Horse Pests and Parasites

Horse Pests and Parasites

Posted by Green Mountain Horse and Tack on 31st May 2022

It is a battle that nearly every horse owner faces with the arrival of longer days and warmer temperatures: the war with pests and parasites. Certainly, the most visible and irritating pest we war with is the fly; difficult to live with and impossible to eradicate, it’s sometimes hard to believe how aggravating these little buggers can be, to both your horse and you. While you cannot wipe them out completely, it’s important to have a plan and implement it at the right time to minimize their brutality in the barn.

In addition to flies, the warm weather also turns on the parasites and these guys can get downright dangerous, robbing your horse of essential nutrients causing diarrhea, depression, lesions in the mouth, internal ruptures and even death. In this article, we’ll cover basic and advanced techniques to keep your horse healthy and happy.

Flies are, unfortunately, simply a part of living when you have horses. Most often, they are just a nuisance, but if your area is producing horseflies or deerflies, you’ll have more than a nuisance on your hands. These little buggers have no fear, they don’t care about fly and pest sprays, and they’ll bite you and your horse hard enough to draw blood.

Fly masks will protect your horses’ eyes and ears, an appealing target of the fly. They typically Velcro a face covering, putting a mesh material over the eyes, which protect the eyes while allowing the horse to see through the mesh. They are available with or without ear protection. Some horses are extremely sensitive about the ears and have difficulty wearing masks with ear protection, but if they’ll take it, it’s good protection.

Other tried and true efforts are the many iterations of fly traps and, of course, the sticky tape rolls to hang from the beams of the barn. The only way to tell if these will be effective for your specific situation is to buy one and try it out. We have found that in our barn, the traps and tape fill up with so many flies so quickly, it’s just throwing money into the manure pile, but it’s worth a try, for sure.

There are many different fly sprays available today and new products seem to hit the market every year. There are purely chemical-based formulas, all-natural formulas, and everything in between. Fly sprays are used directly on the horse, so make sure you’re using a spray that is formulated for horses – do not just spray your horse with a bottle of RAID, as this could prove to be unsafe for your horse. There is also quite a price range when looking at fly spray. More expensive sprays are usually formulated to last longer and do not have to be re-applied as often. Again, experimentation is recommended to find the right product for your situation.

One product we’ve had pretty good luck with are parasitic fly wasps. These tiny wasps do not sting and are harmless to horses and humans, but they use fly pupa to lay their eggs, and this destroys the pupa and the larva, so the reproduction cycle of the flies are interrupted. They will arrive in the larva stage in a plastic bag with sawdust. Simply spread them throughout the barn and horse stalls and even in the pasture – anywhere the flies are heavy. It’s important to understand that you must start this treatment early in the season – after the first 3 days or so of 40-degree weather – and you must maintain with additional monthly doses. We have found a dramatic reduction of flies in our barn for about $200 per season for 5-6 horses. You can find more information on these fly parasites at They’ll generate a dosage schedule based on your geography and how many horses you are covering and ship the doses out automatically. It’s been a good product for our barn.

Parasite Prevention

The domestic horse is particularly susceptible to parasites. Unlike wild horses, they use smaller pastures, paddocks, and stables and if parasites do enter these areas, they can quickly become saturated with parasite eggs and present a real danger to the horse.

It is important to have a two-pronged approach to preventing parasitic illness. Since manure is the vehicle in which parasitic eggs develop, it’s important to regularly clean out stalls. Get that manure out of their living space. If your pasture is small, try to keep it as clean as you can by picking up the manure piles. For larger pastures, drag the pasture with a drag harrow or by pulling an 8–12-foot post or 4x4 across the field to break up the manure so it can dry and kill the larva. It’s a good idea to give the pasture a rest after dragging for 10-14 days as the larva dry out and die.

Types of Parasites
Large Strongyle (Bloodworm)
This bloodworm lives in the horse’s colon and cecum and is small - about an inch and a half – but mighty. The damage it can do to a horse during its migration through the body can be extensive. Blood clots, internal lesions, intestinal colic and gangrene of the bowel as well as liver damage and peritonitis are all potential consequences of the large strongyle. A broad spectrum dewormer such as moxidectin, ivermectin and fenbendazole are all suitable treatments for the migrating phase of this parasite.

Small Strongyle

This threadlike worm also lives in the colon and the cecum and is less than one inch in length. But make no mistake here – this is a nasty beast and is of much concern to horse owners. There are estimated to be twenty-five species of small strongyles and they live in different parts of the intestines.

The damage from this parasite comes as worm emerges from the cyst, releasing the waste product that has been build up inside it. The consequence is a loss of protein in the intestine that cannot be recovered. This will shortchange the horse’s ability to build healthy bone and muscle and the horses’ skin will become unhealthy as well. If the infestation is bad, the horse can also exhibit diarrhea, dehydration, and colic. Up to a million of these parasites can infect a single horse during a bad infestation. While any parasite is bad, this one is quite severe to the horse. Treatment for the adult stage is a broad-spectrum dewormer and moxidectin and fendendazole for the adult and encysted stage.

The tapeworm develops in the cecum, colon and small intestine after a horse eats an infected mite. A light infestation will show no signs, but during a heavy infestation the horse will exhibit signs of colic and will lie down frequently. It is a heavy infestation that you must worry about. Praziquantel, which is in many broad-spectrum dewormers, is the appropriate treatment.

Female pinworms can grow up to two and a half inches long and males will be under a half-inch in length, and they form in the colon and passed into the anus, causing sever itching. This can result in a very perplexing situation as the horse incessantly scratch his tail against a fence post or stall and quickly creates bald spots around the tail. Fortunately, there are rarely any internal effects, but some horses that are particularly sensitive can be restless and exhibit poor appetite. Pinworms can impact any age of horse but are found to be most prevalent in young horses.

The tapeworm develops in the cecum, colon and small intestine and can grow to about two inches in length. They are transferred to the horse when he eats an infected pasture mite. A heavily infested horse will be depressed and show signs of colic. A light infestation will show no outward signs of illness. Tapeworms are treated with praziquantel, which is found in many broad-spectrum dewormers.

There are two varieties of the botfly: the common botfly and the throat botfly. Adult botflies are seen as slightly furry yellow and black striped flies. The female common botfly will lay attach her eggs on the horse’s shoulder and forelegs, and the horse will then lick the eggs off and ingest them. The throat botfly will lay her eggs on the horse’s chin and throat and when they hatch, they will burrow under the skin into the mouth, remaining in the lining of the tongue and cheek for about four weeks before moving into the stomach. Mouth ulcers will be the sign of a bot infestation. Ivermectin and moxidectin are suitable treatments.

Parasites are a moving target as they are constantly evolving and developing resistance to certain chemical treatments. Work with your vet to determine the correct parasitic control schedule for your horse. Your local vet will be intimately familiar with the risks of exposure to these parasites for your specific area and the most effective treatments.