Why Thinking About Neck Threadworms Still Leaves Us Scratching Our Heads

header neck threadworms

Less than 12 months ago, I wrote about neck threadworms and how they might be behind many horses’ frantic itching. The uptake on that article continues to astonish me. It has now had well over 60,000 visitors from all over the world and is still climbing steadily.

Itchy horses, it seems, are a perennial worry for thousands if not hundreds of thousands of horse owners (that’s counting the many who haven’t read the article).

At time of writing, it’s Spring in the northern hemisphere, and the reader numbers are building up again. So are the microfilariae levels.

The itchy horses are starting to scratch.

And the more I think about the neck threadworm issue, and read comments under the articles, the more I’m scratching my head.


Who am I to write about neck threadworms?

Unsurprisingly, there have been a few knock-backs as well. I’m not a vet and, although I read as many papers as possible and discuss my thoughts with research vets before I post my blog articles, my assertions, which are based on observations and backed up with research, have still tickled the inside of a few noses.

To be honest, neck threadworms aren’t exactly an easy subject. Filarial nematode parasites are a complex life form so fundamentally different to those within our normal understanding. They rely on more than one host and undergo several stages in their development – they make the caterpillar to butterfly transformation look unambitious. (What, just two life stages? Onchocerca Cervicalis (neck threadworms) has twice that...)

And we are looking at not two, but three species here. The horse, the neck threadworms, and the biting midge (culicoid fly) that does the middle man part. The microfilariae leave the horse, courtesy of the midge, get flown around, travel through the midge’s body parts and enter different larval stages, and are then returned to the horse when the midge returns to ingest blood.

A lot of people are confused, and I don’t mean just horse owners. There are vets working for manufacturers of worming products who swear that their product can bump off the adult neck threadworms.

This would be welcome news to the World Health Organization, I’m sure, as decades of research have yet to find a chemical that successfully infiltrates the central nervous system of the adult Onchocerca Volvulus (the human version of the parasite, which currently affects an estimated 18 million people throughout Africa, Latin America and the Yemen) and eliminates it.

Moving on…

1. About standard texts on neck threadworms

“Oh, but those are the wrong signs for neck threadworms.” After “we don’t have that problem around here”, a lot of people are hearing this line when asking for professional help with their itchy horse. The problem, apparently, is that their horses don’t have the ‘typical’ itching along the ventral line, ie, the underside of the neck and belly.

I do wish that instead of saying, “it can’t be that, as it’s not described that way in the textbooks”, more people would wonder instead if there were something that could be added to those textbooks.

Textbooks tend to be based on research that other people have done. In the case of neck threadworms, there is relatively little out there. This means that a lot of information that finds its way into books and training is already reasonably dated, and not exactly far ranging.

Here’s how to put together a program of treatment for your horse with neck threadworms (and maybe the Itch) – How to Fight the Big Fight Against Neck Threadworms

2. Neck threadworms may be the same worldwide, but biting midges aren’t

This is something we’re especially aware of here in Australia, where geographical isolation has led to the evolution of some remarkable indigenous species, found nowhere else in the world. Our ticks are different. Our flies are different. We have marsupials.

Most of the research on neck threadworms hails from the US. When findings of research involving vectors – the biting midge ‘middle man’ – are applied worldwide, it’s important to remember that some conditions can be ‘similar but different’.

Indeed, when we look at research on Onchocerca Volvulus, neck threadworms’ well-studied cousin that affects humans, we can come across statements like this one: “Many simuliid species have been incriminated to a greater or lesser degree in the transmission of O. volvulus, their relative vectorial roles contributing to shape diverse transmission patterns across endemic areas.” (Basáñez M-G et al, 2006.)

Say what? Different flies are active in different areas and create different patterns of transmission? I think the point is made well enough.

3. The biting midges must be studied too

If we consider the possibility of different culicoides acting as vectors, then other differences come into consideration as well.

European studies into Sweet Itch (Summer Itch, Queensland Itch) have shown that different types of biting midge have a preference for different parts of the horse’s body.

What? But that’s breaking the rules, surely? Yup. That’s what nature does – makes its own rules.

4. More rule breaking

It doesn’t stop there: studies of cattle in Darwin (a hotbed for livestock research due to its tropical climate) have revealed the presence of more than one type of Onchocerca, with at least two species of the parasite being found in the ‘wrong’ animal.

This suggests that one or more species of biting midge are ‘cross-contaminating’ the parasite by moving from one species of mammal to another. Possibly. But the point here is that variation exists – we can’t assume that because a few studies have identified certain conditions, these are replicated globally.

5. Neck threadworm life cycle: what happens and when

Another point that isn’t clearly answered by existing research is how much the microfilariae travel. If the adult worms are living in the nuchal ligament, that is where they give birth to zillions of microfilariae. Some flies must bite in this area in order to ingest the microfilariae and take off with them. It then follows that there are millions that don’t get ingested, and die off. It’s the die off that causes the itching, both during normal times and after treatment with ivermectin.

So here we have itching that’s not on the ventral midline.

It’s more remarkable that horses do itch on the ventral midline. If adults live in the nuchal ligament, mostly, and microfilariae are dying on the ventral line, either the microfilariae are traveling long distance, subcutaneously, or there are adults living in the ‘white line’ that runs from the the sternum to the umbilical area.

At the very least, we can say that this is problem isn’t confined to the underside of the horse.

If your horse has the Itch, Queensland Itch, Sweet Itch, Summer Itch, Summer Sores… sometimes it’s really neck threadworms. Read the original article, The Disturbing Truth About Neck Threadworms and Your Itchy Horse

6. And look at the behavior of itchy horses

Whatever the country, whatever the culicoid species, the horse that’s going insane with itchiness can only ever scratch the parts that it can reach. Most horses can easily scratch their toplines on branches and fences, whereas far fewer are seen scratching their bellies on the ground. Horses will always break the skin in any area that they scratch in earnest, and consequently the blood attracts the biting midges to that area.

As the immune system goes into stratospheric hypersensitivity, intense itching can become an all-over sensation. Remember having chicken pox, or measles? Or ask any person who has such an uncontrollable immune response.

Let’s not overlook the possibility that horses displaying lesions on their undersides are in the more advanced stages of parasitic infestation, or are rugged (fewer areas are available for the midges to bite), or even – taking the earlier points on board – that they live in areas where local midges prefer that part of the horse’s body.

7. And let’s not forget owner behavior

Who is that calls the vet, and why? Not the horse… An owner who notices their horse scratching its mane and tail head is way less likely to contact a vet than the owner who sees bald, scratched patches on their horse’s underside.

It’s the difference between “dang, my horse has the itch” and “what the heck’s my horse doing, rubbing under there?”

It entirely makes sense that the more advanced cases, with all over itching and rubbed patches, are more likely to be seen by vets, and to make it into veterinary research. (Note: in research, not all horses whose nuchal ligaments were dissected at necropsy displayed ventral lesions, far from it, although many tested positive for Onchocerca Cervicalis.)

Important: Learn why you should never only use mectin wormers, even if your horse has neck threadworms – The Worm That Kills – And Why Only Two Worming Chemicals Can Stop It

Where to next, with neck threadworms?

Well, what do you think? I know what I think. I know what I’ve experienced with my horse.

I trust my observations. I trust the research that I’ve read and that fits closely with my observations. And no, I’m not cherry-picking research to prop up my own emotionally driven ideas.

At present, other explanations aren’t available, but lots of questions remain. Looking at the ones here, it’s easy to understand why research is such a long and painstaking process – it’s not possible to go from A to Z or even A to D without covering the intervening steps.

For the horse owner, just one thing is clear, and that is that we have a horse being driven mad by itching. So, please do stick with ivermectin for identifying the initial reactions. Stick with ivermectin or moxidectin/praziquantel wormers for reducing the problem (moxidectin will not cause the itching response post-worming).

And remember, that not every itchy horse has neck threadworms.



© All text copyright of the author, Jane Clothier, www.thehorsesback.com. No reproduction of partial or entire text without permission. Sharing the link back to this page is fine. Please contact me for more information. Thank you!


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Basáñez M-G, Pion SDS, Churcher TS, Breitling LP, Little MP, et al. (2006) River Blindness: A Success Story under Threat? PLoS Med 3(9): e371. doi:10.1371/journal.pmed.0030371





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  1. Margo Nielsen says:

    Thank you for this update and clarifying why is is so confusing to diagnose NT.
    I started the original treatment in early July 2013. The midges are out now and I am going to do a double/triple Ivermectin series again, since someone here mentioned it might not be a bad idea to prevent a buildup or new infestation.
    Usually they have Ivermectin every month, but I gave them Ivermectin/Moxidectin last month. The itching wasn’t as bad as with just Ivermectin. So far their manes and tail are still growing back, although my 30 something gelding still rubs the top of his tail.
    Is there any mention of the Ivermectin killing the midges that bite the horse? Maybe we can kill two birds with one stone? They are on Simplifly every day to compensate for having no chickens.

    • Yes, I’ve seen papers stating that the high point in numbers occurs in the Spring. It makes sense – that’s birthing in time to get the microfilariae ready and waiting for the incoming midges.

      There is mention of ivermectin knocking out some species of culicoides, but I’ve no idea which ones. Your horses are no doubt feeding more than one species, so it’s not as if we know which specific type is the vector for the larvae. But if it’s helping, that’s got to be a good thing!

  2. Khat Hammond says:

    I was given a yearling with fistulous wither in October 2012. Vets treated him with antibiotics for many months with no success; we had xrays, an ultrasound (which shwed some infection of the nuchal ligament) and surgery, with varying degrees of short-lived relief/hope. Eventually, I decided that, with nothing to lose, I would double-dose him with an ivermectin based wormer, fortnightly. I continued this cycle for a couple of months. Levi has now been symptom free (absolutely no swelling, no lesions, no hair loss, so soreness) for four or five months. I have all exact dates and doses etc. documented in his journal, which I have kept since he first came to live with us, and if anyone’s interested, am happy to share more precise details.
    I realise that Levi’s recovery does not prove causality, but will hopefully be of some help to anyone with an interest in these subjects.

    • Oh wow, Kat. The question of fistulous withers is such a huge one, and I’ve been avoiding it so far.

      As you say, not causal, but co-existing, complicating and compounding for sure. And this is a good example of ‘treatment as diagnosis’.

      I’ve seen research that says there’s no known connection between NT and fistulous withers, but this is definitely a coincidence… at the very, very least, a raw lesion on the withers would be an inbound destination for visiting midges carrying larvae.

      That’s amazing news and I’m so pleased for you!

  3. Great article. You don’t need initials following your name to do legitimate research and intelligent interpretation of the same.
    Question – if the itching is caused by the die off (I presume from toxins released at the time of die off) why is there an itch response with ivermectin but not with moxidectin? This has me scratching my head : )
    Thank you!

  4. Jude Jiggens (Ms) says:

    Having been around horses all my life and never having heard of neck threadworm, I now have a little QH who is being driven insane by the midges that cause this. She has gone from a sleek, shiny little thing to a starey coated thin horse with no top to her tail and no mane whatsoever and hair going from her face and inside her ears. The other horse in paddock isn’t affected at all. I have completed two courses at fortnightly intervals of Ivomectin worming paste, given the mare vinegar, bi-carb soda and mild bleach washes, all to no avail. She has wipe-on insect repellant applied each afternoon and is hooded and rugged for the night too. Vets are absolutely no help at all and I am tearing my hair out too – we will both be bald soon. I did seem to give relief for the mare when I dusted her mane with Prantil powder (human prickly heat itch powder) and have been using that too and I have been spraying her with Citronella solution under her belly and down her legs and in her tail which seems to keep the little buggers off her. Will the mare ever have relief? It is terrible to see what she has become.

    • Margo Nielsen says:

      Jude, Since the immune system has become so compromised by the stress of this affliction, it is important to help the horse with supplementation of the diet. Many people have suggested probiotics after each worming to balance the gut bacteria. My horses are all seniors. I have been adding oil to help regrow the coats, manes and tails, and am about to introduce ground flax seed. I am also giving them a daily herbal mix for “Sweet Itch” which includes Chamomile (calming), Rosemary, Garlic (both bug repellants) and Burdock (anti-inflammatory). This mix has helped immensely. I get it from HerbNHorse online for about $30. It lasts one month for 2 1/2 horses.

  5. Jude Jiggens (Ms) says:

    Margo, thank you for your suggestions. I forgot to mention that I was also putting vege oil, and apple cider vinegar garlic blend into the mare’s feed daily. The cider vinegar/garlic mix is supposed to repel the midges. But I will definitely be trying the mixes that you advised me on too. This mare does have age up on her (20 years), so here is hoping that I can turn things around for the little mare.

    • Margo Nielsen says:

      Jude, And thank YOU for reminding me about Apple Cider Vinegar! I have never used it but many people have mentioned it, so I will start RIGHT AWAY!

  6. Lyndsey Phillips says:

    I live in UK and am stunned and almost overcome by your articles. Has anyone associated neck worms with head shaking, nose rubbing, sinus congestion or coughing? Would be interested to know too whether there is a link between affected horses and an inability to stretch forward on to the bit to work long and low and/or stretch their backs. If the nuchal ligament becomes infested is it still able to stretch and interact effectively with surrounding muscle and ligaments?

    Many thanks


    • Hi Lyndsey

      Lots of thoughts there… and who knows? The irritation factor is certainly significant, but with the microfilariae being subcutaneous, there would certainly be visible dermatological signs present, I’d have thought.

      The adults are in the nuchal ligament. Have you seen this ligament? As in, touched and held it at a dissection clinic? It reminds me of industrial rubber. It’s pretty astonishing that parasites can penetrate it at all. While adult worms are reported to be up to 10cm length, they are extremely thin and the only photograph I’ve seen has a measurement of 5mm across when curled up in a veteran horse’s ligament. Post mortem studies involve slicing the ligament to identify their presence. I have no idea how many it would take to cause thickening and hardening, but again from what I’ve read, and from assessing a pony that we believed him to have neck threadworms (on account of the reaction to ivermectin, with pus ‘eruptions’, and palpable lumps in front of the withers beforehand), I imagine the effect would be minor. I am attending a dissection next week and plan to go exploring…

      In the end, we don’t know! If you read the ‘scratching heads’ article comments, there is somebody there who had pursued veterinary treatment for fistulous withers for a long time, only to have the problem clear up with ivermectin worming and a neck threadworms protocol. Once again, that won’t clear the adult worms, but only the MF.

      Thanks for responding to this!

  7. Jane – I for one am very thankful that you posted this information…my horse has been “itching” for 3 years and the only thing vets would say was ‘give shot of steroid’. They never said anything about anti-histamines or other offer of information; you just gotta ask the ‘right’ questions to get the right answers. no one knew anything about why the horse was itching other than sensitivity to insects and he just got worse with each year. he is only 6 and the first 3 years of his life he was fine…worming just as instructed, which now I know is WRONG. I did the ivermectin test on my boy and immediately got ‘the reaction’ so he is on the initial attack stage of de-wormer and I’ve already given for 3 weeks. I guess I will try one more week of initial stage and then once a month until late fall with the alternate cycle of the de-wormers to rid him of these nasty infant neck threadworms and the ‘gastrointestinal’ parasites I will use the new maintanence de-worming of more in spring/summer and less in fall/winter for the rest of his life. Just because we are not scientist doesn’t mean we don’t know how to use our brains…actually I think that we as horse owners that care for our equine friends are better equipped to know how to use our brains with common since because we aren’t so puffed up in the head with a “degree” on a piece of paper. that is so black and white when the rest of us work with the whole Crayola Box. Thank you again.

  8. Claire lang says:

    It was so refreshing to read this. and i hope i can add to your documentation. I believe i have had 2 cases of NTW, one of which ended up with my TB being blind and PTS. he never really showed any symptoms just had a very itchy spot under his belly but nothing round his neck. It would seem that the microfilea infected his eyes (dead or alive) as an exit point = uveitus. which could not be treated as underlying cause was at that time unknown.

    My mini shetland suddenly had what i initially thought was a bout of sweet itch – never before, he looked a mess but just either side of his neck. so ivermectin treatment given and he recovered and has been ok all summer.

    There are so many people that do think their horses have sweetitch, and will never give a thought to NTW.

    So i will be watching out in the spring for another outburst. i live in france and no one could help me it was my own research that came to this conclusion.

    Also has anyone noticed that after treatment there are what appear to be “lice” type egg/flecks in their manes, which after a few weeks dissappear, been same on both my horses. i believe these are the Dead critters.

    • hey Claire: yes…the flecks, that you talk about. I noticed this in the mane, tail and all over the torso and shoulders as well as the neck where the dermatitis heals after the wormer paste kill off that stage of the N.T.W. it’s like he has dandruff or something. My boy had all this start with a belly itch but didn’t understand it was parasites until it was too late. Now I know better. He had a huge patch of raised bumpy skin on his right rear leg muscle (above the stifle) and now I can’t see it too much and it is almost perfectly smooth to the touch. I just started him on a second cycle of wormer – 3tubes of Ivermectin in 8 days, wait 7 days and 1tube of Quest Plus. It has been 78 days since the last Quest Plus and 95 days since the last round of wormer “zapping” program (like I described above 3&1 in a two week period. The sores on his belly are still bad and I’m working to fix them before we get into winter and this is the reason for the second cycle of wormer program. Then next spring; these critters will not even get a ‘foot hold’ if I have anything to say about it.

  9. Liz Porter says:

    Incredibly interesting reading…would love to keep hearing comments.

  10. Melissa Landriscina says:

    Hi all
    This webpage has opened my eyes to the ‘mysterious’ issue my boy has been having since spring last year. Vet results came back with nothing, no signs of itch however his face, poll and all of sudden his thick luscious mane, has been going bald and itchy. We have had rounds of peddy’s to no avail. He is super touchy with his poll, mane and face. Some days are a battle to wash or go anywhere near his ears. I am currently using specific washes for the itch and oatmeal, coconut oils etc too sooth some new sores I have seen on his mane. There are also odd dandruff patches on his elbows that I have been trying to clear for over a year. He gets in his feed omega 3 oil, turmeric, garlic, marshmallow powder, copra etc and is on a non-grain diet. I have given him 24 hours ago double dose of Ivermectin and will proceed in the following fortnight. I haven’t seen any of the ‘obvious’ symptoms as yet. May happen tomorrow… what if there is no reaction to dying NTWs? Means he doesn’t have any and I need to look for other symptoms?
    Many thanks 🙂

    • Hi Melissa

      I should just progress with the doses. Even if you don’t see an insane reaction to the ivermectin, I am sure that as the owner, you WILL recognise if there’s an improvement after the doses. I do remember my horse responding after a couple of days, as you need time for the wormer to reach the microfilaraie – and it is their die-off that causes the itching response to ivermectin.

      Where do you live? Which season are you in? That is also relevant.

      The other possibility is, of course, that it’s something else altogether. The good thing, though, is that the ivermectin wormers offer a very economical way to rule out this particular parasite.


      • Hi Jane many thanks for your response. It is the bed of summer here Qld, raining everyday after cyclone. Humidity, flys and sand flies are out of control. I live in SE Qld. I am not sure if I noticed much to be honest, he has been a little more unruly. I did treat and wash him mane and as I washed and put maigic itch in and gentle comb hair feel out. It is basically half bis mane left on his rright side. The left has fallen out. Maybe he is allergic to grasses? Feed?

      • Melissa says:

        Thanks Jane,
        Just an FYI
        I have given another double dose again after a fortnight, 24 hours after dose I have noticed his frustration with his ears. Shaking his head a lot to rid of whatever maybe annoying him? I have checked his ears, however, finds it very hard to let me gently look, he pulls away all the time. Head tossing was intense during flat work, still head shaking after taking him pack to the paddock. In saying this, I have noticed his hair on his face is improving and is not as itchy. He gets so frustrated with midges. Did anyone see this type of behaviour after administering ivermectin?

        • Margo Nielsen says:

          Yes. The itching was so bad that my Cherokee ruined the cartilage in his ears before the treatment and continued for months after I began the treatment.

          • Melissa says:

            Wow! Funny enough he has hat developed a minor ulcer in his right eye now but I am noticing a difference to his hair. Thank you for everyone’s great advice and input.

  11. Wendy satara says:

    Hi Jane, this information has blown my mind! My wb 9 year old had a wither infection and lumps on his chest and my younger 31/2 yr old wb has lumps on chest and rubbing his mane above his combo rug , the top third of his mane. Middle of winter, frosts, it just can’t be midges now! Can you tell me the dosage rates I would use ? Thank you , I haven’t seen this information before but it’s certainly interesting…

    • Hi Wendy

      Sorry about the delayed reply. I’ve been inundated with spam on this address – IRO 50-80 spam messages per day! – so haven’t been on top of the notifications.

      The article, ‘Fight the Big Fight’, includes suggestions on how to address this problem. The wither infection sounds like the poster whose horse had fistulous withers and spent thousands of dollars pursuing the problem, only to get confirmation through a couple of doses of ivermectin.

      The midges are the vectors… ie, they move the microscopic larvae of the onchocerca parasite from horse to horse. If your horse has built up a large enough population, then it won’t be dependent on the midges in order to hit peak periods in Spring and Autumn. Where do you live? Have your horses been in a subtropical zone of Aus, or exposed to other horses who have?

  12. Is there any research on other possible insect vectors for the microfilariae, or are they culicoid specific? Here in New Zealand we don’t have a culicoid midge, but we do have plenty of other little bitey buggers right around the country . And more than enough horses are imported from other areas of the world that there MUST be a small population of NT infected horses here, even though ‘common knowledge’ is that they (NT) don’t exist here. Given that small population, would that mean that we are at risk of a slow spread into the rest of the horse population via our local biting insects, or is the lack of a culicoid midge protection against that?
    I’ve known a number of horses with variations on sweet itch or general allergy itchiness that could possibly fit into the NT symptoms (none of them imported) – sadly they are all long gone to the big paddock in the sky or I’ve lost contact of where they are, so there’s no way to try the ivermectin and see what the reaction is, but I’ll be sure to remember it for future cases.
    Claire Vale, New Zealand

    • Thanks Claire! I think there is definitely regional variation in the culicoides and also in the areas of the horse that they bite. I still stand by the questions asked in this post after having done further reading in the literature on the subjects, much of which relates to sweet/Queensland/summer itch.

      As for NTW not being present in a region, well, I’d have presumed it wasn’t prevalent in the UK, yet a clutch of owners have come forward reporting positive repsonses to ivermectin in itchy horses following the floods of recent years. There must be others ‘hidden’ amongst the itching horse population, but with less severe symptoms.

      As for the extent of problematic NTW in more temperate areas, I imagine it occurs where three factors combine: enough horses with NTW to ‘seed’ the population, a large and prevalent number of culidoid vectors, and enough carrier horses to throw up the minority of cases with a severe, highly visible response.

      Meanwhile, ivermectin remains a quick, easy and cheap way to identify or eliminate NTW in the case of severely itching horses.

  13. Elizabeth Elmore says:

    Two things
    The Onchoriea, at least the human ones are dependent on the bacteria Walbachia to reproduce.
    A 2 month run of doxycycline kills the Walbachia and sterilizes the adult worms.

    Also it appears that Auranofin a human arthritis drug, can kill half of the adult worms with each dose.

    • That’s interesting, isn’t it? Obviously if the Auranofin was at trial stage last year, it’s some way off being licensed for medical or veterinary use. If only the equine issue would receive greater recognition, then there might be a greater push for that (ie, a market exists).

      I can picture a trial in the use of doxycycline in equines, though! I must finish that lit review… unfortunately my research area is different.

      Great information, thank you!

  14. Lex Kromhout says:

    Has anyone ever tried to spray ivermectian Cattle worker in low doses on a horse on a daily basis using a conventional bug spray bottle applicator for an extended period of time to rid thread worms?

    • I believe someone brought it up in the comments after the first article, The Disturbing Truth About… and I seem to remember somebody expressing a concern about DMSO content, the skin thicknesses being very different in cattle. What do you see as the advantages of using a spray?

      • On reflection, it was probably the pour-on that was being discussed.

        Just glancing at studies comparing results on gastrointestinal worms, it seems to be that there is some efficacy in reducing fecal egg counts, but that overall effectiveness is lower due to the lower concentration of ivermectin in pour-ons.

        Seeing that more intensive ivermectin administration is needed to knock back the microfilaraie of the neck threadworm (not a gastrointestinal worm), then it could be that a spray-on is even less effective than a pour-on.

  15. Lex Kromhout says:

    Cattle worker Ivermectin that is

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