Research on Environmental Effects of Equine Anthelmintics

Research on equine anthelmintics and their effect on the environment were eye-opening.

The studies identified that ivermectin residues in aged manure retain toxic effects for at least four months affecting fresh-water organisms, beetles (such as the dung beetle pictured above), flies and pasture productivity. Courtesy Bugwood.org

At the 2021 British Equine Veterinary Association (BEVA) Congress, Bryony Sands, Senior Research Associate at the University of Bristol in the United Kingdom, gave an interesting presentation about the environmental effects of anthelmintics and in particular the macrocyclic lactones (ML) such as ivermectin and moxidectin.

Sands pointed out that these anti-parasite control medications are actually insecticides. They pass through the intestines in an unmetabolized form, so they are excreted unchanged in the feces. One of the insects affected is the dung beetle, which has an important role in recycling nutrients in the soil by clearing dung. Two forms of dung beetles exert their work: a) dung beetle dwellers; and b) dung burying beetles. The burying process helps to improve soil structure as well as serving as a natural parasite control by quickly clearing away the manure in which parasitic larvae would otherwise develop.

Two studies conducted by Sands are revealing about the environmental effects of macrocyclic lactones. One study showed that the abundance of beetles was not changed by the anthelmintic, but there was 63% lower species diversity from ivermectin (IVM) deworming. The dung burying beetles were most significantly affected by the dewormer. Comparing effects of the use of synthetic pyrethroids with ivermectin on dung beetle survival, there was a far greater decrease in dung beetle species populations from exposure to ivermectin than to insecticide. Even if the abundance of beetles remains constant, functional communities are affected by the changing proportion of beetles—the loss of burying species renders the ecosystem less functional. Sands reports a 60% decline in dung beetles in Europe.

A second study looked at the duration of toxic effects of ivermectin in manure. Feces treated with ivermectin was stored for four months, and during that time rainwater was also collected from the manure piles. Manure was spread on a field and left for 10 weeks. The study findings noted that:

  • Rain water runoff from ivermectin-treated manure was 100% toxic.
  • Non-treated manure had an initial slight toxicity in the first month but not much effect in the final three months.
  • Insect emergence of flies and beetles was significantly decreased in ivermectin-treated manure-spread fields.
  • Primary pasture productivity decreased by 18% in ivermectin-treated manure compared to pastures spread with non-treated manure or not spread with any manure.

In conclusion, the studies identified that ivermectin residues in aged manure retain toxic effects for at least four months affecting fresh-water organisms, beetles, flies and pasture productivity.

Strategies to mitigate ivermectin toxicity were discussed by Sands and included:

  • Decrease anthelmintic use through targeted, selective deworming treatments informed by fecal egg counts. It is suggested to deworm every horse once in the winter to address encysted larvae not identified in fecal egg counts, then use fecal egg counts to inform further deworming strategies.
  • Sweep paddocks and pick up manure at least twice weekly before larvae have a chance to develop into the infective stage.
  • Use rotational grazing with ruminants, which are dead-end hosts for equine internal parasites.

For more information and how to improve the environment for dung beetles, Sands recommends visiting this website.

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