A study from Spain suggests that parasite-eating fungi may be used to prolong the efficacy of deworming treatments for horses.

The life cycles of many parasites that infect horses, including small strongyles, rely on pastures. When parasite eggs are expelled with manure, a reservoir of larvae develop in the soil and then re-infect horses as they graze. One way to break this cycle is to remove horses from a pasture until the parasites die off, but some can persist for long periods, which makes this method less reliable.

Working at the University of Santiago de Compostela, researchers dewormed 22 mature horses with ivermectin, then divided them into three groups. The first group was kept continually on the same pastures and fed a standard pelleted feed. The second group also received the standard pelleted feed but was rotated among four pastures, allowing each grazing area to “rest” for several weeks at a time. The third group was rotated among four pastures and fed a pelleted grain product infused with spores of two fungi known to eat parasites: Mucor circinelloides and Duddingtonia flagrans.

Subsequent fecal egg count testing showed that the horses fed the fungi-infused pellets had significantly reduced numbers of strongyle eggs in feces for as long as 16 weeks after deworming. By comparison, eggs reappeared 10 weeks after deworming in horses that received standard feed but were rotated among the four paddocks. What’s more, strongyle eggs reappeared only six weeks after deworming in horses that were keep continuously on the same pastures and were not fed the fungi-infused pellets.

The researchers speculate that the M. circinelloides fungus destroys part of the strongyle eggs shed in the feces, and D. flagrans fungi may create filamentous webs that capture and then digest stongyle larvae that develop from the surviving eggs, thus inhibiting the growth of the population. They conclude  that strongyle infection in horses “could be decreased by combining rotational pasturing with feeding pellets containing the spores of parasiticidal fungi.”

Subsequent fecal egg count testing showed that the horses fed the fungi-infused pellets significantly reduced the number of strongyle eggs for as long as 16 weeks after deworming. By comparison, eggs reappeared 10 weeks after deworming in horses that received standard feed but were rotated among the four paddocks. What’s more, strongyle eggs reappeared only six weeks after deworming in horses that were keep continuously on the same pastures and were not fed the fungi-infused pellets.

The researchers speculate that the M. circinelloides fungus destroys part of the strongyle eggs shed in the feces, and D. flagrans fungi may create filamentous webs that capture and then digest strongyle larvae developed from surviving eggs, thus inhibiting the growth of the population. They conclude that strongyle infection in horses “could be decreased by combining rotational pasturing with feeding pellets containing the spores of parasiticidal fungi.”

Reference: “A combined effort to avoid strongyle infection in horses in an oceanic climate region: rotational grazing and parasiticidal fungi,” Parasites and Vectors, July 2018

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