Egg Reappearance Periods (ERPs)!

Written By Malena Hyde

📚 Diagram adapted from: Macdonald et al. (2023), Veterinary Parasitology – ERP study on anthelmintics in horses.
doi.org/10.1016/j.vetpar.2023.109926

Let’s Talk About Egg Reappearance Periods (ERPs)!

Do you know how long after worming your horse you can expect to see worm eggs return in their manure?

 

This is called the Egg Reappearance Period (ERP) — and it varies depending on the type of wormer used and how effective that wormer is on your property.

 

Here’s a general guide to ERP for the main worming drug types:

  • Moxidectin (e.g. Equest) – 10 to 12+ weeks

  • Ivermectin (e.g. Equimax) – 6 to 8 weeks

  • Pyrantel (e.g. Strategy-T) – 4 to 5 weeks

  • Benzimidazoles (e.g. Fenbendazole, Oxibendazole) – 4 to 5 weeks

    Note: Shorter ERPs can indicate developing resistance.

 

Why does this matter?

If eggs are reappearing sooner than expected, it could mean the wormer is no longer fully effective. This is where Worm Egg Counts (FECs) are so valuable — they help you track when worms return and whether your current worming program is still working!

 

At Mal’s Equine Worm Egg Counts, I offer reliable, science-based FEC testing to help you make smarter, more targeted worming decisions. Based in Tassie and run by a qualified medical scientist with parasitology training

 

Message me to book your horse’s next test or to chat more about ERPs and resistance!

Reference: ERP timeframes and resistance interpretation are based on veterinary parasitology research, including guidelines from the American Association of Equine Practitioners (AAEP), World Association for the Advancement of Veterinary Parasitology (WAAVP), and published studies by Nielsen et al. (2012–2023).
See: https://aaep.org/guidelines/internal-parasite-control-guidelines

Malena Hyde
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