Internal and External Parasites in Horses – Bugs and Worms!


We don’t often want to think about our darling horses being cute little snacks for other creatures, but that’s really what they are. Parasite is a big term for anything that lives its life at the expense of others. Little buttheads. Two types of parasites infect horses – internal parasites and external parasites. It’s a matter of where they live – in your horse, or on your horse.


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Internal parasites of horses.


Small strongyles


  • Small strongyles are also called small redworms or cyathostomins. They are in fact small, coming in at about 10mm in length, and some are red, while some are gray. They are short-lived, with a life span of about 7 weeks or so. But they have a secret weapon for survival – the larvae create cysts in the intestinal wall of your horses where they can sort of hibernate a bit, for weeks, months, years. This makes most deworming protocols ineffective. The encysted larvae can also interfere with how well your horse absorbs his nutrients, which often creates a horse that’s just sort of dull, and looks dull as well.


  • These small redworms usually eat on your horse’s intestinal lining in the cecum and large colon, which obvi leads to some problems. Besides ulcers, your horse can develop colic, diarrhea, and overall poor health and condition. The problems get bigger if a large group of encysted small strongyles break out of their cases. This is called larval cyathostominosis, and can be deadly for horses.


  • Other things to know about small strongyles. They are plentiful. And the eggs they pass in your horse’s manure trickle out, a little bit at a time. Then your horse grazes and re-infects himself. I know that you can already predict I’m going to get on a big soapbox and talk about picking your horse’s manure out of the paddocks.


picking up manure in a pasture

Scoopin’ all the poop.


Large strongyles


  • These guys are also known as bloodworms, and are about twice the length of the small strongyle. Their lifecycle is about six months and eat on…you guessed it…blood. Anemia, colic, and death are possible side effects of an out-of-control strongyle infection.


  • Large strongyle eggs are passed in a horse’s manure and become larvae in the field. They like to hang out at the top of blades of grass, where your horse will eat them. Once inside your horse’s digestive system, they congregate in the artery that is the main blood supply for your horse’s intestines – the cranial mesenteric arteries, to be exact. Here they can create clots that release and cause damage and death to your horse, eat so much blood your horse becomes anemic, and even straight-up block flow. And how does it get to that area? It burrows through your horse from the intestinal tract.




  • This is a whole other ball game, folks. Tapeworms are notoriously impossible to detect. And can really muck up your horse. There are three main species of tapeworms in horses, and Anoplocephala perfoliata is the worst of the three. They appear as pumpkin seed-shaped jerks in the small intestine and stomach of your horse. They especially like the area between the small intestine and the cecum, and will attach themselves with hooks to your horse. This can create a blockage in your horse. Not to mention intussusceptions (telescoping intestines), gas colics, and impactions.


  • They do pass eggs, which are then eaten by mites that live in pasture. These mites are then eaten by your horse to carry on the life cycle. You won’t usually find evidence of tapeworms in a fecal egg count, but there is a blood test that can be done, too.



horse worms in a jar

So mean to horses, this worm.


Ascarids (roundworms)


  • Roundworms love young horses, usually under age three. Despite your best efforts, roundworms often take over these youngsters and lead to potbellies, a rough coat, and problems keeping weight.


  • Here’s the bad part of roundworms – one roundworm female can lay about 150,000 eggs per day. And the eggs are super, turbo-strong and can live for 10 years in all sorts of weather. Once eaten, the larva migrate through your horse’s lungs and liver. From there they are coughed up and swallowed. Not usually a problem for the liver, which can regenerate, but the lungs are often permanently damaged.




  • What can I say about these itchy meanies? These worms are usually a gray or white shade, with the males being small at around 10mm and the females reaching lengths of 10mm. They live in your horse’s rectum and literally come out at night to lay eggs around your horse’s anus and under his tail. The worm egg glue, as it is, is really itchy. Most horses with crazy tail tops from itching should be tested for pinworms. When a horse rubs his butt, the eggs are released into feed bins, hay piles, near fences, etc. By the way, the internet at large seems to believe that Listerine will “cure” an itchy tail, and I’m here to say that in the case of pinworms, Listerine won’t do the trick.


  • Luckily (?) horses with a pinworm population usually have no consequences, other than an itchy bum. Can this lead to other problems? Sure! Horses will damage their tails, and possibly rub sores into themselves.





  • While technically not a worm, the bot flies that bother your horse use his body as part of their life cycle. The botfly female will deposit her yellowish eggs on your horse. Usually, the legs, neck, or areas that your horse can easily reach the eggs. Here’s why…as the eggs mature, they will be stimulated to release the larvae when your horse grooms himself or rubs himself.


  • The larvae then make their way to your horse’s mouth, which serves as the larvae’s home for the next month. Your’s horse’s tongue or gums become a little nest for this butthead parasite. It’s pretty obvious how these parasites can damage your horse’s mouth and even create abscesses and loose teeth. He will then burrow through your horse’s body and into the stomach to set up shop. Here they clog things up for almost a year. Ulcers, paralysis of the esophagus, and ruptured stomachs can happen, as well as tumors and anemia.


  • While still inside your horse, they started to pupate, and are passed with manure into fields. There they spend a few months, hatch, mate, and start again. Here’s a handy time to remind you to pick poop out of pastures!



yellow bot eggs on a horse knee

These are botfly eggs. BLECH. You can remove them with a special knife.


Threadworms and Neck Threadworms


  • These parasites often affect young horses, as they are passed from mare to foal via milk. Threadworms live in your horse’s tissues… which is how they reproduce. The threadworm is triggered by your horse’s hormones after birth, and migrate to the mammary tissue so they can pass their eggs along into the milk. Foals can suffer from anemia, diarrhea, and weakness. It was once thought that diarrhea was a telltale sign of threadworm in foals, but there are cases where diarrhea isn’t present.


  • Most adult horses have developed enough immunity and don’t have a threadworm problem.


  • These internal parasites shouldn’t be confused with the neck threadworm, which is an entirely different beast. These parasites, Onchocerca cervicalis, live in the nuchal ligament of your horse. This ligament runs from withers to skull. The worms reproduce by sending out micorfilariae into the loose tissues under your horse’s skin in the neck area.


  • Here’s where it gets interesting. Remember culicoides midges, the no-see-ums that cause sweet itch? Well, these little bugs are part of the threadworm life cycle. And, if your horse is dewormed, and the microfilmiae are killed, there’s usually a skin reaction that looks an awful lot like sweet itch. But isn’t. Either way, your vet will have to confirm that your horse has neck threadworm or sweet itch. Otherwise, you may not be treating the correct issue. Who knew?


  • And, EW.


External parasites of horses.




  • There are too many types of mites. You may have seen a tragic photo of a stray dog without any hair and lots of scabby skin? That’s the work of mites, causing a condition called mange. All types of mites are microscopic, we just see the damage that they do. They are also closely related to ticks, which is a whole other category of blech.


  • For the horses out there with naturally thick feathers, they can often be infested with chorioptic mites, also known as feather mites. Like some other skin conditions like rain rot, feather mites spread via direct contact from horse to horse or via humans sharing stuff between horses. The mites eat skin debris, fat, and the sores that they create. An infection of feather mites is itchy and creates pimple-like sores and hair loss. The itch will cause your horse to chew the area, often to the point of being considered self-mutilation. And then you have the secondary complication of a bacterial infection setting in. These mites are specific to horses, there’s no chance that you can also get them.


  • Then there are the Psoroptes mites. These are commonly known as scab mites, as they (logically) create little scabs. And then these mites poop. And then your horse has an allergic reaction to the mite poop. Which produces exudate (gross slimy stuff like pus), and the skin will harden and thicken, coupled with hair loss. These are also passed among shared things at the barn, although horses can also get this from each other and from the environment.


  • And say hello to the sarcoptes mite. This is the most common, and most tricky, and can be transmitted to humans. Sarcoptes spread the same way as the feather and scab mites. The skin irritation caused by these jerks goes haywire. As these mites dig into your horse’s kin, their saliva creates a huge allergic reaction. You will also see pimples, and the skin will get so thin that it folds over itself. Also, it spreads all over your horse and is wildly itchy, leading to more severe damage and secondary infections.


  • And lastly, the demodex mite, which hangs out in hair follicles. These usually spread on the head and then travel down your horse’s body. A scaly skin texture is the result of this infection.


  • All mite infections need diagnosis from your vet to prescribe the proper treatment. This will be time-intensive, as the skin needs to heal and hair to grow back.



mite photo up close to show legs and body

This is some sort of mite. I can’t say for sure what type, nor do I want to investigate further for fear of being more grossed out.




  • Unlike mites, lice can be seen by the naked eye, although it’s really hard. Most of the time you are just seeing movement. Lice in horses fall into two categories, the sucking variety and the chewing variety.


  • Sucking lice can actually cause anemia from so much feeding on your horse. The chewing lice seem to be more annoying to horses, causing hair loss, rubbings, and even open sores. It’s generally understood that lice tend to overtake horses that are unhealthy, poorly groomed, and have a not-ideal immune system.


  • Like mites, they can be easily transmitted from horse to horse, either with our help in the form of shared tools, or horse to horse contact.


  • Your vet will have to diagnose and prescribe the best treatment for a lice infestation. It could be a dust powder or spray, or oral medication.




  • Not only do mosquitos create itchy bumps, but they also transmit all sorts of horrible (and deadly) diseases amongst horses. You may have heard of EEE, WEE, and West Nile? All nasty, all without cures, all will break your heart.


  • EEE is eastern equine encephalitis, causes raging fevers, neurological issues, and paralysis. There’s an 85% chance your horse won’t make it. WEE is western equine encephalitis, and similar in presentation as EEE, although the death rate is somewhere around 40%. Surviving horses often have permanent neurological damage. There are vaccines available for several mosquito-borne diseases.




  • These monsters are perhaps my greatest parasite nemesis. So gross. Aside from carrying diseases, like Lyme’s, they can cause itchiness, welts, and general unpleasantness. And that’s not ticks that are not buried deep in your horse’s ear.


  • It’s fairly easy to inspect your horse for ticks, and even easier to remove them. It should be part of your daily grooming routine!


tick on a white background




  • There are so many categories of flies that affect horses, each with its own set of things that work, and don’t work, to combat them.


  • There are the large predators, like the giant black bomber horse flies and deer flies and greenheads that feed on your horse’s blood.


  • Then there are the non-vampire flies that poop and vomit all over your horse while irritating the snot out of him.



  • Flies also contribute to hoof problems if your horse is stomping a lot, can create summer sores by transmitting Habronema worms into wounds, transmit EIA, put worms into your horse’s eye, and even create massive abscesses as is the case with pigeon fever.



horse wearing sox for horses on mat

Yes, your horse can wear socks. These are Silver Whinny’s from Sox for Horses.


Parasite control measures for horses.


  • Boy howdy – this is an all-encompassing challenge to try and mitigate the number of parasites that your horse interacts with. I don’t think we can ever, nor should we ever, aim to eliminate all of them. It does help, however, to be sure your horse stays healthy and comfortable.


Fecal egg counts


  • These easy and inexpensive tests, in the spring and fall, give your vet a vague idea of how many strongyles and ascarid eggs your horse is passing through. Low numbers indicate your horse is a low shedder, and likely will not need targeted deworming for those worms. High shedders need to be addressed, as they are passing on such a large volume of eggs.


  • A few thoughts on this – using a rotational deworming system and worming low shedders increases the number of resistant worms in the environment. If this continues, we will reach a point where our current medications are useless against certain parasites.


  • Imagine a scenario where every single non-resistant worm was eradicated. This could be on a farm level, a state level, or even larger. The only worms left are the resistant ones. You would be out of luck with any dewormer, and the populations would increase so greatly that horses are at risk of the complications of worms – colic, impactions, etc – without any way to treat the worms. There are no new dewormers in development right now – what we have is what we have.



  • The other thing to know about fecal egg counts is that they do not catch all of the possible internal parasites your horse may have. This is why it’s absolutely crucial to make sure your vet gives you deworming advice. I see too many “charts” on the internet with rotational schedules – but they don’t fit every horse. For example – a horse in the south that never goes through a freezing cycle in winter won’t have the same deworming needs and schedule as a horse living where winter truly exists. Seasons and climate influence when dewormers should be given.


  • Let’s also blow another myth out of the water here – there is an old school belief that you should deworm a horse when you see worms pass out of his system. The worm life cycle typically only allows EGGS to pass – you may never, ever see a worm. And you will most definitely never, ever see an egg – they are microscopic.


Pasture maintenance


  • If you really want to help your horse out here, make sure his pasture and paddocks are kept clean. Nothing attracts flies and insects like manure, and nothing can deliver millions upon millions of worm eggs into a field like manure. Manure piles in pastures also encourage horses to graze some spots and not the “dirty” ones, so your pasture growth is uneven and can create weed problems.


  • Don’t rely on a freeze or a heatwave to “kill everything” – it’s not how that works. Some parasites and their eggs and larvae have adapted, others have not.


  • If your horse lives in a herd, you need to take that into consideration when you and your vet come up with a testing and medication plan. It won’t do you any good (in the long run) if your horse is tested and treated if he lives with other horses that are not tested and treated.


  • For horses that may not live in a herd, but do switch around paddocks and share with other horses, the entire barn needs to be on the same page with testing and appropriately treating all of the horses. It’s a community responsibility!


  • When picking up pasture manure, you will also get a workout. And you will be able to inspect for all sorts of things, like holes and broken fence.


Grooming and Barn Chores


  • The point of grooming your horse and cleaning his area is not to make him pretty and cute and shiny and Instagram ready. It’s to make sure he’s healthy. The pretty stuff is a by-product.


  • Do you see bot eggs on legs, bellies, flanks, necks?


  • Are there ticks under tails, in elbow skin folds, in between the back legs? In the ears? This is my plan for tick inspections here.


  • Does your horse have an itchy tail? Investigate why before assuming that your mouthwash can fix it.



  • What about the skin and hair deep in the mane? See or feel anything there?


  • Does your horse have welts? Bites? Tiny scabs? You can’t feel all of these with grooming tools – use your hands to inspect every inch.


  • How clean is your horse’s bedding?


  • What’s the fly population in his area? And what types of flies are they?


  • How often are you scrubbing feeders and buckets and water troughs?


  • If your horse eats off the ground, how are you giving him an egg-free plate? On another note, using a feeder or mat will also reduce the volume of sand and dirt your horse eats. Win – win.


  • What’s your stall, paddock, and pasture manure picking routine like?


What’s your fly control plan like?


  • I’m a big fan of preventing an insect from attacking. Most readily available sprays are great – if you can apply them a few times a day. I can’t, so I rely on fly sheets, masks, and boots. You can also use socks to end the incessant stomping.


  • Your fly control plan heavily relies on your manure control plan, too! Composting your horse’s manure is always a good idea. When it’s done correctly, it will not attract flies. Promise.


  • How are you attacking all stages of the fly life cycle? Predator wasps, cleaning manure up, using sprays and fly sheets, fly traps and fly strips all help the fly and insect population stay low. It’s not enough to just toss on a sheet and be done.


It’s a lot of information to digest! Don’t be overwhelmed – you can make adjustments to your routine over time.


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Cashel Quiet Ride Horse Fly Mask with Long Nose and Ears

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