Addressing drug resistance in equine tapeworms

Article by Jacqui Matthews

Tapeworms are important parasites of horses

All horses can be infected with internal parasitic worms, which can cause health issues, including weight loss, diarrhoea, and colic. The most common worms affecting racehorses, and other horses, are the small strongyles (cyathostomins) and the common equine tapeworm, Anoplocephala perfoliata. Horses are infected by ingesting parasites from contaminated grazing, whether it be a field, turn-out paddock or opportunistic grazing on training grounds or racetracks. 

Recent reports of dewormer resistance in A. perfoliata are very concerning, especially as there are few available products to treat these parasites and no new drugs are expected to enter the market soon. These relatively large parasites typically reside at the junction of the small and large intestines and can cause colic. The worms attach in clusters to the intestinal wall, which can cause mechanical obstruction and mucosal damage. 

Blockages can cause impaction, potentially necessitating surgery. Moreover, the presence of tapeworms may lead to intussusception, where one segment of the intestine telescopes into an adjacent segment, also requiring surgery. Studies indicate that having as little as 20 tapeworms can cause significant damage to the intestinal wall (Pavone et al. 2010). Therefore, it is crucial to prevent such burdens from accumulating in horses.

Tapeworm resistance to deworming products 

There are two types of dewormers (anthelmintics) available for treating tapeworms: praziquantel and pyrantel (given at double the dose used for treating roundworms). In the UK and EU, there have been several anecdotal reports of reduced effectiveness of anti-tapeworm drugs. A recent research study on a Thoroughbred farm in the US evaluated the performance of both tapeworm dewormers (Nielsen, 2023). 

The results demonstrated treatment failures in foals and broodmares in which tapeworms survived treatment. This was the first formal report of suspected drug resistance in tapeworms. Resistance occurs when parasites survive deworming treatments and pass on reduced sensitivity to the drugs to subsequent generations. Repeated treatments with the same drug can lead to parasite burdens that cannot be cleared and may result in clinical disease. 

Given the threat of resistance in this species, it is essential to reduce the overuse of anti-tapeworm medications. Implementing more sustainable control methods is now crucial for the long-term effectiveness of these important dewormers. These control methods must include:

• maintaining a clean grazing environment 

• regularly monitoring parasite burdens 

• deworming only those horses that truly need treatment. 

Use grazing management methods to reduce reliance on dewormers

Tapeworms differ from other common equine worms because they develop inside mite intermediate hosts on paddocks (Fig. 1). Horses become infected when they consume hay or grass that contains tapeworm-infected mites. Mites are infected by eating eggs passed in the dung of infected horses.  Where horses have access to grazing paddocks, it is essential to remove dung daily and dispose of it well away from both the grazing area and any water sources. Extra caution should be taken with horses that have grazed away from the yard and newcomers to the yard (see quarantine recommendations below).

Use tests to reduce dewormer treatment frequency

Regular testing is essential for effectively managing tapeworm infections. Faecal egg count (FEC) tests are unsuitable for detecting tapeworms. These detect worm eggs shed in dung, but are not reliable indicators of the overall parasite burden in individuals, particularly since immature worms are not detected. FEC methods are also influenced by the variable release of egg-containing segments from adult tapeworms. 

The main purpose of FEC tests in tapeworm control is to assess the effectiveness of deworming treatments. If tapeworm eggs are detected in dung samples taken two weeks after treatment, this is a significant finding. However, the absence of eggs in a FEC does not mean that tapeworms are not present. If resistance is suspected, this should be discussed with a veterinary surgeon. 

Tests that measure antibodies to tapeworm provide valuable information about levels of infection and should be used to guide treatment decisions. Antibody tests are available in blood and saliva formats. In the blood test, samples are collected by a veterinary surgeon and sent to the laboratory for analysis. 

This test measures levels of tapeworm-specific antibodies in the blood, with results reported back to the veterinary surgeon as "serum scores." These scores are categorised as low, borderline, or moderate/high, and treatment is recommended for horses with results in the borderline or moderate/high categories.  The non-invasive saliva test involves taking a sample from the horse’s mouth using a specially developed swab (Fig. 3) and does not require a veterinary surgeon. 

The swab containing the saliva sample is mailed to the laboratory in a preservative solution, ensuring stability for at least three weeks. At the laboratory, the saliva sample is assessed using a special three-ELISA system that accurately measures tapeworm-specific antibodies, with the results reported as “saliva scores”. Similar to the blood test, the saliva test categorises results as low, borderline, or moderate/high, with treatment recommended for horses that have results in the second two categories. Because antibodies take time to decrease after effective treatment, horses should not be tested again until 4 months after the last deworming for blood tests or 3 months for saliva tests.

By reliably detecting tapeworm burdens, antibody tests enable treatments to be targeted to only those horses that need treating and therefore reduce the risk of dewormer resistance. Results from tapeworm testing have led to significant reductions in the use of dewormers; from 2015 to 2022, over 164,000 horses in the UK were assessed using the saliva test, with only one-third recommended for treatment (Matthews et al. 2024). 

Applying tapeworm testing at racing yards

Tapeworm testing frequency can be determined by conducting a risk assessment. Key risk factors include age and access to contaminated grass, as well as historical test results. These parasites can be long-lived and persist for extended periods, so it is essential to consider each horse’s history during or before training. 

While most horses in training are at low risk due to having limited pasture access, yearlings and two-year-olds may have higher burdens, especially if from breeding farms or other premises where there is a high level of infection. However, all ages of horses are susceptible to tapeworms. Regular assessments with a veterinary surgeon will also identify risk factors in yard management practices, including those associated with activities like short daily turnouts. A comprehensive risk assessment will:

1. Identify which tests to perform (FEC tests, small redworm blood tests, tapeworm tests) and the frequency of testing

2. Highlight the need for treatments for high-risk horses when tests do not provide information for treatment decisions

3. Provide information on worm exposure and ways to minimise infection risks.

If significant risks are detected, such as a high level of tapeworm infection indicated by testing or the frequent introduction of new horses, testing should occur every six months. Once a year testing may be appropriate in low-risk situations where previous testing has shown low evidence of tapeworm infection. 

Testing identifies infected horses that could spread infection to others, allowing for prompt treatment and reducing the risk of colic. If many horses test positive, it is crucial to identify the source of infection and improve management practices to reduce spread. In a recent case study on a UK training yard, 56 horses were tested for tapeworm antibodies. 

The results revealed that only 14% of the horses had tapeworm burdens that required treatment. These horses were turned out in a small paddock for just 30 minutes each day, and because dung was not removed from the area, the paddock was identified as a source of infection. The trainer was advised to remove the dung from the paddock daily and to treat any horses that tested positive for tapeworms. 

This testing protocol not only helped reduce the overall deworming frequency, but also provided the trainer with valuable information about horses at risk of colic. It also highlighted potential areas for improving parasite management practices.

Avoiding the introduction of new or resistant worms

Introducing new horses to racing yards requires proper assessment to determine if they have roundworm (small redworm, ascarid) or tapeworm infections. The traditional method of treating all newcomers with a broad-spectrum dewormer is outdated and should be avoided due to increasing drug resistance in all common parasites. Instead, it is recommended to assess new horses using appropriate tests, specifically;

1. FEC tests to identify if they are shedding eggs such as small redworm and ascarid eggs 

2. Blood tests to detect small redworm stages that may not be detected using FEC tests

3. Tapeworm tests to identify horses that need specific treatment for this parasite. 

If any of these tests return positive results, the appropriate dewormer can be selected to target the parasites present. Furthermore, if a horse tests positive in the initial FEC test, it is advisable to conduct a follow-up FEC test two weeks after treatment to determine whether the dewormer has been effective.

In conclusion

Every horse will encounter parasitic worms at some point in their life, making effective parasite control essential for their health and well-being. While traditional all-group dewormer treatments have been common, rising cases of dewormer resistance reveal that this approach is no longer sustainable, especially as no new anti-tapeworm treatments are expected soon. 

Using tapeworm tests to determine if treatment is needed is crucial to maintain the effectiveness of existing dewormers. Many horses in low-risk environments have minimal or no tapeworm infections, making regular treatments unnecessary. Testing helps identify only those horses that truly need treatment, thus promoting the longer-term efficacy of dewormers. 

In the racing industry, there is significant overuse of dewormers, with few trainers using evidence-based practices. It is essential that the spread of resistant worms is prevented, especially as racehorses move to various environments (breeding farms, sport horse yards, sanctuaries, leisure horse premises) where more vulnerable horses may reside. For this reason, the industry must adopt management-based and test-led methods to control worm populations effectively.

References

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

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

Pavone et al. 2010. Vet. Res. Commun. 34, S53-6.

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.