Last month we introduced you to the major internal parasites which can plague your horse. This month we will discuss management strategies that you can use to decrease the parasite load on your horse, in part through an understanding of their life cycle. We can actually use the horse’s environment to help decrease our reliance on de-wormers and do our part to aid in the battle of anthelmintic resistance.
If you remember the life cycle of our most insidious parasite, the small strongyles, you know that the tiny infective larvae hatch from eggs outside of the horse. They then use the dew or moisture present on the grass to be able to wriggle around in the blades of grass and await your horse to come along and ingest them. Since they need this moisture as part of their life cycle and to be mobile, horses housed in stalls and dry lots are far less likely to be able to pick up infective larvae. It is pasture grazing, therefore, which is the key to the strongyles’ survival. Worm larvae will tend to be located in the thicker grass areas of the pasture and down in the thatch layer, where moisture remains longer. The highest potential for infection will occur if your horse crops the grass close to the ground.
If you observe horses natural feeding patterns, horses tend to graze pastures into areas of roughs and lawns. The lawns, characterized by short grasses, are the areas which are cropped closely to the ground and the roughs, which have longer grass, are the areas where horses choose to defecate and avoid grazing. Obviously the larger the area in which horses are kept, the less likely they are to graze near infective piles of horse manure. This will decrease their chances of picking up larvae. As stocking density of the pasture increases, or vegetative growth decreases, such as in times of draught, the horses will be forced to eat nearer these thick areas of grass just teeming with swarms of larvae. If the grass becomes too short, supplemental hay should be provided to avoid forcing the horse to graze in the roughs. Additionally, the pasture can be mowed to keep the roughs from spreading further into the pasture.
Many people employ dragging the pasture to break up manure piles and spread them through the pasture to prevent the formation of roughs. However, if you use this strategy, you must understand that you are effectively dispersing the eggs and larvae far more thoroughly than they could ever do themselves. Even on their own, larvae can spread 4 to 12 inches from their original pile, and even further if aided by heavy rainfall. Therefore, if you drag the pasture, keep the horses off the pasture for at least two weeks. Preferably the dragging should be done in the hottest part of the year in order to expose the larvae to heat and dehydration. Cool temperatures allow the larvae to survive longer, so it is not advised to drag during the spring and fall. If you must drag in cooler weather prevent the horses from grazing for an even longer period of time. As strongyles larvae are especially hardy and can survive winter quite easily, this is really not a good strategy for trying to kill the larvae. Finally, if you are going to spread manure on pastures as a means of disposal, never spread fresh manure. Make sure it has been thoroughly composted before applying it to your pasture.
In an ideal world, pasture rotation allows the best management strategy to reduce strongyles infestation in your horses. Horses grazing in fresh new pasture will avoid grazing near manure piles, and have a lower chance of re-infesting themselves. Letting pastures lie dormant will also allow any eggs or larvae present to die before horses are introduced. If space and equipment allows, putting pastures into hay production will allow parasites to die as well. Finally, if you own multiple species of animals, grazing pastures alternatively between cattle, goats and sheep will reduce your parasite burden, as the worms are host specific. Obviously all of these strategies do require a significant amount of acreage and fencing to be effective and may not work for everyone.
Remember, for strongyles elimination, heat is your friend. Only drag pastures during the hottest part of the year, and do not allow horses back onto the pasture for at least two weeks. Use separate pastures for winter pasture and summer pasture. Remember, winter does not kill the parasites. In cooler climates, parasites will not die after emerging from their dormant state until about June, May in hotter climates. If you do have a clean pasture, before you turn horses onto it, chemical deworming can prevent parasite infestation. Horses that are dewormed should be held on dry lots for several days before turning them out. This will allow all the eggs that the mature female has deposited to pass through your horse’s digestive tract. When your horses enter their new pasture, they won’t be bringing any “friends” with them!
What about the other parasites in your horses life other than strongyles? There are certainly management strategies which will help control their populations as well. For ascarid control, remember that these worms are primarily a problem for young horses. If possible and space allows, rotate which pastures house young horses with adult horses. However, even this may not be completely effective as ascarids can remain alive in the environment for several years. Essentially, if foals and young horses have been housed in a pasture, it is fairly likely that ascarids are present. Unfortunately, as ascarids don’t involve the same strategy for survival as strongyles, they can also infest the young horse in stalls and dry lots. This is typically why young horses are dewormed more frequently than older horses.
Stomach bot larvae and adult fly control are unfortunately only going to be controlled through the use of anthelmentics. The adult form can fly for miles so even if you have a great deworming program, if your neighbors do not, their flies will simply fly over to your property to lay eggs on your horse.
Tapeworms are relative newcomers when discussing parasites in horses. While not new to the horse, they are new to us, so not as much is known about them. They are believed to have a similar susceptibility to climate as the small strongyles, but may be hardier. More horses in northern climates have been exposed to tapeworms, which would indicate that these parasites are relatively cold resistant, but may have a susceptibility to heat. Therefore, follow similar management protocols as you do for small strongyles control.
From looking at the parasites life cycle and their means of infesting horses, it is clear that horses are often dewormed more frequently than is really necessary. As anthelmintic resistance becomes a growing issue in horses, we need to understand the ways in which we can manage horses to reduce their parasite burden. Next month we will tackle the issue of anthelmintic resistance and discuss which deworming strategies might be the most correct option for your horse.
This month we will discuss other aspects of horse management that directly affect the nutritional status of your horse. While most horse owners are familiar with deworming their horses regularly, current recommendations from many equine practitioners are to be much more strategic with our deworming. There is a growing concern that parasite populations are developing resistance to almost all types of anthelmentics (drugs used to eliminate internal parasites). As no new anthelmentics will soon be offered to the public, this could represent a real risk to the health of our horses. In order to understand these issues, we will begin with a review of the major parasite classes in horses.
While there are many types of worms which infest horses, we will address the major classes that represent the most health risk to your horse; ascarids, strongyles, tapeworms, bots and pinworms. Ascarids, or Parascaris equorum, are a type of round worm which grow to a substantial size of 8-15 inches within the intestine. They are yellowish in color and may be occasionally seen in the feces. Despite their robust size, much of the damage created by these parasites involves their life cycle and migratory journey through the horse. Adult females pass eggs into the horse’s feces, where they spend 1-2 weeks in the environment before they are capable of infecting a new host. Horses ingest the infective eggs by grazing or eating in contaminated areas. Once inside, the larvae burrow through the intestinal lining and enter the bloodstream, where they travel to the liver. They then travel to the heart and then the lungs. Ultimately they enter the alveoli of the lungs where the horse coughs them into the oral cavity and then are swallowed back down into the stomach and intestines. The entire life cycle of the ascarid takes about three months and the journey these parasites take can cause significant damage and scarring of the tissues. A heavy parasite load of adult worms can even lead to blockage of intestines. Young horses are the most susceptible group of horses to acquire ascarids, as well as weak, or malnourished horses. Coughing and nasal discharge in young horses may actually be a sign of ascarid infection. Older horses eventually develop an immunity to these parasites, so ascarids are primarily an issue with horses under two years of age.
Strongyles exist as both large and small strongyles, with many sub-species. The three main species of the large strongyles are Strongylus vulgaris, Strongylus edentus, and Strongylus equinus. Small strongyles actually have about 50 different species. Strongyles are also the most damaging of the parasites that horses will encounter. Similar to the ascarid, the females lay eggs which are shed in the feces. Unlike ascarids, they hatch into infective larvae that the horse ingests. The larvae molts three times before it is ready to infect the horse. The larvae actually crawl up the blades of grass in the dew. The larvae can crawl up or down multiple times waiting for a host, or even burrow into the ground when the weather isn’t favorable. Unfortunately for the horse owner, these parasites are extremely hardy and can persist through the winter.
The characteristics of the large and small strogyles life cycle make them particularly damaging. Large stronglye’s life cycle involves two stages where they migrate through the arterioles and arteries which supply blood to the intestine. Unfortunately, wherever these larvae burrow through the intestinal wall to migrate, all of them will return to one single location, the cranial mesenteric artery. Here they congregate and can cause immense damage. They can cause hemorrhaging, blood clots, or even rupture. The blood clots themselves can break free and travel further down through the blood supply to where they block blood flow and create a thromboembolic colic and even death. Oddly, enough lameness can also result from blood clots traveling to the legs as well.
Small strongyles have an additional strategy to help them survive. As they pass through the horse’s intestinal wall, the horse’s immune system is also trying to wage war against the larvae. However, the larvae are too big and travel too fast to be eliminated. The final migration of the larvae and complete maturation is actually held in check by the presence of adult strongyles in the lumen of the intestine. Essentially the adults provide feedback to the larvae that there is no room at the inn. When the larvae get that message and slow their migration, they become encysted within the intestinal wall by the immune cells. Here they can lie in wait for several years to take their turn at being the adult worms in the intestine. The horrifying reality is that when the adults die of either natural causes or by our purge deworming of the horse, the encysted larvae “wake up” and emerge to replace the newly vacated intestine. Within 6-8 weeks they will have matured and begin laying their own eggs to begin the cycle anew. Again, it is the pattern of traveling through the tissue that can cause a great deal of damage to the horse.
Relative to those bad boys, the rest of the worms which typically invade horses are mild in nature. The other major parasite classes which trouble horse owners are pinworms, stomach bots and tapeworms. Pinworms have a very simple life style compared to ascarids and strongyles. Adult females have a rather interesting feature, however. Not content to just shed her eggs into the feces, she actually deposits the eggs on the horse’s anus. This causes irritation to the horse who then scratches on anything available in the environment, effectively dispersing them. The horse then incidentally ingests the eggs, which hatch in the intestine where the larvae mature. Thankfully, these worms do little damage to the horse because their life cycle does not involve migrating through sensitive tissues. However, they can cause great irritation to the horse and robust itching of the tail head.
Tapeworms in horses can also cause reduced nutrition and potential blockages due to the preferred location in the horse’s gastrointestinal tract. The main species of tapeworm which inhabits the horse fixes at the ileocecal junction, or where the terminus of the small intestine joins into the cecum. A heavy parasite load can result in blockages, thickening of the ileocecal valve or even intusussecption, when the intestine rolls over itself due to regular peristaltic action. The tapeworm also has a separate host for part of its life cycle. While the adult parasite resides in the horse, the eggs of the tapeworm are actually ingested by a type of mite, which the horse then later ingests while grazing. There does not appear to be any age related immunity to tapeworms, as they are found in all ages of horses.
Finally, stomach bots are frequently seen in horses as well. The stomach bot, or Gasterophilus, also has subspecies, which include the horse bot fly, the throat latch bot, and the nose bot fly. The adult fly form can actually fly for several miles in search of a suitable subject on which to lay its eggs. The female hovers near the horse and deposits single eggs on one hair at a time. The eggs actually hatch into larvae within 7-10 days of being deposited. They then wait to emerge until the horse licks or scratches at the eggs. The larvae then enter the mouth and bury themselves in the gums, tongue or lining of the mouth where they hang out for a month. As they mature to later stages of larvae, they move into the stomach where they attach to the non-glandular or upper part of the stomach. The larvae live in the horse’s stomach for 9-12 months, before they and pass out into the feces. This typically occurs in late winter to early spring. There the larvae pupate and remain in the feces for several months. The flies then emerge in late summer or early fall, find mates and renew their life cycle. The damage the bots cause to the horse can occur in the mouth where they cause great irritation and even form pus pockets or cause the teeth to loosen. Large numbers of larvae in the stomach can cause blockages and erosion of the stomach lining. They, like all internal parasites, can result in reduced nutrition being delivered to the horse. An important heads up to horsemen: when handling horses with bot fly eggs on their hair, use caution. While rare, the larvae are capable of burrowing into human skin, and if one rubs their eye after handling bot eggs, they larvae can actually invade the eye. I’m quite sure the last thing anyone wants is a bot larvae living in your eye!
Next month we will use what we know about these parasites to develop management strategies to reduce their ability to infect our horses. After that, we will discuss strategic methods in using anthelmentics in order to reduce our reliance on medications and reduce the spread of resistance in parasites which invade our horses.
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