Diagnostics to aid worm control
Article by Prof. Jacqui Matthews, BVMS PhD FRSE FRCVS, RCVS Recognised Specialist (Veterinary Parasitology).
Introduction
Horses in training typically have a low risk of parasitic worm infections due to their limited access to pasture. However, young horses, especially those newly arrived from breeding farms, may harbour significant burdens and experience related health issues. Once established full-time on racing yards, horses are not generally at risk of worm infection. However, during training, they may be exposed to worms in certain situations, such as during rest periods at grass or regular short turnouts to contaminated paddocks, or when allowed to graze in training areas. For these reasons, it is crucial to regularly monitor horses in training for worm infections.
Parasitic worms and their clinical impact
Nearly all horses are exposed to worms at some point. The most common are the small redworms (Fig. 1) and tapeworms (Fig. 2). Horses become infected with these parasites when they ingest contaminated grass (Fig. 3). Large burdens of small redworms can cause weight loss, diarrhoea or colic, particularly in horses under five [1]. This is because some horses can accumulate considerable burdens (10,000s-1,000,000s) of small redworm larvae in their gut wall, and when these larvae emerge in large numbers, they cause severe damage. Tapeworms can also cause colic [2]. Immunity to both parasites develops gradually, and some horses remain susceptible throughout life, especially if they graze heavily contaminated fields or have weakened immune systems. Ascarids, a type of roundworm, affect foals and are commonly found on breeding farms [3]. While these worms are less of an issue as foals mature, infections can cause long-lasting lung damage due to the ascarid migratory life cycle.
Other parasites that infect horses include large redworms and pinworms; these are infrequently seen in horses in training due to their sensitivity to wormers (large redworms) and management practices [pinworms]. It's important to remain vigilant, though, especially when introducing new horses; large redworms can cause a life-threatening colic requiring surgery, while pinworms can cause severe itching around the tail and are challenging to treat.
The impact of worms on athletic performance in horses in training remains unclear. One study conducted on healthy Standardbred trotters found that high strongyle (small redworm) faecal egg counts (FEC) did not significantly affect race performance [4]. However, since very few horses had high counts in this study, the evidence was inconclusive. High worm burdens, which can cause long-lasting tissue damage in the lungs and gut, will certainly affect performance.
Wormer resistance is a significant threat to sustainable parasite control
There is a limited selection of medications available for treating equine worm infections, and unfortunately, resistance has been reported against all types of wormers (Table 1). It is therefore essential to balance the need for worming with the preservation of these medicines for horses that have high parasite burdens that require treatment. To effectively manage this balance, evidence-based programmes are necessary. These programmes must include risk assessments, diagnostic testing and the avoidance of practices that contribute to the spread of worms.
Risk assessments
Risk assessments, ideally, conducted with a veterinary surgeon, will evaluate an individual's likelihood of worm infection. The assessment results can then be used to guide decisions on management, testing and treatment. The online tool “What’s Your Worm Risk” (whatsyourwormrisk.com) simplifies the risk assessment process by asking nine straightforward questions about each horse's profile. Based on the answers given, this tool categorises worm infection risk as low, moderate or high, and offers recommendations for improving parasite control in the individual and their cohort, including:
How to use monitoring tools to determine the need for treatment and assess resistance.
Management strategies to reduce the risk of infection.
Diagnostic tests
Most decisions regarding worming should be based on diagnostic testing. However, in certain high-risk situations, a strategic worming treatment approach may be more suitable as the available tests do not provide enough information to guide treatment decisions. Such situations include administering strategic treatments to young foals for ascarid infections and treating horses identified as being at high risk of significant small redworm encysted larval burdens. In all other cases, diagnostic tests should be used to inform treatment decisions. These tests include those that estimate worm egg numbers in dung (FECs, Fig. 4) and those that measure antibody responses to worms (Fig. 5).
FEC tests mainly determine a horse’s potential for contaminating paddocks with ascarid or strongyle eggs, making these tests especially useful for horses that have access to grazing. In low-risk groups, 80% or more of horses shed only 20% of the eggs being shed, indicating that many horses do not require treatment. Horses should be FEC tested every 12 weeks. Treatment thresholds are indicated in Table 2. For horses that need worming, a follow-up FEC test should be done two weeks after treatment to evaluate the effectiveness of the wormer. FEC tests only detect the presence of egg-laying adult female worms; they do not provide information about the number of immature or male worms present and are greatly affected by the sporadic egg shedding behaviour of tapeworms. To address these limitations, antibody tests are available for detecting small redworm and tapeworm infections.
It was previously recommended that all horses be treated for encysted small redworm larvae during late autumn/winter to reduce the clinical impact of these parasite stages. However, this treatment is now advised only for high-risk horses. Routine treatments in low-risk horses contribute to resistance. Veterinary surgeons can assess small redworm burdens in individuals using the Small Redworm Blood Test (Austin Davis Biologics). This test measures specific antibodies to determine if horses have low worm counts that do not require treatment. Studies indicate that, using this test, high proportions of low-risk horses, such as sport horses, harbour low burdens of small redworms [6].
Tapeworm antibody tests are available in two formats: the EquiSal saliva test and the Tapeworm Blood Test (both Austin Davis Biologics). These measure specific antibodies that indicate the level of tapeworm infection and reliably identify horses with over 20 tapeworms (burdens above this level have the potential to cause disease [7]). Horses should be tested for tapeworm once or twice a year, depending on the assessed risk level and ongoing test results. Many horses do not need regular tapeworm treatments; analysis of over 160,000 EquiSal results revealed that over two-thirds of horses tested did not need worming [6]. If multiple horses do test positive on a yard, it is important to identify the source of the infection and improve management practices to reduce transmission. All horses should be tested at the same time. It is advantageous to combine small redworm and tapeworm testing, as this will provide the best information for selecting the appropriate worming product. Since antibodies take time to decrease after treatment, horses should not be tested until four months (blood tests) or three months (saliva test) have passed.
Management Strategies
For horses with access to grazing, it is essential to implement management practices that minimise the risk of infection. Daily removal of dung from paddocks is recommended, and dung should be disposed of well away from the grazing area and water sources. Maintaining a low stocking density will further reduce worm transmission, with at least one acre of grazing land recommended per horse. For groups of horses that have rest periods at grass (for example, national hunt horses in summer), it is essential to evaluate their worm load before they are turned out to grazing and when they return to training. This can be done by conducting FEC or small redworm testing and tapeworm testing.
Quarantine Recommendations
A high-risk circumstance for training yards is when new horses are introduced or horses return from periods at other sites. The traditional practice of treating all arrivals with a broad-spectrum wormer is outdated and should be avoided. Instead, new horses should be isolated, tested (using FEC or small redworm blood tests and tapeworm tests), and wormed based on the results. The effectiveness of treatments should be assessed by conducting a FEC test two weeks after worming. Arrivals should be kept off grazing areas until test results are known and, if treated, for three days after worming.
In conclusion
Studies have demonstrated that there is widespread and unnecessary overuse of worming products in the racing sector [8]. This needs to change because of the increased threat of wormer resistance in the most important parasites of horses. Working closely with their veterinary surgeon, trainers can develop effective worm control strategies that help safeguard the effectiveness of these important medicines, especially as no new wormers are coming to market in the foreseeable future.
References
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