Curious bacterial finding unearthed in headshaking horses

Researchers in California detected high levels of a certain bacteria in the caecum of horses affected by headshaking.
Researchers in California detected high levels of a certain bacteria in the caecum of horses affected by headshaking. Picture by Man Dy

An interesting connection between high levels of methane-producing bacteria in the digestive system of horses and headshaking have been identified by researchers, but they are unsure what it means.

Monica Aleman and her fellow researchers at the University of California, Davis, found that horses with trigeminal-mediated headshaking had a greater abundance of Methanocorpusculum bacteria in their caecum – the large muscular sac located at the junction of the small and large intestines.

The caecum is considered to be the start of the large intestine.

The study involved 10 geldings – five diagnosed with headshaking involving the trigeminal nerve and five neurologically normal controls.

The study team, writing in the journal Veterinary Medicine and Science, said the caecal microbiota of horses with headshaking was similar to the control horses in terms of diversity. However, they differed significantly, with Methanocorpusculum species being more abundant in horses with headshaking.

The researchers said all horses used in the study were donated to the university for chronic non-treatable medical conditions requiring euthanasia. They were kept under the same husbandry and dietary conditions and were fed orchard grass hay for 30 days before being euthanized. Their conditions included headshaking and orthopedic disease (in the control group).

Caecal samples for microbiota analysis were collected within 20 minutes of death and were subjected to molecular-based testing.

While Methanocorpusculum bacteria were more abundant in the headshaking horses, their role in the condition is unknown, they said.

The authors cautioned that it could also represent an incidental finding because of the small number of horses involved in the study.

Trigeminal-mediated headshaking in horses is a form of neuropathic facial pain that compromises performance and quality of life, often resulting in euthanasia. It commonly manifests as sudden violent vertical shakes, snorting, rubbing of the nose and anxious facial expression, suggesting pain.

A variety of treatments with different mechanisms of action and variable results have been used in attempts to alleviate pain in suspected cases. Treatments have included antidepressants, anticonvulsants, channel blockers, antihistamines, corticosteroids, dietary management, facial physical devices, and nerve surgery.

Discussing their findings, the researchers said Methanocorpusculum has been shown to be the main bacterial inhabitant of the equine cecum, part of the highest two archaeal clades in the faecal microbiota in horses. Methanocorpusculum reduces carbon dioxide to methane using hydrogen.

The researchers noted that the microbiota in horses varies greatly among the different compartments of the gastrointestinal tract, with less variation between adjacent compartments and the largest diversity in the lower gut.

“The higher number of Methanocorpusculum in caecal samples from horses with trigeminal-mediated headshaking does not prove cause and effect,” the study team said.

Seasonal dietary changes can occur as a result of soil conditions, forage composition and harvest management on which forage from winter crop to spring crop is different. These, they noted, can coincide with the onset or worsening of headshaking.

Dietary components influence changes in blood pH and ionized electrolytes such as calcium and magnesium which are essential for regulating nerve transmission.

“Whether microbiota adaptation to dietary changes and season of the year occurs in horses with trigeminal-mediated headshaking is uncertain,” they said.

The study team noted that the contribution of microbiota in neurologic disease and pain in humans is well documented.

The authors stressed that caution is required in interpreting the study results because of the small number of horses involved, and variability in breed and age in the control group. “Therefore, the role of Methanocorpusculum in disease is unknown.

“Understanding the role of microbiota in trigeminal-mediated headshaking throughout the seasons of the year might result in dietary and environmental modification in attempts to prevent clinical manifestations of disease, improve quality of life and avoid possible euthanasia.”

The study team comprised Aleman, Shara A. Sheldon, Guillaume Jospin, David Coil and Jonathan Eisen, all with the University of California, Davis; and Meri Stratton-Phelps, with All Creatures Nutrition in Fairfield, California.

Caecal microbiota in horses with trigeminal-mediated headshaking
Monica Aleman, Shara. A. Sheldon, Guillaume Jospin, David Coil, Meri Stratton-Phelps, Jonathan Eisen
Veterinary Medicine and Science, 21 January, 2022,

The study, published under a Creative Commons License, can be read here

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