Notes: Faecal egg count (FEC) tests evaluate the extent of worm egg contamination on pastures but do not provide accurate information on the internal burden levels in horses, particularly, the presence of immature (larval) worms.
Tapeworm tests can be conducted using saliva or blood samples. Small Redworm Tests are performed using blood samples. The presence of worm-specific antibodies, as detected by these tests, correlates with the levels of worm infection at certain burden thresholds.
It is essential to test all horses in a group to identify and treat those with high levels of egg shedding or elevated antibody levels. For worm control programmes targeting young horses and foals, seek guidance from a veterinary surgeon or a Suitably Qualified Person (SQP).
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Background to worm control in horses:
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Intestinal worms are common in horses. The commonest types are:
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Small redworms (cyathostomins)
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Tapeworms (Anoplocephala perfoliata)​
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Horses become infected with worms by consuming immature worms, or larvae, while grazing. Small redworm larvae are directly ingested along with the grass, whereas tapeworm larvae are consumed within mite intermediate hosts that live on paddocks. The following diagram outlines the lifecycle of parasitic worms and the crucial factors for their management.
While worms can lead to severe illness, only horses with significant worm infections typically exhibit clinical symptoms. The majority of well-cared-for, healthy adult horses over five years-old carry low worm burdens and remain disease-free. Consequently, most adult horses (about two-thirds) do not require frequent worming treatments. This is crucial since worms can develop resistance to wormers through persistent use, allowing them to withstand the lethal effects of these medicines. Resistance is a problem across all prevalent worm species, as outlined below:
As no new wormers are being introduced to the market, control strategies cannot depend solely on these medicines. Wormers should be administered only when necessary, and worm control programmes must incorporate paddock management strategies to lower worm infection levels in the environment and diagnostic testing to determine which horses need treatment.
Management strategies:
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Effective parasite control relies on management strategies that minimize contamination of worms and mites on pastures. The most effective method to reduce contamination is complete removal of dung from paddocks at least twice weekly. It is crucial not to deposit dung near paddocks, as mites infected with worms and worm larvae can travel from the dung heaps back to the grazing areas. Fresh dung should never be spread on grazing paddocks, and dung heaps must be situated far from watercourses. In the UK, harrowing paddocks for worm control is not advised.
Maintaining low stocking densities is an important component of effective worm control, with more than one acre allocated per horse being optimal. Utilizing cattle and sheep can aid in lowering infection levels on paddocks. Using these animals during the first half of the grazing season to rest the paddocks from horses is beneficial. However, it is vital to be mindful of liver fluke, as this parasite can transfer between different animals. If this is a concern, consult your veterinary surgeon.
Pasture resting can be effective without sheep or cattle because many small redworm larvae that infect paddocks in one season will die by the middle of the next summer. However, this does not apply to tapeworm larvae in mites or eggs of other worms, such as ascarids (commonly referred to as roundworm), which can survive for more than a year.
Diagnostic testing:
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Diagnostic tests that are useful for guiding worming treatment decisions include:
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- Faecal egg counts (FEC) which estimate the level of egg shedding in the dung of individuals. These tests can be used to assess shedding of eggs of small redworms and other worms such as ascaris (roundworm).
- Antibody tests that provide information on the burden of tapeworms and small redworms.
Implementing testing can significantly decrease the frequency of worming treatments. Through integration of excellent pasture management and worming treatments guided by diagnostics, substantial reductions in the usage of wormers are achievable. This approach reduces the likelihood of resistance developing within worm populations.
The relationship between management practices, diagnostic measures, wormer applications, and selection for resistance is detailed in the diagram below.​
Good worming practice:
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When administering wormers to horses, it is crucial to calculate the dosage accurately based on the horse's weight to prevent accelerating resistance due to under-dosing.
It is important to use the appropriate wormer for the type of worms present. For instance, if a tapeworm infection is confirmed when there is no evidence to treat for other worms such as small redworm, a wormer that specifically targets tapeworms, such as praziquantel, should be used rather than a combination wormer or a wormer that has an effect on non tapeworm species. In this way, unnecessary resistance will be avoided.