All horses are exposed to worms while grazing, but how we control these parasites is essential to horse health and performance.

 

Most horse owners are aware of testing to determine whether their horse needs deworming. The tests comprise of faecal worm egg counts (FEC) for redworm/roundworm detection and EquiSal Tapeworm saliva testing to detect tapeworm infections (standard FEC methods are unreliable for tapeworm). Until now encysted small redworm larvae have remained undetectable as FEC only determines the presence of egg laying adult worms. This has meant that routine winter moxidectin treatment has become recommended practice to target potentially life-threatening burdens of small redworm encysted larvae. Excitingly a new small redworm blood test is being commercialised* which detects all stages of the small redworm life cycle, including the all-important encysted larval phase. Together, these tests offer a complete worm control programme for common horse worms using diagnostic information, this is known as ‘diagnostic-led worm control’ (see Figure 1 for common worms in horses). Essentially, testing is used to tell you whether your horse needs deworming or not.Most horse owners are aware of testing to determine whether their horse needs deworming. The tests comprise of faecal worm egg counts (FEC) for redworm/roundworm detection and EquiSal Tapeworm saliva testing to detect tapeworm infections (standard FEC methods are unreliable for tapeworm). Until now encysted small redworm larvae have remained undetectable as FEC only determines the presence of egg laying adult worms. This has meant that routine winter moxidectin treatment has become recommended practice to target potentially life-threatening burdens of small redworm encysted larvae. Excitingly a new small redworm blood test is being commercialised* which detects all stages of the small redworm life cycle, including the all-important encysted larval phase. Together, these tests offer a complete worm control programme for common horse worms using diagnostic information, this is known as ‘diagnostic-led worm control’ (see Figure 1 for common worms in horses). Essentially, testing is used to tell you whether your horse needs deworming or not.

Why should you use testing to determine whether you should use dewormers or not?

Gone are the days of routinely administering dewormers to every horse and hoping for the best. This strategy is out-dated and has caused widespread drug resistance in worms - meaning that worms are able to survive the killing effects of dewormers, which can lead to disease and in worst cases, death. To reduce the risk of further resistance occurring, we need to ensure that dewormers are only used when they are genuinely needed – when testing detects that horses have a worm burden requiring treatment. Regular testing also helps identify horses likely to be more susceptible to infection and thus at risk of disease in the future.

How to plan your horse’s worm control programme

Tapeworm programme

Recently published UK Vet deworming guidelines state that blood or EquiSal Tapeworm saliva testing should be conducted to detect tapeworm burdens. The publication states, “The traditional approach to routinely treat for tapeworms annually or six-monthly without diagnostic testing, is obsolete. “Treatment should only be administered to adult horses in response to positive serum or salivary antibody testing.” Figure 2 highlights that testing should be carried out every six months, preferably during spring and autumn. The saliva test provides a low, borderline or moderate/high diagnosis and deworming is recommended for horses diagnosed as borderline or moderate/high. 

Redworm and roundworm programme

FEC analysis should be carried out for measuring worm egg shedding throughout March to October (Figure 2) for detection of adult redworm (strongyles, including small redworm) and roundworm (ascarids). It is recommended that at least three FEC tests should be conducted to monitor for egg shedding in spring and summer periods. Horses with high egg shedding (for example, those excreting >200 eggs per gram of dung) should be treated with a dewormer, ideally effective against adult strongyles.The new small redworm blood test can now be used to detect the presence of small redworm (including encysted larval stages) at a time when previously an annual treatment with moxidectin was recommended. The optimum testing period for these small redworm stages is September to December, but if this is missed, testing should still be considered until April the following year (Figure 2). 

Before the small redworm blood test can be carried out, your veterinarian will assess each horse’s risk of small redworm infection. If signs of active infection are evident, such as previous and consistent high FEC results, or is in a high-risk environment (for example, high herd turnover, high exposure levels and poor paddock management), they may recommend treating with moxidectin without testing. To get your horse blood tested, contact your veterinary practice for advice. They can sign up for the testing service by contacting This email address is being protected from spambots. You need JavaScript enabled to view it. 

worm

Figure 2. Seasonal diagnostic-led worm control programme that can be put into place for most adult horses here in the UK

The most important worms that infect horses are the strongyles (redworms), classified as large and small redworm. Large redworms used to be important as they can cause life-threatening colic, but in the UK, these are now uncommon in adult horses as they are very sensitive to the most widely used dewormers, moxidectin and ivermectin. On the other hand, small redworms (cyathostomins) are extremely common and most grazing horses are infected. Healthy adult horses usually have low burdens, but when large burdens (several million worms) occur, horses can develop a fatal colitis which is difficult to treat. Unfortunately, small redworms are expert at developing resistance to dewormers. Control plans now need to balance the requirement to ensure horses do not build up extreme burdens with the need to preserve dewormer effectiveness. It is especially important to preserve moxidectin dewormer for strategic use against the encysted small redworm larvae that cause life-threatening disease as this is the only compound that has a high effect against these larvae. The most important worms that infect horses are the strongyles (redworms), classified as large and small redworm. Large redworms used to be important as they can cause life-threatening colic, but in the UK, these are now uncommon in adult horses as they are very sensitive to the most widely used dewormers, moxidectin and ivermectin. On the other hand, small redworms (cyathostomins) are extremely common and most grazing horses are infected. Healthy adult horses usually have low burdens, but when large burdens (several million worms) occur, horses can develop a fatal colitis which is difficult to treat. Unfortunately, small redworms are expert at developing resistance to dewormers. Control plans now need to balance the requirement to ensure horses do not build up extreme burdens with the need to preserve dewormer effectiveness. It is especially important to preserve moxidectin dewormer for strategic use against the encysted small redworm larvae that cause life-threatening disease as this is the only compound that has a high effect against these larvae. 

The most common horse tapeworm, Anoplocephala perfoliata, predominantly attaches to the ileocaecal junction in the gut, a narrow section between the small intestine and the caecum, so it is no surprise that large burdens can cause health problems, including significant gut inflammation and blockages, leading to either acute or low grade recurring colic symptoms. 

Roundworms (ascarids) are more common in young stock and can be detected using FEC.

As no new dewormers are being developed for horses in the foreseeable future, control plans must aim to maintain horse health whilst protecting the currently effective dewormers. You should work with your veterinary surgeon or a Suitably Qualified Person to design and implement an appropriate worm control plan.

For More Information Please Visit:www.equisal.com or www.austindavis.co.uk 

Note: Moxidectin is the only licenced drug to effectively kill encysted stages. Although 5-day fenbendazole treatment is licensed to kill encysted stages, there is widespread resistance to this product so without knowing the worm population sensitivity status to this type of dewormer, its use would not be recommended.*Commercialised by Austin Davis Biologics Ltd, providers of EquiSal Tapeworm testing and developed by Prof. Jacqui Matthews’ group at Moredun Research Institute (funded by The Horse Trust).

Article By Dr Corrine Austin By Dr Corrine Austin - Austin Davis Biologics & Prof Jacqui Matthews - Roslin Technologies