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Internal Parasites of Horses


This section discusses the internal parasites of the horse from clinical, epidemiological and control perspectives.

A so-called "wormy" horse is one that is thin, long-haired, has a "dull" coat and is lethargic, but beware of blaming everything on worms. There are a lot of equine diseases and problems that produce a horse that looks like this.

In descending order of importance, the common internal parasites of horses are:

  • large strongyles
  • small strongyles
  • ascarids
  • bots
  • pinworms
  • strongyloides
  • stomach worms
  • tapeworms
  • eyeworms
  • filarids

    Clinical Disease

    The clinical effects produced by these parasites range across a whole spectrum, from no detectable clinical signs, to general unthriftiness, to diarrhea and life-threatening colic.

    The seriousness of the clinical signs will largely depend on 3 factors:
  • - Number of parasites present
  • - Age of the infected horse (young are more susceptible)
  • - Pathogenic potential of the infecting species.

    Control of Internal Parasites

    Control of internal parasites of the horse is directed primarily against the strongyles, in particular the large strongyles. Effective parasite control is one of the keys to a successful horse operation.
    In addition to the large strongyles, there are 4 other groups of parasites that we can classify as major - small strongyles, the ascarid Parascaris equorum, bots, and Strongyloides westeri.
    Every parasite control program must be designed for effective control of these 5 major groups. If you are successful in doing that then adequate care will have been taken of the others because the drugs you will use are of sufficient broad spectrum to include good activity against the other minor horse internal parasites.
    In the first six months of a horse's life, parasite control is directed primarily against Strongyloides and Parascaris. Following this, strongyle infections become the focal point for parasite control programs.

    Parasite control in horses is also based primarily on a chemotherapeutic approach, i.e. the use of effective drugs. However, it is important not to rely exclusively on drugs because there are some effective management practices that are useful in limiting the build-up of pasture populations of infective larvae.

    1 ] Avoid overcrowding of horses on pastures - leads to high concentrations of fecal deposits and therefore increased pasture contamination with L3's.
  • Manual removal of feces - very useful but labor intensive.
  • Composting of feces - high temps kill eggs and larvae.
  • Harrow pastures to break up feces and expose larvae to the elements.
  • Periodically plough and re-seed

    2 ] Mixed grazing with cattle and sheep. This works because cross transmission between ruminants and horses is limited to one species, T. axei. However, intensive use of mixed grazing may preferentially select out T. axei on a pasture and cause it to be a problem on all grazing host species.
    While these management and sanitation measures are very useful the ubiquitous nature of the major internal parasites of horses, particularly the strongyles, necessitates that drugs play an important part in any parasite control program for horses.

    Dewormers (Anthelminics) in Horses

    For practical purposes dewormers can be separated into three groups:
  • additives that are mixed into the daily feed
  • pastes that kill migrating larvae
  • pastes that kill only the adults in the bowel

    Recent advances now make it possible to convert some hard keepers to easy keepers and without too much effort to eradicate most parasites. This article discusses the pros and cons of the various products available including Strongid, Stongid P, Ivermectin, and the new Equest. Also discussed are the relative merits and problems with tube worming, pelleted dewormers, and paste wormers.

    General Considerations

    Under the old recommendation of deworming every 2 to 3 months with ivermectin- based products, parasites are given time to recover and begin egg production again.
    Not all parasites are sensitive to the same drugs. In fact, the same parasites may be sensitive to different drugs at different times of their life cycles.
    Prevention of drug resistance is important. Ivermectin manufacturers thought that there are no worms resistant to ivermectin (which is true); however, if we do not rotate dewormers, it is just a matter of time before resistance will develop.In fact, some early signs of developing resistance have already been seen as decreasing prepatency times. Many parasitologists believe that rotating products every deworming worsens the resistance problem. Yearly rotational programs are recommended. There are only two chemicals worth using in a regular paste deworming program--ivermectin (in numerous products) and milbemycins (Moxidectin). The only worms not controlled by this program are tapes for which a double dose of pyrantel paste is given yearly.

    We recommend you follow these instructions :

  • If in doubt about your horse's weight, ask your veterinarian to approximate the horse's weight.
  • Paste dewormers must be given when the mouth is empty and should be placed as far back in the mouth as possible. The horse should be watched for a few minutes to be sure he does not spit it back out.
  • A good time to have your deworming program checked is at his yearly vaccination and should be just before a regularly scheduled deworming .
  • Do not deworm your horse at mealtime. Best is to give the dewormer 12 hours after the last meal and withhold food for 1 more hour after deworming. This may deliver as much as twice as much medicine to the parasite as worming after feeding.
  • If you have a foal that has an uncertain deworming history or appears to be suffering from poor care. Best to start slowly with the deworming program. Obstruction due to a massive kill of of round worms and inflammatory bowel disease can result from deworming foals with heavy burdens of parasites. Consider a dose of strongid paste first followed in a week or two with an ivermectin based dewormer.

    Management will Help Control Parasites

    One of the most important points of any control program is to reduce the horse's contact with the larvae-infested feces. If at all possible do not feed on the ground; this greatly increases the rate of infestation. Every time I have found a significant parasite problem in a horse that has been dewormed regularly, his hay was thrown out in front of the barn onto the ground. If the horses must be fed on the ground, pick a clean spot that does not receive run-off water. If the horse is stalled for any length of time, the manure should be picked daily. Other important management practices are:
  • Do not spread uncomposted manure on the pastures.
  • Avoid use of poorly drained pasture.
  • Clean paddocks at least every 5 days.

    A debate over whether it is best to drag pastures with horses in them is ongoing. Breaking up the manure allows for quicker killing of the parasite larvae but spreads them over the whole pasture. The larvae are sensitive to heat and drying. Pastures with horses on good deworming programs probable benefit from dragging during hot, dry weather.

    Testing for Parasites

  • The best and quickest way to find out if your horse has intestinal parasites is by having a lab test performed on the feces for the parasites eggs. In this test a small amount of stool is placed in a super saturated solution of salts or sugars and the eggs, which are ligher than the surronding solution, float to the top. There they are collected on to a microscope slid which is examined under a microscope. Adult round worms and hook worms, usually called strongyles, are easily detected.
    It is important to realize that a fecal test alone is not diagnostic for parasites. The test is not perfect and so must be interpreted in light of the history. Since the test only picks up the eggs layed by adults it will miss the larval forms. This might happen if a horse was recently dewormed with a good product, so now the adults have all been killed, but is on a pasture that is heavily contminated with parasite larvae. Though recently dewormed, the horse now has both the encysted larvae that were resistant to the dewormer and the new larvae picked up from the pasture after the deworming.

  • This should lead you to the conclusion that fecals are best done just prior to a deworming and gives you this vital piece of information: is your parasite program good enough to keep your pastures free of parasite eggs? It does not give you this vital piece of information: is my horse being exposed to large numbers of larvae. This is an important point as the larvae do much of the serious damage to the horse prior to becoming adults.

  • If your horses are on well manged pastures where a good deworming program has been in effect for over a year, as evidenced by several negative fecals, you have little to worry about. If however you are on pastures that are poorly manged with respect to parasite control and with horses that are on a spotty deworming program, but your horse is being dewormed well he may have both a high larval parasite burden and negative fecals.

  • Fecal tests do have one other weakness: they are a poor indicator of tape worms because they produce relatively few eggs. Recently a blood test has been developed to detect the prescense of tapes but is not widely available as of early 2002. Fecal tests must be interpreted in light of the history and are best when several have been performed a year.


    The Avermectins: Ivermectin

    and the other products that contain ivermectin kill adult parasites and most circulating larvae (they both contain the active ingredient ivermectin). They,will not, however, kill the encysted L4 stages. With ivermectin-based dewormers it takes 42 to 63 days before egg production begins. This class of dewormer does not kill tape worms.

    Equest: Moxidectin

    A recent introduction, Equest (moxidectin), has been effective at killing the 4th stage larvae. Equest is a new type of avermectin. Because of this L4 efficacy, the time between deworming and egg production is 84 days. The product is safe in pregnant mares and stallions and is approved for all horses over 4 months of age. If dosing horses less than 500 lbs we recommend you premeasure the dose into another syringe then administer.

  • Note that neither the ivermectins or equest will kill tape worms.
  • Also note that equest does not do as good a job of killing all the stages of bots that ivermectin does.

    There have been several reports of miniature horses and small foals developing coma after huge overdoses of this product. Typically the locking hub fails and a small equine receives a 5 times overdose. If dosing horses less than 500 lbs we recommend you premeasure the dose into another syringe then administer. You need to read the instructions carefully and practice working the locking hub before administration. Occassionally death will result from greater than 5 times overdose, but most animals recover within 36 hours. Fluid support, urinary catherization, and treatment for hyperthermia are generally needed to ensure the best possible outcome.

    The Benzimadazoles:

    Look for the name of the active ingredient to end in dazole. All of this class of deworming pastes have little effect on the larvae at the recommended dose and many have resistance problems making there use questionable in a good deworming program. These products also tend to be more toxic and some (cambendazole) are not suitable for pregnancy. Recently a high dose five day treatment of Panacur has become available which will kill 90% of all the larval stages in the horse. This includes a good efficacy against the encysted dormant larvae. These have been difficult to kill with any other deworming product. They can be a cause of diarrhea, weight loss, and chronic colic. The treatment is expensive and really does not figure in well with well managed healthy horses. This may be useful for treating horses with signs of anthelminic larval disease and horses with a poor history of parasite management before introducing them to a herd.

    Pyrantel Pamoate Paste and Pyrantel Tartrate Granules:

    Strongid pastes

    Pyrantel can be bought in paste or pelleted form. The paste should not be the mainstay of any deworming program as egg production of the strongyles is suppressed for just a short time. It is effective at a double or triple dose for tape worms. Most horses on paste dewormers during the year need a-once-a-year deworming for tape worms whether they show up in the fecal or not. The deworming is given right in between the regular deworming pastes and not substituted for them. At this time cold weather seems to be the best time to deworm, but this is uncertain and any time of the year will do. If tapeworms are established as a problem twice yearly dosing with a 3X dose of pyrantel and switching over to pyrantel pellets (see below) is warranted.

    Strongid Pellets

    The pellets are more effective than the pyrantel based pastes for regular deworming and control of strongyles and round worms. Pyrantel tartrate pellets (Strongid and now there are generics), is a feed-additive dewormer that kills larvae as they are ingested. The larvae never have a chance to take hold. Before beginning the pellets, you must give a good deworming with ivermectin or moxidectin to eliminate the parasites that are already encysted. You will also need to deworm once or twice a year for bots as this product is not effective against them. Fecals should be run just prior to the bot treatments as breakthroughs will occur if any treatments are missed. There is reduced efficacy of this product in immature horses so frequent fecals and more frequent use of good pastes is suggested.

    Research is showing that the daily use of pyrantel tartrate pellets is very effective at controling the common tape worm infections of horses. It effectively suppresses egg production so that in several years the infective larvae die off the pastures. It is still uncertain if it kills the adults though we suspect it. It tapes are a known problem you should give a 2X or 3X treatment with pyrantel paste. Otherwise no other treatment is required for them than the pellets.
    This is an excellent product but expensive, and you still need one or two ivermectin dewormings each year to control bots. For people who have horses on pastures where they do not control all the horses' deworming programs this product is worth every penny. This is the only product that will protect your horse from a contaminated pasture. The result will be fewer colics, a better hair coat, and less feed required.
    Many owners report significant improvements in weight and appearance after beginning Strongid pellets. This occurs even in some horses with good deworming programs.


    This dewormer has been available since the 1960's and has been used around the world for tapeworm infections of animals and man including horses. Though not available in a horse approved product it has approved for use in dogs, cats, and humans. Name brands are Yomesan, Mansonil, Fenasal, and Lintex Safety limits in the horse have not been established. Experimentation, practical experience, and the observation that it is very poorly absorbed from the GI tract in the species it has been studied strongly suggest it is safe in all mammalian species at the recommended doses and has a wide margin of safety. Currently the product is available as tablets that would have to be crushed made into a paste and syringed or tubed into the horse. Dose is 100 mg / kg (45 mg / lb ). This product gives the horse owner an alternative to double or triple doses of pyrantel based pastes for tapes.

    With the new products, we may soon see the end of the small strongyle parasite in well-managed herds of horses. But this creates a problem, particularly with the young horses. Naturally-acquired immunity is an important mechanism to keep horses from getting very ill from parasite exposure. To get this immunity, the horses must be exposed to the parasites. Young adult horses without it, when exposed to even moderate to low levels of parasites, develop severe enteritis and colitis. So now that it is in our power to eradicate intestinal parasites, is it best? Remember that even low-grade exposure to parasites results in some health problems.

    Deworming Programs

  • 1] An excellent deworming program for most conditions that probably will not completely eliminate small strongyles:

    First year: Equest every 3 months with a once yearly double dose of Strongid paste between the equest treatments.
    Second year: Ivermectin every 6 weeks with a double dose of Strongid between two of the ivermectin treatments.

    The Best time to rotate is just after the main bot season.
    Rotate back to the equest the third year.
    Have fecals checked yearly just before a deworming.

  • 2] An excellent program for difficult keepers or situations where horses are put out on contaminated pastures:

    Treat the horse with Equest the day you begin the Strongid pellets.
    Treat summer and fall with equest or ivermectin products. Rotate between the two from year to year.
    This program will, in time, control rounds, tapes and strongyles if no new source of pasture contamination is allowed.
    Have fecals checked just before deworming for bots.

  • 3] If the aim is to eliminate strongyles:

    Equest every 2 months for 2 years.
    Treat all new horses with equest 2 weeks before coming on the pasture, and make sure they are not exposed to larvae. After the two years have a fecal checked 3 months after the last deworming and then twice a year thereafter and deworm if parasites return. You should still treat for tapes (double dose of Strongid) yearly and bots (ivermectin) as recommended above.

    If you have foals:
    Early experimental work has shown some breakthrough of large strongyles in foals on Strongid pellets. Also known is that all classes of dewormers are not as effective in foals. Using ivermectin monthly prior to weaning and equest every other month or ivermectin monthly after weaning for the first year may be important. Note recommendations above on dosing animals less than 500 lbs with equest.

    What the Experts Recommend

    Parasitologists have long recommended that the best way to deworm is to pay attention to the management considerations above and base the deworming practises on the results of fecal tests. If the fecals are negative have them rechecked 3 months later if negative have them checked every 6 months and as long as negative don't deworm. This is a good practice under certain conditions:

  • A stable population of horses where there is no exposure to new horses.
  • Manure and pasture managnement is excellent.
  • New Horses are not being added
  • All horses have been on an excellent program for the past year.

    By avoiding the frequent use of dewormers it is thought that resistance will develop slower. Though it requires diligence this system has a lot to recommend it and it saves money if the horses are not becoming rapidly reinfested. The biggest short fall is that tape worms may not show up on standard fecals. Perhaps a once yearly double dose of strongid paste should be given no matter what the results.

    Parasite glossary

    Strongyloides westeri

    Foals are born worm-free. The first parasite foals are exposed to is S. westeri. Initial infections are via the mare's milk, and larvae can be found there 4-44 days post-partum.

    With a prepatent period of only 5-7 days, by the time a foal is 2 weeks old, it can be passing S. westeri eggs from parthenogenetic females.

    These eggs will develop, producing infective larvae that will provide a constant source of reinfection for foals (Routes of infection - percutaneous, oral and transmammary guarantee that all foals will be infected). Fecal egg counts can be high, sometimes as high as 20,000 epg by the time a foal is 5 weeks old.

    Despite the fact that many suckling foals are wormy, they often grow well because they compensate by drinking more milk.

    Infections are self-limiting because a strong immunity develops by the time a foal is 4-6 months old.

    The acquisition of this immune response is apparently related to the duration of infection, since treating infected foals with an effective drug such as Fenbendazole at 2 and 4 weeks, and then every 4 weeks thereafter will tend to curtail infections, delay onset of the immune response, and thus prolong susceptibility to S. westeri. A better approach might be to treat at 4 weeks and again at 12 weeks.

    As far as the anthelmintics you might wish to use against S. westeri, a broad spectrum of activity is not an essential property, since foals at this age will only be infected with one helminth, viz. S. westeri. However, as they get older, consideration must be given to the fact that they may be infected with Parascaris equorum.

    Parascaris equorum

    Infections with P. equorum are common in young animals, foals, yearlings, and to a lesser extent in 2 year olds. Acquired resistance is strong, and infections in mature horses are quite uncommon.

    Coughing, accompanied by a copious mucoid nasal discharge is a common clinical sign in infected foals, and this is due to lung migrations of P. equorum larvae. In uncomplicated cases, respiratory disease is usually transient. The presence of immature and mature adults in the small intestine is usually asymptomatic. However, heavy infections may result in rupture of the small intestine at the mesenteric attachment where blood vessels enter and where the musculature is weakest.

    In a recent edition of Veterinary Medicine the following statement is made:

    "Ascarid infections are a common cause of intraluminal obstructions in foals and yearlings. Foals usually develop immunity to the parasite by six months to one year of age; therefore ascarid impactions are uncommon in adult horses. Surgical intervention is often necessary to relieve the obstruction and may necessitate multiple enterotomies to remove the parasites. The prognosis in these cases is guarded, especially if multiple enterotomies have been performed since these will increase the likelihood that intra-abdominal adhesions will form."

    The more usual clinical picture is poor appetite, progressive unthriftiness, rough hair coat, and pot belly.
    Because foals are susceptible to infection, patent infections are often first seen when foals are 10-12 weeks old (PPP of P. equorum = 10-16 weeks).

    Ascarid infections are usually diagnosed by clinical signs, ascarid eggs in the feces and a positive response to treatment with an effective anthelmintic.

    Gasterophilus spp - BOTS

    Bots are larval stages of bot flies, the Gasterophilus species. They are extremely common in horses of all ages.
    The complete life cycle of these species takes 12 months. Adult bot flies actively deposit their eggs between May and October on the forelimbs and head regions. Horses naturally lick themselves, and this licking action stimulates the bot eggs to hatch with the larvae burrowing into the mucosa of the lips and gums and migrating to the stomach in 3-4 weeks, where they remain overwinter before passing out in the feces. They pupate in the ground, emerging as adult flies in 1 or 2 months.
    Bot fly larvae may cause periodontal ulcers as they migrate, or they may cause stomach ulcers which, if they perforate, may cause a fatal peritonitis. Traditionally veterinarians have treated horses for bots 30 days after the first killing frost, the theory being that frost kills any remaining bot eggs attached to hairs and kills the adult flies. The boticide kills larvae in the stomach so that the horse remains bot free until the following spring when adult flies emerge from the pupal cases and begin laying eggs.


    There are 2 species of tapeworms found in horses: Anoplocephala magna and Anoplocephala perfoliata - the commonest of the two and found in clusters around the ileocecal valve in the cecum.
    The frequency of infection averages about 14% in random sampling of horses via fecal examination for eggs. However, localized heavy infection may be seen on several farms within a locality. It is on farms such as these that acute obstructive colic resulting from Anoplocephala infections is most likely to be seen.
    Although not frequent in occurrence, serious results from tapeworms in horses have been reported in several clinical reports in the veterinary literature.

    These include reports of an association between A. perfoliata infections and Intussusceptions, cecal perforations and peritonitis.

    Stomach Worms

    The three stomach worms, Habronema muscae, Draschia megastoma and Trichostrongylus axei are widespread in occurrence and quite common, being found in an average of 50% of randomly sampled horses in several studies.

    Trichostrongylus Axei

    Remember that the Spirurids (Habronema and Draschia), require an intermediate host for completion of their life cycles, either the house fly (Musca domestica) or the stable fly (Stomoxys calcitrans).

    Deposition of infective larvae in skin abrasions result in the production of lesions called summer sores. These lesions tend to heal spontaneously during winter but often recur in the subsequent spring and summer. They can often be distressing to horses because of the accompanying pruritis.

    Adult Draschia megastoma provoke the formation of large tumor-like growths in the stomachs of infected animals. Occasionally these may rupture leading to an often fatal peritonitis.

    Infections with T. axei often occur where horses and donkeys are grazing with cattle.

    Infections are usually inapparent but may be responsible for an acute hemorrhagic gastritis in heavy infections.

    Adult spirurids in the stomach are refractory to treatment with many of our anthelmintics. CS2 has been used. The organophosphates, Dichlorvos and Trichlorfon are active against Habronema. However, the treatment of choice is Ivermectin. It is highly effective against both Habronema and Draschia at the dose rate effective against the strongyles.

    Treatment of the summer sore lesions has usually involved local applications to the lesion site. Dichlorvos and Trichlorfon have been used successfully as topical applications. The organophosphate Fenthion, used in cattle against warbles as a pour on product has been shown to be highly effective in clearing up summer sore lesions.

    However, Ivermectin is highly effective against Spirurid larvae, the cause of summer sores. So the treatment of choice for summer sores and adult worms is Ivermectin.

    As for T. axei, little is known about the efficacy of anthelmintics against T. axei in the horse and there are few specific label claims on horse products except for Cambendazole and Ivermectin. However, we can safely assume that compounds effective against ruminant Trichostrongyles will be effective against this nematode in horses and donkeys.

    Control programs that include Ivermectin in at least 2 of the treatments in any one year will be effective in successfully controlling stomach worms.


    Adult Oxyuris equi are found in the large intestine. Gravid females migrate to the anus and deposit their eggs there. These adhere to the peri-anal skin in masses and become attached to blankets, stable walls, fixture, fences bedding etc.

    Oxyuris egg

    The usual effect of pinworm infection is anal irritation and pruritis inducing affected horses to tail rub causing loss of hair and a "rat tail" appearance.

    Almost all of our modern anthelmintics have good activity against pinworms. Incorporating these into a parasite control program should be more than adequate in controlling pimworms. The exceptions are: Piperazine and Pyrantel with less than desirable activity against pinworms.

    adult Oxyuris


    Onchocerca adults produce large numbers of microfilariae in the skin of infected animals. Surveys have shown that prevalence of infections is high, ranging from 22% to as high as 98% in some areas. Peak infections are apparently reached by the time a horse is 6 years old and these are maintained at high levels throughout the rest of a horse's life.

    Intensely itchy skin lesions have often been associated with the presence of microfilariae in the skin of horses, but it is possible that these lesions are hypersensitivity reactions to the bites of Culicoides, the intermediate host for Onchocerca.

    Eye lesions may be produced when microfilariae invade the eye.

    Diagnosis is usually made on the basis of skin lesions with hair thinning, scaly patches of dermatitis on the neck, head, chest and ventral midline, and confirmed by finding microfilariae in biopsy samples.

    The drug of choice for treating filariid infections in horses, is undoubtedly Ivermectin. The use of Ivermectin in routine control programs may sometimes be associated with a ventral abdominal edema, which if severe may require corticosteroid therapy.

    This edema is believed to be caused by a hypersensitivity reaction resulting from the death of large numbers of Onchocerca microfilariae. Use of Ivermectin in routine control programs is usually sufficient to control filarids. The drug should be used at least twice in any yearly program to ensure adequate control of filarids. Ivermectin is the reason why these infections are less common today than in previous years.


    The lungworm Dictyocaulus arnfieldi occurs in donkeys as the natural hosts. Infections appear to be very common in donkeys, as high as 50%. Infections in horses are low and sporadic in occurrence - as low as 2-3%.

    The usual epidemiological picture where lungworms become problems in horses is the common grazing of horses and donkeys on the same pasture.

    Most infections are asymptomatic, but persistent coughing is the usual sign that lungworms may be a problem.

    Infections can be confirmed by detecting the characteristic spike-tailed larvae in the feces of infected horses and donkeys.

    Treatment is effective if some of the benzimidazoles are used - larvicidal doses of TBZ- 10X. Fenbendazole at 2X normal dose is also reported to be effective in horses, but 6X FBZ is apparently not effective in donkeys.

    Levamisole is very effective but is not approved for use against lungworms in horses, and remember that Levamisole has a narrow safety margin in horses.

    Ivermectin at normal therapeutic doses for other equine nematodes is extremely effective against Dictyocaulus arnfieldi. It is the drug of choice for lungworms and its routine use in nematode control programs will be more than sufficient to control D. arnfieldi.

    The Strongyles

    The strongyles are found wherever horses graze, so that all horses become infected early in life.

    No significant age or acquired immunity appears to operate, at least under field conditions, and as a result large worm burdens can accumulate.

    With heavy burdens, emaciation, debility, and death occur, and even light infections can seriously affect development and performance

    In terms of clinical disease, adult cyathostomes, or small strongyles, are of secondary clinical importance. They are plug feeders but their buccal capsules are so shallow that they are probably more correctly described as mucosal grazers than plug feeders. The larvae are much more pathogenic and are relatively unaffected by many anthelmintics, and even when they are, it usually requires higher dosages.

    In the late 1970's a number of cases of clinical disease due to cyathostomes came to light. These were primarily of diarrhea and loss of condition in spring caused by cyathostome larvae emerging in large numbers from the gut wall.

    Epidemiological evidence would suggest that these larvae were acquired in the previous grazing season and overwintered in the gut wall as arrested larvae.
    Infective larvae on pasture are scarce during winter and spring. By late May-June any larvae overwintering on pasture finally die out. In June, the egg output of mares begins to increase sharply with environmental conditions favoring the rapid development of infective larvae with peak levels of pasture larvae in September and October, followed by a sharp decline to a low level in the following winter.

    If we look at seasonal variations in the populations of small strongyles in horses, we see that 4th stage larvae emerge from the gut wall and develop to adults. These latter clearly contribute to the rise in fecal egg counts in horses at this time. These eggs develop to third stage larvae by late summer, and are responsible for the high level of larval contamination - the most important source of infection for grazing foals.

    The large strongyles are very, very important wherever horses are raised. S. vulgaris is the most important because it is the most pathogenic. The high degree of pathogenicity of S. vulgaris is because of their extensive migrations in the mesenteric arterial system where they are associated with thrombus formation and arterial pathology producing interference with blood supply to major parts of the alimentary tract.

    S. Vulgaris worms

    Anemia, unthriftiness and poor performance are attributed to the plug-feeding activities of the adult large strongyles. They feed by withdrawing plugs of mucosa resulting in the formation of ulcers.
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