Rotational deworming--dosing horses with different classes of dewormers in rotation--is often recommended for controlling equine internal parasites. The theory is that by using all of the available effective deworming drug classes, we combine their benefits into a maximally effective program. However, some researchers say rotation isn't a good idea because it contributes to parasite resistance and will reduce the effectiveness of available deworming drugs.

At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., two different viewpoints on rotational deworming were presented.

Regardless of their positions on anthelmintic rotation, both presenters (Swiderski and Nichols) agree that sound management practices should be an integral part of equine parasite control programs. Several recommended practices include the following:

• Keep pens and pastures as free of feces as possible.
• Swiderski recommends the following strategy for new arrivals to the herd: If a horse will be staying less than six weeks, deworm him with ivermectin because it takes six to eight weeks for eggs to reappear in the feces after using this drug, and resistance is rare. If a horse will be staying six weeks or longer, quarantine him and use high-dose fenbendazole to kill encysted small strongyle larvae, followed by ivermectin or moxidectin after two weeks. Release him from quarantine if comparisons of FEC testing from before and two weeks after deworming confirm appropriate reduction in egg counts (at least 80-90% FECR for all nonmacrocyclic lactones, and greater than 98% for the macrocylic lactones).
• Swiderski also recommended singling out horses with high fecal egg counts at the beginning of the deworming season (>500 epg) for treatment with either moxidectin or larvicidal fenbendazole therapy (10 mg/kg once daily for five days), because these individuals tend to mount poor immune responses to small strongyles and harbor large numbers of encysted small strongyle larvae. These are also the individuals primarily responsible for contaminating pastures with parasites.
• Avoid ground feeding.
• Don't overcrowd pastures (avoids forcing horses to graze close to infected manure piles, where worms are usually plentiful) or spread manure on them.
• Disc pastures only when climatic conditions (hot summer temperatures) will kill eggs and when pastures can be left unoccupied for two weeks in warm Southern climates or four weeks in cooler Northern climates. Pastures should not be harrowed after Oct. 1 in the United States because parasite larvae dispersed by harrowing will not undergo the climate extremes required to kill them.
• Rotate livestock species in pastures when possible.
• Leave pastures vacant for at least two months during the warm season when possible, or allow horses to graze fields that have recently produced hay.
• Use scales/weight tapes to prevent inaccurate horse weight estimation and, thereby, underdosing when anthelmintics are used.
• Run FEC and FECR tests; as Nichols commented, "If you can't measure what you're doing, you can't measure improvement."

This article condensed from an online article by Christy West, TheHorse.com Webmaster.  Go to www.thehorse.com/ViewArticle  for the complete article.