Equis ISSN 2398-2977

Therapeutics: parasiticides

Contributor(s): Byron Blagburn, Maggie Fisher, Sheelagh Lloyd


Anthelmintic resistance

  • ​Resistance to the benzimidazoles and probenzimidazoles among the small cyathostomin strongyles is widespread.
  • Resistance to pyrantel   Pyrantel  and to 5-day fenbendazole   Fenbendazole   has been seen in some populations.
  • Resistance to ivermectin   Ivermectin   has been recorded in Parascaris equorumand a possible shortening of the strongyle egg reappearance period after ivermectin treatment may be an indication of early resistance.
  • As resistance to all 3 classes of anthelmintics now is recorded among strongyles of ruminants, cyathostomin resistance to all 3 classes seems inevitable.
  • It is important to slow the rate of development of triple resistance in horse parasites. Triple resistance now is seen on sheep farms making control of sheep parasites impossible and farms have had to be depopulated in the UK and elsewhere. By the time resistance is detected by the fecal egg count reduction test, resistance may have reached 25% among the worm population.
  • The dose rates and demonstrated spectrum of activity of the individual drugs do vary from country to country. Datasheets should be consulted.
  • The activity of the various drugs against cyathostomins does vary   Anthelmintics: efficacy - graph  .
Print off the Owner factsheets All about worms and Worm control to give to your clients.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Stratford C H et al (2014) An investigation of anthelmintic efficacy against strongyles on equine yards in Scotland. Equine Vet J 46 (1), 17-24 PubMed.
  • Geurden T et al (2013) Determination of anthelmintic efficacy against equine cyathostomins and Parascaris equorumin France. Equine Vet Educ 25 (6), 304-307 VetMedResource.
  • Allison K, Taylor N M, Wilsmore A J & Garforth C (2011) Equine anthelmintics: survey of the patterns of use, beliefs and attitudes among horse owners in the UK. Vet Rec 168 (18), 483 PubMed.
  • Nieto J E et al (2009) Effect of lipopolysaccharide infusion on gene expression of inflammatory cytokines in normal horses in vivo. Equine Vet J 41 (7), 717 PubMed.
  • Molento M B, Antunes J, Bentes R N & Coles G C (2008) Anthelmintic resistant nematodes in Brazilian horses. Vet Rec 162 (12), 384-385 PubMed.
  • Rehbein S, Visser M, Yoon S & Marley S E (2007) Efficacy of a combination ivermectin/praziquantel paste against nematodes, cestodes and bots in naturally infected ponies. Vet Rec 161 (21), 722-724 PubMed.
  • Comer K C, Hillyer M H & Coles G C (2006) Anthelmintic use and resistance on thoroughbred training yards in the UK. Vet Rec 158 (17), 596-598 PubMed.
  • Esposito M et al (2005) In vitro efficacies of nitazoxanide and other thiazolides against Neospora caninum tachyzoites reveal antiparasitic activity independent of the nitro group. Antimicrob Agents Chemother 49 (9), 3715-3723 PubMed.
  • Barrett E J, Farlam J & Proudman C J (2004) Field trial of the efficacy of a combination of ivermectin and praziquantel in horses infected with roundworms and tapeworms. Vet Rec 154 (11), 323-325 PubMed.
  • Pook J R et al (2002) Evaluation of tests for anthelmintic resistance in cyathostomes. Vet Parasitol 106, (4) 331-343 PubMed.

Other sources of information

  • Vercruysse J & Rew R S (2002) Macrocyclic Lactones in Antiparasitic Therapy. CABI Publishing Co. pp 323-337.
  • Colahan P T et al (1999) Equine Medicine and Surgery. 5th edn (2 Vol). Mosby, USA. ISBN: 0815117434.