Internal parasites, also known colloquially as worms, are a common concern for many horse owners.
Parasites are organisms that live on the horse’s skin or infiltrate the intestinal tract to gain nutrients. They can cause inflammation, immune problems, ulcers, and, in serious cases, impaction of the intestines.
All horses are susceptible to parasites and infestations can lead to a number of negative health consequences. Signs of equine internal parasites include weight loss, colic, impaired growth, poor coat quality, lethargy, and a potbelly.
The level of parasitic infection for individual horses will depend on a variety of factors. These include feeding and grazing conditions, pasture management, and deworming practices. 
If you suspect your horse has a high parasite load, consult with your veterinarian to receive a diagnosis and for treatment options. Making changes to your horse’s feeding program and management can help to reduce the risk of internal parasites in horses.
How Horses Get Internal Parasites
Under natural conditions where horses are allowed to roam freely, internal parasites are rarely a problem.
This is because horses live a nomadic lifestyle and graze over large areas. In this environment, they don’t eat near their piles of manure.
However, the confinement of horses to small pastures has led to the problem of high parasite burdens as worm egg populations become concentrated in small areas.
Eggs that are shed in manure will hatch into larvae which make a home in the surrounding grass and soil. From there, they can be ingested by the horse or other grazing animals.
Once ingested, different parasites move to different internal organs in the horse where they may stay for weeks or even months.
The parasites mature through several life stages to become adult worms. The female worms then lay eggs which are passed in the feces back into the environment.
If climate conditions are favorable, the eggs develop into infective larvae and the cycle continues.
Unless something is done to interrupt this cycle, parasites will continue to reproduce, infecting any horse that grazes in the pasture.
High parasite loads are common in young horses, though adult horses do gain some immunity to certain types of parasites.
However, some horses remain susceptible to carrying higher parasite loads throughout their lifetime. In severe instances, internal parasites can be deadly for horses.
Types of Internal Parasites Affecting Horses
Over 150 species of internal parasites can affect the horse. The seven most common ones include:
- Large Strongyles
- Small Strongyles
Large Strongyles (Strongylus vulgaris)
Historically, large strongyles were considered the most dangerous internal parasite for horses. They infected their host by migrating through the blood vessels in the intestines, causing significant bleeding and damage.
A high parasite load often resulted in rapid weight loss, diarrhea, and caused blockages that led to colic. Severe infections also led to death for many horses.
The good news is that, since the 1970s, frequent deworming practices have greatly reduced the prevalence of this parasite.  As a result, large strongyles are rarely found in domestic horses today.
Small Strongyles (Cyathostomin spp.)
Small strongyles are the most common internal parasite in horses today. Once the larvae of this parasite are ingested, they burrow into the gut lining where they continue to develop.
During the winter months, small strongyles can hibernate within the gut wall. These are often referred to as encysted strongyles.
Encysted strongyles can be dangerous to the horse in spring when large numbers of larvae emerge. This is known as larval cyathostominosis.
Most horses do not show clinical signs of infection, however larval cyathostominosis has been reported to be fatal in 50% of all cases. 
Young horses between the age of 1-4 are the most sensitive to strongyles since they are still developing immunity to this parasite. 
Roundworms (Parascaris equorum)
Roundworms, also known as ascarids, can grow to be 50 cm in length and are especially dangerous to foals and young horses.
Once ingested, roundworm larvae move through the gut wall to the liver and then the lungs where they are coughed up and swallowed. Then, the larvae mature to lay eggs in the horse’s intestines.
Horses with high roundworm loads may have a cough and nasal discharge while the larvae move through the lungs. They may also have weight loss, a pot belly, and/or diarrhea when the larvae are in the intestines.
Tapeworms (Anoplocephala spp.)
Tapeworms are another large and potentially dangerous species of internal parasite, growing up to 20 cm in length.
They are transmitted through an intermediate host, the orbatid mite. This mite is found in grass, harvested forage, and even grain.
When the mite is ingested by a horse, the tapeworm larvae develop into adult tapeworms. Within 6-10 weeks, the females shed eggs.
Tapeworms gather in clusters at the junction between the small and large intestine where they can cause digestive problems and impaction colic in some instances.
Young horses and those over 15 years of age appear to be most at risk of tapeworm infection.
Pinworms (Oxyuris equi)
Pinworms lay eggs around the outside of the horse’s anus, causing itching and irritation. Horses will rub against trees, stall walls, or anything they can to relieve the itching.
This frequent rubbing can lead to hair loss and create sores around the anus which may become infected.
Threadworms (Strongyloides Westeri)
Most horses develop natural immunity to threadworms by six months of age.
However, threadworms can remain dormant in adult horses and then be transferred to foals through the mare’s milk.
High threadworm loads can lead to lethargy, diarrhea, and anemia. A foal’s growth rate can also be affected by high levels of infestation.
Foals should be treated for threadworms at or after four weeks of age. Deworming the mare during pregnancy can also help to reduce threadworm transferral to the foal.
Bot flies are a seasonal parasite, leaving eggs on the horse’s hair coat during the summer grazing months. Some of the eggs are later ingested as the horse grooms itself.
Once in the mouth, bot eggs hatch into larvae that migrate to the stomach to attach themselves to the lining and continue to develop.
They will then detach and pass through the manure where they will pupate into flies.
Symptoms of Parasite Infection in Horses
If horses become infected with high numbers of parasites, they may show clinical symptoms. These can include:
- Poor body condition
- Reduced exercise performance
- Poor reproductive performance
- Slow growth in young horses
- Scratching (with pinworms especially)
However, it is also possible for a horse to have a high infection rate and show no clinical symptoms.
Diagnosing Internal Parasites in Horses
It’s important to note that all horses normally have some level of internal parasite infection.
However, having a high number of parasites is not normal and can pose a health threat to horses.
Until recently, it was recommended to keep all horses on a routine deworming schedule to prevent high parasite numbers. Due to growing anthelmintic drug resistance on the part of the parasites, this approach is no longer recommended.
Instead, a targeted approach should be used to treat horses with high infection rates. The first step is to consult with your veterinarian for a diagnosis and to determine the type of parasite present.
Fecal Egg Counts
Since 1999, the use of Fecal Egg Counts (FECs) has become important for Selective Anthelmintic Therapy (SAT). SAT is a means of reducing drug resistance in internal parasites.
With this approach, only horses that exceed a certain FEC threshold are treated .
FECs have been especially helpful in treating small strongyles, the most common type of equine parasite and the parasite most likely to reach levels of dewormer resistance.
By using FECs, horse owners can help preserve the effectiveness of dewormers and minimize unnecessary treatment in animals excreting a low number of eggs.
SAT aims to keep parasite levels below that which could cause disease in horses. According to common consensus, horses shedding less than 200 eggs per gram (epg) do not need treatment. 
Most veterinarians can perform an FEC for horse owners. There are also several laboratories that take mail-in samples which owners can collect themselves.
Several techniques for performing an FEC exist, but the Mini-FLOTAC technique is known for both its high precision and accuracy rates, especially in detecting strongyle and ascarid eggs. 
Consistent use of FECs can also show which horses on a farm are high shedders or which horses are carrying the highest parasite burdens. One study showed that egg shedding remains fairly consistent in adult horses.
Targeting anthelmintic treatment to high shedders can help to reduce pasture contamination and worm burdens in the whole pasture herd. 
Diagnosing Tapeworm Infection
Tapeworms are difficult to detect through an FEC. Blood tests and a new saliva test are available to help make a diagnosis.
The saliva test works by identifying levels of tapeworm-specific antibodies in the horse’s saliva. A veterinarian can perform the saliva test or horse owners can order a swab kit and send it to a lab to be processed.
Anthelmintic Drugs (Dewormers)
Anthelmintic drugs, also known as dewormers, are the most common option for reducing parasite loads in horses. However, due to growing drug resistance, they must be used judiciously.
Anthelmintic drugs can be given orally as a gel, paste, granule, or powdered feed additive, or they can be given by nasogastric route (performed by a veterinarian). 
Modern anthelmintic drugs can be classified into four main categories according to their chemical makeup and how they work in the horse:
- Macrocyclic lactones
Each class of anthelmintic has varying effects on different types of parasites.
Benzimadazoles have commonly been used for the treatment of mature strongyles. Pyrantels are often used for treating adult ascarids, large and small strongyles, and pinworms.
Macrocyclic lactones are used to treat adult and migrating large and small strongyles, as well as bots. Praziquantel is used to treat tapeworms.
Double or triple combination anthelmintic products have also been developed to provide effective control against parasites that are drug resistant. 
It is often recommended give probiotics during and after deworming to help re-establish healthy colonies of bacterial microorganisms.
Please note: Equine dewormers are not designed for use in non-equine species and can be toxic to livestock, cats, dogs, and humans. Keep dewormers out of reach from animals, and children; carefully dispose of used dewormers. Appropriate parasite control for your livestock and pets should be discussed with your veterinarian. 
One of the biggest concerns with anthelmintic drugs is resistance due to the over-treatment of internal parasites.
Strongyles have developed widespread resistance to benzimidazoles and some farms have reported multi-drug resistance. 
Large roundworms have developed resistance against macrocyclic lactones (ivermectin and moxidectin), and both Europe and the U.S. have reported drug resistance with some of the combination drugs as well. 
There is also evidence that worms are developing resistance to benzimadzoles and pyrantels.
Alternative Deworming Treatments
Increased anthelmintic drug resistance and some horse owners’ desire to avoid chemical dewormers has resulted in a growing market for herbal and alternative deworming products.
It should be noted that little research has been conducted to support the efficacy of these products, but many people anecdotally report reduced FECs in their horses while using them.
Anthelmintic herbs are classified into two main categories: vermicides, which kill parasites, and vermifuges, which expel dead parasites from the body.
There are also herbs that are said to act as parasite preventatives. Most herbal deworming products include a blend of herbs with all of these properties.
Some examples of anthelmintic herbs include:
- Cayenne pepper
- Pumpkin seeds
- Red clover
Many of these herbs have been used for centuries in human traditional medicine practices to support digestive health and combat intestinal parasites. However, further research is required to determine their efficacy in horses.
Another alternative deworming treatment that many holistically-minded horse owners prefer to use is Diatomaceous Earth (DE).
This product is a chalky powder made up of fossilized diatoms (a type of single-cell algae) found in ancient sea beds around the world.
Diatomaceous Earth is said to bind to parasites and toxins, helping to remove them from the gastrointestinal tract. The microscopically sharp edges of the diatoms perforate the outer protective layer of parasites, causing them to dehydrate and die.
There is little research to back the efficacy of DE. To our knowledge no studies have been done in horses. Research in cattle and sheep has shown no effect on FEC or growth and performance measures.  One study in hens showed that those fed DE had lower FECs than the control group. 
Long-term use of DE as part of your parasite management program is not recommended due to the risk of secondary nutrient deficiencies. DE can also bind important nutrients in the horse’s diet and inhibit their absorption.
DE is often used in yards and households as a means of insect control. Likewise, it is added to many commercial feeds and supplements as an insect deterrent.
If purchasing DE to feed to your horse, only use food grade DE and not the pool filter grade DE which has been chemically treated. Pool grade DE is toxic if consumed by humans and animals.
With any herbal or alternative deworming treatment, it’s important to conduct regular FEC tests to assess their effectiveness.
Management Strategies for Controlling Internal Parasites
In addition to anthelmintic medications, several equine management strategies are effective in reducing parasite loads in horses. They include the following:
First and foremost, it’s important not to overstock pastures. When too many horses are kept in a small pasture, it increases the amount of manure and leads to overgrazing.
Both of these factors can increase parasite transmission.
The recommendations for stocking pastures will vary depending on the type of terrain and grass species present. If you are unsure what is recommended for your area, check with your local county extension agent or veterinarian.
Another strategy for managing internal parasites is rotational grazing, where horses are moved from one pasture to another over the course of a year.
This decreases the likelihood that horses will graze the grass close to the ground where strongyle larvae tend to stay. This strategy helps reduce parasite transmission.
A similar strategy is strip grazing in which pasture is sectioned off and horses are moved from one section to another throughout the grazing season.
Co-grazing horses with other livestock can also reduce the parasite burden in horses. One study found that young horses grazing in the same pasture as cattle had reduced FECs. 
Co-grazing helps to reduce parasites in horses since some equine-specific larvae are ingested by cattle. These parasites can’t continue their life cycle with cattle as a host and no harm is done to the cattle.
Alternate Grazing and Haying Fields
Another way to break the parasitic cycle is allowing horses to graze a field in spring and then cutting hay on that same field during the summer.
The parasite larvae likely won’t survive on the drying forage. You can allow horses to graze on the pasture again in late summer or fall.
Depending on the climate you live in, harrowing can either be helpful or harmful as a parasite management strategy. Harrowing involves using an implement pulled behind a tractor to break up and spread manure piles in a field.
In warm, dry climates, harrowing a pasture (and keeping horses off of it for several weeks) can cause existing larvae to die off and may be helpful in parasite management.
In moderate, moist climates, this tactic likely won’t kill parasite larvae. Instead, it will make transmission to your horse easier.
As a general rule, the hotter the climate, the better harrowing works as a means of parasite control.
However, keep in mind that it is mainly effective for strongyles. Ascarids, which are still in egg form in manure, can survive harrowing.
The average horse produces between 60 to 70 pounds of waste per day. Regular clean-up of manure in pastures is a crucial part of parasite control.
This is especially important on small farms. Collected manure can either be hauled off-site or composted.
The concept behind manure management is simple: less manure in the pasture means fewer parasite larvae available for your horse to ingest.
A high parasite burden in the gut can negatively affect nutrient absorption and your overall horse’s health. This is apparent when symptoms such as weight loss and diarrhea are seen.
Certain types of parasites, such as roundworms, can affect organs beyond the gut including the liver and lungs. If your horse tests positive for roundworms, you may want to provide additional support for liver and respiratory function.
To support liver function, consider providing milk thistle supplementation during and after deworming. You can support respiratory health by ensuring you are feeding low-dust hay, soaking the hay or providing it in elevated hay nets.
Following deworming, it is important to re-establish healthy hindgut function to recover from the negative impacts of parasites. Using highly digestible fibre sources such as beet pulp or soybean hulls can nourish the beneficial microbes of the hindgut and support hindgut fermentation.
Probiotic supplements, which are live beneficial microbes, can be given after deworming to help support fibre digestion in the hindgut. Look for supplements that have a high CFU count which ensures that enough microbes reach the hindgut to have an appreciable effect on digestion.
Internal parasites are here to stay, but understanding how they affect our horses and implementing appropriate treatment and management strategies can help keep parasite numbers in check and decrease negative effects on your horse’s health.
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