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Why Thinking About Neck Threadworms Still Leaves Us Scratching Our Heads

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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.

All text (c) Jane Clothier, https://thehorsesback.com No reproduction without permission. Thanks!

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.

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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.

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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.)

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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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Reference

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

 

 


 

 

How to Fight the Big Fight against Neck Threadworms

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This follow-up to ‘The Disturbing Truth About Neck Threadworms and your Itchy Horse’ discusses how to manage the parasites, once you know your horse has them.

The original article was aimed at my Australian bodywork clients here in NSW, yet it quickly worked its way through other networks – I haven’t a clue what or where, but it started running and it’s still running today.

All text (c) Jane Clothier, https://thehorsesback.com No reproduction without permission. Thanks!

Astonishingly, five years later, it’s been read by over 500,000 people in nearly every  country in the world.

I make no claim to be the first to write about neck threadworms. For making it a more widely known issue, the forum members of The Chronicle of the Horse website must take credit (and I recommend anyone with specific questions about their horse to head over there).

I guess my post succeeded in bringing the available information together in one place – in that it simply reflected my own experience of researching the subject on behalf of my newly itchy, increasingly unhappy horse.

‘The Disturbing Truth’ has certainly offered a possible answer to many owners whose horses are besieged by the living hell that is variously known as Sweet Itch, Summer Itch, or Queensland Itch.

That’s because in a certain percentage of cases – not in all of them – the cause is onchercerciasis, a hypersensitive reaction to the larvae of the Onchercerca cervicalis parasite.

Ignore the nay-sayers – it’s an international battle

Imagine your itchy horse with a regrown mane...
Imagine your itchy horse with a regrown mane…

One thing that’s become really, really clear from responses to the first article is that onchercerciasis is a problem for horses worldwide. It’s obviously worse in some climatic areas than others, but that doesn’t mean it doesn’t exist in cooler zones.

Reading the comments triggered by the article tells us all we need to know. Horses travel within countries and internationally – the larvae can then be transmitted to new host horses in the destination region, providing there are enough local culicoid flies to transmit them.

As another reader pointed out, warmer and wetter winter and summers are giving the flies a better hit at it than ever before.

Research has shown the presence of neck threadworms in horses in such non-tropical countries as Canada, the UK and Poland. In fact, the UK research was conducted in the early 1900s, showing that air transport of horses has little to do with it and that this parasite has been in cooler climes for a long time.

Dragging neck threadworms into the daylight

The comments at the bottom of ‘The Disturbing Truth’ are illuminating. Some people have battled their horses’ itch, often so bad that it prevents their horses from being ridden, for a decade or more.

They have struggled with different diagnoses, with various medications, with investigations into suspected colon disorders and testing for allergen sensitivities, all to no avail. Everyone involved certainly meant well, but the problem still remained.

Astonishingly, more than one person has previously mentioned neck threadworms to their vet, only to have it dismissed out of hand, because “that’s not a problem around here”.

Despite this, some of those same horse owners have started treating their horses and are already some way down the road to reducing the larval population and are resolving the issue. Some are seeing manes grow back on their horses for the first time in years.

This isn’t to say that all itching is caused by the larval stages of Onchocerca cervicalis. I don’t want to be grandiose about this. But it’s certainly affecting enough horses for it to be worth ruling out. For the price of a couple of tubes of ivermectin wormer, why on earth not do just that?

Culicoid fly in close-up
Culicoid fly in close-up

It’s baffling that this problem has ceased being common knowledge and so often slips under the radar. It’s true that the companies producing wormers certainly know about neck threadworms, but quite naturally focus on promoting their products’ effectiveness against far more serious gastrointestinal worms.

(And let’s be honest, many horse owners feel distrust towards drugs companies, whether involved in human or veterinary medicine.)

Even so, in regions such as the Australian tropical and subtropical zones, you’d think there’d be a bit more awareness, wouldn’t you? But then perhaps we’ve all been so hung up on Queensland Itch, we’ve been unable to see the wood for the gumtrees.

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Down to business: How to win at worming

With your initial burst of worming, either on a fortnightly or monthly basis, single or double dose (as described in ‘The Disturbing Truth’), you’ll have established that your horse does indeed have neck threadworms.

Using ivermectin, you’ll have seen a temporary worsening of the problem and, possibly, the increased itching and/or eruption of pus-filled lumps in the mane.

Moving on from that point , it’s important that you establish a strategy for dealing with the ongoing presence of the neck threadworms in future months and years.

Regretfully, when I say ‘win at worming’, what I actually mean is ‘win at managing the worms’. As we know, you can’t kill off the adults, only the microfilariae. Hence you can only strive to bring it all under control.

That’s why tackling neck threadworms in your horse is a numbers game. It’s safe to assume that most horses can deal with a relatively small number of microfilariae, so it follows that the more you can bring those numbers down – and keep them down – the more likely it is that your horse will be able to deal them without experiencing hypersensitivity.

In other words, the itching will ease.

A calcified lump in a dissected nuchal ligament
A calcified lump in a dissected nuchal ligament

Just to repeat: you can’t bump off the adults. Once they’re in your horse’s nuchal ligament, they’re there to stay for 10 years or more. And then even when they die, they remain present, entombed in a small calcified lump in the ligament.

You can see one in the picture on the right. ‘M’ marks the center of a lump nearly 2x4cm in a veteran horse’s nuchal ligament.

How frequently should you worm?

At different times of the year, the microfilariae being introduced to your horse will vary in numbers, depending on the level of culicoid flies around. This means that with evenly spaced wormings, you’re going to have different results at different times.

As the warmer months are going to see faster increases of the population, it makes sense to worm more frequently in Spring and Summer. The microfilariae are picked up from a horse by the biting fly, after which they develop in the fly for 15-21 days (research seems to differ on this point).

The flies are the vectors that take the larvae through an additional life stage. After this time period has lapsed, they deposit them back into a horse. This is going to happen faster with horses that live in herds within a kilometer or so of standing water.

  • Some owners feel that monthly worming during Spring and Summer is sufficient to interrupt the pre-adult lifecycle. Check with your equine vet if you’re not sure (although one reader’s vet suggested worming at 6-monthly intervals, for some mysterious reason).
  • Once the microfilariae numbers are down, possibly after the first year, some owners come down to 6-weekly and 8-weekly wormings.
  • Others prefer to leave the horse from mid-Autumn until mid-Spring, following only a ‘normal’ worming schedule during this time, before worming more frequently during the fly season.

In the end, it’s a matter of individual choice. All you can do is observe your horse and keep at it until you see a significantly reduced reaction in terms of itching.

The longer your horse has been living with neck threadworms, the longer it’s likely to take. And the more tropical and humid your regional climate, the more biting flies there are going to be. Even so, you might be pleasantly surprised by the speed of the positive change.

Include gastrointestinal worming in your plan

Once you’ve used an ivermectin wormer to identify the presence of neck threadworms in your horse (see ‘The Disturbing Truth‘), it’s sensible to also worm intermittently with moxidectin, found in Quest/Equest. The reason is that this is the only wormer to hit Onchocerca microfilariae AND encysted small strongyles.

Encysted small strongyles in the colon. (c) Vetnext.com
Encysted small strongyles in the colon. (c) vetnext.com

While neck threadworms can make life highly unpleasant for your horse, encysted small strongyles can kill him or her if they suddenly erupt.

They would have to build up to a high level first, but if your horse has been malnourished before coming to you, or is a rescue horse, or if you’ve only used ‘natural’ wormers for some years, you have the perfect conditions for such an event.

The best approach is to address the intestinal worms before moving onto the neck threadworms. So if it’s unlikely your horse has had Quest/Equest or Panacur for some years, use one of these first.

In my region, Strategy is also valuable in addressing pinworms, which are resistant to many other products.

Support your horse’s intestinal tract

Shop for the Best Discounted Pet, Equine, & Livestock Supplies!Worming for neck threadworms is always going to be a necessary evil. None of us would wish to be following such a rigorous worming schedule if we had the option, so we must do our best to mitigate any negative effects.

If you’re following a heavy worming schedule, you should also be supporting your horse’s gut, which is going to be taking a chemical hammering.

The best place to start is with probiotics, either added to the feed or administered through a plunger tube (although your horse may be a little off plungers at the moment…).

The reason for using a probiotic, which is essentially a supply of beneficial flora for the hind gut, is that the existing ‘beneficial bacteria’ will have been reduced by the wormer. It’s then more likely that pathogens (‘bad bacteria’) will proliferate, potentially leading to intestinal inflammation and diarrhoea.

This is not only uncomfortable and painful for your horse, but the affected digestive processes will reduce nutritional uptake, which in turn leads to an impaired immune system.

And what happens with an impaired immune system? The horse will be itchier.

Using probiotics means that in supporting your horse’s intestinal function during worming, you will also be supporting your horse’s immune system. It’s a double-win .

You can also help to restore damaged mucosal lining. Various plant sources are ‘mucilages’, meaning that when mixed with water, they form a slippery substance that lines and soothes the intestine. You can consider using aloe vera juice, slippery elm, marshmallow root, liquorice root and chia seeds.

Itching elsewhere on the body

Some owners are saying that their horses, which have had all the signs of neck threadworms, have also started itching on the tail head and along the back. This doesn’t fit with the usual oOnchocerciasis locations of head, neck, chest and ventral areas, and could be due to:

  • Pinworms, which lay eggs around the anus and cause itching of the tail head,
  • The horse reacting to dying microfilariae previously delivered by culicoid flies biting in ‘atypical’ areas of the body (it’s interesting to note that bite locations may vary according to the particular species of fly, which differ from country to country),
  • The horse reacting to the saliva of all culicoid flies, having become hypersensitive to microfilariae that have entered their current life stage within the flies’ saliva. In other words, the horse has developed the Itch.

It’s worth noting that there are two different types of Onchocerca worms also affecting horses and donkeys, and that these cause problems in other areas of the body.

  • Itching on the legs can be due to Onchocerca reticulata, as the adult worms are found in the connective tissue of the flexor tendons and suspensory ligament of the fetlock, mostly in the forelimbs.
  • In donkeys, Onchocerca raillieti are also found in the nuchal ligament but also, nastily, in subcutaneous cysts on the penis and in the perimuscular connective tissue. 

Can neck threadworms lead to the dreaded ‘Itch’?

Itching along the topline
Itching along the topline

Now, this is where things get a bit curly. Once your horse has started reacting to Onchocerca microfilariae, they may develop The Itch – aka Queensland, Summer or Sweet Itch (take your pick).

If their system is reacting to the microscopic larvae carried in saliva, why not react to the saliva itself? The immune system is nothing if not intelligent: it learns and it acts on the information it receives.

I’ll be honest here. I’ve not read any research into this, but hooray for Dr Carl Eden BVM&S MRCVS of Virbac, who writes:

Internal parasites such as Onchocerca cervicalis… may also cause pruritus and in the case of O. cervicalis, its involvement in cases of Queensland itch must not be underestimated.

Let’s look at this for a moment.

The Itch is a clinical syndrome, being a hypersensitivity to certain allergens. When the allergen is the saliva of the culicoid flies, the outcome is an inflammatory response.

This in turn leads to a pruritic (itching) skin condition. We’ve all seen its awful progression from hair loss, bumps and wheals, to alopecia, crusting, broken skin and worse.

So I’m speculating here: some of the horses with neck threadworm symptoms develop the Itch as they develop a heightening, acute sensitivity.

The thing is, whether this supposition is right or wrong, you can do no wrong by managing your horse as if it has both neck threadworms and the Itch.

Managing the itching side of neck threadworms

Furiously rubbing the tail head leads to hair loss (c) hyfeed.com.au
Furiously rubbing the tail head leads to hair loss (c) hyfeed.com.au

While you’re working on reducing the microfilariae numbers, there’s plenty you can do to make your horse more comfortable. There’s nothing worse than seeing your horse breaking his or her skin while rubbing an itch that won’t ease, whether it’s caused by neck threadworms alone, by the parasite and a developing itch problem, or by the parasite and a pre-existing itch problem.

I’m not going to go into lots of detail as to what you can do, but here, in no particular order, is a starter list.

  • At the heavier end, a short course of glucocorticoid therapy (ie, steroids) will reduce the inflammatory response. Some owners have used oral prednisolone – Preddy®-granules –  to support their horses during the initial stages of ivermectin worming. It’s not good for pregnant mares, suspected or actual laminitics (steroids can trigger an onset) and horses with internal issues.
  • Antihistamine preparations work on the chemical response, but may lead to occasional behavioral changes or even have a sedative effect.

There are changes you can make to your horses diet and nutrition:

  • Supplementing your horse’s diet with Essential Fatty Acid (EFA) – Omega 3s – has been shown to reduce insect hypersensitivity. This can be a shop-bought product, or cold pressed linseed oil, or cod liver oil added to the feed.
  • Improving mineral supplementation helps to boost the immune system. This doesn’t mean using an off-the-shelf standard product, although that is better than none. Ideally, identify the deficiencies or imbalances in your horse’s forage and diet, and then supplement accordingly.
  • Some people are using a combination of chondroitin sulphate, spirulina, and ground linseed (flax) in the daily diet, with some success.
  • Probiotics also help to boost the immune system by improving gut function, as we saw earlier.

There are also measures you can take to keep the culicoid flies away from your horse in the first place:

  • A fly rug keeps the carriers, the culicoid flies, away from the horse (c) sweetitchtreatments blog
    A fly rug keeps the carriers, the culicoid flies, away from the horse (c) sweetitchtreatments blog

    External and topical protection can help to keep the culicoid flies at bay: fly rugs, wipe-on insecticides, and oil-based creams and lotions, especially those containing camphor, menthol or thymol, act as barriers while providing relief and repelling flies.

  • If you stable your horse, try to do so between 4pm and 8am, when the biting flies are most active in subtropical and tropical zones. Fans will help circulate air and discourage flies, while mosquito netting on windows will also help.
  • Bathing in dermatological shampoos containing chlorhexidine gluconate bring relief while working against localized bacterial infection.

Do it your way: it’s an individual fight

Ultimately, the line you take when tackling neck threadworms in your horse is going to be an individual one.

You’ll be taking into account the severity of your horse’s reaction to the microfilariae, the duration of the problem, and the intensity of the fly presence in your area. And in managing the itching side, different treatments will work with different horses.

Let’s not forget, there’s also the matter for what you can do in practical terms – eg, if you’re never home from work before 6pm, you’ll be unable to stable your horse from 4pm every day.

And then there’s what you can afford to do on a financial level, especially if you have multiple horses affected (one reader of ‘The Disturbing Truth’ had four).

Please feel free to add your comments and share experiences below. We’re all learning and the contributions are fantastic. Meanwhile, if you’re after specific advice for your individual horse, I recommend talking to the guys over at the Chronicle of the Horse forum.

 

Queensland Itch, Dr Carl Eden BVM&S MRCVS

Studies on Onchocerca cervicalis Railliet and Henry 1910: I. Onchocerca cervicalis in British Horses* Philip S. Mellora1 p1 , J Helminthol. 1973;47(1):97-110.

Prevalence of Onchocerca cervicalis in equids in the Gulf Coast region. Klei TR, Torbert B, Chapman MR, Foil L.  1984 Aug;45(8):1646-7.

 

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The Disturbing Truth About Neck Threadworms and Your Itchy Horse

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Look on any ivermectin or moxidectin-based wormer packet and you’ll see a long list of parasites. Tucked in neatly at the end – it’s nearly always at the end – you’ll see the words Onchocerca cervicalis, otherwise known as neck threadworms.

Also known as neck threadworms, these critters vary in length from 6cm to 30cm (think the length of a regular ruler). Astonishingly, they live in the horse’s nuchal ligament.

Yes, the nuchal ligament. It runs the full length of the neck, from poll to withers, with a flat ligament part connecting with the cervical vertebrae.

Apparently, most horses have Onchocerca. For many they’re not a problem, but some horses develop a reaction to their microscopic larvae (the microfilariae). This is known as Onchocerciasis. The horses become itchy, mostly around the head, neck, chest, shoulders and underside of the belly. That’s why owners often make the understandable assumption that their horse has Queensland itch or sweet itch.

 

A quick introduction to neck threadworms

Original article by Jane Clothier, posted on https://thehorsesback.com, June 2013. All text and photographs (c) Jane Clothier. No reproduction without permission. Links to this page are fine.

Onchocerca is what’s known as a parasitic filarial worm (nematode). One reason these worms get relatively little attention is that they never live in the intestines. The microscopic larval form live in the horse’s skin, mostly around the head, neck, shoulders, chest and underside of the belly. It is the adult worm that later makes its home in the nuchal ligament.

The problem is global and horses in most countries have been found to have this parasite. Unfortunately for those of us who keep horses in warmer, humid climates, it’s more frequent here. The biting insect that serves as a carrier is the Culicoides fly, which is also connected to Queensland Itch (aka Sweet Itch, Summer Itch, etc.).

It’s an unfortunate coincidence of environment that leads to many cases of neck threadworms being missed, because they’re assumed to be Itch.

 

Does your horse have “the itch” – or neck threadworms?

It’s a humdinger of a thought. If your horse is itchy, something different could be happening to what you think is happening.

  •  Your horse has the ‘regular’ itch (ie, Queensland, sweet, whatever it’s called in your region) and are reacting to midge spit – and nothing else. (The point of this article certainly isn’t to try and say that all itch cases are due to neck threadworms. Just some.)
  •  Your horse has neck threadworms and its inflammatory reaction to them has increased its sensitivity, so it’s now reacting to fly bites everywhere – in other words, Queensland/sweet itch has been triggered as a secondary response.
  • Your horse only has neck threadworms, in which case they’re probably rubbing along the mane and particularly the base of mane, around the neck and face, under the chest and down the ventral line (under the belly), but not on the tail head – or at least, relatively little.

Are you by any chance now thinking other horses you know? If so, they might be suffering from Onchocerciasis. There’s a lot of it about.

 

So how do we identify neck threadworms?

Neck threadworms have a distinctive life cycle, but as is so often the case, the problem presents in  different ways, depending on the individual.

In my brumby Colo, it started with him scratching the underside of his neck on posts. That was about 3 months before I had an inkling it might be neck threadworms. How I wish I’d known  what it was at that point, so that I could have nipped the problem in the bud…

A pony with the Itch and neck threadworms. It's Autumn and she's stopped rubbing out her entire mane, but is still itching that tell-tale area in front of the withers. Her coat has raised in a temporary histamine reaction to the ivermectin wormer.
A pony with the Itch and neck threadworms. It’s Autumn and she’s stopped rubbing out her entire mane – it has grown back – but is still itching that tell-tale area in front of the withers. Her coat is raised in a temporary histamine reaction to the ivermectin wormer.

I’ve also seen it manifest as a new, previously unseen itchy and scurfy patch on the lower part of the neck of a horse who’d never been itchy. And I’ve heard of a local horse who suddenly started furiously itching his face, bang in the middle of the forehead, to the point that it bled. He had never been itchy before.

These are the classic early signs, usually recognised by the owner only through miserable hindsight. Other signs include small lumps forming along the underside of the horse and on its neck and face, weeping spots, and a scaly crest to an area of the mane through rubbing.

The base of the mane, just in front of the withers, seems to be party central where neck threadworms are concerned.

 

 

The real nastiness of neck threadworms

The microscopic larvae can travel to the eye, although this is rare.
The microscopic larvae can travel to the eye, although this is rare.

It just gets better: the larvae can travel to the horse’s eyes, where they can cause untold damage. This cheering sentence from Scott and Miller’s Equine Dermatology sums it up: “O. cervicalis microfilariae may also invade ocular tissues, where they may be associated with keratitis, uveitis, peripapillary choroidal sclerosis, and vitiligo of the bulbar conjunctiva of the lateral limbus.”

Oh heck. Nobody’s sure how common this is. All I know is that I don’t want to find out the hard way.

Consider this: in humans, a slightly different strain of Onchocerca infestation is known as River Blindness.

Please remember this detail when you’re deciding whether to worm for neck threadworms or not.


The very strange lifecycle of the neck threadworm

These worms have a complicated existence. They’re among the shapeshifters of the parasitic worm world, developing through several larval stages before reaching adulthood.

The first stage microfilariae live in the horse, close to the skin. Their numbers are highest in the spring and decrease to their lowest point in mid-winter. They live in clusters, which is why you may first notice patches of scurfy skin where the horse has started itching. This is a reaction to the dead or dying larvae.

Itching down the midline. Mine have itched neck and shoulders only - so far. (Photo courtesy of blog, Baba Yaga's Mirror)
Itching down the midline. Mine have itched neck and shoulders only – so far. (Photo courtesy of blog, Baba Yaga’s Mirror)

At this point, our good friends the culicoid flies make a contribution, by biting the horse and ingesting a good number of microfilariae along with blood. Within the fly, the larvae then develop through a further stage (or two). They are then deposited back into a horse when the flies bite. The flies can do this for an impressive 20 to 25 days after first hoovering up the larvae.

Back in a host horse, the larvae then make their way via the bloodstream to the connective tissue of the nuchal ligament, which runs along the crest of the neck. Here they moult and develop into adult worms. The adults live for around 10 years and in this time, the females release thousands of microfilariae (larvae) very year.

Original article by Jane Clothier, posted on www.thehorsesback.com, June 2013. All text and photographs (c) Jane Clothier. No reproduction without permission, sorry. Links to this page are fine.

No matter where the adult worms settle, the itchiness is caused by the microfilariae that aren’t lucky enough to be consumed by a fly and are instead left to die off.

The next part’s really not fair. The more the horse itches and breaks the skin, the more the flies will bite exactly where the microfilariae are located, before transporting them to the same or another horse, to start all over again.

Unsurprisingly, horses with most lesions have higher microfilariae counts – it’s a perfect ascending spiral of parasite-induced discomfort.

The Onchecerca life cycle lasts for 4 to 5 months.

 

Can we test for neck threadworms?

The microfilariae can be identified in the living horse through a biopsy of the nuchal ligament. Published veterinary research shows you won’t get any indication within 34 days of worming, so the timing is critical.

Worming with ivermectin can lead to weeping spots in the mane. This was after they'd cleared.
Worming with ivermectin can lead to weeping spots in the mane. These can be painful. This photo was taken after they’d cleared, leaving bald areas. Sometimes the hair grows back white.

A dose of ivermectin-based wormer is the quickest way to tell if your horse has them. If the microfilariae are present, the horse usually responds with intense itching – and I mean, manically intense, demented itching – around 48 to 72 hours after worming.

It may develop weeping, gunky spots at the base of the mane. (If you live in a paralysis tick area, it’s similar to the localised reaction you see in response to the ticks.) These are very specific spots around 1cm in diameter, with hair loss after they’ve erupted.

My brumby responded this way, rolling furiously and rubbing vigorously against posts. Unsurprisingly, he was also hard to handle for a few days. He was definitely sore at the base of the neck, where the weeping eruptions came out, and didn’t want to be touched there. I have to say that the scale of his reaction came as a shock to me, so take heed and be prepared with some soothing salves.

 

What can we do about adult neck threadworms?

Here’s the depressing answer: not much. But we can manage them.

The nuchal ligament runs from poll to wither and links with the vertebrae. Yellow = funicular part, home to neck threadworms.
The nuchal ligament runs from poll to wither and links with the vertebrae. Yellow = funicular part, home to neck threadworms. (Image copyright Sustainable Dressage.)

The adults live for 10-12 years and happily inhabit the nuchal ligament. What often happens is that the horse’s body throws down calcification around the adult worms in an attempt to isolate the foreign body. In some horses, you can feel a collection of  pea-like bumps in the nuchal ligament. In the ones that I’ve checked, this was just in front of the withers.

The slightly better news it that the worms are so fine and the lumps so small that it doesn’t seem to affect the function of the ligament, which is tough and fundamentally taut anyway. However, I’ve not yet knowingly seen a horse with a long history of neck threadworms – I’d be interested in doing so.

Heavier calcification is usually most prevalent in horses in their late teens. It figures, as the adult wormers are older, and longer. Apparently they intertwine and live in small clumps. Mid-aged horses have mainly shown inflamed tissue around live parasites.

In horses less than 5yo, the parasites can be present but there’s relatively little immunological response. So if your horse has suddenly developed itchiness at the age of 5 or 6, you could be looking at the presence of this parasite.

Original article by Jane Clothier, posted on www.thehorsesback.com, June 2013. All text and photographs (c) Jane Clothier. No reproduction without permission, sorry. Links to this page are fine.

 

Managing the initial outbreak

Do you worm your horses? Do you want to reduce the itching at the cost of having to worm more? I know I do, but I realise that some people can’t abide the thought of chemical wormers, or their increased use. But here’s what you can do if you want to reduce that dreadful itching and virtually eliminate the possibility of eye damage.

Unfortunately, there’s no single recommended protocol for worming against neck threadworms, so you’re in fairly uncharted territory.

  • wormerTo address the initial outbreak, the advice ‘out in the field’ is to use a regular dosage of an ivermectin-based wormer, multiple times until symptoms subside. The recommended interval I’ve seen is a week, but do check with your *equine* vet first.
  • I’ve also read forum posts by US horse owners stating that a double dosage at fortnightly intervals is the most effective treatment. It’s usually around three doses, or until symptoms subside. One reason is that lower doses do not kill off enough larvae, allowing resistance to develop amongst those that remain. Wormers are certainly tested as safe at higher dosages, but again, horses are individuals, so always check with your *equine* vet first.
  • I’ve read that an injection of ivermectin can be more effective, with off-label use of a product such as Dectomax being recommended as the heavy artillery when all else has failed. Again, do check with your *equine* vet.

Some say that an ivermectin and praziquantel wormer is more effective. One small comfort is that these wormers are available in the lower price ranges. It’s a consideration, because if you’re worming multiple horses, this won’t be a cheap time. It may even be worth looking at the large bottles of liquid wormer used by studs for greater economy.

Published research has shown that moxidectin-based wormers are equally as effective in addressing the microfilariae (but don’t double-dose with this one – only with ivermectin).

Whichever option you follow, it’s worth following this worming protocol with prebiotics, probiotics and ‘buffers’ such as aloe vera to support a healthy gut lining.

Reducing the larval population

After the initial worming, it’s a matter of management. What you’re trying to do is keep the numbers of microfilaraie low, so that the horse’s itching is reduced. Remember, most horses show little reaction, although the parasites are present. The aim has to be to bring them down to levels the horses’ systems can deal with, while taking other measures to boost the horses’ immune system.

  • Some vets say a single dose every 6-8 weeks during the fly season.
  • Others say every 3 months, timed in accordance with the larval lifecycle, which is 4 to 5 months.
  • In humid sub-tropical zones, where all parasite burdens are dramatically higher, I’ve heard of people doing it as frequently as once a month.

Beyond that, you’re back to the barrier treatments – fly rugs, lotions and potions to deflect the flies and to insulate the skin, lotions to soften the skin and heal the lesions, fly screens on shelters during the day, etc. And don’t forget about boosting your horse’s immune system generally through sound nutritional approaches.

 

And if we do nothing?

If we don’t address the problem one way or another, we have very itchy horses, for their entire lives.

Researchers say that the calcification in the ligaments has no effect, but you’ve got to wonder. There’s no guarantee that those scientists had a highly developed understanding of equine biomechanics. Maybe they did, but… who knows. A lot of the small amount of research available is over 20 years old and the knowledge base has since grown.

There’s a small but serious risk of damage to the eyes.

On the plus side, Onchocerciasis hasn’t been found to have any association with fistulous withers.

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

To recap…

Onchocerciasis is so often masked by the itch that awareness, even in the regions where it’s rife, is low.

And in those same regions, there are so many highly prevalent and deadly parasites – the worms that cause colic, that drag down the horse’s condition, that can kill through spontaneous mass emergence from encysted larval stages – that the neck threadworm larvae simply doesn’t get much of a look-in.

To repeat, I’m not saying that all cases of itch are neck threadworms. Just that these parasites may be involved and can be a contributory factor in a heightened immunological response that leads to Queensland itch (or sweet itch, or whatever you know it as).

However, some horses definitely have neck threadworms. The earlier we can identify and manage it, the better.

We can’t eliminate the neck threadworms, but we can certainly manage the effects and make our horses’ lives more comfortable.

(c) Jane Clothier – no reproduction without permission – jane@thehorsesback.com

 

Here are some of the links I’ve been accessing for this article.

 

Equine Onchocerciasis: Lesions in the Nuchal Ligament of Midwestern US Horses. G. M. Schmidt, J. D. Krehbiel, S. C. Coley and R. W. Leid. Vet Pathol 1982 19: 16.

Efficacy of ivermectin against Onchocerca cervicalis microfilarial dermatitis in horses. Herd RP, Donham JC, Am J Vet Res. 1983 Jun;44(6):1102-5. http://www.ncbi.nlm.nih.gov/pubmed/6688162

Onchocerciasis, Submitted by EquiMed Staff. http://equimed.com/diseases-and-conditions/reference/onchocerciasis

Onchocerciasis, the Neck Thread Worms and Midline Dermatitis in Horses.

The Hypersensitivity of Horses to Culicoides Bites in British Columbia. Gail S. Anderson, Peter Belton and Nicholas Kleider. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1680856/pdf/canvetj00574-0044.pdf

Onchocerca in Horses from Western Canada and the Northwestern United States: An Abattoir Survey of the Prevalence of Infection, L Polley. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1790494/

Research of skin microfilariae on 160 horses from Poland, France and Spain. M.T. FRANCK et al. Am J Vet Res. 1983 Jun;44(6):1102-5. http://www.revmedvet.com/2006/RMV157_323_325.pdf

Efficacy of ivermectin against Onchocerca cervicalis microfilarial dermatitis in horses. Herd RP, Donham JC. http://www.ncbi.nlm.nih.gov/pubmed/6688162

Equine Dermatology. By Danny W. Scott, William H. Miller Jr. via Google Books [link now unavailable]

Baba Yaga’s Mirror. Personal blog. [link now unavailable]