The Worm That Kills – And Why Only Two Worming Chemicals Can Stop It


Guest Post: After worming with standard wormers, some horses become critically ill with colic. Some even die. This post by Ann Nyland explains why this can happen in horses that haven’t received chemical wormers for a long time. 


New guy in town: the encysted small strongyle

The old idea of worming in rotation lingers on from the early days, when it was first put forward in 1966. In the 1960s, the dangerous worm was the large strongyle (Strongylus vulgaris) and worming treatment in the 1980s and 1990s targeted this worm.

Yet today, the problem worm is the small strongyle (cyathostome).

Rotation is no longer advocated by equine parasitologists. At any rate, no amount of rotating will help against encysted cyathostomes.

Unfortunately, most advice given today is, sad to say, still aimed at the old way designed to  eradicate the large strongyle – even though this worm is no longer the biggest problem.


Too much information – and it’s often wrong

Misinformation about equine worms is all over the net, from natural therapists to chemical companies. Horses have died because of this misinformation.

Don’t make the mistake of thinking that if something is said about a product, even on companies’ advertising or seminars, it must be true.

It isn’t. On the contrary, It’s common to find statements on ads about wormers such as:

  • “100% effective against all worms of horses and bots.”
  • “(Non-moxidectin, Non-fenbendazole product) safely and effectively rids horses of all major internal parasites, including tapeworms, in a single dose.”
  • “(Chemical) eliminates all common horse worms and bots.”
  • “(Non moxidectin, Non fenbendazole product) has the capacity to treat all common types of parasitic worms (including tapeworms) and bots.”
  • “(Non moxidectin, Non fenbendazole product) has the best combined efficacy and the broadest spectrum of activity of any wormer.”
  • “(Product) completely protects young horses from ascarids, as well as all other worms.”

All of the above statements are misleading.  There is no one product on the market that can be 100% effective against all worms.

In fact, only moxidectin or a single dose of fenbendazole for five consecutive days are effective against this encysted parasite.

So, moxidectin in Equest / Quest / Farnam ComboCare, and fenbendazole in Panacur 100 (or WSB Fenbendazole in Australia), will do the job. If in the US, you need to double the dose of WSB Fenbendazole.


Exactly why are encysted small strongyles so deadly?

Firstly, only two chemicals of all the available wormers out there can kill them. No other wormers will have any effect whatsoever.

What’s more, these encysted worms can stay encysted for years. (Encysted means that it is enclosed in a cyst in your horse’s intestine, after the larvae have burrowed into the intestinal wall.)

Now, encysted worms are normal part of the small strongyles’ life cycle. It’s a normal stage for them. When they finally develop into 4th stage larvae, they emerge from the cyst and enter the large colon. They then become adults, and the cycle starts again.

Now we come to the problem with them: the process of emerging. If there’s a huge number of them, the process of emerging may kill a horse. Even if there are fewer, but still a lot, the horse is likely get colic and/or scour and/or get edema. Or, they may be found dead in the paddock. In Australia, the owner usually assumes it is a snakebite.

The reason for this is that when they emerge, they release toxins from accumulated larval waste products.


What happens if a different wormer is used instead?

When a horse who has a lot of encysted small strongyles is wormed with a standard wormer (in other words, a wormer that isn’t moxidectin or fenbendazole based), the small strongyles encysted in the lumen (lining) of the intestine aren’t affected. Instead, the standard wormer kills the worms that are not encysted.

These worms then die and are passed out of the horse in manure.

This isn’t too bad if the horse doesn’t have many of them. However, if the horse has a lot, this is what happens next.

Because a lot of small strongyles have been killed in one go, those nasty encysted small strongyles, who have been sitting safety inside the horse untouched by the standard wormer, are given the signal to emerge all at once. They do so in big numbers, ready to replace the ones that the standard wormer has killed.

In emerging, they come right through the wall of the horse’s large intestine. They bring with them a large amount of toxins. This is what can kill your horse or give it colic.

If instead you worm with moxidectin (Equest) or a five-day dose of fenbendazole (Panacur 100 or WSD Fenbendazole), not only are the non-encysted adult small strongyles killed, but you will also kill a whole bunch of the encysted ones who were waiting to replace them. There will be no mass emergence of strongyles and no release of toxins.

These wormers are the only safe ones to use on horses with a suspected heavy worm burden.

To recap:

1) Moxidectin and fenbendazole wormers are the only ones that can kill the encysted small strongyles.

2) With other wormers, horses with a heavy worm burden can get sick or die when the encysted strongyles, which haven’t been killed, emerge through the colon wall to replace the non-encysted strongyles that have been killed.

This process is called Larval Cyathostomosis and it is damaging to your horse. The symptoms are colic, weight loss, diarrhea and/or subcutaneous odema. In the worst cases, the outcome is death.


How can you tell if your horse has encysted small strongyles?

Answer: you can’t. Horses can look very well and still be full of encysted worms.

Just imagine that encysted worms have been sitting there, possibly for up to three years. You can be worming regularly with standard wormers and they’re not affected in the slightest. What’s more, your horse can be looking and even performing very well.

Your standard wormer does nothing against the encysted small strongyles, so just ignore advertising claims that these wormers ‘kill all worms’.

If you have never given moxidectin, or fenbendazole for five consecutive days, well, that could be a cause for concern, depending on your geographical location and your horse’s individual circumstances.

Of course, some horses are more prone to worm infestation than others – the old saying “Some of the horses have most of the worms” is correct. In any group of horses, 20% will carry 80% of the worm burden. Many healthy horses have an effective immune response to worms, which keeps the numbers low.

That is, until the horse becomes sick, is badly fed or gets a large number of worms.


But don’t encysted small strongyles show up in fecals?

Now here’s a problem: encysted small strongyles don’t lay eggs.

Repeat: they do not lay eggs.

If you have an egg count done on your horse’s manure, it will not show how badly your horse is infested with encysted small strongyles. This means the egg count could be zero, but your horse could still be infested with these worms. It’s simply impossible for fecals to show how many encysted small strongyles are encysted within your horse’s intestine.

Your horse may not even look ‘poor’. In fact, your horse could appear to be in glowing health and still have a heavy worm infestation. Fat, shiny horses have died from worms. Don’t be fooled into thinking a horse is not heavily infested with these worms just by appearance.

Encysted small strongyles can’t be seen by the naked eye. You won’t see them, ever.

So, if your horse has ever had colic, has scoured or has been ‘off’ after worming, consider treating for encysted strongyles.

Here’s how to put together a program of treatment for your horse with neck threadworms (and maybe the Itch) that includes treatment for encysted small strongyles as well as the ivermectin treatment – How to Fight the Big Fight Against Neck Threadworms

Using moxidectin or fenbendazole for encysted small strongyles

Equest (moxidectin) is perfectly safe for horses, despite the scare-mongering and other such utter nonsense to be found on the net. Trials have been conducted where foals were highly overdosed (details and references are in my book – see below.)

If you have minis or small ponies and don’t have an accurate knowledge of their weight, you can use Panacur 100 (fenbendazole) for five consecutive days.

I immediately worm all rescue horses arriving here with the full dose of Equest for their weight, and then follow up at two weeks, and then again two weeks later. I don’t wait for the horse to gain weight until I worm – that road is paved with dead horses. And nor do I give them half a dose – again, that is a serious mistake.


Ann Nyland lives in New South Wales, Australia. With a PhD in equine physiology, she has published several books on worming for horses.


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  1. Lisel O'Dwyer says:

    Thanks for this info, very interesting and useful.

  2. Dr K Van Laeren says:

    I dont buy your sentiments regarding fenbendazole. Fenbendazole resistance is well documented. If the worms in refugia are the fenbendazole resistant type then a five day or longer course will still be ineffective against encysted larvi.

    Also moxidectin has been showed to cause far less intestinal damage after killing of encysted larvi than the five day course of fenbendazole. However it works slower than Fenbendazole at killing thes worms.

    • I’ve passed your comment on to the author.

    • Ann Nyland says:

      Obviously, any worming chemical to which there is resistance will not work. Fenbendazole resistance is well documented which is why parasitologists suggested to use it for 5 days in row. These are NOT my sentiments: these are accepted, non-contentious facts fully supported by references to peer-reviewed, scholarly papers by equine parasitologists in academic journals. Moxidectin is also shown to have resistance now. This is a blog article not a fully referenced book like my worms book nor is it a research paper – it is a summary for laypersons. . Please refer to my book.

      • I’d also like to add on that if there are alreadyalready dead strongyles in the horses stomach or intestines, and you worm with only ONE strong dose that kills fast instead of worming for several days small doses which kills slowly, chances are that the intestines become so obstructed that they rupture and kill the horse. So I think personally I’d take the slower route and make sure nothing like that can happen. Especially with youngsters these precautions need to be taken.

      • I recently attended a veterinarian conference where this very issue was discussed in a parasite seminar. . The 5 day course if fenbendazole is far less effective than moxidectin.

        • Thank you.

        • As much as that might be the case but horses also haven’t died yet from using panacur but with moxidectin that’s a complete different story.

          • Hi, please can you direct me to the reports of this? I’d like to read them.

            • Luv2Train81 says:

              Just Google it. Should pop right up. It’s not to be used in minis and foals. I read a few incidents of yearlings dying as well. You gotta be extremely careful bout the dosage. Also I your horse has been on a consistent worming schedule or not. If the horse carries a very high worm load or if it’s older. Weak immune system, sensitivity to the ingredient etc.

  3. Emily Phillips says:

    I wish I had read your information before I had a problem. My new mare had badly overgrown splayed feet when I bought her. She didn’t look wormy and I wormed her with a wormer that did not deal with encysted worms. 3 weeks after this she got acute laminitis. A few months later because of another horse who was sickly, I ran a fecal and tested her because it wasn’t much money to do a second horse. The sickly horse showed very little eggs however she was heavily overloaded. 1470 small strongyles eggs per gram. This confirmed that she had had mechanical laminitis and the inflammatory response caused by worming tipped her over into acute laminitis. 6.5 months on now she is doing much better but it has been a long and painful journey.

    We cannot get Equest from retailers in Australia. Only Equest Plus and I have heard that there is a risk of inflammatory response associated with Praziquantel when that wears off?

    Given her history should I continue giving her anti-inflammatory and antihistamine when I worm her?

  4. Thank you for the info I have a rescue horse being delivered in a few weeks I will be dosing her as soon as she gets here and 2 and 4 weeks after to make sure I have her ready and on the road to happy healthier lifestyle

  5. Our specialist equine vet and I had a chat about this the other day (he brought it up when visiting).

    He said at any point in time, 20% of the worms in a horse’s body are dormant (eg attached to the gut wall, that’s what encysted means), and these dormant ones aren’t killed by worming paste that lasts in the horse’s body 24 hours. The only reason the Moxodectin-based pastes work is they last in the horse’s body for 5 days as the dose is a lot higher. That can be potentially good and bad.

    He also said that having some worms in a horse’s body is natural and needed, and actually helps the auto-immune response. With some immune diseases in people eg crohns, they have actually used introduced worms to help with patient care. And that faecal counts are fine and a good predictor of actual baseline worm counts as all vets know at any one time 20% of the worms are at the stage in their life cycle where they are dormant and attached (ie encysted).

    There are worms that are resistant to both Moxodectin and Ivermectin, and by using 2 different chemicals, we will just end up with worms that are resistant to both chemicals.

    He also agreed that Equest is not currently licensed in Australia for foals under the age of 6 months, as it is too easy to overdose them with the Moxodectin and the long acting drug can be too much of a shock for their system. Equest don’t print that on the package in Australia. He said to use something without Moxodectin for the first 12 months of life to be on the safe side.

    Just an alternative view …

    • Thanks for the reminder🙂 It caused me to remember listening to a radio program here in the US called NPR where this man used hookworms to cure his severe allergies. I was repulsed listening to it but it gave me a new perspective!

  6. Jessica Ryan says:

    Hi Ann,
    I’ve just been reading through your extremely interesting article on the issues of encysted small strongyles -very alarming I have to say!
    You say that there are no eggs to show up in a faecal count, so how are they actually spread through a horse population? I’ve tried googling it, and there is lots of references to their eggs, so finding it all a little confusing… Thanks

    • because the encysted do not lay eggs, they are the eggs in a sense, they are the portion of larva that burrow in and wait until the adults are gone to emerge, SO, once you deworm with a lesser wormer and have never killed the encysteds, its a signal to come out… and they do, all at once.

      • Luv2Train81 says:

        Not quite. Yes they are the larva but they do develop into the adult stage during the encysted period and do lay eggs. But the eggs usually are passed with the feces and then usually develop there into the larva again which are ingested again by the horse to burrow back in the intestinal lining. The adults usually lay the eggs inside but they die after that.

      • I lost 3 in January from this. Devastated and heartbroken. I am so worried about my other 5.

  7. RICHARD GREER says:

    How many times do you want to mention dead horses? In over 20 years as a pro’ horse trainer with experience on 3 continents dealing with many neglected horses some suffering worm induced colic I have never seen one or ever heard of one killed by worms, I don’t deny it happens but really ‘Roads paved with dead horses’ A scare mongering bad joke.

    • Hi Richard, it’s not my research, but I believe the point here is that these horses would mostly be classified as colic deaths, unless a necropsy and further tests were completed. So, it’s not a bad joke, although the language may be emotive.

      • My horse just went into new Bolton for what most would think colic signs to the point they almost cut him open….. I requested a fecal count for small strongyles and sure enough the colic symptoms have been from that! I definitely see where it can oil horses quick as my guy is very stoic and he was very close to not pulling through. We will treat him with Quest plus. I’m doing ulcer guard 28 days , bentonite clay, BVC minerals and aloe juce ….I’m just so worried for what we can’t see ( encysted). Im now wondering f I should do quest plus tomorrow and then again in 2 weeks then in 4????

    • Its definitely no joke….this has happened to 2 horses that I know of in the last couple of years….the vet gave us a vague discription about this 3 years ago when it killed my niece’s 10 yr old horse she had bought and only had for about a year… and then a couple weeks ago same thing happened with a friends 11 month old colt….both went down within hours after being wormed….thanks so much for this article, it really helped me to understand why this happens

    • It is horrible to watch. They suffer and the Vets can’t help stop it. It’s massive amount of diarrhea and loss of balance and it’s like they have a heart attack. Been around horses for 60 years and never seen anything like this before.

    • Kylie Graham says:

      I lost a valuable yearling yesterday from this very reason, I too have not seen this before, & have been breeding all my life. But it certainly can happen & I thought I was a good manager of my horses. I think conditions have been perfect for these worms in this in seasonally wet winter here at Taroom Qld.

  8. Leslie Gilley says:

    I normally do not stall horses in the barn…It is kept for ill or hurt horses….w/that said…I brought a gelding in that I was trying….put him in one of those stalls…he seemed to have issues hurting turning the first barrel…I treated him for ulcers…kept him on ulcer meds….about 3mos..he colicked soooo severly that he had to be put down…an autopsy was performed and the Vet said he had worms not from our area (Ft Worth, TX)….He was insured….but because he wasnt mine
    I never got to see the report…he looked perfect on the outside…shiny coat…carry good weight….

    My question….how contagious are these worms…can they be picked up out of a stall? Reason being…Ive lost 2 more from being in and out of that stall since then….(stall cleaned but not stripped)….Ive lost 2 more.. to colic…no other colics of any of the horses….Can they oick them up like that???? If so putting lime in the stall….before stripping…will kill them??? OR how to kill them….I need the use of that stall…


    • leslie gilley says:

      I sure would appreciate an answer from uall…about other horses picking up these worms from a stall infected by a previous horse…I have not used this stall in 2 yrs….what do u suggest to kill all worms who have been voided in this stall before it is stripped

      • Luv2Train81 says:

        I highly doubt worms can survive for two years in a stall. Most worms die the second they are exposed to fresh air. Only way to survive for a short time would be in the manure pile. I dont think you have anything to worry about there. I’d just strip it clean. Spray it with bleach or trifectant and let it soak for a while. Btw saw dust will kill them too.

  9. Connie P says:

    I had a mare that almost died from these worms. She kept losing weight, started having cow pies instead of normal manure, she also became very lethargic and would fall asleep and almost fell over waiting for the farrier one day. Someone happened to mention this type of worms so we talked to our vet who put her on ivermectin for 5 daily doses. Then 30 days later we wormed with Quest. Amazing change, she gained all her weight back and was in excellent health after that.

    • Connie P. This sounds like a better idea for those that have preciously used ivermection. Had ivermectin been your wormer of choice previously? And are you concerned about her developing a resistance to ivermectin after doing 5 does daily? And what worming schedule are you currently using now? I have rhis issue with one of ny aged mares and I’m very concern about using both wormers. Im starting with a fecal egg counts, but not sure how to proced after that.

  10. I will never use Equest with moxidectin ever again as my horse became very unwell with 24 hours of being wormed with this wormer. She did end up with colic and the vet operated only to find her bowel had twisted and we had to put her to sleep on the operating table. One very upset owner and a vet clinic with upset staff. I had used pancur 100 in the past and never had any issues but Equest was the new go to wormer which everyone including the vet were recommending. Have found out since that moxidectin can cause allergic reactions in some animals (it is also used in some dog wormers with similar results). I found out that year I was not the only person to lose a horse by using Equest wormer. I have never lost a horse or pony before or since this experience and I never again use any wormer with moxidectin in it.

  11. When worming for five days in a row with Ivermectin is the recommended dose given each day?

    • Follow the dosages on the label. There’s no special protocol suggested in this article, just the choice of product.

      The five day dosage is Panacur 100 (fenbendazole).

  12. Lauren Paytes says:

    With documented cases being AGAINST using Quest in donkeys, what would be the correct course of deworm in for them?

  13. I just got a mini mare and when I dewormed her with Strongid she passed a glorious cacophony of worms. I saw some round things; I thought maybe enteroliths but I picked it out of the poo and it was full of worms. She also had a worm that was 18 inches long, sort of flat and segmented.

    Now she has loose stool, green cow pies. She had regular poo before I dewormed her.

    After reading this article, I don’t know what to do with her. I was going to do another dose of Strongid today but think I will do the fenbendozole instead.

    The day after she was dewormed she was feeling bad. I gave her a dose of Banamine and she was better the next day. She acted like she had mild colic. I am worried about killing her now that I read this article.

    • Follow the advice and you’ll be on safe ground.

      Also, Ann’s book is now available free of charge:

      • I talked to my vet and she suggested I deworm with Quest so I got the Quest +tapeworm medicine because of that 18 inch long thing that came out of her. I did that and was disappointed that I did not see any wild and crazy worms in her poop this time. She did not get sick or have a tummy ache. The first time I dewormed her she was in a lot of tummy pain so I gave her a dose of Banamine and she was fine. This time no upset tum for her.

      • checked on the book and it said no longer published.

        • I got a notification regarding a recent reply on this thread but now that I followed the link to make a reply, the post seems to be gone.
          So here is a copy/paste of the contents of the notification email:-
          “in response to Jane @ THB:
          Follow the advice and you’ll be on safe ground. Also, Ann’s book is now available free of charge:
          checked on the book and it said no longer published.”

          I also clicked on that link as I wanted that book too & yes it is not published anymore.
          I went to Dr Nyland’s Amazon page but the book is not there, even for free download.
          I did purchase the other book “What you didn’t know about worms will surprise you”.
          Then when I added it too my Horses folder on my PC I discovered I already purchased that one last year! oh well no worries 😀

          Anyway it would still be good to locate the book “The Worm that kills.”
          Does Dr Nyland have her own website?

          • Hi Clarissa, I”m not sure what’s happened with previous posts. Occasionally one pops in that is obviously spam or a linkback for a spammy site, so I don’t let those through. I don’t always register which post they’ve been added under.

            I’ll check in with the author of the post and the books and see what’s happened on the publishing side… I’m sorry the link is out of date.

      • Pat Gauvreau says:

        Book no longer available ?

  14. Rebecca says:

    Hello and thank you for a very informative article. I have a question regarding lungworms (Dictyocaulus arnfieldi). First a back ground story:

    A mare moved to our barn about 12 months ago, in March. At the time, I gave her an Ivermectin based paste and isolated her 1 week. She was then put on the same deworming schedule as everyone else, every 8-10 weeks rotating active ingredients. I was informed that the horse had received regular deworming paste/shots where she was before.
    After about 2-3 months, the horse started coughing. The owner did not call a vet but relied on homeopathic drops. The coughing got worse. Also turned out she was in foal, and in September she gave birth. After foaling, she very rapidly deteriorated, and in December I talked the owner in to calling the vet. He diagnosed her with COPD. A few days after he left I found a worm in her feces, and sent him a photo. I asked if it was possible this was lungworms, he said no, that there is no such thing where we are. So we treated the horse with antibiotics, bromhexin, and inhalers, all for the diagnosed COPD.
    In February I found worms again, asked the vet again, he still said no way it was lungworms.
    We called another vet who did fecal analysis and we dosvoered it was indeed lungworms.

    This past year I gave ALL the horses in my barn regular dewormers (Normectin, Equest duo, Normecton,x2, Panacur). The Panacur was given 2 months before the analysis confirmed lungworms.

    Here is my question:
    Is it possible that the mare brought the lungworms with her from where she was before, and that they somehow survived the first worming I gave her? We have never had lungworms. The mare had been in pasture with several donkeys, some of which died of unknown cause and were buried in a field nearby. Does deworming rid you of the worms or do just some die, and then there is a reemergence of worms later? If she did not bring hte worms from the other barn, how could she have them now, one year later, in a place where we have never had htem before?

    Also, 3 months after she arrived, we put down a mare who had suffered from emphysema for years (rescued horse, retired at my barn). The vet said she suddenly went in to respiratory distress from her emphysema and there was nothing we could do. Is it possible she had been infected with lungworms, and that together with the emphysema it just became too much?

    • Hi, I’m sorry but the author who wrote this article isn’t able to answer individual questions on cases. She used to do that, but was inundated with queries. She has made her book available for free though – I’ve added the link to the end of the article.

  15. I thought I’d add in my experience to this very helpful article/thread. I adopted a yearling two years ago that had a heavy load of small stronglys. For us, neither Quest nor Panacur solved the problem. I could get a clean fecal 2-3 weeks after worming her, but a couple months later, the heavy load was back.

    I think the problem is either one or two things (or both): our particular strain is immune to the wormers. She is reinfecting after the wormer wears off.

    What has helped is a daily wormer. She is still infected, but the count is very, very low. It works by killing the newly consumed larvae and the adult stage larvae. So little-by-little, it cleans her system out. After two years of dealing with this, it’s the only thing that has kept her count down over time.

  16. Marilyn C says:

    Hello I have a 7 year old mini jack donkey and a 25 year old gelding horse, I have both on the daily wormer strongid c2 and their fecals have been low to zero. My vet has recommended that I worm twice a year with Quest but I continue to hear that this is not a good idea and I’m torn between listening to my vet, whom I love, and friends and articles online ugh…

    • I should go with your vet on this one. Just my two cents worth 🙂

    • Debbie g says:

      I was worming quarterly w equimax and power dosing once a year. I had decals done trying to change to checking decals and worming accordingly. My mare showed low count, vet recommended quest plus, almost lost her. A week later her poop showed heavily infested worms, of all kinds. I would never take chance w wurst agsin. I went back to quarterly equimax and annual pore dose safeguard.

  17. Very interesting article.
    Just yesterday I wormed mine after leaving them all winter in a new paddock without worming.
    Previous to using the new paddock I did a 3x2wk worm program as I have a mare who may have NTW.
    The volume of worms that came out this morning was amazing. I’m so glad I wormed when I did & I will do them again in 2wks then 2wks later. I’ll be sure to watch for unwellness signs of hatching encysted worms. I did have a horse that coliced for no apparent reason 2yrs ago about 3wks after being wormed but vet said it was too long since worming to be as a result of the chemical. Maybe not, afterall?
    I use an ivermectin wormer usually but sometimes one with praziquantel in it as it seems I have some resistant worms here now too (SE Qld, Aust).
    So even though the paddock was uninfected, they must have already had encysted worms which had a party while my back was turned!! lol
    I fixed ’em!

  18. Justin Howard DVM says:

    This is a loaded article. There are a myriad of partial truths and anecdotal information here. Please leave the practice of medicine up to veterinarians. Herd health of horses is not a one size fits all. The author neglects to mention parasite transmission, life cycles, and regional variations. She also manages to neglect the triage of patients concerning parasite management. I will not treat another doctor’s patients I have not seen over the phone and I resent her making blanket recommendations to the public based on a limited reading and anecdotal experience. Please contact your veterinarian who has years of experience scholastically as well as attends continuing education seminars annually to properly assess and make those judgement calls concerning herd health.

    • Thank you. I appreciate your comments and quite understand the frustration that vets must feel with blog articles that make recommendations based on non-researched and anecdotal information.

      However, allow me to provide some background: I decided to include this article after hearing a Veterinary Professor of Parasitology giving a talk at an Australian Equine Science conference about issues in worming that included the very same information about the dangers of encysted small strongyles. (I am a postgrad researcher based in the agriculture dept at CSIRO, the Australian science agency, so this kind of thing matters to me too.) I’ve also been into the author’s book, which opposes ‘natural’ worming, and checked the references – she’s a double PhD holder, including equine physiology, so also knows how to do her academic research. The journal article titles are enormous, of course, and unfortunately I don’t have them in electronic format so can’t reproduce them here. However, the sources include Veterinary Parasitology, Veterinary Quarterly, and the Merck Veterinary Manual (and I know that’s not a heavyweight source, but not exactly frivolous either).

      I’ll see if I can get the full list of references in electronic format as it would be useful to publish them here, once and for all.

  19. Please tell Dr. Nyland Thank You for offering this book for free. I shall certainly take advantage of the information.

  20. Last January my gelding coliced. Wasn’t at all acute, but he ended up at University of Georgia. The good news is it was small strongyles. He’s rallied right around to his wonderful normal self after treatment.
    I just treated both of my horses with Ivermectin in mid-October for all of the other nasties.
    My question is, is there a best time to treat him for strongyles this, and subsequent, year(s)?

  21. I just want to add that not all vets are as good as they are cracked up to be.
    I have recently had a situation where I ended up treating the horse myself after 2 home visits from vets who were completely on the wrong track but still billed me massively.
    There are very few trades or skills where even if the practitioner is wrong they can still force you to pay & not come back to do a proper job.
    For example a carpenter could not get away with such shoddy workmanship without coming back to do the job correctly or alternatively withdrawing the bill!
    Vets need to update their knowledge regularly & religiously to the latest available because with the advent of the digital age, we the consumers get to know this new stuff within days or even hours & since we are the ones living with our precious horses, we see the minute signs much sooner.
    Then when we go online searching for info (rather than pay a $squillion for outdated info at a home visit), we see the latest research & think that might be what’s going on here.
    You can’t blame people for taking up the latest research info particularly when their horse is sick.

  22. Cristy Shore says:

    I’ve been using Wsd Fenbendazole 100… The label dosage is listed at 10ml per 100kg. There is no mention of a 5 day treatment. Do you recommend 5 days at 10ml per 100kg? Fenbendazole is relatively safe and is showing up in all sorts of research, not the least of which is as a treatment for cancer 😉

    • Panacur is a 5-day treatment. Please consider purchasing the low cost book on worming by Dr Ann Nyland – everything you need to know is in there.

      • Wahchar – I’d think with two sudden deaths like that, you wouldn’t want to do anything without your vets approval. And then second to that, you don’t want to do anything without knowing exactly what kind of worm you are dealing with (if your vet doesn’t do fecal testing, you can send it in to a lab via the mail). Colic was not an issue with the Strongyls I battled (for 18 months), because the worms are microscopic. They are too small to block a gut. The real issue was that my pasture was infected so even if I hit them hard with a chemical wormer, they’d just reinfect a month later (a daily wormer solved the problem).

        Unless you are saying you think the colic was caused by the worming product itself?

        • Hi, I can’t move your post as you requested – your best option is to copy the text and repost it.

          I agree with your advice about talking to a vet.

          Ann’s point about encysted strongyles is not that they block the gut, but after sufficient build-up they cause colic – a gut doesn’t need to be blocked for that.

      • Where can you get the book?

  23. I just lost a horse two hours ago. She was fine yesterday and this all happened with the diahrea and vet couldn’t save her. Another very healthy looking mare died weeks ago the same way, very fast. I am broken-hearted and scared because I have 7 more mares. I have read so much about chemical wormers causing colic and to do the natural way with Dianatecous Earth. What is really safe?

    • I’m so sorry to hear about your horse. Sadly, DE will not work with encysted small strongyles. Not all information sources are reliable – all I can say is that not worming with chemicals is more likely to lead to worm burdens and associated colic than doing so.

    • You may want to look into typhlocolitis or Colitis-X….an otherwise healthy horse that dies very suddenly with gastric symptoms may be due to this. It does not necessarily have to be parasites.

  24. A bit off topic.
    Yes we must worm our horses. I am in the process of seeking certified organic status for my property but there is one huge sticking point.
    Worming the horses & chooks!
    It takes years to get to this stage of the certification process.
    The guy said to me if I kept the horses out of the house, garden & orchard area he could give me a cert on just that part of the property. Which would mean produce from my garden & orchard would be certified but I wouldn’t be able to grow a bigger crop down the paddock or use my own manures. My chooks would also have to be moved out of the orchard into the horse paddock & I wouldn’t be able to use their manure either.
    It sort of defeats the purpose doesn’t it?
    So everything is ready for certification except I must worm my animals.
    People think organic free range chook eggs & meat is the best thing since sliced bread. But those chooks can only be treated by being killed. They can’t be wormed or given antibiotics if they get the flu or a bacterial infection goes through the flock for example. The only way to treat it is to send the whole flock for processing. Meat birds don’t live very long anyway but layers could go for 2 seasons if their health could be handled better.
    In my case I could jump on the band wagon of restocking with fresh birds every year therefore not vetting them at all during their life & just leaving them to nature. Or I can keep the same birds for years but not have them, their manure & eggs or their paddocks certified.

    • It is stated that encysted worms will not be killed by anything other than moxidectin or fenbendazole, and will emerge en masse upon ineffective worming with other products, releasing toxins from accumulated larval waste products, potentially killing the horse…

      My question is, if you worm as suggested, effectively killing all encysted strongyles too, aren’t the toxins they contain suddenly released in the horse just as well ?

      • That is a really good question. Being just a simple horse owner & not a scientist, I can only give my personal opinion.
        I think that is why people recommend Fenbendazole which is a 5day course as it keeps killing the little blighters as fast as they emerge.

        Also, the assumption is that by repeating the worming within 2wks of the initial worming, you actually get in first so the chemical is already present in the system when the next lot hatch.
        One has to hope that the horse is healthy enough to withstand the onslaught of all those encysted worms emerging along with their manky toxicity.

        The premise then, is that by co-ordinating a more thoughtful worming procedure, you prevent more worms encysting in future.
        Well that’s my take on it anyway.

        • Thank you Clarissa,
          In fact my question was not really meant as a reaction to your <>.
          I suppose I was not so handy when I posted my question. I will try again and see if the author could answer my question 🙂


  25. Hello,
    Thanks for that important article. It is stated that encysted worms will not be killed by anything other than moxidectin or fenbendazole, and will emerge en masse upon ineffective worming with other products, releasing toxins from accumulated larval waste products, potentially killing the horse…

    My question is, if you worm as suggested, effectively killing all encysted strongyles too, aren’t the toxins they contain suddenly released in the horse just as well, creating as dangerous a situation ?

  26. To Justin Howard, DVM: Ill cut right to the chase. I have met veterinarians who seem to have a “God complex” and resent people, even educated ones such as myself and the author of this well supported and written article, for finding answers to our concerns of our beloved horses. The reason for this resentment is two-fold: We are taking profit away from the vets AND, and second, they are arrogant. Period. I have a master’s degree is biomedical science plus a myriad of background and experience in medicine and animal health care. I am able to read through the literature, as the author of this article has done, and make intelligent decisions on scientific issues the EXACT same way that you do. You don’t have a magic wand that makes you and other vets privy to technical information. I recently worked with USDA veterinarians on the Avian Bird Flu epidemic this last fall; I was hired as an animal health technician. My colleagues quickly learned that I was more knowledgeable on many topics that were covered in our training seminars than most of the veterinarians there were. Please rethink your arrogance before posting such insulting and demeaning statements. Colette Reich, MS

    • Thank you Colette… although I doubt the commenter requested notifications of replies to his post. What saddens me is the thought that it may have been one of his clients who recommended looking at the article.

      I didn’t write this article, but I did write the neck threadworms articles – as a lay person. I am now a PhD candidate in Animal Science and have greater access to the literature than previously, and all the Onchocerca Cervicalis research simply reinforces the viewpoints I expressed in the third article on unanswered questions.

      I too get frustrated at the increasing reliance on “Dr Google” and the passing around of “Facebook Facts”, characterised by the uncritical cherry-picking of information to support existing beliefs. It matters to me that the articles on this site do not do that, and I won’t accept guest articles that do! That’s probably why it’s a bit quiet round here! 🙂

      • Jane, Your writing skills are excellent…I read your neck threadworm article and not only do you support all of your claims with sound science, you also have a talent with creative prose writing, making the information not only educational and reliable, but also interesting and even entertaining! Thank you!

  27. I live in new Zealand and have a 6yr old tb mare who has recently become itchy – especially under her belly, she has hyper-sensitive skin and hallucinates as well. I have tried toxin binders, crushed linseed, extra salt and several other things recommended for grass affected horses but they do not work. Yesterday after a light lunge I saw lines under her skin in the upper shoulder/neck base area and if I had not read your article would have thought they were veins except they were straight. I find it hard to believe that we do not have NTW if Australia have them given the amount of horses that travel back and forth. I tried to get ivermectin but you have to get it from the vets now so gave her a dose of equest yesterday. My question is how long should I wait before starting the DD of ivermectin 2 weeks apart for 3wks and is this the correct treatment for NTW?

    • Hi, thanks for your question about neck threadworms. I have to ask, first of all, what do you mean by her hallucinating – what are you basing this on? Also, do you mean that you can only buy the most basic ivermectin wormer tubes from the vets?

      • Hi, yes farmlands told me any ivermectin must come from the vet now even if its for cattle. The hallucination thing is where she is rooted to the spot staring at something in the distance or even close by and it takes forever to get past and also when grazing sometimes she cocks her ears sideways, stops chewing stares at something I cant see and then leaps sideways. I had another little horse who would stare at the cattle at the end of my paddocks and flip out, marching up and down and stressing.

        • Interesting. I spoke to a colleague in NZ about this and she says she had just bought ivermectin wormers over the counter.

          • Hi, I assume if you go into a vet clinic you could buy ivermectin and I see the saddlery… warehouse has some with about 4.0 mg/ml. Is this strong enough? I am not sure she has NTW but her itching and bald spots are driving me insane also today after I worked her her belly was sore and I could not touch it. I hosed her but couldn’t dry it very well but now that it is dry its itchy again. How long after the equest dose should I wait to before I give her the ivermectin please?

            • michelle says:

              Hi Diane, did you ever get to the bottom of your mares issues? I have just come across this article and finding parallels with my own horses. Just over a year on it would be interesting to see what happened

              • Hi Michelle, I think I am on the right track; she has an imbalance of minerals caused by the high potassium on my property. The magnesium and calcium are also quite high here so getting her balance right is very hard. It is incredible how sensitive she is to additives and how quickly you notice changes. If it was possible to get all the minerals in separate containers she would probably fix herself. I have also been told that activated charcoal can help with horses like her but read that it can pull minerals out of them along with toxins. The toxin binders I have tried to date don’t work that well and one of them made her worse if I put her on the recommended dose, as does graze-ezy if shes on more than 10 grams 2x a day.

      • Hi, I think it may bee like here in Sweden, that you now adays only can get horse wormers by vet prescripsion.
        I have a 17 yo gelding that get all the symptoms of NTW after wormers that are non Moxidektin. Allso have not figured out what program to run. Last time I gave 1 sigle dose and one more after 2 weeks. After that I felt hi was happier and got his “go” back. But how to minister to keep this worm at bey, is still a mystery to me. Allso it would be easier to go trough all the adwise if just sticking to talking about the chemical substens in wormers, instead of the different brands.

        • Thanks for your comments. This is a guest post – I didn’t write it – and the author isn’t available to answer all the queries. However I’d just say to include those wormers in your annual routine and a build up of the ESS will be avoidable. It is the sudden emergence of a large number that cause the problem.

          Thanks for your tips on how to improve the article 😉 It’s a sad truth of blog writing that people can usually tell you how you should have written it, once they have read it! My response is that everyone should go ahead and write their own articles – there is room for us all 🙂

  28. Totally agree. The only problem is that clearly they haven’t done enough testing on the moxidectin. I used it on a horse with a burden and it got terribly sick for months after as encrypted strongyles passed through. I did it a second time 3 months later and still more came through and he continued to constantly have small strongyles in the manure. He is a young horse and clearly run down struggling to build his immune system. I put him on a diet to address his gut health. High fibre and probiotics and have treated him with 5 days of panacur. Finally he is looking and acting healthy. The moxidectin did not even touch the burden and the tube claimed to have 14 weeks post protection.

    • Thank you for this. I’ve a client whose horse also appears to be ‘retaining’ a small strongyles worm burden after using Equest. I’ll pass this information on to the owner.

  29. I have a hard time accepting anything the author writes since we don’t “worm” horses and we do not give them “wormer”. We de-worm them with de-wormer.

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