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Parasitic Roundworms (Equine)

About Parasitic Roundworms (Equine)

Almost all grazing horses and ponies are exposed to intestinal worms (roundworms and tapeworms). The commonest types of worms that infect equids are the small strongyles (also known as cyathostomins). Although most horses have relatively low numbers of worms, certain individuals can develop high burdens. Animals that are not on an effective worm control programme can develop high levels of these parasites and, even in well managed populations, young, adolescent and geriatric horses are at risk of developing high burdens.

Horses and ponies with high burdens can develop clinical disease, including, weight loss, colic, and diarrhoea. In severe cases, some animals can die of worm related disease. It is therefore important that high worm burdens in individuals are avoided.

Effective dewormers (anthelmintics) have been available to treat and control worms for many decades; but drug resistance, particularly in the small strongyle group, to a number of these medicines is now widespread. This means that worm control needs to be balanced between the need to avoid high levels of worm infection and the requirement to maintain effectiveness of the dewormers that are currently effective.

The use of anthelmintics is only part of a best practice worm control programme. As worms are primarily transferred via dung and pasture, good pasture management is essential. More information on pasture management and the use of anthelmintics can be found in the ‘Further Information’ section below.

Watch our War of the Worms animation.

Key points

  • Intestinal worms are extremely common in grazing horses and ponies. Almost all grazing individuals become infected with parasitic worms; but in managed populations, most animals have low worm burdens.
  • Foals, adolescent and geriatric horses tend to develop higher worm burdens. Generally, the higher the burden, the higher the risk of clinical disease developing.
  • Dewormer resistance is an escalating issue, particularly in small strongyles and P. equorum
  • Good pasture management is essential and should always be used alongside chemical control measures
  • Practice a targeted treatment control strategy that utilises both FWEC-directed treatments (spring and summer) and specific (strategic) treatments to target worm stages not detectable by FWEC analysis (usually autumn and winter).
  • Enforce the correct quarantine policy with all new arrivals or those returning from other sites.
  • Work with a veterinarian or appropriately qualified animal health advisor to develop a site-specific worm control programme.

Research at Moredun

Current research:

  • Development of diagnostic blood and saliva tests for small strongyle worms
  • Development of decision support systems and end-user guidelines to support sustainable worm control in practice
  • Understanding hurdles to the responsible use of anthelmintics
    Anthelmintic resistance: prevalence, mechanisms and mitigation

Future work:

Equine Helminth Control Guidelines in collaboration with The Horse Trust

Equine Helminth Control Decision Support System in collaboration with The Horse Trust

Research funding

We thank the following organisations for generously funding the equine parasitology research via the following projects:

Horse Trust – A test for detection of equine small strongyles; supporting sustainable worm control in practice.

Horse Trust – Taking evidence-based control into the field: tools and training initiatives aimed at mitigating helminth-associated disease in horses.

DEFRA/Veterinary Medicines Directorate – A study to consider prescribing groups and distribution channels to ensure appropriate use of anthelmintics in the UK.

Horserace Betting Levy Board (PhD scholarship) – Development of tools to promote best practice parasite control on Thoroughbred studs.

Donkey Sanctuary – Improved sustainable parasite control programmes for donkeys: creation and validation of a general monitoring and control system.

Elise Pilkington Trust (PhD scholarship) – WormTrust: a decision support system for sustainable parasite control.

Horserace Betting Levy Board – Further development of an immunoassay for cyathostomin infection.

Donkey Sanctuary – Out of Africa: evaluation of African and UK plant extracts for control of GI nematodes.

Horserace Betting Levy Boar (PhD scholarship) – Immunology of cyathostomin infection.

Donkey Sanctuary – Quantifying comparative efficacy of sustainable control programmes using robust estimates of donkey faecal egg count variability.

Horserace Betting Levy Board – Investigation into the status of anthelmintic resistance in breeding Thoroughbreds in the UK.

Horse Trust – A survey of equine helminth control practices and anthelmintic efficacy.

Horse Trust – Moxidectin resistance in the Cyathostominae.

Donkey Sanctuary – Sustainable control of donkey helminth infections.

Horserace Betting Levy Board – Development of a immunoassay for larval cyathostominosis.

Home of Rest for Horses – Phenotypic and genotypic characterisation of moxidectin resistance in the Cyathostominae.

Horserace Betting Levy Board – Role of beta tubulin gene mutations in BZ resistance in cyathostomins.

Home of Rest for Horses – Proteomic characterisation of two antigen complexes for use in an immunoassay for larval cyathostomins.

BBSRC (PhD scholarship) – Molecular analysis of benzimidazole resistance in Cyathostominae populations.

Home of Rest for Horses – Mechanisms of arrest and reactivation in cyathostomes.

Home of Rest for Horses – Development of an immunoassay for the diagnosis of cyathostomes.

Horserace Betting Levy Board – Molecular studies on cyathostome species.

Home of Rest for Horses – Investigation into the role of pro- and anti-inflammatory cytokines in helminth-associated equine colitis. JM PI, with CJ Proundman (Liverpool).

Horserace Betting Levy Board – The development of a PCR-based assay for the diagnosis of cyathostome species.

Home of Rest for Horses – The development of species-specific DNA probes for the diagnosis of equine cyathostomosis.

Further information

The use of anthelmintics

Effective dewormers (anthelmintics) have been available to treat and control worms for many decades; but drug resistance, particularly in the small strongyle group, to a number of these medicines is now widespread. The work that we have performed previously has identified levels of resistance across different types of equine populations to different wormers in the UK and for some of the wormers, the likely genetic mechanisms behind resistance. Generally, benzimidazole resistance is almost ubiquitous in small strongyle populations and there is also moderate levels of resistance to pyrantel in these worms. Emerging resistance to the commonly used dewormers, moxidectin and ivermectin, is emerging. Resistance to the last two types of dewormers was also detected in the large roundworm of foals, Parascaris equorum. This means that worm control needs to be balanced between the need to avoid high levels of worm infection and the requirement to maintain effectiveness of the dewormers that are currently effective. Year-round, all-group interval treatments must be avoided.

It can be difficult to discriminate which horses or ponies are carrying higher worm burdens. Faecal worm egg counts (FWECs) can be performed to assess which horses are shedding the highest levels of eggs. The work that we (and others) have performed previously has identified how FWEC tests can be best run in practice to ensure that the optimum value can be obtained from them in practice. It is important to remember that an animal’s overall burden also includes immature developing worms, and these stages cannot be detected by FWEC analysis. The work that we have performed previously has identified a diagnostic test which has been developed and validated to enable detection of the small strongyle worm stages that cannot be detected by FWEC testing. This blood test is commercially available through AustinDavis Biologics Ltd, who also market the equine tapeworm diagnostic, EquiSal.

Using a targeted treatment control programme will substantially decrease dewormer usage and this will reduce selection pressure for drug resistance. FWEC analysis should be carried out through spring and summer to identify horses or ponies that require treatment to reducing egg shedding. Individuals with negative or low counts (usually <200 eggs per gram) can be left untreated particularly on yards where good pasture management is adhered to. Work that we performed previously demonstrated that, in addition to the benefits of these protocols in identifying animals likely to shed higher numbers of eggs/have higher worm burdens and help preserve dewormer effectiveness, this type control programme can confer financial savings.

At the current time, the recommendation is to administer all horses/ponies in late autumn/winter with an anthelmintic effective against strongyle developing larvae.

Tapeworm control can be integrated into the programme either by applying strategic anti-tapeworm treatments in autumn (and/or spring) or, preferably, using a specific diagnostic test to inform the need to treat for these parasites. Tapeworm diagnostic tests are offered by EquiSal (saliva test) or Diagnosteq (blood test).

In general terms, for all types of worm control, always ensure that dewormer doses are administered as recommended and work from an accurate weight gauge by using scales or a well-calibrated girth tape. If possible, avoid giving the same ‘class’ of dewormer year after year (note that ivermectin and moxidectin dewormers are in the same dewormer class). Test dewormer effectiveness once a year by performing a FWEC reduction test; speak to a veterinarian or appropriately-qualified animal health advisor about performing these tests. Enforce a quarantine policy; ensure that any incoming horses or ponies that will co-graze with residents are administered an appropriate anthelmintic. Ensure that these animals do not have access to grazing until 72 hours after the quarantine treatment.

Pasture management

The use of anthelmintics is only part of a best practice worm control programme. As worms are primarily transferred via dung and pasture, good pasture management is essential. Horse numbers per acre should be kept as low as possible to prevent overgrazing and to reduce worm contamination levels. Work that we (and others) have performed previously has identified the importance of effective dung removal in the control of equine strongyles. Dung should be removed regularly (at least twice a week in summer) and disposed of away from grazing.

Grazing can be rotated with sheep or cattle, to interrupt the worm life cycle (most worm species to not cross-infect different hosts), BUT liver fluke (a flatworm) can be transmitted between ruminants and equids (via snails/pasture), so ensure appropriate control of liver fluke in ruminants if the grazing is likely to encompass snail habitats or do not co- or cross-graze where the risk of liver fluke infection is high (i.e. marshy pastures, or those that encompass streams). Pasture can also be rested: most worm eggs and larvae deposited in one grazing season will be substantially reduced in number by the second half of the next grazing season.

In all cases, work with a veterinarian or animal health advisor (in the UK, Suitably Qualified Person) to develop an sustainable worm control strategy.

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