Horses have a vagus nerve that functions similarly to the human vagus nerve — it is the longest cranial nerve, it innervates the heart, lungs, and gastrointestinal tract, and its dysfunction produces recognisable clinical syndromes in equine medicine. Understanding vagal function in horses matters because many common equine conditions (colic, laryngeal hemiplegia, cardiac arrhythmias, and gastric ulcers) involve vagal pathways.
Equine Vagal Anatomy
The horse’s vagus nerve follows the same general course as in humans — it exits the skull through the jugular foramen, descends in the neck within the carotid sheath alongside the common carotid artery and internal jugular vein, and branches to innervate the pharynx, larynx, heart, lungs, and abdominal viscera. In the horse, the vagal trunks are proportionally larger and longer given the animal’s size, with the left recurrent laryngeal nerve travelling a particularly long course — looping under the aortic arch and ascending back to the larynx, a distance of over a metre in a full-sized horse.
This extraordinary length of the left recurrent laryngeal nerve is clinically significant. The longer a nerve fibre, the more vulnerable it is to degeneration — and the left recurrent laryngeal nerve in horses is one of the longest peripheral nerves in any mammal. This anatomical fact underlies one of the most common vagal conditions in horses: recurrent laryngeal neuropathy.
Recurrent Laryngeal Neuropathy (Roaring)
Recurrent laryngeal neuropathy (RLN), historically called “roaring,” affects an estimated 2-8% of horses, with higher prevalence in large breeds (Thoroughbreds, Warmbloods, draft breeds). The condition involves progressive degeneration of the left recurrent laryngeal nerve, leading to paralysis or paresis of the left arytenoid cartilage and vocal fold. The paralysed arytenoid fails to abduct during inspiration, obstructing airflow and producing the characteristic inspiratory noise during exercise.
The condition is graded on a I-IV scale (Havemeyer grading system) based on laryngoscopic examination — Grade I is normal, Grade IV is complete left arytenoid paralysis with no voluntary movement. The primary treatment for clinically significant RLN is laryngoplasty (“tie-back” surgery), in which a prosthesis is placed to hold the arytenoid cartilage in an abducted position. Ventriculocordectomy (removal of the vocal cord and ventricle) is often performed simultaneously to reduce airway turbulence.
RLN in horses is primarily a distal axonopathy — the longest, largest-diameter nerve fibres degenerate first, which is why the left side (longer recurrent laryngeal nerve) is affected far more often than the right. The underlying cause appears to be a combination of the nerve’s extreme length, its large fibre diameter, and possibly genetic susceptibility. It is not caused by trauma or infection in most cases, making it distinct from acquired recurrent laryngeal nerve paralysis in humans (which is typically caused by surgery, tumour, or compression).
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Vagal Influence on Equine Colic
Colic — abdominal pain from gastrointestinal dysfunction — is the leading cause of non-traumatic death in horses. The vagus nerve is central to equine GI motility, and vagal dysfunction contributes to several forms of colic.
The equine GI tract is innervated by the vagus nerve from the oesophagus to the right dorsal colon (approximately the equivalent of the splenic flexure boundary in humans). The vagus nerve coordinates oesophageal peristalsis, gastric emptying, small intestinal motility, and caecal contractions. Beyond the right dorsal colon, GI motility is controlled by the pelvic nerves (sacral parasympathetic outflow), similar to humans.
Ileus (functional cessation of gut motility) is a common post-surgical complication in horses and involves vagal dysfunction. Following abdominal surgery, the vagal motor output to the GI tract is suppressed by a combination of inflammatory mediators, sympathetic hyperactivity, and direct handling of the bowel. Post-operative ileus in horses is treated with prokinetic agents (lidocaine infusion, metoclopramide, erythromycin) that work partly through vagal pathways — lidocaine reduces sympathetic inhibition of vagal motor output, while metoclopramide and erythromycin act on receptors in the enteric and vagal circuits.
Grass sickness (equine dysautonomia) is a devastating condition primarily seen in horses in the UK and northern Europe. It involves degeneration of autonomic neurons, including vagal neurons in the dorsal motor nucleus and the enteric nervous system. The disease produces severe GI stasis, dysphagia, and autonomic dysfunction. Acute grass sickness has a mortality rate exceeding 95%. The cause remains uncertain, though Clostridium botulinum toxin acting on autonomic neurons is the leading hypothesis. Grass sickness is essentially a generalised autonomic neuropathy with prominent vagal involvement.
Cardiac Vagal Function in Horses
Horses have exceptionally high resting vagal tone compared to most mammals. The resting heart rate of a healthy adult horse is 28-40 beats per minute — much lower than predicted by body-mass scaling alone. This reflects strong vagal cardiac output to the sinoatrial node.
Second-degree atrioventricular (AV) block is common in horses at rest, occurring in up to 40% of healthy horses. This is considered a normal finding, reflecting the high resting vagal tone that slows AV conduction. The block resolves with exercise (vagal withdrawal and sympathetic activation) and is not pathological unless it persists during exercise. First-degree AV block and sinus arrhythmia are also common vagal-mediated findings in resting horses.
Atrial fibrillation (AF) is the most clinically significant cardiac arrhythmia in horses and is more common in horses than in most other species (prevalence estimated at 0.3-2.5% of horses, higher in large breeds and racehorses). The high vagal tone in horses is thought to contribute to AF susceptibility — vagal activation shortens the atrial refractory period and increases the dispersion of refractoriness, creating the substrate for re-entrant circuits. This is analogous to vagal AF in humans, though in horses the large atrial mass is an additional predisposing factor. Treatment is typically quinidine sulfate administered via nasogastric tube, with a conversion success rate of approximately 85-90% for recent-onset AF.
Vagal Reflexes in Equine Practice
Several vagal reflexes are clinically relevant in equine practice. The oculocardiac reflex (pressure on the eye triggering vagal bradycardia) is used diagnostically and can be triggered inadvertently during ocular examination. The laryngeal adductor reflex (slapping the saddle area over the thorax and observing arytenoid adduction via endoscopy — the “slap test”) tests the integrity of the vagal laryngeal innervation and is a standard component of pre-purchase examinations. Gastric distension triggers vagal reflexes that can produce profound bradycardia and cardiovascular collapse — a recognised complication of gastric rupture or severe gastric dilation in horses.
Vagal Stimulation in Equine Medicine
Vagus nerve stimulation has not been widely adopted in equine medicine as a therapeutic intervention. However, the principles are relevant — the prokinetic agents used to treat equine ileus work partly through vagal pathways, and the vagal contribution to equine cardiac arrhythmias is well recognised in equine cardiology. Research into non-invasive vagal modulation in horses is limited but emerging, with interest in whether techniques used in human vagal medicine could have applications in equine GI motility disorders and stress-related conditions.
The vagus nerve’s role in equine health — from roaring to colic to cardiac arrhythmias — parallels many of the conditions discussed in human vagal medicine. My Vagus Nerve & Me covers the human vagus nerve comprehensively. Get My Vagus Nerve & Me eBook and understand the nerve that connects brain to body.
