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Springtime marks the peak season for strongylid infestations. This is why deworming is traditionally conducted during this period. But what exactly does this entail?


Strongylids are a type of worm that reside in the intestines of horses. They come in two main varieties: large (up to 2.5 cm long and reddish) and small (up to 1.5 cm long and white) strongylids, belonging to various subfamilies.

These parasites inhabit the large intestine of the horse, where they lay eggs that are then excreted in the feces. Larvae hatch in the external environment, undergo several developmental stages, and are subsequently ingested by the horse during grazing. These larvae can be found on grass, hay, bedding, and even stable walls, although they are too small to be visible to the naked eye.

Once ingested, the larvae burrow into the intestinal mucosa of the large intestine. While small strongylids typically remain in the intestinal mucosa to continue their development, large strongylids often migrate into the blood vessels, potentially reaching the liver or heart.

During migration, these worms cause tissue damage and increase the risk of blood clots, leading to circulatory issues such as unusual lameness or stroke-like symptoms.

Throughout winter, larvae remain dormant in the horse’s intestines. However, as temperatures rise in spring, they undergo mass migration from the intestinal mucosa and mature into adult worms within the large intestine, where they lay eggs that are subsequently expelled with the feces. This completes the cycle.

The mass migration of strongylid larvae in spring can induce diarrhea or colic-like symptoms in some horses and severely damage the intestinal mucosa, disrupting the delicate balance within the large intestine.

To prevent horses from experiencing mass re-infestation, many stables routinely administer deworming treatments targeting roundworms, including strongyles, during spring.

Worm infestation

However, long-term studies conducted over the past few years have revealed that not every horse automatically harbors a pathological worm infestation. Approximately 70-80% of horses show no signs of problematic worm infestations at all (although a few worms may inhabit the horse, which might sound unpleasant to us but is entirely normal for horses).

Several factors influence a horse’s susceptibility to worm infections. These factors include the horse’s age (young horses are more susceptible than older ones), the condition of its immune system (a healthier immune system correlates with fewer worms), the environment within the large intestine (greater disturbance, such as incorrect feeding, increases susceptibility to worm infestation), husbandry conditions (higher levels of stress lead to greater susceptibility), and the hygiene conditions in the stable (factors such as mattress litter, inadequate mucking out of turnouts and pastures, and fertilization of pastures with horse manure promote worm infestation).


It is also important to note that deworming “prophylactically” is not effective, as dewormers only target existing worm infestations and do not prevent future infestations. This means that administering a deworming treatment today does not guarantee protection against worm infestations next week.

The practice of “prophylactic” deworming still prevalent in many stables today operates under a flawed logic similar to administering antibiotics to horses as a “prophylactic” measure in spring and autumn.

Horse droppings on the street
You can’t deworm “prophylactically” because the dewormers only work against existing worms. © Adobe Stock /

This approach assumes that the horse could potentially be infected with some type of bacterium, which is nonsensical and only serves to foster the development of resistance. Unfortunately, this has been the outcome of decades of “prophylactic” deworming practices. Resistance has now been observed to all active substances available on the market, with the degree of resistance varying depending on the preferred deworming preparations in a stable or region.

“Selective” Deworming

Research has indicated that approximately 60-80% of strongylid strains have developed resistance to certain active substances. As a horse owner, this means that effective treatment options may no longer be available in cases of severe worm infestations. In order to mitigate further resistance development, a strategy known as “selective deworming” or “modern parasite control” is now recommended.

Under this deworming approach, the first step is to assess whether the horse has any worms at all. This is achieved by collecting faecal samples and submitting them for examination. The faecal samples are analyzed to count the number of strongylid eggs present, providing insights into the level of infestation.

A minimal quantity of eggs is considered normal, as low-level infestations typically do not pose a significant threat to horses. Only when the egg count surpasses a defined threshold should it be inferred that the horse has a worm infestation beyond the capacity of its immune system to manage independently, necessitating deworming intervention.

Alternatives to Medication

When egg counts fall within the borderline range, exploring alternative therapeutic methods may be a viable option. In traditional medicine, certain plants and bee resin, known as propolis, are recognized for their worm-repellent properties. Currently, a study is underway to investigate the worm-repellent effects of sainfoin, a plant long utilized for this purpose in small ruminants such as goats and sheep.

Preliminary results from the study appear promising. The efficacy of all alternative therapeutic deworming agents primarily revolves around creating an inhospitable intestinal environment for parasites while bolstering the immune system to enable the horse to fend off unwelcome inhabitants.

These methods tend to yield success in fundamentally healthy horses with robust immune systems, which may have only encountered a few worms due to short-term stressful situations, such as attending events in unfamiliar stables.

However, in cases where the horse’s health is significantly compromised or the infestation is severe, chemical deworming is typically unavoidable.

Monitoring Success

It’s crucial to conduct a “success control” approximately two weeks after deworming, whether it’s chemical or “natural.”

During this process, fresh fecal samples are collected and sent in for examination. Even with alternative deworming products, there’s a possibility that the horse may not effectively fend off the parasites. If the infestation levels do not decrease, it may necessitate resorting to chemical deworming treatment.

However, due to widespread resistance, it’s common for horses to exhibit similar or even higher worm counts after chemical deworming than before. Thus, while trust is important, thorough monitoring ensures a more accurate assessment of the treatment’s efficacy.

More on this topic: Selective or Prophylactic Deworming – What’s the Right Way?