Ask The Vet: Deworming - April 07

I would discuss your concerns with your veterinarian first. A fecal test will give you a good idea of whether or not you need to do any additional deworming. I have not found probiotics to be necessary during a Powerpac treatment because the dewormer does not tend to disturb the bacteria in the intestine the way antibiotics due. I would be hesitant to jump right to a Powerpac treatment in a horse that is not feeling 100% though.

There are other daily dewormers on the market that are very similar to Strongid C (Contiuex, CW, and Nu-Image Guardian to name a few). I have not had much experience with them to vouch for their effectiveness, but many people have used them.

The biggest concern with a horse his age would be roundworms. A dose of Safe-guard should have been effective at treating them, but to be sure you can run a fecal test on him. The hair pattern you describe does not fit with any worm related issues I have heard of, so it may just be the way he is shedding or he may naturally be “naked” there (I have seen this in many thin-skinned breeds like Arabians, saddlebreds, etc.). As an aside, Safe-Guard is not an ideal dewormer to use in combination with a daily dewormer. It is certainly safe to use, but it does not treat the parasites daily dewormers most often miss (like bots). An ivermectin or moxidectin would be more appropriate.

Black walnut can cause laminits and colic in horses. Although it is only certain oils in the tree that cause the problem I would not be comfortable using any product containing black walnut in a horse (especially since herbal product are not regulated for safety or content).

Furthermore, all of the “natural” dewormers on the market (herbal, diatomaceous earth, etc.) have been shown to be completely ineffective time and again, so they are not worth the risk to your horse.

Part of the roundworm life cycle involves migration of larvae through the lungs, where they are then coughed up, swallowed and enter the digestive tract to eventually be passed in the feces. Respiratory problems in young horses often have a roundworm component since younger horses are particularly susceptible to roundworm infections. The damage caused by the roundworms also leaves the horses more susceptible to viral and bacterial infections. Roundworms alone would not cause a nasal discharge, it would require a secondary infection to cause the symptoms you were seeing.

If your deworming program was effective, the nasal discharge was most likely a viral or bacterial infection that your foals were more prone to getting due to the stress of weaning. If you aren’t sure about the efficacy of your deworming schedule, you can have your vet perform fecal exams on your young foals just prior to your normal deworming time.

Almost all intestinal parasites could result in a horse’s death if there was heavy enough contamination. Death could result from stomach perforation (from bots), intestinal blockage from the mass of worms (from roundworms, tapeworms, etc.), damage to the intestinal blood vessels resulting in death of the intestines themselves (strongyles), intussusception –telescoping of the intestines on themselves- (from tapeworms, etc.), lung damage from worm migration (roundworms) and many other causes.

See the previous questions for an in depth discussion on the various types of dewormers to choose.

Check the previous questions for further details, but in short: probably not. You should still deworm at least twice yearly with ivermectin/praziquantel to prevent bots and tapeworms, which are spread without direct contact and do not show up on fecal tests. You may need to treat more often than that depending on the climate you live in. Periodic fecal exams by your veterinarian will also help you determine a more appropriate deworming routine.

Ringworm is not actually a worm or parasite, but rather a fungus. Treatment with topical antifungals is usually effective in most horses. If your horse is not responding appropriately, you should have him evaluated by your vet to ensure that the problem is in fact ringworm, and if so, that his immune system is healthy.

I have not heard of headshaking being reported as a side-effect of using Quest. That said, it is alway possible to develop an allergy to any substance, which could result in head shaking. If the head shaking resolved after a few days/weeks, I would be concerned about a possible allergy.

If however, the head shaking continued longer, it may have been coincidence that it developed at the same time as the Quest. Causes of head shaking include dental problems, arthritis in the bones in the throat/back of head area (called the stylohyoid bones), flies or ticks in the ear or a syndrome called photic headshaking. The last syndrome often is worse during warm sunny weather, so your comment that it started last spring makes me suspicious.

I would have your veterinarian complete a full workup for the source of your horse's headshaking.

Drier climates are not as friendly to parasites as wetter ones, but contamination is still possible. Your schedule should be adequate unless you have a particularly heavy parasite burden. If you have a closed herd, you might even be able to go every three months or so. The best way to determine this is to have a fecal exam done right before your next scheduled deworming. Tapeworms are much less common in your area, but you may want to occasionally deworm for them with a ivermectin/praziquantel combination product (tapeworms do not show up on fecal exams). This is even more important if your hay is brought in from out of state since the mite that spreads tapeworms can live in hay.

There is no such thing as a single chart that would be appropriate for every horse in every location and every situation. The simplest schedule for adult horses would involve ivermectin for each deworming and twice yearly ivermectin with praziquantel to treat tapeworms (the timing is not important, but they should be spaced out evenly). To date, ivermectin resistance has not been noted for any of the major parasite in adult horses, so rotating may not actually be necessary. Occasionally, pinworms seem to have some ivermectin resistance, so a dose of pyrantel or a benzamidazole can be used if your horses start rubbing their tails.

Younger horses (less than 2) should be treated more often with pyrantel or oxibendazole to better control roundworms, but an occasional dose of ivermectin or ivermectin/praziquantel should still be given. The ideal time for the ivermectin doses would be after the first killing frost (when there are no more bot flies) and again in the spring after the first good thaw (when strongyle migration is at it’s peak).

Warm, wet climates will require more frequent deworming than areas with hard winters or drier air. If this is your foal’s first deworming I would recommend a product like pyrantel (strongid, etc) or oxibendazole (anthelcide). The parasite of most concern in young horses is roundworms (Ascarids) and ivermectin is not as effective as the previous two products at treating them. As the foal matures (around age 2), roundworms are less of a concern and strongyle are a bigger issue, so ivermectin is then more appropriate. Young horses should still be dewormed with ivermectins on occasion, to prevent bots and prevent early strongyle infections.

Strongyloides (a different type of parasite than strongyles or roundworms) is passed in the milk and is often cited as a cause for diarrhea in young foals. It is easily treated with any of the dewormers on the market.

An exact schedule would depend on your horses’ exposure levels (new horses coming in and out, etc.) and prior contamination of your pastures. The best way to determine the best schedule is to have your vet perform periodic fecal exams.

In warmer weather like Florida, deworming will be required at least 3 to 4 times yearly. It may be needed more often than that depending on the prior contamination level of your pastures. Neighboring horses should not be a big problem unless their feces occasionally make it across your fence line. The best way to determine an appropriate deworming schedule is to perform periodic fecal tests to check the level of parasite exposure your horses have.

Deworming the mare after foaling does not help the foal build immunity to the parasites, but it kills off any parasites the mare may have so she is not infecting her foal with them. I generally recommend deworming all mares within 24 hours of foaling. The stress of foaling can trigger the development of encysted worms, so that is another reason to treat right after foaling.

Coprophagy (eating manure) is a normal foal behavior. Many experts believe it helps colonize the foal’s intestines with “good” bacteria from the mothers intestines. If your mare has been on a good deworming program and the pasture they are in is not heavily infected you should not need to deworm any more often than what you are. I usually deworm mares on the same schedule as their foals until the foals are weaned.

If it is not possible to deworm the other horses at the same time as your mare, putting her on a daily dewormer would help minimize the effect of any parasites she might pick up from the other horses. Alternatively, you may not need to be deworming your horse as often, if there is not a heavy parasite load on the pasture right now. To determine that you could have a fecal test run just prior to your normal deworming time.

Ideally, any horses that are sharing pastures, paddocks, stalls, etc. should be dewormed at the same time. In a perfect world, they would be dewormed on the same day, but deworming within a few days of each other is a reasonable compromise. Waiting more than a week would allow recently dewormed horses to be reinfected by horses that had not yet been treated.

The five day SafeGuard PowerDose is a pretty strong dewormer to start with. Since you already gave her some, however, you have probably killed off a lot of the parasites in her. If it has not been very long since you dewormed her, it is probably safe to give her a regular dose at this time.

Generally, in horses with heavy parasite loads, I recommend dosing with a half dose of pyrantel pamoate or oxibendazole, then dosing with a full dose of the SAME product in two weeks. It is important to not let more time pass between the 2 doses, and you should not repeat the low dose, or you can promote parasite resistance. Following the full dose at 2 weeks, I administer a full dose of ivermectin 4 weeks later. After that time, the horse can generally go on a regular deworming schedule if their environment has not been contaminated. Fecal checks can help determine how well the initial dewormings worked.

If you have ruled out parasites, there are several other possibilities that could be causing the diarrhea. Possible causes could include: food allergy, excessive sand/soil ingestion, malabsorption (the inability of her intestinines to get the nutrients out of the food she eats), etc.

The next best step is to seek the advice of a board certified Internal Medicine specialist. Even if your veterinarian is very experienced, a specialist may be more familiar with the less common conditions that might be affecting your foal.

Yes, all the dewormers on the market today have a wide margin of safety and can be reused shortly after one dose without any side effects. Some heavily infested barns deworm as often as every 3-4 weeks if needed.

The plan you propose is definitely an acceptable one. The parasites to be concerned about (because they don’t show up on fecal exams) would be bots and tapeworms. These should be treated at least twice yearly, regardless of fecal results, but for the rest of the year it is totally acceptable to base your deworming on fecal exams (it is also better because it decreases the chance of resistance developing). Many owners are not aware of this method of deworming, but it has been gaining popularity recently.

It varies depending on the drug, but ivermectin does not cause any signs of toxicity until about 10 times the normal dose. Pyrantel pamoate (Strongid, etc.) has been shown to cause no toxic effects at doses as high as 20 times the normal dose.

Mares can be dewormed within 24 hours of foaling to minimize parasite exposure to the foal. Essentially, all of the dewormers currently on the market are safe to give to a lactating mare, though only a few have this actually printed on the label. Also, at about 4 to 6 weeks of age, your foal should receive it’s first deworming treatment (pyrantel or oxibendazole dosed appropriate to it’s weight is my general recommendation).

Although there are not any specific interactions between dewormers and antibiotics that I am aware of, I would hold off on deworming any debilitated horse until they are feeling better. Unless there is a specific concern about parasites, it won’t hurt to wait a week or two until her temperature, appetite, etc. is back to normal.

The one exception to this would be in young horses where roundworm infection is more likely. Roundworm migration can cause lung damage, which can complicate respiratory infections even further.

I have never heard of deworming causing a flare-up of uveitis, and I have treated several patients that had recurrent uveitis for years (including two of my own horses) that were all dewormed on a regular basis. Because of the cyclical nature of uveitis, some people may have associated it with deworming only because deworming was done every few months and flare-ups happened to occur every few months (this is the same reason it is called “Moon Blindness” because people used to think the flare-ups were associated with phases of the moon).

Certain parasites (Onchocercha) can actually cause uveitis and they are easily preventable with regular ivermectin deworming, so I would definitely recommend a regular deworming routine in a horse with recurrent uveitis.

There have been document cases of large strongyle resistance to Strongic C. For that reason, it is always wise to get periodic fecal exams to ensure your horses are being protected. Assuming that you do not have a resistant strain on your property, it is not necessary for all the horses on a pasture to be on the daily dewormer, since any parasites your horses pick up will be quickly eliminated by your daily dewormer. They should all be dewormed at the same time, however, you should also treat them for tapeworms twice yearly. That way, an infected horse won’t be contaminating a recently dewormed one.

Don’t worry, deworming is not as complicated as it may seem!

Although deworming needs will vary by location, it is impossible to set one specific guideline for an area because there is also a huge variation in deworming needs based on a horse’s exposure, lifestyle, age, etc. That is why you’re having such a hard time finding charts specific to your region. Individual deworming companies have released their own deworming schedules, but they are obviously biased toward their own products and more frequent administration. The best way to determine the ideal deworming program for you and your horses is to consult with your local vet who knows your horses’ lifestyles, ages and medical history. I have mentioned a few basic guidelines in previous questions to help you get started.

In regards to small strongyles: the 5-day dose of Fenbendazole (Panacur, Safe-Guard, etc) will kill all the encysted stages of small strongyles. Moxidecin (Quest, ComboCare, etc.), is also very effective at killing mot stages of encysted strongyles (the brief period when it is not effective only accounts for a very small number of encysted strongyles). If a horse is on an effective deworming program its entire life, it will not have a heavy burden of encysted strongyles. The trouble arises when a horse is purchased later on in life and it’s previous deworming history is unknown. In that case, it is possible for a horse that has a negative fecal test (and has been negative for a while) to still harbor a large number of encysted larvae as they do not show up on fecal exams until they “hatch.” These encysted larvae may or may not cause problems in the future, but if they do, signs like chronic diarrhea, recurrent colic and general “unthriftiness” are typical. If these signs are seen in a horse, and other causes of the symptoms have been ruled out, many veterinarians will prescribe a course of Panacur or Quest to see if that solves the problem. Eliminating all the encysted parasites is not without risk, however. If a horse is heavily infected and a large die-off occurs, it can result in severe intestinal inflammation resulting in colic, diarrhea, etc. For that reason, many vets do not recommend treating for encysted strongyles unless they are causing a problem (making sure the benefits of treatment out-weigh the risks). Over several years (3 to 5, generally) encysted strongyles will slowly “hatch” and be killed by regular deworming programs, so a horse that has been thoroughly dewormed for that amount of time is unlikely to have many encysted strongyles.

As far as ivermectins and resistance goes, it is true that resistance is very rare. Recently, a few reports have come out about ivermectin resistant pinworms in adult horses, and ascarids (roundworms) in young foals on large breeding farms. It’s important to remember that these are very isolated cases. At this time, you probably do not need to rotate between classes of dewormers (other than to use a product with praziquantel twice yearly for tapeworms). Occasional fecal exams can verify that resistances have not developed. To further support this method, some parasitologists have shown that rotating too frequently can result in parasites that are resistant to more than one drug. They recommend that if you are going to rotate, you should rotate on a yearly basis (strongid one year, ivermectin another, etc.). Since avermectins are the only effective boticide at this time, you would still need to use them at least twice yearly (once after the first hard freeze, and again mid fly season).

I would definitely deworm your mare again at this time, since the stress of foaling can often lead to an increased shedding of parasite eggs (which will then infect the foal). After the initial deworming, I would recommend deworming her every two months while she has her foal at her side, assuming you do not have any major parasite issues on your farm. Any other horses sharing their pasture (even if at different times of the day or different days) should be on the same deworming schedule.

See the previous questions for an appropriate deworming schedule for your foal.

On farms with an average or below average risk for parasites, I recommend deworming foals with a full strength (for their body weight) dewormer at one month, two months, four months and six months. After that time, they can begin the same schedule as the adult horses. On farms with known parasite problems, or for foals that appear heavily infested, we will often treat them at 2 to 3 weeks with a half-dose of pyrantel or oxibendazole, and then follow up with a full dose of the same dewormer at one month of age. These foals should be dewormed monthly for the first six months, at which time they can begin the same schedule as the adult horses. Young foals are most susceptible to round worm infections, so I usually recommend the first two dewormings be done with pyrantel pamoate or oxibendazole. Subsequent deworming can be done with any of the dewormers other than moxidectin (not recommended in foals less than six months of age).

Unless a farm has a severe roundworm problem, deworming every week would be excessive, though probably not harmful.

All the standard over the counter dewormers currently on the market are considered safe for pregnant and lactating mares, but only a few actually have the indication on the label.

In general, I recommend deworming a mare one month before her due date and again shortly after foaling. I generally use an ivermectin product at those times, but most dewormers would be effective as long as the mare was on a good parasite prevention program prior to that time. Young foals are most susceptible to round worm infections, so I usually recommend the first two dewormings be done with pyrantel pamoate or oxibendazole. Subsequent deworming can be done with any of the dewormers other than moxidectin (not recommended in foals less than 6 months of age). On farms with an average or below average risk for parasites, I recommend deworming foals with a full strength (for their body weight) dewormer at one month, two months, four months and six months. After that time, they can begin the same schedule as the adult horses. On farms with known parasite problems, or foals that appear heavily infested, we will often treat them at 2 to 3 weeks with a half-dose of pyrantel or oxibendazole, and then follow up with a full dose of the same dewormer at one month of age. These foals should be dewormed monthly for the first six months, at which time they can begin the same schedule as the adult horses.

An occasional missed day here and there is unlikely to affect the overall effectiveness of the daily dewormer. If several days in a row are missed, however, it is possible that your horse could develop an established strongyle larvae infection. If you are concerned about the effectiveness of the dewormer, you can submit a fecal sample to your veterinarian, or you can administer a standard dewormer occasionally (twice yearly treatments for bots and tapeworms should still be given while on a daily dewormer).

Since your horse is by itself, it is at lower risk for infection than many horses. Although he shares his pasture with cattle, there are not very many parasites that are shared between the two species.

At a minimum, your horse should be dewormed twice yearly for bots and tapeworms using an avermectin (ivermectin or moxidectin) and praziquantel. These can be spread without direct horse to horse exposure, so your horse is still susceptible to them. Depending on where you live and any prior pasture contamination, additional deworming may also be necessary. The best way to determine the ideal deworming frequency for your horse would be to submit periodic fecal samples to your veterinarian. In most areas of the country with a distinct winter, horses should be dewormed at least 3 to 4 times yearly. In warmer, moist climates more frequent deworming may be needed.

At most boarding facilities, horses are at higher risk for parasite infection because all the horses are often on different deworming schedules and there are lots of different horses coming and going all the time. For that reason, I would recommend continuing to deworm your horse every 2 to 3 months. Ivermectin alone is fine for most dewormings, but you will want to use a product with praziquantel in it to treat tapeworms twice yearly. Also, it is important to use a boticide (ivermectin or moxidectin) AFTER the first good freeze to kill off any bots your horse may have picked up in the fall.

As you know, there are benefits and disadvantages to either type of deworming.

Daily dewormer does keep the parasite burden down at a lower level all the time (it does not actually PREVENT infection though, it just treats it at an early stage). Daily dewormers are very safe, and are no more “poisonous” than any other medication we give our horses (even salt is poisonous in excessive amounts). Daily dewormers do not kill every type of parasite, however (namely bots and tapeworms), so it is important to treat your horses with a product containing an avermectin (ivermectin or moxidectin) and praziquantel at least twice yearly. One downside to daily dewormers, is that occasionally horses that have been on it their entire lives do not develop some of the natural immunity against parasites that a normal horse would have. If you then stop the daily dewormer, some of these horses are more susceptible to parasite infestations for several months/years until their natural immunity develops. This is not a big issue as long as owners are aware of it and are more thorough in their deworming program during that time. For this reason, and because some parasites (like strongyles) can encyst inside the wall of the intestine where they are not affected by daily dewormers, I recommend using daily dewormers year-round if you are going to use them. Although rare, it is possible for resistance to develop against daily dewormers so periodic fecal exams should also be done. On the plus side, Pfizer, the company that makes the Strongid C2X you are using , has a program that helps pay for colic surgery (of any cause, not just parasites), if your horse colics while on their product. To be eligible, you must purchase your Strongid through your veterinarian (so they can verify that you are using the correct amounts), and your horse must have regular checkups (twice yearly) from your vet. This is a nice added “insurance,” especially if you are planning to feed the product anyway. Contact your veterinarian for more details and to see if they participate.

Periodic deworming, every 4-12 weeks depending on the situation, is also effective at controlling intestinal parasites. Some advantages include a cheaper overall cost (depending on the type and brand of dewormers used), ability to use varying classes of dewormers to minimize resistance, and the ability to tailor the deworming program to treat parasites seen on fecal exams. Periodic deworming does allow parasites to remain in the horse for a slightly longer period of time, but if timed correctly, the deworming will take place before the parasites can cause damage to the horse. This method also allows horses to develop some natural immunity to certain parasites as well, so they are less likely to have as severe a response as a naïve horse would have. Disadvantages include the difficulty in administering paste dewormers (though newer, easier to administer forms are now being released), the need to remember to administer the dewormer at the correct time, and the fact that a higher (though still very safe) dose is given at one time.

Neither method is right or wrong, and I have found both methods to be equally safe and effective.

From your description, it sounds like your horse may have “rain rot”, a skin condition caused by a bacteria called dermatophilus congolensis, rather than a parasite problem. The bacteria thrives in warm moist conditions, so spring rains, winter hair that traps moisture along the back, and excessive blanketing that causes the horses to sweat on his topline have all been associated with rain rot. Removing loose hair, clipping if it is excessively long, avoiding heavy blankets (and washing them thoroughly), and providing cover from heavy rains will help alleviate the problem. Horses with weakened immune systems are at increased risk, so if your horse does not have some of the predisposing conditions I’ve previously mentioned, you should contact your veterinarian for a check up.

Parasite problems (like lice and pinworms), usually result in mane, tail and chest hair loss, since these are the easiest areas for horses to scratch.

If your horse is alone is his pasture and his fecals have been negative he is at very low risk for most of the common intestinal parasites. Be sure to monitor his fecals closely over the next few months as the weather warms back up though, as many parasites are dormant in the winter months. If they continue to be negative my biggest concern would be bots and tapeworms, which do not need direct contact with other horses to be spread, and do not show up on fecal exams.

I would still recommend deworming your horse twice yearly to prevent bot and tapeworm infestation. Most dewormers are very safe, but it is always possible for reactions to occur in individual horses. It is impossible to tell what ingredient your horse might have reacted to (often horses are not reacting to the main drug, but rather the other components that the drug is administered in). To be safe, I would use a different brand and drug to deworm your horse. If he reacted to ivermectin, I would use moxidectin instead to treat the bots. If there was praziquantel in the dewormer he reacted to then I would use a double dose of pyrantel pamoate in addition to the boticide (ivermectin or moxidectin) to treat tapeworms. If you find you do need to deworm more often you can also experiment with some of the newer forms of dewormers (Ivermectin now comes in a pellet, for example), to see if that prevents a reaction. Anytime you are trying a new type of paste dewormer you might want to “spot” check a small area of your horse’s mouth with a dab of wormer first, then wait a few days for signs of a reaction.

There really are not any chemicals out there that will treat an infected pasture. Removing any manure and harrowing the pasture will minimize contamination and break up the mat of organic material that many eggs and larvae live in. Exposure to heat, sun and freezing temperature will kill many of the parasites in a pasture, but some (like ascarid eggs) are extremely resilient and can survive for several years in the right environment. The best treatment is prevention, but when that is no longer possible, maintaining your horses on a regular deworming schedule will break the cycle of parasite infection before it can cause any damage to your horse. Be sure to treat all the horses at the same time to minimize horse to horse contamination. Regular fecal exams will show if your deworming program is effective.

It’s impossible to guess how heavily infected these horse might be. If they were on a fairly consistent and effective deworming program initially and new horses were not coming in and out of the herd, they may not be severely infected now. Since all the horses are now adults, they are less likely to have the severe roundworm infestations we often see in young horses that are the most common cause of impactions, but it is still possible. If, however, the original deworming program was less than adequate, these horses could now have quite the heavy worm burden. If the horses appear to be in good health, it is less likely that they are severely infested, but that is never a guarantee.

The safest way to develop a deworming plan would be based on fecal samples. You don’t have to get an individual sample from each horse, a few random “community samples” from each pasture will give you a good idea of the overall status of the herd. If there are specific horses that you are more concerned about (or ones that were housed in higher risk situations, i.e. small paddocks), it would be ideal to get one from them, but since they have all been in a fairly close environment, they should all have pretty similar parasite loads. Shoot for about 20 to 30% of the horses (so if you have 30 horses try to get 6 to 9 individual samples).

If the fecal samples show a high worm burden, or if you can’t get the samples and want to play it safe, I would start with a half dose of a weaker dewormer like pyrantel pamoate (Strongid, etc.) or Fenbendazole/oxibendazole (Panacur, Anthelcide, etc). If that goes well, follow-up in two weeks with a full dose of the same medication. Two weeks later, use a full dose of ivermectin. To be safe, I would not use the combination Ivermectin/praziquantel (Zimectrin Gold, Equimax, etc.) until they have had a normal ivermectin dose first. Since the praziquantel will kill off tapeworms, it would be better to be sure the horse is clear of other worms first so you have less species of worms dying off at once. Four weeks after the ivermectin dose, you could safely administer the combination product. I would continue deworming every four weeks for the next 3 to 6 months depending on your weather. Very hot, dry weather or a hard freeze will help kill many of the parasites in the new pastures that may be reinfecting the horses. Warm, humid weather, on the other hand, will allow the parasites to last much longer outside the horse, so you will want to continue with the more frequent deworming.

I would avoid moxidectin (Quest, Combo Care) or the high dose fenbendazole (Panacur PowerPac, Safe-Guard Power Dose) for the first year or so unless specifically directed to use it by your veterinarian. Although it is tempting to try to wipe out any encysted strongyles, you can’t be sure of how severely infected your horses were (even a fecal now would not tell you if they were more severely infected previously). Horses with a high number of encysted larvae are at increased risk for complications if the larvae are all killed off at once. In about a year (around next spring), the number of encysted larvae should have decreased significantly and you can more safely use these products (and I would certainly recommend them at that time).

Rotating dewormers is a hot topic these days. Resistance to Avermectins (ivermectin and moxidectin) is extremely uncommon so far (though a few cases of resistant pinworms have been reported in adult horses and resistant roundworms have been seen in young horses on some large breeding farms), and it is the only class of dewormer that is effective against bots, so you could just use ivermectin (and the occasional ivermectin/praziquantel combination for tapeworms) and be pretty safe. Periodic fecal checks (again, a few representative samples would be fine) would help confirm that resistance has not developed. Some people advocate rotating dewormers so that resistant parasites can not develop. The dilemma is if dewormers are rotated too often, you can actually develop “super resistant” parasites that are resistant to more than one class of dewormer. There is no good answer to the rotation debate yet. Regardless of what you do, the most important way to avoid the development of resistance is to use the full dose of dewormer recommended. Using half-doses should only be done in extreme situations (as described above) and should not be repeated. It is also important to not underestimate the weight of your horses. A weight tape is a cheap purchase to help with a better estimation, and when in doubt, guess high. Also, try to ensure your horse receives the full dose and doesn’t spit half of it out on the ground. All the dewormers on the market have a wide margin of safety for overdosing in adult horses (moxidectin has a narrower range, so should be used cautiously in young horses, small ponies and minis).

If your horse is the only horse that ever uses that paddock and his stall, he is at very low risk for many of the common intestinal parasites. Most parasites require horse to horse transmission (generally via feces). Some types of parasites, however, are spread by other means. Certain types of tapeworms, for example, are actually spread by a grass mite that can survive the hay making process, and stomach bots are spread by flies that lay eggs on the horse’s hair coat. These types of worms do not generally show up on regular fecal exams because they do not shed their eggs in the horse’s feces. For that reason, every horse should still be dewormed at least twice yearly with a product that treats both bots and tapeworms. Avermectins (ivermectin and moxidectin) are the only approved over-the-counter dewormers for bot control and praziquantel is the most effective drug for treating tapeworms (a double dose of pyrantel pamoate was the preferred method in the past, but praziquantel has been shown to be more effective and is the only drug licensed to treat tapeworms in horses).

There are several commercial products that contain both of these dewormers, so a single treatment can control both types of worms. Periodic fecal counts for the other types of intestinal worms (ascarids, strongyles, etc.) and appropriate treatment only when needed will help ensure your horse is well protected without exposing him to unneeded dewormings and will minimize the risk of developing parasites that are resistant to the common dewormers.

Small strongyles, a specific type of intestinal parasite, go through several stages as they develop from eggs to larvae to adult worms. Two of these stages (called the L3 and L4 stages) “encyst” within the lining of the intestines. These encysted larvae are protected from most deworming products because they are not actually within the intestines and they are not particularly active so they are not affected by many of the drugs we traditionally use. Over time, these encysted larvae can “hatch” and develop into adult worms, which will then begin to shed eggs in the feces. The length of time it takes for a larva to hatch varies considerably, sometimes taking as long as 2 to 3 years. Since strongyles can live a “protected” life for so long in the intestine, it only takes occasional exposure (every 2 to 3 years) to keep a horse infected. While the larvae are encysted, they do not produce any eggs, so they will not show up in a fecal. There are currently only two drugs on the market that have been shown to be effective against encysted larvae. Fenbendazole, given at a double-dose for 5 days or moxidectin, given in a single regular dose, treat for late L3 and L4 stages.

Your horse is probably at low risk for small strongyle infection from other horses, but since strongyles can “hide out” for so long, it would be possible for him to re-infect himself occasionally. Using a combination moxidectin/praziquantel product twice yearly would treat for encysted small strongyles, bots and tapeworms all at the same time. Periodic fecal exams in between deworming would ensure that your horse has not picked up any parasites in the meantime.

If the worm was fairly large and seen in fresh manure it is most likely an ascarid (also called a roundworm). Ascarids are very common in young horses, even on well managed farms. As horses mature, most seem to become more resistant to them, so it is unusual to see ascarids in adult horses unless they are exposed to them in very high numbers. Strongyles, bots and tapeworms can also be seen in the feces on occasion, but they are usually more difficult to notice unless high numbers are seen.

Here are a few sites that have pictures of the more common intestinal parasites in horses:

www.animalrangeextension.montana.edu/articles/equine/intlparasites/background.htm

www.valleyvet.com/si_worm_facts.html

nz.merial.com/horse_owners/disease_info.html

The best way to determine the type (or types) of worms your horses are infected with is to take a fresh fecal sample to your vet. The majority of worms that can be present in horses are only passed as eggs in the feces and cannot be seen by the naked eye. A fecal sample will also confirm that your new ponies are not shedding any eggs that will infect the rest of your horses. Keep in mind though, parasite eggs can be shed for several days AFTER you have dewormed your horse, and ascarid eggs in particular can live a very long time in the soil (up to several years). The area your ponies have been living in right now should be considered contaminated, and any horses that are housed on it will need to have fecal samples checked or receive regular dewormings.