Reading time 14 minutes

For decades, the dogma in stables has been: horses receive a deworming treatment four times a year. With the increasing emergence of resistance to various anthelmintics (deworming treatments), measures were scaled back. However, deworming horses in spring and autumn remains a routine part of life in many stables, much like grazing or mucking out.

For some years now, however, there has been an increasing call to move away from ‘prophylactic’ deworming in favour of ‘selective’ deworming. The Scandinavians are leading the way in this regard; in Denmark, it has been forbidden since 1999 to administer deworming treatment to horses without first diagnosing a worm infestation. This targeted use of anthelmintics aims to prevent further progression of resistance in worms, thereby averting a disaster akin to that caused by antibiotics.

Antibiotics have been distributed to humans and animals like candy for decades, leading to the emergence of increasingly multi-resistant germs—bacteria that no longer respond to antibiotics. Infection with such a germ can be fatal. With our horses, we are also on the path to achieving this. Studies have shown that oxyurid parasites are now practically completely resistant to all active substances. Up to 80% of strongyles, the most common worms, are already resistant to various active substances. It is therefore high time to rethink and deworm only after diagnosis.

But how does selective deworming work?

Numerous veterinarians and laboratories provide services for testing faecal samples sent to them, using the results to advise whether or not deworming your horse is necessary. Some even offer monitoring services, allowing you to register and receive timely reminders for the next scheduled check. Essentially, you would routinely collect faecal samples two to four times a year, submit them for analysis, and subsequently receive the results. If there is an infestation necessitating treatment, the laboratory will also advise you on the appropriate active ingredient to use. If the horse requires deworming due to an infestation, another faecal sample is submitted 14 days later to verify the effectiveness of the treatment. Due to the resistance situation, it’s entirely possible that a deworming treatment may be administered, which proves ineffective, leaving the horse with either the same amount or even more worms than before. Therefore, it’s crucial to always verify the effectiveness of the treatment.

The 8 most common counter-arguments against selective deworming and why they are invalid

“Faecal samples may not always provide reliable information about a worm infestation; a negative result does not guarantee the absence of worms in the horse.”

The faecal samples must be collected correctly, comprising 120-150g of freshly deposited faeces placed in a plastic bag and promptly sent to the laboratory by post, with a cool pack during the summer months. If faeces are collected from areas where they have been exposed for an extended period or from any pile found in the open yard, the result may not be reliable. It’s advisable to keep a small bag, such as a dog faeces bag or a freezer bag, in your pocket when you’re with your horse. Most horses defecate during grooming or at the beginning of a riding session, providing a fresh pile that can be reliably attributed to the horse being examined. It’s preferable to collect the faecal sample at the beginning of the week. If you collect the sample on Friday, drive it around with you in the car until Saturday, and then post it at midday, it might not reach the laboratory until Monday or Tuesday, potentially affecting the accuracy of the result. This is because larvae typically hatch within 48 hours at the latest. Therefore, it’s essential for the faecal sample to reach the laboratory before this time has elapsed.

Studies have demonstrated that, on average, approximately 80% of horses do not have a worm infestation requiring treatment and remain stable for years. Of the horses with a worm infestation, approximately 80% are infested with strongyles. The remaining 20% is distributed among all other species of worms. So, out of every 100 horses in the stable, 80 faecal samples will yield negative results. Out of the 20 horses requiring treatment, 16 are infested with strongyles, while the remaining 4 have other types of worm infestations.

Strongylids are reliably identifiable in the faecal sample if it has been collected correctly. Roundworms, a common ‘childhood disease’ in horses, can also be reliably detected in the faecal sample. After the age of 5, the risk of roundworm infestation decreases significantly. Stomach worms (Habronema, not to be confused with Dassel larvae) can also be reliably detected in the faecal sample, as can dwarf threadworms, which can be a problem in breeding stock but are not significant in adult horses. Tapeworms cannot always be reliably detected as they do not continuously excrete eggs. Conversely, the presence of tapeworms in a horse’s large intestine does not typically result in immediate death. If there is a significant infestation, this can also be evident in the faecal sample. Awl tails are also not consistently visible in the faecal sample, as they do not continuously excrete eggs either. However, as they deposit their eggs in chains in the anal region, egg remnants are always found there. If an awl tail infestation is suspected (often characterized by the typical ‘brush tail’ appearance, with protruding hairs as horses rub their tails), diagnosis can be confirmed by taking a swab sample from the anal region, which is reliable.

Botflies cannot be detected through faecal samples, but such detection is unnecessary. If the horse resides in an area where botflies are prevalent, this can be identified during summer by the presence of yellow eggs adhering to the fur of the front legs or shoulders. If you have horses in the group with such eggs in their coats during summer, it’s advisable to deworm the entire group in winter once the insects have ceased flying, ideally after the first significant frost. This interrupts the reproductive cycle of the botflies, as no new flies can hatch from the larvae excreted in spring. If consistently done, this practice can effectively eradicate the botfly from your region, eliminating the need for deworming against it.

Horses can also become infested with lungworms, but this typically occurs only when they are or have been kept together with infested donkeys, the main host. These can be explicitly requested in the faeces as an examination. If a liver fluke infestation is suspected (in which case the horse is also a secondary host, with the primary hosts being ruminants), horses typically become infected if they are or have been kept on pastures where infected sheep or cows are or have been present, diagnosis can be confirmed using an antibody test on blood.

In summary, the most important types of worms can generally be detected in the faecal sample, while in special cases such as awl tails or liver flukes, specific tests provide additional information. With a reliable diagnosis, you can administer deworming treatments sensibly, avoiding overkill.

Horse droppings on the street
All the primary worm species can be identified in the faecal sample. © Adobe Stock /

“One faecal sample per group is enough, if one is infected, so are the others.”

Unfortunately, this is not true. Of course, worms are always infectious; if there are no worms present, no horse can become infected. But similar to bacteria, fungi, or viruses, not all horses in a group are affected to the same extent. Just because one horse in the group has a cold, you don’t administer antibiotics to the entire group, but only to the affected horse. Therefore, it’s important to test every horse in a group. It’s often surprising to find that while most horses in a group consistently have negative results, individual horses may repeatedly test positive. In horses with a healthy gut, the immune system can effectively defend against worm infestation. Therefore, not necessarily all horses in a group have to be infected with worms; instead, each case must be assessed individually. If you are familiar with your group and have consistently submitted faecal samples from all horses over an extended period, you can eventually consider ‘pooling’ samples. This involves collecting faeces from three to five horses in a single bag, labelling it, and maintaining records of which horses’ faeces were included in each bag. The usual suspects, which almost always yield positive results, should be collected separately and sent in as a single sample. Pooled samples save money because you now only need one test for several horses. On the other hand, if the result comes back positive, you have double the work, as you then need to collect and analyze faeces from all the horses whose samples were pooled in the bag. It is therefore only worthwhile to pool samples if you are familiar with your group and know which horses consistently have clean results.

Six horses grazing in the pasture
Not all horses in a group are always infested with worms; it’s important to assess each horse individually. © Adobe Stock / Alexia Khruscheva

“Selective deworming is far too time-consuming.”

The most time-consuming aspect is collecting the faecal samples. Instead of instructing all owners to administer worming pastes into their horses’ mouths that week (which can be challenging with uncooperative horses), you can also instruct everyone to collect and send in a faecal sample. That’s a few extra minutes for every owner while they’re already with their horse, which should be manageable for everyone. And many horses (as well as owners) are relieved to avoid the hassle of administering the paste every six months, with all the associated struggles, arguments, and spitting.

“It’s far too expensive.”

A complete blood panel is expensive, whereas a faecal sample analysis is not. Prices may vary between laboratories, but a reputable, certified laboratory will generally charge around 30 euros. This cost is not significantly higher than the price of deworming treatment. Moreover, by opting for faecal sample analysis, you contribute to environmental conservation. The residues of deworming paste often end up on dung heaps along with faeces, eventually reaching fields and bodies of water, where they can cause significant damage. Additionally, in the long term, you help to preserve the effectiveness of anthelmintics, ensuring they remain effective in the event of a severe infestation in your horse. Consider purchasing one less saddle pad and investing in a faecal sample analysis instead. Of course, it’s possible that a faecal sample may come back with a positive result, requiring payment for a worming treatment followed by another faecal sample (along with associated costs). But a worm infestation is a disease, and we all know the expenses involved if a horse’s health deteriorates. The cost of two faecal samples and a worming treatment remains relatively inexpensive compared to treating conditions like laminitis or colic.

“Giving a worming treatment is much safer.”

Given the well-documented resistance situation, which has been described in scientific literature since the 1960s, and the widespread high-level resistance against all active substances, a faecal sample analysis is much more reliable than a deworming treatment. Many owners truly comprehend this concept only when they no longer administer deworming treatment blindly but instead monitor their horse’s gut health by regularly taking faecal samples. The results indicate an infestation of 350 eggs per gram (EpG), and it is recommended to proceed with deworming. And so does the owner, lulled into the belief that all is now well. Two weeks later, the follow-up sample is sent in, and lo and behold, suddenly over 1,000 eggs per gram (EpG) are detectable. How is that possible? This phenomenon is indicative of resistance. It is now understood that deworming treatments have a significant impact on the microbiome, or “intestinal flora,” of the large intestine. Disorders in the microbiome of the large intestine can often coincide with a weakening of the immune system. If the horse harbors a worm strain resistant to the administered active ingredient, the deworming treatment doesn’t affect the worms, but it can significantly disrupt the intestinal microbiome, thereby compromising the immune system. This disruption can lead to further colonization by worms, resulting in a higher infestation evident in the follow-up sample compared to the initial one. Old wives’ tales? Not at all, there are numerous documented cases where precisely that scenario has been proven to occur. Therefore, a faecal sample analysis is always a safer option than a “prophylactic” deworming treatment.

“But once a year you have to give a ‘big one’. “

The “big one,” commonly administered as standard in stables during winter, typically consists of two active ingredients—one effective against roundworms (and thus most types of worms), and the other against flatworms (such as tapeworms). High levels of resistance are also known to develop against these two active substances, primarily due to the practice of administering “prophylactic” deworming treatments to horses—without an indication of disease and without verifying the efficacy of the treatment. So if we continue with this practice, it’s reasonable to assume that we’ll soon encounter 100% resistance to “the big one.” This essentially means that if a horse does indeed have an infestation, there will be no effective treatment left in our arsenal.

“Selective deworming requires full stable participation.”

Worm infestation should be assessed individually for each horse. Monitoring and managing an entire stock simplifies the process. Not every horse is automatically infested with worms, so not everyone in the stable needs to follow the same worm management system. Those who, in consultation with the stable manager, opt for selective treatment after prior diagnosis are acting responsibly, both for themselves and for the overall stock. This approach helps prevent resistance. By following this procedure, other horses in the stable won’t have more or fewer worms. Faecal samples enable you to monitor whether horses not receiving prophylactic deworming pose a risk to others in the stable.

“People who deworm selectively are just “parasites” at the expense of those who deworm their horses regularly.”

The argument arises from the notion that horses regularly dewormed are presumed to be entirely free of worms, thus reducing the risk of infection. This reduces the risk of infection in non-dewormed horses. This would indeed be the case if deworming treatments still retained their original effectiveness. But those days are long gone; for the past 20 years, resistance to all active substances has been acknowledged, in some cases to a significant extent. Hence, it’s not safe to assume that a horse dewormed without a prior faecal sample is truly worm-free. On the contrary, “prophylactic” deworming promotes resistance and often results in horses that are frequently and regularly dewormed having the highest and most persistent infestations in faecal samples. It’s quite the opposite: those who continue to deworm “prophylactically” are exploiting the efficacy of dewormers, to the detriment of those who practice selective deworming. Selective deworming helps reduce the development of resistance.

“Horses must be dewormed before being turned out to pasture.”

A horse that is free of worms cannot excrete any onto the pasture, thus preventing contamination for other horses. However, this does not justify allowing horses to be turned out to pasture in spring without any control. Selective deworming involves using a faecal sample to diagnose whether a horse requires treatment for an infestation. Only horses confirmed to be free of infestations should be turned out to pasture. For horses with a positive spring test result, the following protocol should be followed: deworm first, verify success, and then allow them out to pasture.

Haflinger grazing on the pasture; grass warts
For horses with a positive spring test result, the following protocol should be followed: deworm first, verify success, and then allow them out to pasture. © Adobe Stock / ba11istic


The conclusion drawn from long-term studies analysing horse populations over many years is as follows:

More crucial than the continual administration of deworming treatments, which merely fosters resistance and exposes many worm-free horses to unnecessary chemicals, is the regular monitoring of horses with faecal samples and maintaining proper stable and pasture hygiene. Thus, the spectre of ‘intestinal parasites’ quickly loses its horror.