As grazing animals, horses can be infected by a wide range of gastrointestinal parasites including roundworms, tapeworms and bots. Horses grazing on pasture will be repeatedly exposed to these infections and even horses kept indoors without access to grass may be exposed to intestinal roundworm infection. As a result, the monitoring, treatment and reduction of parasite burdens in horses is a vital part of equine preventative health care. Successful parasite management in horses requires cooperation between veterinary professionals, stable managers and owners. Several parasites need to be considered. This article will discuss the parasites, diagnosis and treatment for each.
Small strongyles (red worm)
Infections with small strongyles is common, with horses becoming infected on pasture through the uptake of infective third-stage larvae (L3). The larvae then develop in the intestinal mucosa before they re-enter the intestinal lumen. Infections in horses kept indoors are rare and when they do occur, red worms are not present in sufficient numbers to cause clinical disease. Small burdens are generally well tolerated but large numbers of red worms can damage the intestinal mucosa and result in emaciation, diarrhoea and colic. When large numbers of larvae lie dormant in the mucosa then larval cyathostominosis can occur, a syndrome in which synchronised remergence of numerous L3 larvae results in massive tissue destruction. This disease is mostly seen in animals up to 6 years of age, and results in acute and persistent diarrhoea, which is sometimes accompanied by colic, weight loss or fever. Death is oft en the result.
Diagnosis
Diagnosis of patent small strongyle infections is performed by faecal examination and identification of strongyle-type eggs that measure approximately 80–100 μm in length (Figure 1). Oft en larvae and adult worms are found in large numbers in the faeces of treated horses (Figure 2). There are various faecal examination methods for the identification of strongyle eggs and quantifying their numbers.
Faecal flotation allows the presence of eggs to be determined. There are many different flotation methodologies but one example is:
- The faecal sample is mixed with water (approximately 10 ml per gram of faeces) and stirred to form a suspension
- The suspension is then poured through a strainer to remove very large particles and decanted into centrifuge tubes. These can either be centrifuged at 1500 rpm for 15 minutes or allowed to stand for 20 minutes
- The water is then removed and the tubes half filled with flotation solution and mixed
- Each tube is filled with flotation solution until a meniscus is formed at the top of the tube
- A cover slip is then added, contacting the flotation solution
- The tubes are again centrifuged for 10 minutes or allowed to stand again for 30 minutes. When tubes are spun with cover slips in place, care should be taken not to open the centrifuge before it stops spinning, or the cover slips can shift and ruin the preparation
- Once these steps are complete the cover slips are lifted off the tubes and examined under the microscope.
The flotation solution can either be sugar solution or zinc sulphate (specific gravity = 1.2). Zinc sulphate is less sticky and easier to handle but slides tend to dry out and crystallise very quickly. Distortion of ova can also occur making identification more difficult.
The McMaster technique allows quantification of the number of eggs per gram (epg) of faeces. Again, there are many different variations, but one example is:
- Mix 3 g faeces per 15 ml of water through a strainer
- Pour the strained material into a 15 ml centrifuge tube and centrifuge at 1500 rpm for 2 minutes
- Mix the sediment in 10 ml of flotation solution and pour into a beaker. Then add an additional 32 ml of flotation solution
- With a pipette, transfer the suspension to a McMaster counting chamber and fill both chambers
- Transfer the slide to a microscope and count all the eggs inside the ruled squares
- Multiply the numbers of eggs in both chambers by 50 for the total eggs per gram of faeces.
Samples may also be sent to commercial labs for flotation. Several recent developments have provided methods or protocols with improved sensitivity including the FLOTAC and mini-FLOTAC methods, which have a sensitivity of 1 and 5 epg, respectively.
Faecal testing should be performed on all horses showing relevant clinical signs that may indicate small strongyle infection. It is also useful to monitor horses' faeces for eggs to identify those shedding large numbers in their faeces. Although there is no evidence correlating numbers of strongyle eggs with the number of adult worms, calculating the number of eggs per gram of faeces gives an indication of the extent to which individual horses may be heavily contaminating pasture with strongyle eggs. Horses shedding large numbers of eggs (>200 epg) and those clinically affected should be treated for infection.
Annual faecal egg reduction tests are important to monitor for resistance. These consist of testing faecal egg counts before and after treatment to monitor its efficacy.
Treatment
Horses first become infected with small strongyles as soon as they start grazing and start to shed strongyle eggs 6–14 weeks after infection. Therefore, foals should first be treated at 2-3 months of age. In the UK, where the intensity of small strongyle infection is low to moderate, a further treatment 2–3 months later should be sufficient to prevent clinical signs developing. In adult horses it may be feasible to treat only once yearly, provided that faecal monitoring does not indicate further treatment.
Horses with larval cyathostominosis should be treated symptomatically for diarrhoea, gut inflammation and dehydration, if required. Irrespective of clinical status, all horses of the same group should receive anthelmintic treatment against the mucosal worm burden using a licensed product containing moxidectin (0.4 mg/kg bodyweight once orally only in horses more than 4 months old) or fenbendazole (7.5 mg/kg bodyweight orally once daily for 5 days). At the end of the grazing season, horses up to 6 years of age should be treated with moxidectin for encysted larval small stronglyes. Some UK studies have shown small strongyle anthelmintic resistance to benzimadazoles (eg fenbendazole) and pyrantel (Relf et al, 2014) and some evidence for reduction of efficacy in ivermectin and moxidectin as well (Tzelos et al, 2017; Molena et al, 2018). Therefore, it is advisable to regularly test the efficacy of any anthelmintic drug class used by performing annual faecal egg count reduction tests.
Large strongyles
The large strongyles Strongylus vulgaris, Strongylus edentates and Strongylus equinus are migratory worms whose adults are found in the large intestine. This migration includes the anterior mesenteric and nearby arteries (S. vulgaris), the liver to the subperitoneal connective tissues (S. edentatus) and the liver, pancreatic and renal region (S. equinus). The damage caused by the migrating larvae leads to severe pathological consequences and clinical signs which differ depending on the Strongylus species involved. S. vulgaris is the most severe, with thromboembolic colic caused by larvae migrating to the cranial mesenteric artery. Depending on the numbers of migrating worms, initial clinical signs of nonstrangulating intestinal infarctions may be mild with recurrent colic, fever and peritonitis. If the infarcted intestine is not recognised and surgically resected, the intestine will necrotise and rupture with a fatal outcome. Sometimes even horses with severe intestinal necrosis caused by thrombosis do not show signs of serious pain and peritonitis is often the only sign advocating surgical intervention. Adult Strongylus species can cause damage to the intestinal mucosa leading to diarrhoea, weakness, emaciation and sometimes anaemia.
Diagnosis and treatment
Small and large strongyle eggs cannot be distinguished morphologically. Faecal cultures of strongyle eggs to obtain L3 stages can be performed with faeces being placed in plastic cups and incubated at a temperature of 25–27ºC and relative humidity of 80–100%. After 14 days, L3 larvae can be identified to genus/species level by using morphological keys, or taking images and sending to experts for identification. If culture is not carried out, the presence of strongyle eggs along with relevant clinical signs warrants treatment. Given the severity of infection, routine preventative treatments are desirable to prevent clinical infections occurring. To date there is no evidence of drug resistance in large strongyles and treatment twice a year with a macrocyclic lactone should be sufficient for control.
Equine roundworms
Parascaris equorum is a an ascarid parasite living in the small intestine. Like most ascarids, adult worms are large, reaching up to 50 cm in length, and are found predominantly in foals and young horses. Female worms shed the eggs via the faeces into the environment, and can shed hundreds of thousands of eggs per day, leading to large numbers rapidly building up in the environment, which can persist for many years. Once in the environment the eggs mature into the infective embryonated stage containing an L3 larva. When these eggs are ingested larvae are released and penetrate the small intestinal wall. They then migrate through the liver, heart and lungs, before returning to the small intestine. The parasite is often well tolerated with no clinical signs developing. The migration though the lungs, however, can lead to coughing, reduced weight gain and secondary bacterial infections. Large intestinal burdens can also lead to a reduced appetite, rough coat and intermittent colic. Rarely, more severe colic and intestinal obstruction can occur. Adult mares also occasionally shed eggs in their faeces, acting as a source of infection for subsequent generations of foals.
Diagnosis and treatment
In heavy infections, adult worms may be expelled in the faeces. Otherwise, eggs can be detected by faecal flotation. Eggs are typically ascarid, with thick shells, brown colour and approximately 100 μm in diameter. Because clinical signs are a function of worm burden and eggs rapidly build up in the environment, all horses found to be shedding eggs must be treated. Macrocyclic lactones are effective against migrating larvae as well as adults, and traditional recommendations have been to use these products every 6–8 weeks until horses are 1 year old. This prevents environmental contamination and the accumulation of large worm burdens. However, resistance to macrocyclic lactones is now becoming widespread (Relf et al, 2014). As a result, regular feecal monitoring is advised alongside treatment with fenbendazole or pyrantel at 2–3 months of age and then at 4-5 months old. The drug class used for each treatment should be alternated and supported by good stable and pasture hygiene. Clinically affected animals should also be treated with fenbendazole or pyrantel. Care must be taken in cases of heavy worm burdens because obstruction of the small intestine can occur as the worms die.
Tapeworms
Anoplocephala perfoliata and Anoplocephala magna are tapeworms of clinical significance in horses, with A. perfoliata being the most common. Infection occurs mainly in the second half of the grazing season on pasture though the accidental ingestion of tiny oribatid mites. Adult tapeworms are 4–8 cm in length (Figure 3) and are found in the caecum close to the ileocaecal junction (A. perfoliata) and small intestine (A. magna). Large A. perfoliata burdens can lead to colic as a result of bowel inflammation, ileac impaction and in intussusception. Infections with A. magna are well tolerated with few, if any, clinical signs.
Diagnosis and treatment
Praziquantel is the treatment of choice for tapeworm in equines. Horses should be tested every 6–12 months. If clinical signs are present it must be determined whether treatment is required. The sensitivity of faecal flotation for detection of tapeworm eggs is poor and if faecal examination is to be performed, combined centrifugal sedimentation-flotation techniques are required. These process large volumes of faeces (15–50 g). If faecal examination is used to detect tapeworms, all horses with access to the same pasture should be tested and all given treatment if tapeworm eggs are found in any samples.
Commercial ELISA tests are now available that detect antibodies to A. perfoliata in serum or saliva. These tests are more sensitive than faecal examination for detection of the parasite but can generate false positives as antibodies to the parasite can last up to 4 months after exposure. However, as long as this is taken into account these tests are very useful for screening horses for parasite exposure and testing individuals with relevant clinical signs.
Bot flies
Gasterophilus species are known as bot flies and Gasterophilus intestinalis, G. haemorrhoidalis and G. nasalis frequently infest grazing horses. The larvae of these flies in the digestive tract of horses is known as gastrointestinal myiasis. The adult flies resemble bees and lay eggs in late summer on the hairs of horses around the shoulders, forelimbs, flanks and head. The eggs are yellowish in colour, small (1–2 mm), operculate and visible to the naked eye. L1 larvae emerge from the egg and are ingested through licking of the coat or they migrate though the soft tissue of the oral cavity. The larvae then moults in the stomach (G. intestinalis), duodenum (G. nasalis, G. haemorrhoidalis) or rectum (G. haemorrhoidalis) twice before the L3 larva is passed in the faeces. The larvae pupate in the soil and adults emerge in June/July, remaining active until October or November depending on environmental conditions.
Even large burdens of bots can be well tolerated without clinical signs but nevertheless pathological changes with subsequent clinical signs can occur. The larvae can cause focal superficial ulceration where they attach and feed on the mucosal lining of the stomach or intestine. Migrating larvae in the oral cavity and throat can also cause gingivitis, localised pain and difficulty swallowing. In rare cases, volvulus, rectal prolapses, rupture of the gastrointestinal tract, peritonitis and anaemia can occur.
Diagnosis and treatment
Gasterophilus eggs can easily be detected in the coat and larvae confirmed in the gut of of horses demonstrating relevant clinical signs by endoscopy. The larvae are particularly susceptible to macrocyclic lactones and if this drug class is not already being used for helminth control, then a strategic treatment in late autumn should remove all larvae present. The removal of the eggs with a special bot comb knife is also useful. If specialist equipment is not available, then thoroughly washing the coat with warm water mixed with an insecticide can be useful. The low buzzing noise of adult flies can cause significance distress to horses. If this is the case, fly repellents are required.
Threadworms
Strongyloides westeri are roundworms of the small intestine and patent infections are most commonly found in foals under 6 months old. Older horses can harbour the infection with mares acting as a source of infection for their foals via their milk. This parasite is unusual in that only female worms are parasitic and it has a complex reproductive free living cycle outside of the host.
In the intestines, small adult female worms (up to 1 cm long) pass thin-shelled, embryonated eggs (40–50 μm x 30–40 μm) containing first-stage larvae (L1). These hatch in the environment and develop into either infective L3 larvae or free living adult males and females. Adult Strongyloides westeri in the environment will reproduce, producing infective L3 larvae. Horses may be infected via transmammary infection (the most common route of infection in foals), ingestion of L3 larvae on pasture or via percutaneous infection. If percutaneous infection occurs in immune competent adult horses, larvae rarely migrate to the digestive tract but instead migrate to somatic tissue where they act as a reservoir of infection for lactating mares.
Infection is often well tolerated but large burdens can lead to diarrhoea, anorexia and lethargy in horses. Large percutaneous challenge can also lead to dermatitis and loss of coat quality.
Diagnosis and treatment
Eggs can be detected by faecal flotation and S. westeri should be considered as a differential when investigating diarrhoea and skin disease in young horses.
Treatment and control of infections consists of both anthelmintic treatment and good horse and environmental hygiene. Where stables and breeding establishments have a history of clinical problems associated with S. westeri infection, mares should be treated with an anthelmintic before or shortly after parturition to reduce the number of larvae in milk. Where clinical signs occur, foals can be treated with ivermectin or fenbendazole. Fenbendazole doses are much higher for treatment of S. westeri than for other intestinal roundworms, with doses up to 50 mg/kg required. Good stable and pasture hygiene are important to reduce environmental larvae, as well as cleaning mares udders to help reduce larval transfer.
Pinworms
Infections of pinworm (Oxyuris equi) are common in stabled horses but can also occur sporadically on pasture. They live in the large intestine from where adult female worms deposit tens of thousands of eggs onto the skin of the perianal region. Infection is well tolerated although large burdens may lead to fatigue and failure to thrive. The most important consequence of infection is that the eggs on the perineum can lead to intense pruritus leading to excoriation and alopecia around the tail head.
Diagnosis and treatment
Faecal flotation carries a poor sensitivity for diagnosing infection but adhesive tape applied to the skin of the perianal region and then examined under the microscope will reveal numerous pinworm eggs in clinically affected horses. These are very distinctive with an operculum and flattened on one side.
Treatment consists of an anthelmintic and washing the perianal region of infected horses with warm water to prevent dissemination of eggs into the environment, and to relieve irritation. Petroleum jelly applied to the perianal region is useful to make worm movement and egg laying more difficult. Macrocylic lactones and benzimadazoles are both affective at treating infected horses.
Horse roundworm control
Most roundworm pathology in horses is a function of the worm burden present and so measures to limit worm burdens are important, while not promoting anthelminitic resistance. This is achieved by environmental measures to reduce parasite burdens in the environment and preventative treatment of horses to reduce egg shedding.
Treating the environment
Stable and pasture hygiene are both important to reduce parasite eggs and larvae in the environment while minimizing the requirement for anthelmintics. Stables should be cleaned daily, mechanically steam cleaned annually and treated with a disinfectant that is effective against ascarid eggs. All other worm eggs and larvae are adversely affected by low humidity and desiccation, so stables should be kept as dry as possible. Faeces must be picked up from the pasture at least twice a week and sometimes daily, depending on the number of horses. Stocking density should be kept to a minimum where possible and rotational/cograzing with ruminants is useful in reducing parasite contamination of pasture. Because of the longevity of ascarid eggs, foals should not be grazed on the same pasture for consecutive seasons.
Preventative anthelmintic treatment
Underdosing and frequent treatment with anthelmintics are thought to be the most common reasons for anthelmintic resistance developing. The aim of any preventative treatment strategy in horses should be to minimise treatments needed while preventing disease. To prevent novel or resistant parasites being introduced to stables, new horses should be treated and quarantined on arrival. Faeces should then be tested 10–14 days later to ensure that no egg shedding is occurring before the horses are released onto pasture.
Selective or strategic treatment approaches can be used to control worm burdens in horses. Selective treatment uses faecal egg counts to ascertain which horses to focus treatment on. Strategic dosing treats all horses but at a minimum requirement to prevent disease based on the lifecycle, likely immunity and climatic conditions. Both strategies are effective in preventing clinical disease. Selective strategies are used against small strongyles in adult horses. A combination of strategic and selective treatment is used in foals and young horses where immunity to helminths is still developing but limiting anthelmintic resistance is still important.
Selective treatment for small strongyles in adult horses
This strategy involves testing the faeces of all adult horses on pasture or in stable and treating those shedding high numbers of strongyle eggs in the faeces (>200 epg).
Faecal samples from each horse should be examined at least four times in the first year to identify high shedders. In subsequent years this can then reduced to three times (beginning, middle and end of the grazing season). The aim of selective treatment is to reduce environmental contamination by treating horses shedding large numbers of eggs while allowing small numbers of eggs to contaminate pasture. The presence of environmental contamination with parasitic life stages not exposed to anthelmintic is known as refugia and limits resistance by reducing anthelmintic selection pressure on the worm population. However, care must be taken when using selective treatment with large strongyle infections. These are uncommon since frequent anthelmintic treatment in horses was widely adopted but are still thought to be present in the UK. Even small numbers of large strongyles can cause significant pathology and therefore all horses should be treated with a macrocyclic lactone or fenbendazole in the autumn for large strongyle control. Horses up to 6 years of age should be treated with moxidectin at the end of the grazing season to limit the risk of larval cyathostomosis. This treatment also helps to limit the risk of large strongyle infection.
Selective treatment strategies reduce the number or horses treated and in theory should limit resistance through increased refugia. However, it has yet to be proven that applying this strategy to adult horses alone has a significant impact, as foals and yearlings are the source of the majority of strongyle egg output.
Treating foals and young horses
Treatment of foals and young horses every 4–8 weeks is no longer appropriate because of the risk of anthelmintic resistance. Strategic treatments with fenbendazole or pyrantel at 2–3 months old and then 4–5 months old are recommended, followed by faecal monitoring to allow selective treatment of young horses that are shedding large numbers of ascarid and/or strongyle eggs.
Advice for vet nurses, suitably qualified persons and owners
Parasite control decisions need to be made in discussion with stables and horse owners to maximise compliance and ensure that all parties are able to carry out advice and understand the benefits of doing so. Part of this process is a consistent message from veterinary practices and drug suppliers. It is therefore vital that veterinary nurses and suitably qualified persons (SQPs) understand the reasoning behind control advice so this can be communicated effectively. Recommended preventive measures, routine monitoring and regular deworming practices should be made clear to horse owners by veterinarians, veterinary nurses and SQPs. Surveys of horse owners indicate an interest in deworming strategies but that horse owners are more likely to take preventative treatment guidance from a variety of sources, such as fellow horse owners and social media rather than professional structured advice (Stratford et al, 2014).
Anthelmintics are available from a variety of outlets and, contrary to guidelines, are often sold without investigation of which diagnostics have been performed and which product is most appropriate. There is an appreciation of anthelmintic resistance risk among horse owners but many do not believe it applies to their horses (Rose Vineer et al, 2017). Most horse owners will engage with faecal testing for parasites because of concern about the environmental impact of anthelmintics and the effect of worms on performance, and so it is important for nurses and SQPs to find reasons for testing and treatment that resonate with horse owners. It is important that owners appreciate the importance of dosing correctly according to weight and not ‘guestimating’. Holding conversations about preferred routes of administration and ensuring that the advice being given can be carried out by the client are vital in maximising compliance.
Conclusions
Horse health and welfare can be compromised by a wide range of gastrointestinal parasites. Anthelmintics have formed the basis of intestinal worm control in horses through routine use for many years but the development of anthelmintic resistance now means that strategies are required to limit anthelmintic use while maintaining minimal parasitic disease. Environmental management both in stable and on pasture form a vital component of this strategy. Convincing owners of the importance of accurate dosing, faecal testing and selective treatment are also vital, and veterinary nurses and SQPs have a vital role in communicating with horse owners and stable managers. Through vet nurses, SQPs and vets having the same fundamental understanding of the parasites involved and building relationships with stable managers and owners, worm control strategies can be implemented effectively.
KEY POINTS
- A wide variety of gastrointestinal parasites can impact on equine health.
- Preventative strategies traditionally relied on frequent routine treatment of horses with anthelmintics, now made unsustainable by anthelmintic resistance.
- New strategies to limit anthelmintic use while maintaining horse welfare rely on environmental management, accurate dosing and selective treating though faecal testing.
- Communicating the importance of these components in parasite control is vital if compliance is to be maximised.
- Veterinary nurses and suitably qualified persons (SQPs) play an essential role in parasite control plans and communicating key messages to clients.