Animal Care… Parasites and Deworming Update


Parasite control has been changing over the past few years. The old, traditional methods of rotating dewormers at regular intervals are actually based on concepts from over 40 years ago. These old methods were to control Strongylus vulgaris (large strongyles), which were successful at the time, but now these parasites are very rare in well managed horse herds. Now small strongyles are the primary equine parasite of concern in adult horses and Parascaris equorum (roundworm) is the main parasite in foals and weanlings. Longtime frequent deworming has lead to drug resistance in small strongyles and roundworms, which allows the resistant parasites to preferentially survive.

Quantitative fecal exams can be done to determine current parasite loads. These will determine the number of parasite eggs per gram (EPG) in individual horses. Horses can be classified as low, moderate, or high shedders. Low contaminators are horses with a fecal egg count of 200 EPG, moderate contaminators have egg counts of 200-500 EPG, and high contaminators shed greater than 500 EPG.

As parasite resistance to dewormers is continuing to increase we need to be more aware with how, when, and with what dewormer we used. A Fecal Egg Count Reduction Test (FECRT) can be done to detect resistance to a specific dewormer. This will let you know if a particular dewormer is still effective. These horses should not have been dewormed for 8-12 weeks before performing a FECRT. A fecal sample is collected prior to deworming, the dewormer is administered, and then another fecal sample is collected 14 days after treatment. The egg counts should decrease if the dewormer is effective, but if the egg counts don’t decrease enough then resistance has developed. If resistance is noted in a specific dewormer, that dewormer should no longer be used in that herd.

Horses grazing on the same pasture share the same parasite population, but they can have large differences in the amount of strongyle eggs shed. It has been shown that usually only 20% of adult horses shed about 80% of the eggs in a given horse population.

Adult horses that are classified as low or moderate shedders should get dewormed twice a year to target common parasites. Those that are high shedders should get additional treatments throughout the year. Tapeworm infections are difficult to diagnosis with common egg counting methods, so usually a single annual tapeworm treatment at the correct time of the year is sufficient for most adult horses. Young horses (foals, weanlings, yearlings) need to be treated differently. During their first year of life they should get at least 4 deworming (2-3 mo, at weaning, 9 mo, and 12 mo). Yearlings and two year olds should be treated as high shedders and dewormed at least 3-4 times a year.

Pasture management is also an important aspect of parasite control. Horses will generally divide pasture space into lawns (uncontaminated areas, where they graze) and roughs (areas containing manure piles) if they have enough pasture space. Because of this they typically will avoid the areas of heavy parasite loads. If the pasture is overstocked with too many horses, there will not be enough pasture space and will be forced to graze too close to the manure and acquire larger parasite loads.

The true goal of a deworming protocol is to limit parasite infections so clinical illness does not develop. Total eradication of parasites from an individual is not the goal. We are also trying to control parasite egg shedding and maintain efficacious dewormers and prevent further dewormer resistance.

Common Dewormers:


  • Fenbendazole (Panacur)
  • Oxibendazole (Anthelcide EQ)


  • Pyrantel (Strongid)

Macrocyclic lactones:

  • Ivermectin
  • Moxidectin


  • Praziquantel



AAEP Parasite Control Guidelines,
Parasite Control Guidelines AAEP – Click here to read the AAEP guidelines

Reinemeyer CR, Nielson MK., Handbook of Equine Parasite Control, Wiley-Blackwell, 2012