Mare Care

Deworming the Pregnant Mare

Strategic deworming is another essential ingredient of preventive health care. The major gastrointestinal parasites of concern in the mare are large and small strongyles and, in some instances, tapeworms. Another parasite, Strongyloides, can be passed from dam to foal in the milk.

Any deworming program should include active ingredients that are effective against mature parasites and migrating or encysted larvae. In general, it’s wise to avoid any unnecessary drug administration, including dewormers, during the first 60 days of pregnancy since this is the time when major organ systems are developing in the fetus (organogenesis).


After you determine the shedding status of your mare through a fecal egg count examination, deworm her prior to breeding with a drug appropriate for the time of year, her parasite susceptibility and risk of exposure. Large strongyles and small strongyles, as well as tapeworms, are typically acquired from pasture and any environment contaminated by horse manure. Since broodmares spend most of their time on pasture, they are at constant risk for parasite infection.

  • High shedders require 3-4 dewormings per year
  • Low shedders require 2-3 dewormings per year

Avoid administering dewormers during peak hot, dry summer months since risk of strongyle transmission is greatly reduced. Late spring/early summer or early fall are more optimal times to deworm in most regions. Excessive drought conditions will also reduce the risk of strongyle (and possibly tapeworm) transmission and should be taken into consideration.

If you have a large breeding operation with many mares grazing limited pastures, you’ll have an increased threat of heavy worm burdens accumulating during the grazing season. Remember, if your mares are shipped off to be re-bred, they will be exposed to other parasites on other farms. All incoming or returning mares should have fecals performed and may benefit from deworming prior to exposing them to your pastures and resident mares.


If mare was not dewormed pre-foaling with a drug having an egg reappearance period still effective during post-foaling period, then deworm soon after delivery with broad spectrum dewormer effective against Strongyloides westeri (threadworm).

Periodically monitor fecal egg counts (FEC) to evaluate efficacy of drugs used and to confirm the mare’s shedding status.


  • Low shedders: Ivermectin, pyrantel or fenbendazole
  • High shedders: Larvicidal fenbendazole or moxidectin
  • Spring/post foaling: Ivermectin

End of first trimester/second trimester

late winter/early spring

  • Monitor fecal egg count (FEC) as needed to evaluate efficacy of dewormers
  • Administer a broad-spectrum dewormer (e.g., fenbendazole, ivermectin, pyrantel)
  • High shedders may require a second treatment in early fall

Third trimester

late fall/early winter

  • Treat for bots, tapeworms, large and encysted small strongyle larvae (if indicated as in the case of high shedders)


Treatment Options

Consult your veterinarian for the most effective deworming schedule for your mare and region.

1. Adapted from AAEP core and risk-based vaccination guidelines.