Deworming is a key component of equine health. It helps protect horses from gastrointestinal parasites such as strongyles, roundworms, and bot fly larvae, which can cause colic, diarrhea, and reduced performance when infestations are severe.
Deworming plays an important role in maintaining a horse’s overall health and well-being. Gastrointestinal parasites are common in horses worldwide, and most horses carry small numbers of them without showing obvious signs of illness.
Problems arise when this natural balance is disrupted. Higher parasite burdens can place significant strain on the digestive system and may contribute to issues such as colic, diarrhea, weight loss, and reduced performance.
Modern parasite control is about more than routine deworming at fixed intervals. Today, leading veterinary guidelines—both in the United States and in Europe—emphasize targeted treatments, informed decision-making, and good management practices to keep parasite levels under control while helping to slow the development of dewormer resistance.
Because parasite pressure, climate, and management conditions vary by region, recommendations are not identical everywhere. In this article, we therefore take into account both commonly used guidelines in the United States (such as those from the American Association of Equine Practitioners) and established European recommendations, highlighting where approaches align and where they may differ. This allows horse owners to better understand the reasoning behind modern parasite control and make informed decisions suited to their individual situation.
Common signs of significant parasite burden:
Horses can be affected by several types of gastrointestinal parasites. While the importance of individual parasite species may vary depending on region, climate, and management, the following groups are considered relevant in both North America and Europe.
Understanding which parasites matter—and why—helps horse owners make informed decisions and supports a more targeted, responsible approach to parasite control.
|
Parasite |
Relevance | Notes | |
|
Small strongyles (Cyathostomins) |
Most common parasites, also found in clinically healthy horses | Primary target of selective deworming programs | |
| Large strongyles (Strongylus vulgaris) | Rare today, but potentially dangerous due to migration through blood vessels | Historically the main target of interval deworming | |
| Roundworms (Parascaris spp.) | Mainly affect foals and young horses | Increasing resistance to ivermectin, especially in young horses | |
| Tapeworms (Anoplocephala perfoliata) | Associated with pasture access, particularly under poor hygiene conditions | Linked to certain types of colic | |
| Other parasites (stomach worms, neck threadworms, pinworms) | Generally lower clinical relevance |
|
Small strongyles (Cyathostomins), sometimes referred to as “red worms,” are the most common intestinal parasites in horses worldwide. They are found in almost all horse populations and are often harmless when present in low numbers.
Adult horses usually develop a certain level of immunity, allowing them to tolerate small strongyle infections up to a critical threshold without obvious clinical signs.
However, in foals and young horses—generally up to around six years of age—small strongyles can pose a significantly higher risk, particularly when parasite burdens increase or large numbers of larvae are present.
In simple terms, horses become infected with small strongyles by ingesting larvae while grazing.
After ingestion, the larvae penetrate the lining of the large intestine, where they can remain encysted for several months and, in some cases, for more than a year. During this encysted phase, they are largely inactive and difficult to detect.
When conditions become favorable—often in spring, but also during periods of stress or management changes—large numbers of larvae may emerge from the intestinal wall and migrate back into the gut lumen.
In cases of heavy infestation, this mass emergence can trigger inflammation of the intestinal lining and lead to acute diarrhea, fever, colic, and general deterioration of the horse’s condition. Only once the larvae mature into adult worms and begin producing eggs can they be detected through fecal testing.
Fecal egg counts provide information about the number of eggs shed in the manure, but they do not always reflect the true parasite burden.
Even with a negative fecal result, a significant infection may already be present if large numbers of small strongyles are still encysted within the intestinal wall and not producing eggs.
For this reason, a single fecal sample offers limited insight. Regular monitoring over time is far more meaningful than isolated test results.
Additional diagnostic tools (primarily Europe / UK)
In some regions, modern blood tests such as the cyathostomin ELISA are available. These tests measure antibodies produced by the horse’s immune system in response to current or previous exposure to small strongyles.
While the ELISA does not determine the exact worm burden, it can indicate whether a horse has been in contact with encysted larvae and may therefore be at increased risk of a significant larval burden. This is particularly useful because encysted larvae are largely undetectable using standard fecal egg counts.
At present, this test is not widely used in all regions. In practice, it is best viewed as a complement to fecal testing, not a replacement—helping to build a more complete picture as part of a well-considered parasite management program.
United States perspective
In the United States, fecal egg counts are the primary monitoring tool used in routine parasite control. While the limitations regarding encysted larvae are well recognized, alternative diagnostics such as antibody tests are currently less commonly used in everyday practice.
As a result, interpretation of fecal egg counts in the US relies heavily on repeated testing, clinical context, and knowledge of farm-specific parasite pressure.
Because small strongyles are the parasite group with the highest levels of documented anthelmintic resistance, their management is a central element of modern parasite control programs worldwide.
Current recommendations in both the United States and Europe emphasize:
Large strongyles (Strongylus spp.) are considered rare today, but they remain potentially dangerous parasites. In particular, Strongylus vulgaris is of major clinical relevance.
While adult worms mature in the large intestine, the greatest harm occurs during the larval migration phase. Over a period of several months, larvae migrate through blood vessels and organs before returning to the large intestine. During this migration, they can cause significant damage to arteries and surrounding tissues.
Severe infections have been associated with intestinal infarctions, serious colic, and, in rare cases, fatal outcomes.
Large strongyles are difficult to detect using routine fecal egg counts.
Their eggs cannot be reliably distinguished from those of small strongyles until the larvae reach the third larval stage. To confirm the presence of large strongyles, eggs must therefore be cultured in vitro under laboratory conditions until differentiation is possible.
Because this diagnostic step is not routinely performed, large strongyle infections may remain undetected unless specific larval culture is used in addition to standard fecal testing.
United States perspective
In the United States, large strongyles are currently considered rare due to decades of intensive parasite control. As a result, routine targeted diagnostics such as larval culture are not commonly recommended for most adult horses.
Instead, fecal egg counts are primarily used to:
They are not intended as a species-specific monitoring tool for large strongyles.
Europe / UK (ESCCAP-aligned)
The aim is to prevent large strongyles from persisting or re-establishing within horse populations.
A serious risk for foals and young horses
Roundworms (Parascaris spp.) are one of the most important and potentially dangerous parasites in foals and young horses. While adult horses usually develop immunity and are rarely affected, infections in young horses can become severe.
European field studies report prevalence rates ranging from 20% to over 80% in foals. While this wide range highlights how common roundworms are, it offers limited practical guidance for individual horse owners—especially because infection risk depends heavily on management, hygiene, and previous contamination of the environment.
Roundworms are the largest intestinal parasites found in horses. Adult worms can grow up to 50 cm (20 inches) in length, and female worms may shed hundreds of thousands of eggs per day. As a result, parasite burdens can increase rapidly.
Clinical signs may include coughing, poor weight gain, dull coat, colic, intestinal obstruction, and in severe cases, life-threatening complications.
Stables and pastures with a history of roundworm infections may remain long-term sources of infection and should be managed with particular caution.
After ingestion, roundworm larvae hatch in the small intestine and penetrate the intestinal wall.
From there, they migrate via the bloodstream to the liver, heart, and lungs. After reaching the lungs, larvae travel up the airways to the throat, where they are swallowed again. Only after this migration do they return to the small intestine, where they mature into adult worms.
Although this migration often causes surprisingly few visible signs, it can also lead to significant secondary problems—particularly involving the respiratory system or secondary bacterial and viral infections.
Possible clinical signs during migration include:
Roundworm infections are diagnosed by detecting roundworm eggs in the manure using a fecal sample.
A positive result means that adult roundworms are present in the intestine and actively producing eggs, so an established infection exists.
However, this method has important limitations:
Because roundworm eggs survive for long periods in the environment and contamination levels can be high, a confirmed infection in one foal usually indicates an increased risk for other young horses in the same stable or pasture. For this reason, treatment decisions often need to consider the group and environment, not just the individual test result.
Regular fecal testing of individual foals and young horses helps monitor infection pressure over time, but results must always be interpreted in context.
Roundworm control is fundamentally different from adult-horse strongyle control.
Key points shared across regions:
Roundworms mainly affect foals and young horses
Environmental contamination plays a major role
Heavy infections can develop quickly
Treatment itself can trigger complications if many worms die at the same time
For this reason, roundworm control must be planned carefully, not handled casually or purely by routine.
Good hygiene, regular monitoring, and age-appropriate treatment schedules are essential.
In Europe, foals are typically treated strategically, starting at around two months of age.
During the first year of life, treatments are often given every two to three months, sometimes with rotation of active ingredients.
Important safety note:
In foals with heavy roundworm burdens, some dewormers can cause sudden mass worm death, increasing the risk of colic or intestinal blockage. In such cases, treatment should be planned carefully and, if necessary, adjusted or staged.
US: The AAEP places strong emphasis on the fact that roundworms in the US often show resistance to certain dewormers, particularly macrocyclic lactones.
For horse owners, this means:
Tapeworms (Anoplocephala perfoliata) are a common intestinal parasite in grazing horses. They require an intermediate host in the form of infected oribatid mites (soil-dwelling pasture mites).
Horses become infected while grazing, which is why tapeworm infections are closely linked to pasture access. In many regions, infections are most relevant during the second half of the grazing season.
Tapeworms are associated with inflammation and thickening of the intestinal wall and have been linked to certain types of colic and, in severe cases, intestinal obstruction.
Tapeworms do not migrate through the horse’s body.
Their larval development takes place entirely outside the horse, within the intermediate host (oribatid mites). Horses ingest infected mites while grazing. Once ingested, the larvae are released in the small intestine and attach at the junction between the small and large intestine (ileocecal region).
There, they mature into adult tapeworms.
Although this life cycle sounds less invasive than that of other parasites, the attachment site is sensitive. Chronic irritation can lead to inflammation, thickening of the intestinal wall, impaired gut motility, and an increased risk of colic.
Tapeworms shed eggs intermittently, which makes them difficult to detect in routine fecal samples. As a result, a fecal test can be negative even when tapeworms are present.
Some laboratories use specialized flotation techniques to improve detection rates, but sensitivity remains limited.
If tapeworms are detected in one horse within a group, it is often recommended to treat all horses with similar pasture exposure, as infection risk is usually shared.
Serological tests (saliva or blood ELISA) that detect antibodies against tapeworms can provide additional information about exposure. However, these tests are not yet widely used in all regions and are generally considered a supplementary tool, not a standalone diagnostic.
Bot flies are easily recognized by the yellowish eggs they lay on a horse’s coat during the summer months—most commonly on the legs, shoulders, or around the head.
Bot flies are widespread throughout Europe and North America and primarily affect horses with regular pasture access.
After ingestion, the bot fly larvae migrate into the stomach and the upper part of the small intestine (duodenum), where they can cause irritation of the mucosa, superficial injuries, inflammation, and in some cases even gastric ulcers.
Bot fly infestations are often first recognized by the visible yellow eggs on the horse’s coat (typically on legs, shoulders, or around the head).
A definitive confirmation of bot fly larvae in the stomach is possible via endoscopy—more specifically, gastroscopy (endoscopy of the stomach), where larvae may be seen attached to the gastric lining.
There are also antibody-based tests (ELISA) that can indicate exposure, but they are not widely used in everyday practice.
US note: AAEP considers bots very mild pathogens, meaning they typically should not drive treatment decisions on their own.
In practice, this means most owners do not “chase a diagnosis” with testing—bots are usually managed seasonally as part of the overall parasite program.
Bot control is seasonal and predictable.
A single, well-timed treatment with ivermectin—the drug of choice for bot fly larvae—in late autumn or early winter is usually sufficient (i.e., after the bot-fly season).
Removing eggs from the coat during the season (bot knife, etc.) helps reduce exposure.
In addition to the parasites discussed above, there are several other parasite species that may affect horses. In most cases, these parasites are considered of minor clinical relevance, both in Europe and in the United States.
According to current European recommendations and the AAEP Internal Parasite Control Guidelines, these parasites are not primary targets of internal parasite control programs. Where they do cause problems, this is usually related to local irritation or skin reactions, rather than significant gastrointestinal disease.
Good hygiene, pasture management, and targeted treatment when needed are generally sufficient to keep them under control.
Threadworms (Strongyloides westeri)
Threadworms primarily affect young foals and are usually transmitted via the mare’s milk shortly after birth.
Infections are typically mild or completely asymptomatic. If clinical signs occur, they are most often limited to mild diarrhea in very young foals.
Because clinical relevance is generally low, threadworms are not a major concern in modern parasite control programs when hygiene and management are adequate.
Pinworms (Oxyuris equi)
Pinworms are best known for causing itching around the anus and tail rubbing.
While they can be uncomfortable and annoying, they are not considered a serious health threat. Systemic illness or digestive problems are rare.
Control focuses on hygiene measures (cleaning the tail area, stable surfaces, and equipment) combined with targeted treatment if needed. Pinworms are regarded as a management issue rather than a gastrointestinal parasite problem.
Stomach Worms (Habronema and Draschia spp.)
Stomach worms are transmitted by flies and occur mainly in horses with pasture access, especially in warmer conditions.
Adult worms live in the stomach and usually cause little to no gastrointestinal disease. From an internal parasite control perspective, they are therefore considered of low relevance.
However, larval stages deposited in wounds or moist skin areas can lead to summer sores (cutaneous habronemiasis). For this reason, their main clinical relevance lies outside the gastrointestinal tract, and prevention focuses on fly control and wound management, rather than routine deworming.
Neck Threadworm (Onchocerca cervicalis)
Neck threadworms are transmitted by biting midges and are commonly found in many horse populations.
Most infected horses show no obvious clinical signs. In some cases, larval stages may contribute to skin irritation, particularly along the neck or ventral midline.
As with stomach worms, the primary relevance of Onchocerca cervicalis is related to skin reactions, not intestinal disease. Control therefore centers on vector management and overall parasite control, rather than parasite-specific treatment.
These parasites are generally regarded as secondary in importance compared to strongyles, roundworms, tapeworms, and bot flies.
Both European recommendations and AAEP guidelines agree that:
Targeted control, primarily for small strongyles
Selective deworming focuses mainly on small strongyles (cyathostomins). Research has shown that most adult horses are able to tolerate low to moderate infections without clinical problems, thanks to a mature immune system.
Only when parasite levels exceed certain thresholds does treatment become necessary.
European guidelines are relatively explicit and structured when it comes to selective deworming.
Key principles:
Adult horses (usually 6 years and older) are monitored regularly using fecal egg counts (FEC).
A commonly used treatment threshold is 200 eggs per gram (EPG) for small strongyles.
Only horses exceeding this threshold are treated.
Recommended monitoring (Year 1):
4 fecal egg counts per grazing season (approximately April/May to October/November)
Treatment of horses above the threshold
Follow-up fecal testing after treatment to confirm efficacy
Additional considerations:
Regular testing increases the chance of detecting tapeworm infections, although sensitivity of single samples remains limited.
At least one annual check for large strongyles (e.g. larval culture or PCR) is recommended.
A baseline treatment in late autumn (“large deworming”) is usually included.
From the second year onward, monitoring may be reduced to 3 fecal tests per season if the herd remains stable and results are consistent.
Practical limitations:
Selective deworming works best in small, well-organized herds with good veterinary support. In large boarding barns with frequent horse turnover, consistent implementation is often difficult.
The AAEP follows the same core idea—but frames it differently.
Key principles:
Fecal egg counts are used to classify horses as low, moderate, or high shedders.
There is no fixed universal EPG threshold defined by AAEP.
The focus is on identifying consistently high shedders, not on single test results.
AAEP strongly emphasizes:
Repeated testing over time, not isolated samples
Fecal Egg Count Reduction Tests (FECRT) to verify that dewormers are still effective
Avoiding unnecessary treatments in low shedders
Importantly, AAEP integrates selective deworming into a two-tier system:
Surveillance-based treatments (driven by FEC results)
Baseline treatments (applied to all adult horses once or twice per year)
This baseline element is a key difference from many European programs.
Selective deworming is not a single fecal test, but a structured program.
One negative fecal result does not mean “no parasites.”
Poor sample collection or inconsistent timing can invalidate results.
Selective deworming only works if the concept is applied completely and consistently.
Important exception:
Farms with confirmed large strongyle (Strongylus vulgaris) infections should use strategic deworming for at least two years before considering a selective approach.
Accurate parasite control depends on good sample quality.
Best practice:
Samples should be fresh, ideally collected immediately after defecation or rectally.
Composite samples over 2–3 days improve sensitivity.
Samples should be stored cool (approx. 4–8 °C) until shipment.
In practice, selective deworming often fails not because the concept is wrong—but because consistent, coordinated sample collection is unrealistic in large facilities.
When selective programs are not practical
Strategic deworming follows a fixed schedule, with treatments timed to interrupt parasite life cycles and reduce pasture contamination.European recommendations typically combine:
Strategic treatments
Periodic monitoring using group or pooled fecal samples
General principles include:
More frequent treatments in young horses
Alternation of drug classes to slow resistance development
A commonly used seasonal logic:
Early season: benzimidazoles or pyrantel
Mid-season: macrocyclic lactones
Late season: praziquantel if tapeworm risk is present
Autumn/winter: macrocyclic lactone for bots and strongyles
Choosing the right dewormer is not about using “the strongest product,” but about selecting an appropriate drug class for the parasite in question and verifying that it still works.
Both European and US guidelines emphasize that parasite control should be guided by diagnostics and professional advice, not by routine or convenience.
Because parasite resistance patterns, age-related risks, and management conditions vary widely, working with a veterinarian is strongly recommended when selecting and evaluating dewormers—especially for foals, young horses, and farms with known parasite challenges.
| Drug class | Effective against | Important notes |
|
Macrocyclic lactones (ivermectin, moxidectin) |
Broad spectrum of roundworms (e.g. strongyles, ascarids*, pinworms) and bot fly larvae |
Highly effective; widespread resistance in Parascaris spp. (roundworms) reported, especially in the US. Ivermectin is the drug of choice for bots. |
|
Benzimidazoles (fenbendazole, oxibendazole) |
Roundworms, especially strongyles and ascarids |
Resistance common, particularly in small strongyles. Efficacy should not be assumed without testing. |
|
Pyrantel |
Roundworms; at higher doses also tapeworms |
Generally well tolerated; resistance reported in some strongyle populations. |
|
Praziquantel |
Tapeworms only |
No resistance reported to date; usually combined with ivermectin or moxidectin. |
* Note: In many regions, macrocyclic lactones are no longer reliable against roundworms in foals.
Interpreting FECRT results correctly often requires veterinary expertise, as timing, parasite biology, and test variability all influence results.
Important reminder for horse owners
Modern parasite control is not a “do-it-yourself checklist.”
Effective and responsible deworming involves:
choosing the right drug class for the situation
using treatments only when justified
confirming efficacy through testing
and seeking veterinary advice, especially in young horses, large herds, or when problems persist
This approach protects not only the individual horse, but also helps preserve the effectiveness of available dewormers for the future.
Deworming alone is not sufficient to control parasites long term. Many parasite eggs and larvae survive for months or even years in the environment. Good stable and pasture hygiene is therefore an essential part of parasite control.
Key measures
Regardless of whether a selective or strategic deworming program is used, the treatment must be administered correctly and at the proper dose. What sounds simple can be challenging in everyday practice.
Underdosing must be avoided, as it contributes to the development of parasite resistance.
For this reason, horse owners should have a realistic estimate of their horse’s body weight. Visual estimates are often inaccurate—even among experienced horse people. Periodic weighing (or using a weight tape consistently) helps establish reliable reference values.
Special caution is needed with large warmbloods or draft horses. Many dewormer syringes are calibrated for up to 700 kg (≈1,540 lb), which may be insufficient for heavier or very muscular horses.
Dewormers should be administered directly into the mouth, ensuring that the entire dose is swallowed. Horses are often surprisingly skilled at avoiding or expelling paste.
If a horse resists oral administration, this should be trained in advance (for example using applesauce or juice).
Administering dewormers mixed with feed is not recommended, as intake cannot be reliably controlled and incomplete dosing increases the risk of treatment failure and resistance.
Time of administration:
Dewormers are best given in the morning, allowing the horse to be monitored closely during the following hours.
Observation:
Horses should be observed carefully after treatment so that early signs of discomfort or colic can be recognized and addressed promptly.
Avoid unnecessary treatments:
Using selective or risk-based deworming strategies helps reduce the frequency of treatments and lowers the digestive burden—particularly important for sensitive horses.
AAEP, Internal Parasite Control Guidelines, https://aaep.org/wp-content/uploads/2024/05/Internal-Parasite-Guidelines_Updated.pdf
ESCCAP, Empfehlungen zur Behandlung und Kontrolle gastrointestinaler Parasiten bei Pferden und anderen Equiden, https://www.esccap.org/uploads/docs/7yd2oo9c_22022PferdeEmpfehlung8.pdf
Matthews, J. B., & Mair, T. S. (2025). Sustainable control of cyathostomin infections in practice. Equine Veterinary Education, 37(3), 129–138. https://doi.org/10.1111/eve.14182
Lightbody, K. L., Austin, A., Lambert, P. A., von Samson-Himmelstjerna, G., Jürgenschellert, L., Krücken, J., Nielsen, M. K., Sallé, G., Reigner, F., Donnelly, C. G., Finno, C. J., Walshe, N., Mulcahy, G., Housby-Skeggs, N., Grice, S., Geyer, K. K., Austin, C. J., & Matthews, J. B. (2024). Validation of a serum ELISA test for cyathostomin infection in equines. International journal for parasitology, 54(1), 23–32. https://doi.org/10.1016/j.ijpara.2023.07.001
Jürgenschellert, L., Nielsen, M. K., Barutzki, D., Schaper, R., & von Samson-Himmelstjerna, G. (2020). Investigations on the occurrence of tapeworm infections in German horse populations using different diagnostic methods. Parasites & Vectors, 13(1), 421. https://doi.org/10.1186/s13071-020-04318-5
Nielsen, M. K. (2022). Anthelmintic resistance in equine nematodes: Current status and emerging trends. International Journal for Parasitology: Drugs and Drug Resistance, 20, 64–72. https://doi.org/10.1016/j.ijpddr.2022.10.005
On our topic page, you’ll find additional information and further resources on gastric ulcers in horses. We also outline practical options for the prevention of gastric ulcers.
Equine 74 Gastric
Buffers the excess acid in the horse's stomach instead of blocking it.
Equine 74 Stomach Calm Relax
Supports the nervous horse stomach in stressful situations.
