to be thin or has been experiencing colic and diarrhea.
In the past, the use of specific techniques to increase the effectiveness of deworming drugs was necessary because many of these original drugs were not very effective. These specific techniques included the administration of very large quantities of a drug, which was believed to be more effective because, although most of these early drugs were not absorbed from the gastrointestinal tract, it was believed that some amount of the drug might be absorbed if a very large quantity could be given.
In order to administer these large quantities, the veterinarian had to pass a nasogastric or stomach tube and pour the required volume of deworming drug straight into the stomach through the tube. This practice was known as "tube worming" and it became very popular. At the present time, deworming drugs are extremely effective at killing parasites throughout the body and the need for "tube worming" can no longer be justified.
Another strategy involved mixing together a number of different agents in large volume and passing them into the stomach to enhance the effectiveness of each agent. Veterinarians no longer use mixtures of weak deworming drugs in parasite control programs for horses.
It should be noted that many horses are dewormed routinely at times when there may not be any parasites in their gastrointestinal tract to be killed; therefore, not all deworming activities are necessary. For maximal effect, deworming treatments should be timed and coordinated to be given at times that are likely to have the most effect at killing parasites in the horse. Simply treating adult horses on a regular (but unregulated) basis with deworming agents certainly does not compensate for poor horse management practices.
It should be remembered that the most damage inflicted by the common horse worm parasites in North America is done by the larval stages of the parasites. The adult worms in the intestine certainly produce eggs that lead to pasture contamination and infection of other horses, but adult worms do not cause any appreciable damage themselves. It makes sense that deworming programs should be designed to prevent larval damage in the horse.
Generally speaking, the tests used to determine whether a horse is currently parasitized have limitations. It should be remembered that a given horse might be very significantly parasitized but "tests" (for the presence or absence of parasites based on the presence or absence of parasite eggs) are very often negative due to long prepatent periods (several weeks). A prepatent period is the time that transpires between the ingestion of a parasite egg and the appearance of new eggs in the horse's feces. Therefore, negative fecal tests for parasites should not be interpreted to mean that there are not any larvae in the horse. Furthermore, damage inflicted by parasites may cause intestinal disturbances long after the parasites have been eliminated.
The age of your horse is important in determining the best method for preventing parasitism. Young horses are more prone to parasitism than older adults. Adult horses may have very high worm egg counts (in feces) and contain tens of thousands of worms, but will appear in better physical shape than a similarly parasitized yearling. With age, the immune system of older horses helps prevent parasite migration and the damage inflicted by parasites in the gastrointestinal tract. Different species of parasites are important for horses less than one year of age compared with those with medical significance after the first year of life.
How Do Pastures Become Infested with Parasites for Horses?
Until proven innocent, all adult horses should be considered a threat to the pastures, by passing parasite eggs in their manure. Following a deworming treatment, the adult parasites in the intestine are killed and the passage of eggs in the manure stops for a while. In the meantime, new adult worms develop from larval stages in the horse's intestinal wall and, after a delayed period, start to produce eggs again. These eggs are passed onto the grass. The eggs are NOT immediately infective for other horses; they must undergo a phase of maturation on the ground. The egg must develop into an infective 3rd-stage larva to be infective. The maturation of eggs into 3rd-stage larvae is affected by the climatic condition.
How do Climatic Conditions Affect the Development of the Parasites?
The higher the temperature, the more rapid the maturation of the egg. While colder temperatures lead to "hibernation" of the eggs, larvae which mature at higher environmental temperatures quickly die (they deplete their energy reserves). Although horse owners have traditionally believed that parasites are killed by cold winter weather, the exact opposite actually happens. The parasite eggs survive the cold weather much better than hot summer weather. Horses can be readily infected by digging through snow to graze the underlying grass. The clever horse owner can thus use the weather patterns in a given area to help eliminate parasites from the horse pastures.
Can Stabled Horses be Infected with Parasites?
As a rule, the important parasites of adult horses (cyathastomes) cannot infect horses in the stable. The manure should be cleared away before the eggs develop into infective 3rd-stage larvae so that there is no risk of additional exposure to these parasites. Furthermore, the parasite eggs tend to be neutralized by the ammonia produced in soiled bedding. Therefore, when horses are stabled in the winter (as occurs in the northern USA), there is little risk of new parasite infection. However, following turnout in the spring, the risk of new infection is very high. Spring pastures contain an abundance of infective 3rd-stage larvae that are "ready to go" – having matured from the last grazing season (and surviving through the winter).
Pasture contamination larval levels decline in the warming weather and are quite low by early June. The number of larvae will remain low into summer if re-contamination of the pasture is not allowed. To that end, horses should not be allowed onto pastures if they are passing parasite eggs in their feces.
How Can Horses and Pastures be Managed to Minimize or Eliminate Parasites from the Pastures?
The infective parasite larvae must move away from the piles of manure in order to be eaten by horses because most horses will not eat grass that has been soiled by manure ("fecal avoidance behavior"). Parasites are aided and assisted in their ability to move away from piles of manure by the action of horses' hooves, farm machinery and rainfall. The areas of the pasture that are contaminated with manure are known as "roughs" and the uncontaminated areas are known as "lawns". If space is limited, horses that are low in the "peck" order (low dominance) are often forced to graze in the roughs and are therefore exposed to higher levels of parasites. If practical, the regular removal of manure from the pasture will help to reduce the number of parasites to which grazing horses are exposed. Whether or not regular manure removal is practicable for a given farm typically depends on the size of the pasture and the number of horses on the pasture. However, this technique can be used to great advantage in some farm settings.
The horse owner should take advantage of the fact that parasites in the pasture are killed by periods of hot dry weather. It should also be remembered that, although parasite larvae cannot mature in very cold weather, they are fully capable of persisting and being ready to infect new horses as the weather warms up. Local conditions obviously differ in different parts of the USA. In the southern states, the summer weather can be relied upon to effectively eliminate the majority of pasture parasites. However, in the more northern states, the "summer kill" is not so effective.
Which Drugs are Available for Deworming Horses?
There are currently a variety of drugs available for the treatment of gastrointestinal parasites. The drugs are divided into a variety of different classifications, including piperazine.
Fenbendazole (Panacur, Safeguard)
Oxibendazole (Anthelcide EQ)
Ivermectin (Eqvalan, Phoenectrin, Zimectrin)
Ivermectin and Praziquantel( Equimax)
Pyrantel pamoate (Strongid paste and suspension, Anthelban)
Pyrantel tartrate (Strongid C)
Unless resistance has developed in a parasitic worm population, all these currently marketed compounds have a good level of effectiveness against adults and larval stages in the horse's intestine. Only ivermectin and moxidectin have good effectiveness against larval stages of parasites migrating in the wall of the intestine or in the blood supply to the intestinal tract. High levels of fenbendazole and moxidectin also have effectiveness against the hypobiotic (encysted) cyathastomes in the wall of the intestines.
What is Parasite Resistance?
In time, all species adapt and change to meet the needs of survival. During the preceding 30+ years, equine parasites have changed and the drugs have changed. Parasites now reproduce FASTER than they did in 1966! Over the years, parasites have tended to develop ability to survive different parasite-killing drugs. If a worm parasite develops the ability to survive the effects of a parasite killing anthelmintic drug, it is said to have developed "resistance" to that drug. Broad resistance to many of the older deworming drugs is also now evident. The worms are able to evolve and to pass along resistance mechanisms to their offspring. No single parasite control recommendation should be regarded as perfect and permanent. There does not exist a simple single strategy for minimizing parasites that will work for every horse in every different circumstance. Local factors must be considered. If necessary, local factors must be investigated locally and monitored over time. Fortunately, at this time, parasite resistance has not been reported in the equine parasites for the very effective drugs, ivermectin and moxidectin.
It used to be believed that, by switching between different deworming drugs on successive treatments, the likelihood of parasite resistance could be lessened. The practice of "anthelmintic rotation" was designed to try and reduce the development of parasite resistance and to ensure that, if one deworming treatment "missed" some parasites, they would be killed by the next (different) treatment. Whether or not "anthelmintic rotation" practices truly ever did confer any protection against the development of parasite resistance has never been conclusively demonstrated. There have even been suggestions that this technique might increase the risk for parasite resistance!
If a parasite develops resistance to any single drug in a given category (eg. fenbendazole – in the benzimidazole family), the parasite will also be resistant to all the drugs in the same family. Properly documented parasite resistance to anthelmintic drugs is an important problem and warrants thorough investigation by the veterinarian. Special measures may be needed to provide protection against resistant strains of parasites. The indiscriminate use of anthelmintic drugs without veterinary supervision is an important causative factor for the development of parasite resistance.
The veterinarian will determine which anthelmintic drugs are effective or ineffective on a given farm (there is widespread, but not universal, resistance to benzimidazoles). If resistance to a family of drugs is identified, drugs in that family should not be used. A routine monitoring program should be arranged to ensure that any selected anthelmintic drugs maintain their efficacy on a given premises.
What is the Egg Re-appearance Period?
If an anthelmintic drug is effectively killing adult parasites, the fecal egg count should decrease by more than 90 percent. After a defined period, egg production starts again as encysted (hypobiotic) larvae "emerge" and mature and start producing more eggs. The interval between anthelmintic treatment and resumption of significant egg production is known as the "egg re-appearance period (ERP)". The ERP is different for the different anthelmintic drugs.
Piperazine = 4 weeks
Benzimidazoles = 4 weeks
Strongid suspension or paste = 4 weeks
Ivermectin = 6 to 8 weeks
Moxidectin = 12 weeks
Knowledge of the ERP allows the horse owner and veterinarian to know when the horse should be dewormed again if pasture contamination by further parasite eggs is to be prevented. In order to prevent pasture contamination by parasite eggs, horses should not be allowed onto pastures if they are passing parasite eggs in their manure. By strategically deworming horses over a period of 2 to 4 years, the resident population of parasites (encysted larvae) in the horse's intestine and the parasites in the pasture can be progressively depleted.
What is a Daily Deworming Program?
In recent years, a popular and effective program for the prevention of parasite problems in horses has entailed the administration of a specific anthelmintic drug every day. The drug used in this program is pyrantel and it is marketed as Strongid-C. Horses in this program must first be dewormed using either ivermectin or moxidectin (the most highly effective anthelmintic drugs). Immediately following this primary treatment, the horse receives a small dose of pyrantel in the feed every day. This dose of pyrantel is all that is needed to kill new incoming larvae before they invade the intestinal tract. The use of daily pyrantel in this manner is an effective preventive strategy for the strongyle parasite group (the most important group) but it is probably not effective against other categories of parasites. This program is also very good for minimizing problems with roundworms (ascarids) in young (< 10 months of age) horses. Not all horses are managed in a system that readily allows for individual horses to be treated with a dose of pyrantel every day. This is a good method for reducing parasitism in those horses that are looked after as individuals and are fed individually each day.
In the southern US, the timing is different. The high risk for infection occurs at the end of the summer. In spring-summer, it is too hot for eggs to develop into infective 3rd-stage larvae (even if there are lots of eggs). Risk of infection increases during fall – a result of newly-deposited eggs which are now able to mature. Therefore, pasture contamination should be prevented by deworming just prior to the time pasture contamination will start occurring (Sept-Oct).
The practice of SUPPRESSIVE DEWORMING is intended to prevent pasture contamination. The climate is allowed to "clean-up" the pasture at a time when all grazing by horses is done only by horses which are not shedding eggs. The frequency must be based on the ERP (see above) and you must deworm within the ERP of the previously-used drug!
By using this SUPPRESSIVE DEWORMING practice, fecal egg counts remain low and the pastures stay clean. Eventually, the cycle of reinfection is broken and the transmission of small strongyles is attenuated. It takes 2 to 3 years to deplete horses of their pool of encysted (hypobiotic) cyathastomes.
In the northern US, wintered horses should be dewormed just prior to being turned out onto pastures in the spring. For horses in the northern US, it is only necessary to suppress contamination past the beginning of July. After this time, there is insufficient grazing time left for significant pasture infectivity to develop. A final deworming is advocated when the horses are stabled for winter (in combination with a bot treatment).
In the southern US, the transmission season begins sometime around September, at which point horses should be dewormed to prevent pasture contamination. Repeated suppressive treatments should be continued until late winter (February/March) and then suspended until the following September. Some eggs will be passed by horses after March, but the weather takes care of them. Hot dry summer prevents eggs from developing into infective 3rd-stage larvae.
Resistance to the benzimidazoles and pyrantel has been reported (sometimes to both). Resistance (which occurs naturally) is promoted by the exclusive use of one drug (or group). Although controversial, the practice of rotating between different anthelmintics has been advocated to minimize the risk of promoting resistant strains. Rapid rotation entails changing the drug every time a deworming is done. Slow rotation entails changing the drug every 6 – 12 months. You should be rotating between classes (do not rotate between 2 drugs in the same class).
Ungrazed areas ("roughs") should not be dispersed into grazing areas ("lawns") by use of harrowing or dragging pastures, unless the pasture is to be left unoccupied by horses for a long period.
Other Strategies – Integrated Control
Incorporate pasture management with deworming program. Remember that there are 2 parasite populations on every farm – those in the ground and those in the horse. Remember that a pool of encysted ("hypobiotic") larvae always survives each deworming treatment. Obviously, there are limitations based on the availability of space.