Diagnostic-led worm control

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Notes:  FEC (faecal egg count) tests assess levels of egg contamination onto pasture and do not provide information on the level of burden within horses, especially when there are high proportions of immature (larval) worms present.

Tapeworm tests are available in saliva and blood formats. Small Redworm Tests are available in blood format. Worm-specific antibodies measured by these tests have been shown to correlate with worm levels at certain burden thresholds. 

All horses in a group should be tested so that all individuals with high egg shedding or high antibody levels can be identified for treatment. For worm control programmes for young horses and foals, please consult with your veterinary surgeon or SQP. 

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Intestinal worms are common in horses. The commonest types are:

  • Small redworms (cyathostomins)
  • Tapeworms (Anoplocephala perfoliata)

Horses are infected with worms when they eat immature worms (larvae) whilst grazing – small redworm larvae are ingested directly with grass, whilst tapeworm larvae are ingested within mite intermediate hosts that live on grass. The diagram below summarises a lifecycle of parasitic worms and key factors relevant to their control.

 
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Although worms can cause serious disease, clinical signs are only observed in horses with higher worm burdens. Most well managed, healthy horses (5-20 years old) have low worm burdens without disease. This means that most horses (~2/3) in this age group do not need repeated ('interval') worming treatments. Horses older than 20 may be more susceptible to developing larger worm burdens and therefore be at increased risk of disease.

It is important to consider distributions of worms in horses of different ages because worms become resistant to wormers with overuse, meaning that they survive the killing effect. This is an issue in all common worm types as summarised below: 

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No new wormers are coming to market so control must not rely on these medicines alone. Wormers should only be used when needed and worm control programmes must include:

  • management strategies to reduce worm infection levels in the environment
  • diagnostic testing to identify which horses require treatment

 

Management strategies

As indicated above, management strategies that reduce contamination with worms and worms in mites on pasture are key to effective parasite control. The most effective way to reduce contamination is to fully remove dung from paddocks at least once a week. Dung should not be placed in adjacent areas to paddocks as worm-infected mites and worm larvae can migrate from heaps back onto grazing areas. Fresh dung must not be spread onto grazing paddocks and dung heaps should be placed well away from water courses. Harrowing paddocks for worm control is not recommended in the UK.

It is important to maintain low stocking densities; ideally, more than one acre per horse. Cattle and sheep can be used to help reduce infection levels on paddocks. It is of most value to use these species in the first half of the grazing season whilst resting the paddock from horses, but it is important to consider liver fluke as this worm can cross between animal species. Speak to your veterinary surgeon if this is a concern.

Pasture resting methods can be used in the absence of sheep or cattle as many (small redworm) larvae that contaminate paddocks in one season die out by the middle of the following summer.  This is not the case for tapeworm larvae in mites or eggs of other worms (i.e. ascarids), which may persist for over a year.

 

Diagnostic testing

Diagnostic tests that can be used to inform worming treatments include:

  • faecal egg counts (FEC) for estimating egg shedding levels in dung of small redworm and other nematodes (other types of roundworms)
  • antibody tests for providing information on tapeworm and small redworm burdens.

Use of testing can considerably reduce worming treatment frequency. Tests must be used as part of an integrated worm control programme. By combining excellent management with diagnostic-led treatments, considerable savings in wormer use can be made. This will reduce the risk of resistance emerging in the worm populations on site. The interactions between management, diagnostics, wormer use and resistance selection are summarised in the diagram below.

 
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Good worming practice

When worming horses, always ensure that the dose is calculated based on an accurate estimate of the animal’s weight as under-dosing can speed up the development of resistance. It is important to ensure that the correct type of wormer is given for the worms being targeted. For example, if only tapeworm infection is diagnosed, use a wormer specific for tapeworm (i.e. praziquantel) and not a combination wormer (containing more than one drug type) or a broad spectrum wormer that also target small redworm.