How Dewormers Affect Horses’ Gut Microbiome


Targeted worm treatments (anthelmintics) are important for controlling parasitic worm infections in horses. However, a study by French researcher Michel Boisseau and his team at the University of Tours shows that these treatments may also have unwanted effects on the gut microbiome. The researchers studied the interactions between the worms, especially the small strongyles, and the gut microbiome of their host animals.

Study setting and results

The study was conducted on ponies naturally infected with worms. Changes can be observed over time – before and after treatment with the active ingredient pyrantel. Pyrantel targets the adult small strongyles but leaves the larvae in the intestinal wall unaffected. The results were published in the journal Science Science Published.

studied 40 Welsh pony mares who were divided into four groups based on the severity of worm infestation and whether they had received pyrantel treatment. In ponies with high worm infestations, the researchers identified 14 different species of small strongyles, of which Silicocyclus nasatus was the most common.

Interestingly, ponies with high worm burdens had a more diverse and dynamic gut microbiota than those with less severe infections. The presence of butyric acid-producing bacteria (clostridia) played an important role in stabilizing the intestinal ecosystem and increased the animal’s tolerance to parasites. Butyric acid, also known as butyrate, has been shown to have anti-inflammatory and immune-modulating properties and supports good gut function and an intact gut barrier.

Effects of anthelmintics

Pyrantel treatment induced significant changes in the intestinal environment and microbial communities. A significant local and systemic inflammatory response was also observed following removal of the parasites. These changes are visible after seven days of treatment and, to a lesser extent, after 15 days.

Unnecessary deworming weakens the body

The researchers conclude: “These observations demonstrate how anthelmintic treatments alter the three-way relationship between parasite, host and gut microbiota.”

The complex intestinal ecosystem showed remarkable long-term stability, indicating a well-adapted and balanced relationship between the host, microbiota and parasites. These findings are consistent with growing evidence that there is a permissive relationship between intestinal parasites, the microbiota, and the host immune system.

After deworming and parasite removal, environmental instability was noted. This situation is comparable to the “Anna Karenina principle”, in which chaotic societies are different from stable societies, and environmental changes can easily upset the equilibrium. This is especially true of older horses, which often have recurrent infections and are routinely dewormed as a preventative measure.

The research team warns that routine (prophylactic) deworming not only boosts the immune system, but weakens the body’s ability to cope with other stressors. This increases the risk of additional health problems as the diversity and stability of the gut microbiome and other indirect bodily functions are affected.

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