To worm or not to worm? Addressing the dilemma of worming treatment decisions for horses in training

Article by Jacqui Mathews

All horses are exposed to parasitic worms at some point in their lives. It is not possible to eradicate all worms from all horses, nor completely avoid the risk of worm-associated disease, so some level of parasite control is necessary in any environment where horses are kept.   Traditionally, regular all-group wormer (anthelmintic) treatments were used to control these parasites, regardless of the management conditions. Increasing reports of wormer resistance over the last two decades [1] indicate this is no longer sustainable and will only act to worsen the situation, especially as no new wormers are coming to market any time soon. It is essential to take an approach that safeguards the effectiveness of anthelmintics. As common equine worms are spread via grass (Fig. 1), and horses in training do not routinely graze for significant periods (so are at lower risk of infection), they represent ideal candidates for diagnostic-led programmes.  

The worms that turned

The main worms of concern for horses in training are small redworms and tapeworms. Young horses (<2 years-old) may also be infected with ascarids. Small redworms can cause weight loss; in heavy infections (10,000s-1,000,000’s worms), this can be severe and accompanied by diarrhoea and/or colic. Tapeworms can cause colic but at a lower infection level; burdens of >20 tapeworms have been shown to cause gut damage. Ascarids are more likely to be problematical on studs; infections usually peak in 4-8 month-old foals, with a gradual reduction in susceptibility due to immunity. Immunity takes longer to develop against small redworms and tapeworms and a few horses remain susceptible throughout life, especially when exposed to heavily-contaminated paddocks and/or have medical conditions that affect their immunity. 

Wormers available include fenbendazole, pyrantel salts (double dose for tapeworms), ivermectin, moxidectin and praziquantel (tapeworms only). Resistance to these wormers has been reported in small redworms (benzimidazole resistance is ubiquitous, with reports of resistance to all other wormers), ascarids (especially resistance to ivermectin) and tapeworms (pyrantel and praziquantel resistance was recently reported [2]). If effective worm killing is not achieved due to the presence of resistance, a situation could occur where veterinarians are unable to effectively treat horses that present with disease due to heavy burdens. It is therefore essential to reduce the amount of wormers administered and only treat horses when an assessment indicates that worming is necessary.

Risk assess to consider if horses are likely to be infected with worms

Be aware of the risk factors for worm infection, with age and access to contaminated grass key features. As most horses in training have no/limited access to pasture, they should be at low risk of infection, especially horses >4 years. Yearlings, 2- and 3-year-olds are more likely to have higher burdens, especially small redworm; this should be taken into account when planning testing and treatment options (see below). Older horses (>15 years), used as riding horses or companions, may also have higher burdens so can act as potential sources of contamination. 

Regular assessment with your veterinarian of the risk of infection to the individual or group enables danger zones in management practices to be identified, addressed, and the impact of improvements monitored over time. Include sufficient detail in the assessment so that seemingly innocuous practices that increase risk (for example, short daily turn-outs) can be identified and action taken. Risk assessment will:

1. Inform which tests to perform, test frequency and which horses to include 

2. Indicate the need for strategic treatments; for instance, small redworm larvicidal therapy in high-risk (younger) horses where tests cannot be used to guide treatment decisions

3. Provide information on potential worm exposure and the need to reduce the opportunity of horses being infected (at the yard or elsewhere).  

Tests provide information to help treatment decisions

Diagnostics are essential for making informed decisions about worming and for selecting which product to use, whilst reducing selection for resistance. Tests available include faecal egg count (FEC) and antibody-based assays. 

FEC tests estimate the number of worm eggs a horse is passing in dung (a measure of contamination potential) and provide information on the type of eggs excreted. On racing yards, testing is recommended every 12-16 weeks.  Usually, ~80% of horses excrete ~20% of the eggs passed [3], meaning that many individuals have no/low worm egg shedding and will not need treatment, thus preserving wormers. Horses estimated as passing >200 to >500 worm eggs per gram (epg) dung are recommended for treatment. When collecting a dung sample, select at least three balls from the pile, with a minimum of 5 grams placed in a pot/bag with all air excluded and the samples kept cool. FEC reduction tests should be conducted once a year to provide information on effectiveness of the wormers being used to target small redworm. 

FEC tests only detect the products of egg-laying adult worms and are not reliable indicators of the burden within an individual, especially as male and immature worms are not detected. In the case of tapeworm, FEC methods are also affected by inconsistent release of egg-containing segments from adult worms so are not recommended for identifying infection with this parasite. Instead, tests that detect antibodies can be utilised to provide information on the level of tapeworm or small redworm infection in individuals.

Tapeworm antibody tests are available in saliva and blood formats. Both work on the principle of measuring worm-specific antibodies, levels of which show a strong positive relationship with tapeworm burden. The tests have been shown to accurately identify all horses that harbour clinically-relevant burdens of >20 tapeworms [4]. Testing identifies horses that will contaminate areas where horses graze, as well as those harbouring burdens that may put them at risk of colic. All horses should be tested at the same time to identify those that need anti-tapeworm treatment; ideally, in combination with tests that detect small redworm infection (FECs or small redworm blood test). By doing this, the correct worming product can be selected based on the test data (Fig. 2). Testing can be performed once or twice a year, depending on the level of risk identified at the initial assessment and informed by ongoing data. Tapeworm testing results in large reductions in anthelmintic use; from 2015-2022, >164,000 UK horses were assessed using the saliva test and only 1/3 were recommended for treatment [5]. In the unlikely event where many horses test tapeworm-positive on a yard, the source of infection needs to be identified and management rectified to reduce transmission via oribatid mites.

It was previously recommended to treat all horses for small redworm encysted larvae in late autumn/winter. As it acts to select resistance, routine all-group treatment is no longer advised for horses at low risk of infection. Horses in training will usually fall into this category. For low-risk horses, the options are to not administer this treatment, or use the Small Redworm Blood Test. Similar to the tapeworm tests, this measures worm-specific antibodies and demonstrates high sensitivity in identifying horses with low small redworm burdens that do not require treatment. The test can be utilised in autumn/winter when it is more likely that small redworm encysted larvae, that are not detected by FEC tests, are present. Applying the test in low-risk sport horse groups demonstrated that many horses (>60%) fell below the low 1,000-small redworm threshold [5]. 

Horses in training can test positive by any of these methods, despite the fact that they do not graze for significant periods. This is because they can become exposed to worm infections during short turnout periods, or if they are allowed to graze on training grounds or at the race course. Wherever there is dung deposited, there may be worms!   

In the case study (Fig. 3), tapeworm and small redworm serum scores in December are shown from horses based at a training yard in the UK. The results demonstrated negligible burdens (<1,000 worms) of small redworms in ~1/3 of the group, with only 14% of horses recommended for tapeworm treatment. These horses had 30 minutes turnout to a small paddock each day; dung was not removed from this paddock, providing a source of worm infection. The veterinarian subsequently advised the trainer to remove dung daily from the paddock and to treat test-positive horses with a larvicidal anthelmintic and, where indicated, an anti-tapeworm treatment. These horses previously received regular all-group treatments, so although blood testing recommended a proportion to be wormed, this strategy reduced worming frequency overall and, importantly, provided the trainer with insights regarding management procedures. 

Advice for horses new to a yard

The introduction of newcomers or the return of previous residents to a yard risks introduction of ‘new’ parasites. All new arrivals should be isolated, tested (FEC/small redworm blood test, tapeworm test) and wormed based on the results. For small redworm, a FEC reduction test should be performed to assess wormer sensitivity of the parasites the horse is carrying. Ideally, keep the horse away from grazing in the interim, or at least prevent access to turnout paddocks for 3 days after worming to stop transmission of eggs that are excreted after treatment. 

In conclusion 

Few studies have examined worm prevalence, control practices or effectiveness of anthelmintics on training yards. Those that have, indicate industry-wide overuse of wormers, with few trainers using evidence-based methods [6]. The racing industry must avoid the legacy of spreading drug-resistant worms to other parts of the sector.  Once horses retire from training, they enter a spectrum of environments where the introduction of wormer-resistant parasites could prove extremely detrimental, particularly, breeding enterprises where susceptible young animals will co-graze with mares, or retirement homes/sanctuaries containing geriatric horses that may be more prone to worm-associated disease. The introduction of drug-resistant parasites to leisure riding establishments or yards focused on eventing, show jumping or dressage, would be viewed as a negative sequelae of the over-use of anthelmintics in the training sector. Given the amount of attention paid to the health and physiology of racehorses, trainers, working with their veterinarian, are perfectly poised to adopt worm control plans designed to meet the needs of the individual by following a diagnostic-led approach. An exemplar control plan is shown in Fig. 4.

References

1. Nielsen 2022. Int J Parasitol Drugs Drug Resist. 20;76-88.

2. Nielsen 2023. Int J Parasitol Drugs Drug Resist. 22:96-101.

3. Relf et al. 2013. Parasitology 140:641-652. 

4. Lightbody et al. 2016. Vet Clin Pathol. 45:335-346.

5. Matthews et al. 2024. In Practice 46:34-41.

6. Rosanowski et al. 2016. Equine Vet J. 48:387-93.

7 Simple Rules for Parasite Control in Racing Yards

Article by James Gibbons

When Benjamin Franklin wrote, in 1789, ‘in this world nothing can be said to be certain, except death and taxes’, he could, perhaps, have added another certainty to his list – worms in horses. Unlike most other infectious diseases of horses, such as strangles or influenza—which infect a small number of horses relative to the entire horse population at any one time—worms are present in almost all horses all of the time.  This fact leads to two obvious conclusions: first, it is not possible to eradicate parasites and the threat of parasitic disease from our horses; second, worm control is vital in any environment where horses are kept.  

The threat posed by intestinal worms to racehorse performance has long been recognised by horsemen the world over. For many years, regular treatment with anthelmintic drugs (‘dewormers’) was the mainstay of worm control in racing yards, as it was in most in areas of horse breeding and production. The emergence of resistance to these deworming drugs (referred to as ‘anthelmintic resistance’), in the last 10 years in particular, has meant that such regular treatments may no longer be effective and may in fact make the resistance situation in the yard or farm worse. With this in mind, it is important that we consider how best to control worms while preserving the efficacy of the few deworming drugs available to us. This can be achieved using control programs comprised of drug- and non-drug control measures. This article sets out seven points/rules to consider and implement when developing such a worm control programme for racing yards. 

1. Know your enemy

It is not possible to draw up a parasite control program without considering which worms it is you are trying to control. The three main types of worms we are concerned with are strongyles (large and small redworms), ascarids (roundworms) and tapeworms. The role of tapeworms in equine intestinal disease is debated, but they appear to be linked to certain forms of colic when present in high numbers. Notably, there is no evidence of age-related immunity to tapeworms. Ascarids are a cause of disease in foals primarily and yearlings and so are likely to be of less concern in most racing yards. However, unlike redworms, which are transmitted almost exclusively at grass, ascarids can be transmitted in the stable; and ascarid eggs survive for years in the environment so that a single infected animal can infect other young horses for years to come.  

Redworms are the most important parasite of horses and are found in horses of all ages where they can cause anaemia, weight loss, ill-thrift and diarrhoea. Large redworms can burrow into the walls of blood vessels that supply blood to the gut causing a very severe form of colic.  Small red worms can lie dormant in the gut wall for extended periods then emerge en masse to cause an acute shock-like syndrome with severe diarrhoea, which is often fatal.  

Faecal egg count (FEC) testing will identify if redworms or ascarid eggs are present in your horse’s droppings and this, in turn, tells you that the adult worm of that species is present in your horse’s gut. Tapeworm eggs can be detected by FEC testing, but it is not the most reliable method for their detection as the shedding of these eggs is not consistent. As redworms are the most relevant worm present in most yards, most of the information in this article relates specifically to the control of redworms rather than ascarids or tapeworms.

2. Know your horses’ risk

All trainers pride themselves on their knowledge of almost every aspect of their horses’ anatomy and physiology, but not all could tell you which of their horses are at greater or lesser risk of worms. As mentioned above, all horses are likely to carry some worms, but the adult worm burden and the number of worm eggs shed in faeces is far greater in younger horses than in mature stock.  Most horses between the ages of 5 and 15 years will have a lower worm burden, and a lower risk of parasitic disease, than horses below this age due to a degree of age-related immunity. Foals and yearlings can carry particularly high worm burdens and shed large numbers of eggs into the environment to infect other horses. 

It follows that any yard with yearlings—two- or three-year-olds—will need a comprehensive worm control programme; while yards with older stock may get away with less strenuous controls. Horses over the age of 15 may also have higher worm burdens. And while these are unlikely to be in training, they may be used as a riding horse or companion animal and act as a potential source of infection for the string.

3. Know your yard’s weak points

It is not unusual for our lab to get a call from a racehorse trainer wishing to express their disbelief that the faecal egg count test from their horses has tested positive for redworm eggs despite their horses having no access to grass. In these situations, careful questioning as to how the yard operates will usually reveal the use of turn-out paddocks for a short period at some point during the week. Invariably, these are shared, often quite small, paddocks which host many horses over time; and so they are more likely to be contaminated with worm eggs.  

While the intention is for the horse to get ‘a pick of grass’, it may be that it is more ‘a pick of worms’ they are getting in such paddocks! As part of a worm control plan, it is important to first identify high-traffic areas, which may be a pinch-point for worm transmission.  Once identified, the key to reducing the worm burden on such paddocks is ideally through the removal of droppings. This can be a labour-intensive exercise, but it only needs to be done twice weekly rather than daily. And there are now more automated methods for cleaning paddocks than the more traditional wheelbarrow and spade!  

4. Identify high shedders 

FEC testing not only tells you what type of worm is present in your horse’s droppings but also how many eggs there are per gramme of faeces.  Repeated FEC testing allows you to build up a picture of the shedding patterns of the horses within your stable. Horses in racing yards should have FEC testing carried out every three months. It is generally accepted that the shedding of worm eggs in horses follows the 80/20 rule; that is, 20% of the horses shed 80% of the eggs.  

If these high shedders in the group can be identified, then targeted treatment of them may be more beneficial (and cost-effective) than blanket treatment of the entire group.    Horses with a strongyle egg count in excess of 250 eggs per gramme (EPG) on repeated testing may be considered high shedders and require more frequent egg counts and treatment.  It is important to state that this figure of 250EPG is not absolute, and the threshold above which animals are considered high shedders or requiring treatment should be set in conjunction with your vet. Worm eggs are not distributed evenly within the droppings, so when collecting samples for FEC testing, make sure to take at least three faecal balls—each from a different area of the pile. 

FEC testing can only detect egg-laying adult worms, but the egg count is not a reliable indicator of the adult worm burden of the horse, i.e. a horse with a high FEC does not necessarily have a greater worm burden than a horse with a lower FEC; but the horse with the high FEC is more significant in terms of worm transmission to other horses. Immature worms that may be present in the horse are not detected by FEC testing. In recent years, new tests have been developed that can detect antibodies to tapeworms and small redworm in blood and/or saliva. These tests are a useful addition to any worm control programme. Regardless of the type of test used, dewormers are still necessary as part of any worm control programme, in foals and yearlings in particular, but also in high-risk environments, and in order to control the disease risks posed to horses of all ages by large strongyles.  

5. Know which drugs work

While it might seem like there is an endless range of deworming products for horses on the market, the number of active ingredients in these products is very limited with only four drugs (fenbendazole, ivermectin, moxidectin and pyrantel) available for the treatment of redworms and ascarids. With such limited availability, and no prospect of new worm treatments entering the market anytime soon, it is vital that we use the existing drugs judiciously so as to preserve their effectiveness into the future.  

The threat posed by anthelmintic resistance is very real. Already fenbendazole resistance in redworms is widespread, and ivermectin/moxidectin resistance in ascarids is becoming more common. This means that there are only one or two effective treatments remaining for these resistant worms. While thoughts of anthelmintic resistance may not keep any racehorse trainer awake at night, we do expect that the deworming treatments we use to be effective, not least because they cost quite a lot of money!  The only way to determine if our treatments are indeed effective is to carry out a faecal egg count reduction test (FECRT). 

This test follows on from the FEC test; any horse with a FEC of 500EPG or more should be treated with a dewormer and have a follow-up FEC carried out 10–14 days after that treatment. The percentage reduction in egg numbers is calculated and should be greater than 90–95% (depending on the drug). Reductions less than this are suggestive of a resistance problem. Because of the existing resistance situation, FECRTs should be carried out before using fenbendazole to treat redworms or ivermectin/moxidectin to treat ascarids. Equally, if you have any suspicion that a deworming product is not working effectively in your yard, then a FECRT for that drug should be carried out to investigate this.   

6. Don’t overtreat

Aside from the risk of anthelmintic resistance development and the significant costs incurred by frequent or blanket deworming of all horses in the yard, such treatments can have other effects on the horse, which may ultimately impact performance. Research has shown that anthelmintic administration is associated with a decrease in the diversity and abundance of certain bacteria in the horse’s gut, which may impact digestion and other metabolic processes. 

In humans, changes in the composition of the gut bacterial population have been shown to impact physiology, immunity and behaviour; and, while the structure of the gut may differ greatly between human and horses, there is no reason to believe the equine gut bacteria do not play a similarly important role in horse health. In recent years, the role of parasites in human health has been revaluated. And while they are still recognised as an important cause of disease, it is also accepted that they can play a beneficial role in the development of immunity and the prevention of some diseases. While we don’t have similar evidence of a benefit to horse health from parasites, it is not impossible that such benefits do really exist.  

A study of standardbred trotters in Denmark in 2011 found that horses with higher egg counts had better race finishing position than those with lower egg counts. So,  the presence of parasites does not always lead to poor performance! It was standard practice, in some racing yards, to deworm horses every eight weeks, but this seems excessive when one considers the biology of the worms involved. The small redworms take approximately eight weeks to develop to maturity in the horse, while the large redworms take approximately six months to do so. Treatment every eight weeks could only be justified if there were evidence of overwhelming worm exposure on an ongoing basis—something which is unlikely in a modern racing yard. Even if that were the case, other non-pharmaceutical controls would be necessary to get the situation under control.

7. Manage new arrivals

One of the challenges of worm control in a racing yard is the constant turnover of horses with some leaving the yard for rest or recovery from injury, others returning from such breaks and new horses joining the yard. When a horse leaves the yard, they leave the worm control programme as well. Evem if the yard’s deworming programme is followed while they are away, the risk factors in their new environment may be totally different and require an entirely different approach to treatment. More worrisome, the arrival of new horses or the return of others also risks the introduction of new or resistant parasites to the yard. In order to minimise this risk, all new arrivals should be isolated, tested and treated. Ideally, all new arrivals would be isolated for two weeks while a FECRT is carried out to ensure treatment has been effective.  

This may not be practical in all yards, but an isolation of period of at least three days after deworming should be observed to ensure all worms are shed before the horse joins the group. Treatment without isolation is not recommended as treated horses can shed viable eggs for a few days after deworming. There may be a temptation to let horses out to turnout paddocks for exercise or grass during the isolation period, but we have seen cases where this has led to the introduction of new parasites to the yard despite the horse being treated before it was turned out.   

There was a time when parasite control in racing yards was relatively simple and relied primarily on the regular use of deworming drugs. As with many aspects of horse racing, what was once considered acceptable is no longer so. Effective parasite control is not as simple anymore, and an overall plan designed to meet the needs of the individual yard is required. That is not to say that any trainer should be daunted by the prospect of drawing up such a plan. The seven points discussed here provide a basis to work from and your vet and laboratory are well placed to assist in building on this.