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.

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.