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.

Gerald Leigh Lectures 2024 - Optimising Youngstock Growth and Development

Beaufort Cottage Educational Trust hosted the annual Gerald Leigh Memorial Lectures at Tattersalls in Newmarket on June 11th.  These lectures are in association with the Gerald Leigh Charitable Trust as a way of recognising Mr. Leigh’s achievements as a racehorse owner and breeder as well as continuing his passion for scientific knowledge and welfare of horses in the race industry and the equine industry as a whole. 

Dr. Chavatte-Palmer provided a very insightful and interesting talk on the influence of maternal and environmental factors during the prenatal period on athletic performance.  The lecture discussed the phenomenon of Developmental Origins of Health and Disease (DOHAD) which is demonstrated with the increasing evidence of the role in parental nutrition and environmental conditions, from periods preceding conception and throughout gestation, on phenotypic development in horses.  She described how the nutritional environment before birth can affect the postnatal performance of the offspring as it is believed that nutrients can alter the way genes are expressed resulting in developmental trajectories that have repercussions on postnatal development.  With a strong understanding and knowledge in DOHAD, nutritional strategies can be considered and developed to optimise youngstock growth and development.  This is an exciting field as further research will potentially allow veterinarians and breeders to work cooperatively to optimise athleticism as well as the wellbeing and health of foals and mares. 

Dr. Joe Pagan of the Kentucky Equine Research kindly visited from the U.S.A to discuss equine feeding management practices and equine nutrition in sports medicine. His first lecture discussed the effects of season on mare and foal nutrition.  Through various studies some conclusions could be made and these conclusions do appear to be a global trend.  It was found that colts are heavier and taller than fillies.  

After 1 month of age, fillies were fatter than colts.  Foals born in January and February were smaller at birth and grew more slowly in early life but by 150 days of age this lag disappeared.  In addition, mare weight changes and body condition scores were related to season and management factors, as winter-foaling mares lost weight and had lower body condition post foaling than spring-foaling mares. 

Dr. Pagan continued with a lecture on the right balance for optimal growth and development of youngstock.  Recent research has shown that the health of the offspring of overweight mares may also be compromised. In addition, studies have shown that excess maternal nutrition during pregnancy can alter glucose and lipid (fat) metabolism in foals until 160 days of age.  Another study reported a higher incidence of osteochondrosis (OC) in foals born to dams that were fed concentrates during gestation rather than forage. 

A more recent study on the nutrition of the mare during the last trimester and subsequent foal health revealed some interesting observations.  It was found that the growth of foals from 6-24 months of age was not affected by maternal diet and maternal undernutrition appeared to affect bone growth as foals from dams fed forage only had narrower cannon bones than foals from dams fed forage and barley.  In addition, it was found that overfeeding negatively affected yearlings from mares fed barley and forage more than yearlings from broodmares fed only forage.  

Finally, there was a decrease in insulin sensitivity and enhanced insulin levels suggesting insulin dysregulation in yearlings from mares fed both barley and forage and not yearlings from forage-only mares.  Another exciting field of research that will also lead to greater cooperation of breeders and veterinarians in providing good health and wellbeing to foals and mares.   

These lectures had the pleasure of hosting Paul Overton, who is an equine agronomist with a strong understanding of pasture management.  His passion for his area of expertise and in helping with improving management of studs was abundant as he outlined the aims of pasture management that included the need to provide safe grazing pastures from parasites, colic and grass sickness, atypical myopathy and other ailments as well as to provide a safe area to exercise horses of all ages.  In addition, he made clear that fresh leafy bite grass in the paddocks all year round is key.  

We were reminded of the fundamentals of good basic pasture management that included a removal programme for muck, providing a dense leafy grass sward, sharing and mixing the paddock use with other livestock (sheep and cattle) and moving horses to various paddocks around the stud to maximise rest of all paddocks. To optimise paddock health, it should be remembered that all paddock management follows the horses around the seed, be guided by the weather rather than the calendar and work quickly to maximise rest periods. Finally, good simple paddock and grassland management can help a multitude of issues and every marginal gain is helpful. 

Julian Dollar gave us an insight into a stud manager’s approach to soundness.  It was an integral part of the lecture programme as it provided the audience with another perspective of stud management. He made it clear that a variety of factors must be considered when breeding and raising sound horses but two main criteria is a strong team and an attention to detail. 

The team needed is one that is completely dedicated and passionate about their horses and should consist of a veterinary service, farriers, nutrition/feed and land management support. There must be a cooperative work ethic in order to provide the best all round management as possible. This team must strive for excellence and work on marginal gains.
A sound breeding and rearing programme starts with mating focusing on sound parents, especially the mare, to produce sound offspring.  It is important to work with mother nature rather than interfering with it, thus focusing on a healthy environment. This strategy must be flexible that will allow one to provide the horses with a quality paddock. 

The offspring should be between 55-60 kg and they should have a seamless transition from birth to exercise and living out.  In addition, the foals should be monitored and assessed closely in order to achieve timely and maximum beneficial interventions that may include selective surgeries.  Finally, it is believed that the perfect preparation for a yearling to go into training would be some controlled exercise combined with turnout the rest of the time. 

Professor Jacqui Matthews, the director of Austin Davis Biologics, provided some sobering information on parasites and their resistance in the United Kingdom.  She explained that intestinal worms are found in most grazing animals and most breeding premises are at high infection risk due to the presence of young animals grazing permanent pastures as well as having a history of interval deworming treatments. 

In addition, wormer resistance is being reported in all major parasitic worms of horses.  Therefore, it is vital to have control plans that include a risk-assessment approach.  Professor Matthews made it very clear that we need to be concerned regarding parasites and their resistance to wormers on UK studs and we need to act now.  She explained the action that is needed which includes evidence-based worm control.  This approach requires risk assessment that allows one to identify potential issues in the management, the avoidance of all horses regularly being treated, the assessment of wormer effectiveness as well as the effectiveness of interventions. 

With respect to the management, there are some principles behind reducing anthelmintic use.  Good pasture hygiene can substantially reduce infective egg and larval stages in the environment and the lowering of this pasture contamination decreases parasites acquired when grazing.  Pasture management must include: 

• Dung removal works! Full removal of dung at least once per week and place it away from pastures and water courses.  

• Maintain low stocking density 

• Rest pastures – ideally 12 months and avoid grazing foals on same paddocks every year 

• Consider grazing with ruminants between equine groups 

With respect to treatment, make informed choices by using various tests such as faecal worm egg counts, faecal egg count reduction test, antibody-based tests to identify worms, and other tests.  Ultimately, do not blindly treat horses without considering the reason for worming and assessing wormer effectiveness. 

Professor Matthews followed up with a second lecture on integrating diagnostics into sustainable equine helminth control programmes.  An integrated diagnostic-led control programme should consider: 

• Faecal egg worm count in late winter if grazing year-round.  Recommend treatment if 200 or more eggs per gram and select your anthelmintic according to its sensitivity. 

• Tapeworm test in spring to identify individuals with tapeworm burdens and treat infected individuals as these horses will contaminate the paddocks with cestode eggs. 

Late spring/early summer perform faecal egg worm count reduction test in order to assess anthelmintic effectiveness annually.  Worm all horses that have FEC of >200 eggs per gram.  Perform FEC test at day 0 and 14 days after treatment. Calculate the mean percentage.

• Reduction in FEC between 0 and day 14 post treatment. 

• In late summer/early autumn perform tapeworm test to identify individuals that have, over the summer grazing period, accumulated high tapeworm burdens which may put them at risk of colic  

In late autumn perform a small redworm test.  In low infection-risk situations, test in late autumn/winter to identify individuals that do not require treatment for small redworm.  Use in low-risk groups where owners usually apply all-group larvicidal treatments.

With good pasture management one can lower the selection for resistance, leading to a lowering the need for wormer treatments, a lowering in the proportion of horses that test positive and a lowering of paddock contamination; therefore, less horses are likely to be infected and more likely to have lower burdens. 

With his 20 years of buying and selling thoroughbred bloodstock, Jamie Railton gave us insights into the business of foal purchasing.  He discussed all of the variables that must be taken into account when selecting foals for purchase.  Skill and experience are needed in spotting the foal with the potential to increase in value because of its physical improvement and its pedigree.  He explained that a team of people are required as well as having a system to select foals; however, the system must be flexible and one must still want to continue to learn from the process and adapt and later the selection criteria. In addition, he discussed the need to reinvent oneself with the continuous change in the trends seen in the market. 

Dr. Rebecca Mouncey is a veterinary surgeon and post-doctoral research fellow, currently investigating early-life determinants of performance and economics of thoroughbred breeding.  

Dr. Mouncey discussed how musculoskeletal disease and injury remains the greatest barrier to thoroughbreds being retained within the industry and realising their maximum athletic potential, and is likely to have important economic consequences.  She explained that lameness/musculoskeletal injury and disease are the most common barriers to performance.  The causes of these conditions of developmental orthopaedic diseases are fractures and tendon/ligament injuries. 

By improving the foal’s/yearling’s strength to withstand training and resist injury, while avoiding trauma, one can help with the prevention of these diseases.  Unfortunately, there are still important knowledge gaps. It is possible to optimise musculoskeletal health during early-life with exposure during critical stages of growth and development that changes the distribution of cell types, alters metabolic function and ultimately altering the susceptibility and injury in adulthood.  

Postnatally the period of greatest plasticity, response to external stimuli/exposures is during periods of most rapid growth and development.  By 24 months of age, the horse is 96% mature bodyweight and 98% mature height and reached peak bone mass.  There must be a balance in early-life exercise and activity as too little will delay musculoskeletal development and too much will cause tissue damage and trauma.  We must always consider the animal’s growth, nutrition and exercise as a whole and not mutually exclusive.  

Abigail Kent MRCVS from Rossdales Veterinary Surgeons, presented an interesting lecture on the diagnosis and treatment of limb deformities in foals.  Dr. Kent is very knowledgeable in all aspects of equine elective and emergency surgery and has a particular interest in orthopaedics, specifically arthroscopy, angular limb correction and fracture repair.  

Limb deformities in foals are developmental orthopaedic conditions, that can be subdivided into 2 major groups: 1) angular limb deformities, in which there is deviation, primarily in the frontal plane, originating at a joint and/or growth plate and 2) flexural limb deformities, in which there is persistent hyperflexion or hyperextension of the limb. 

These conditions may be acquired or congenital and the aetiology is multifactorial.  Early appropriate intervention is the key to the best outcome. Angular limb deformities are relatively common in foals and strategies for management, including surgical techniques, are well established.  Early recognition, diagnosis and monitoring are essential in order to ensure optimal outcomes. 

In cases of severe deformities that fail to respond adequately to conservative approaches (restricted exercise, splints, corrective hoof trimming and shoeing) other treatments such as timely surgery may be used.  Transphyseal screws are effective for the corrections of both forelimb and hindlimb limb deformities.  Flexural limb deformities can be treated conservatively with the mainstays of limited controlled exercise, nursing care, manipulation of the foot with trimming/shoes, splinting/casting and pain management.

Surgical intervention is typically recommended when conservative treatment is not effective or for severe or rapidly worsening deformities. The goal of surgery is to release tension in the restrictive musculotendinous unit, allowing for greater stretch and more normal range of motion of the affected joint. 

In conclusion, all the contributors provided the participants a very interesting and illuminating day.  This event highlighted the lengths that all stakeholders and the veterinary profession are taking to constantly improve the health and wellbeing of the horses and the health of the race industry.