ISSN 2398-2977      

Parasite control programs


Synonym(s): Parasite control strategies

Factors to consider when designing a program to control worms

  • Drug resistance - mostly in cyathostomins and primarily to benzimidazoles - fecal egg count reduction test required.
  • Stocking density - can be deceptive, some paddocks may be heavily contaminated.
  • Age:
  • Pasture management   Pasture management  .
  • Worm egg count assessment knowledge of parasite population.
  • Climate - severe winters and long dry summers reduce larval survival on pastures.
  • Repeated (years) use of same drug selects for resistant populations.
  • Use of different drugs in rapid rotation within a year selects for resistance.
  • Establish a parasite control program for all horses on the premises.
  • Recall that the age of horses will determine to some degree the kinds and numbers of parasites they harbor.
  • A highly effective worm control program might prevent the development of naturally acquired immunity.

Classes of anthelmintics

Major classes  Therapeutics: parasiticides 

  • Macrocyclic lactones (avermectins/milbemycins - ivermectin   Ivermectin  , moxidectin   Moxidectin  ) - effective against nematodes, bots, some ectoparasites; no against activity tapeworms and flukes.
  • Benzimidazoles and probenzimidazoles (fenbendazole   Fenbendazole  , oxibendazole   Oxibendazole  , febantel   Febantel  , etc) - effective against nematodes with possibly some low activity against tapeworms; no activity against bots, flukes. 
  • Pyrimidines (pyrantel   Pyrantel  ) - effective against nematodes and tapeworms, no activity against bots, flukes.
  • Organophosphates (dichlorvos   Dichlorvos  ) - not commonly available now - effective against nematodes and bots.
  • Piperazines - not commonly available now - effective against nematodes not marked. Also marketed as a combination with benzimidazoles to reduce rate of development of resistance.
  • Isoquinoline-pyrazines (praziquantel) - effective against tapeworms and flukes. Also marketed in combination with avermectins or milbemycins.
  • See also Therapeutics: parasiticides   Therapeutics: parasiticides  .

Egg reappearance period

  • Interval dosing with anthelmintics is based on the egg reappearance period, this being the time after dosing that eggs will reappear in the feces of susceptible horses. This varies from anthelmintic to anthelmintic being based primarily on the age of parasites that are killed in the horse, ie effective only maturing adults and adults, or effective against stages as young as L3.
  • See graph for presentation of estimated efficacy of commonly used anthelmintics   Anthelmintics: efficacy - graph  .

Anthelmintic resistance

  • Benzimidazoles and probenzimidazoles: resistance among the cyathostomin small strongyles to these is increasingly common in stables.
  • 5-day fenbendazole   Fenbendazole   and pyrantel   Pyrantel  : cyathostomin resistance now has been recognized and will spread to other stables unless biosecurity measures are in place.
  • Macrocyclic lactones: resistance of Parascaris equorum  Parascaris equorum   to ivermectin   Ivermectin   is being described and a possible shortening of the egg reappearance period of strongyles after ivermectin   Ivermectin   treatment might be an early manifestation of resistance.
  • Before designing an anthelmintic program carry out a fecal egg count reduction test particularly with benzimidazoles, testing before and 7-14 days after treatment. Reductions of >95% (90% for pyrantel   Pyrantel  ) required. Efficacy of 50% of macrocyclic lactones could be tested to possibly detect resistance early resistance.

Beware of the development of triple resistance.

  • As resistance among horse strongyles to benzimidazoles is widespread and pyrantel   Pyrantel   resistance now is apparent, it seems likely that resistance in horse stables will follow the pattern that has occurred on sheep farms.
  • Trichostrongyles on sheep farms show high levels of resistance to benzimidazoles and resistance to levamisole and the macrocyclic lactones is developing.
  • TRIPLE resistance to ALL three groups now has been described on sheep farms.
  • Unable to cure parasitic disease in their sheep, farmers have had to depopulate sheep farms in the UK and other countries.
  • Development of triple resistance in horses must be retarded.


  • Resistant parasites must not be imported into the stables nor exported.
    Recommendations in place for sheep farms could be followed, although biosecurity will be complicated where stables have large numbers of horse movements.
  • Horses should be wormed, confined for 48-72 h (all feces disposed of) before being turned out.
  • Preferably, fecal egg count before, and 7-10 day after treatment, confining the horses and disposing of feces from the stable and when exercising. Repeat treatment if required.
  • Turn out onto dirty grazing any resistant parasites remaining in the horse will be diluted by interbreeding with the parasites at the stable.
  • Treatment during quarantine should be moxidectin   Moxidectin  . Alternately, following new recommendations for sheep, 2 drugs moxidectin   Moxidectin   followed by pyrantel   Pyrantel    should be used to produce redundant killing little information is available as to how closely these can follow each other in horses but different classes have been used one day later. This will produce redundant killing and kill nematodes resistant both to benzimidazoles as well any resistant to macrocyclic lactones (killed by pyrantel   Pyrantel  ) and pyrantel   Pyrantel   (killed by moxidectin   Moxidectin  )

Remember that horses can acquire parasites if allowed to graze at other stables visited for shows and rallies even for a day. Feces from horses visiting a property should be picked up and disposed of carefully.

As moxidectin is not fully effective against immatures (developing and arrested L3) some horses could pass resistant eggs from worms that developed months or more after the treatmentPrint off the Owner factsheets All about worms, Pasture management and Worm control to give to your clients.

Pasture management

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Targeted dosing with anthelmintics

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Continuous treatment (not available in the UK)

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Annual rotation with interval treatments

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Seasonal/strategic dosing

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Stratford C H et al (2014) A questionnaire study of equine gastrointestinal parasite control in Scotland. Equine Vet J 46 (1), 25-31 PubMed.
  • Lester H E et al (2013) Cost comparison of faecal egg counted-directed anthelmintic delivery versus interval programme treatments in horses. Vet Rec 173 (15), 371 PubMed.
  • Matthews J (2010) Clinical forum: Drug resistance in cyathostomins. UK Vet 15 (3), 9-17 VetMedResource.
  • Fritzen B et al (2010) Endoparasite control management on horse farms - lessons from worm prevalence and questionnaire data. Equine Vet J 42 (1), 79-83 PubMed.
  • Lloyd S (2009) Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs. Vet Rec 164 (4), 108-111 PubMed.
  • Matthews J B (2008) An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet Educ 20 (10), 552-560.
  • Lloyd S, Smith J, Connan R M et al (2000) Parasite control methods used by horse owners: factors predisposing to the development of anthelmintic resistance in nematodes. Vet Rec 146 (17), 487-492 PubMed.
  • Herd R P & Gabel A A (1990) Reduced efficacy of anthelmintics in young compared with adult horses. Equine Vet J 22, 164-169 PubMed.
  • Herd R P (1986) Epidemiology and control of equine strongylosis at Newmarket. Equine Vet J 18, 447-452 PubMed.

Other sources of information

  • Little D & Gardner S Y (2003) Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders. pp 161-164. ISBN: 072169540X.
  • Lloyd S (1997) Gastrointestinal parasites of equines and their control. In: Traction Animal Health and Technology. Universities Federation for Animal Welfare, Potters Bar. Ed: Hall S J G. pp 51-66.
  • Klei T R (1997) Current Therapy in Equine Medicine 4. Ed: Robinson N E. W B Saunders. ISBN: 0721626335.

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