Other Names
GPM, guttural pouch fungal infection.
What is Guttural Pouch Mycosis?
Guttural pouch mycosis (GPM) is a rare but potentially life-threatening fungal infection of the guttural pouches — two large, air-filled diverticula of the Eustachian tubes unique to horses and other equids. Each guttural pouch holds approximately 300–500 ml of air and is located immediately below the skull base in the parotid region of the head.
The guttural pouches are closely associated with several critical anatomical structures including the internal carotid artery, external carotid artery, cranial nerves IX, X, XI, and XII, the sympathetic trunk, and the stylohyoid bones. This anatomical proximity is what makes mycosis of the guttural pouch so dangerous — fungal plaques eroding through the pouch wall can cause catastrophic arterial haemorrhage or neurological damage.
The condition occurs sporadically — there are no known outbreaks — and any age or breed of horse may be affected, though adult horses appear to be most commonly affected in reported cases.
What Causes Guttural Pouch Mycosis?
The causative organism is almost exclusively Aspergillus species, most commonly Aspergillus fumigatus. Aspergillus spores are ubiquitous in the environment — found in hay, bedding, soil, and decaying vegetation. The reason why Aspergillus establishes invasive infection in the guttural pouch of some horses while the vast majority remain unaffected is not well understood. Local factors — trauma, poor ventilation, or other disruption to normal pouch physiology — may play a role.
The fungus forms a characteristic plaque or mat of mycelium on the dorsal aspect of the guttural pouch mucosa, typically in close proximity to the internal carotid artery. As the plaque expands and invades deeper tissues, it erodes through blood vessel walls, cranial nerve sheaths, or cartilage.
Symptoms and Signs
The most dramatic and characteristic presenting sign is spontaneous epistaxis — nosebleed. Unlike most other causes of equine epistaxis, haemorrhage from GPM is typically:
- Unilateral — blood from one nostril only (corresponding to the affected side)
- Unprovoked — not associated with exercise or head trauma
- Recurrent and potentially massive — episodes may begin small (a few spots) but can escalate to life-threatening haemorrhage as arterial erosion progresses
Other signs depending on structures affected include:
- Difficulty swallowing (dysphagia) — cranial nerve IX/X damage
- Laryngeal dysfunction or roaring — recurrent laryngeal nerve involvement
- Facial nerve palsy — drooping ear, drooping lip, or asymmetric facial expression
- Horner’s syndrome — ptosis (drooping eyelid), miosis (small pupil), sweating on one side of the face — sympathetic trunk involvement
- Head shaking or neck pain
- Nasal discharge — may be blood-tinged or purulent
- Guttural pouch empyema or chondroids as secondary complications
Any horse presenting with repeated, unprovoked unilateral epistaxis must be assumed to have guttural pouch mycosis until proved otherwise — the risk of fatal arterial haemorrhage is real.
Diagnosis
Diagnosis requires endoscopic examination of the guttural pouches. A flexible endoscope is passed through the nasal passage and into the guttural pouch opening (Viborg’s triangle). Direct visualisation of the characteristic dark, grey-green fungal plaque on the dorsal pouch lining is diagnostic. The plaque typically overlies the internal carotid artery or other major vessels.
Samples of the plaque may be taken for fungal culture and sensitivity testing to confirm the species, though the endoscopic appearance is often sufficiently characteristic.
Radiography and CT imaging of the head may be used to assess the extent of bony involvement, assess the stylohyoid apparatus, and plan surgical intervention. CT provides superior detail and is increasingly available at equine referral centres.
See also our entry on equine protozoal myeloencephalitis for another condition causing neurological signs in horses.
Treatment
Treatment of GPM is complex and should be managed by an equine specialist. The immediate priority is prevention of fatal haemorrhage.
Surgical arterial occlusion: The definitive treatment for most cases involving the internal or external carotid artery is surgical or transarterial occlusion (balloons, coils, or detachable balloons placed under fluoroscopic guidance to block blood flow through the affected vessel). This prevents fatal haemorrhage even if fungal erosion continues. Several techniques have been described; the optimal approach depends on which vessel is affected and the surgical expertise available.
Antifungal treatment: Systemic antifungal drugs (itraconazole, voriconazole) and local application of antifungal agents directly into the guttural pouch via catheter or endoscope are used to treat the fungal infection. Topical treatment options include enilconazole, nystatin, and natamycin preparations instilled through an indwelling catheter. Systemic antifungal treatment alone, without arterial occlusion, does not adequately protect against haemorrhage.
Supportive care: Blood transfusion may be required for horses that have suffered significant haemorrhage. Neurological deficits (dysphagia, laryngeal paralysis) may require supportive nutritional management including mash feeding or nasogastric tube feeding.
Long-term management: Neurological deficits from cranial nerve damage may persist or be permanent even after the fungal infection is resolved. Dysphagia in particular may require permanent management modifications. Some horses are unable to return to full competitive work following severe neurological involvement.
Prevention
No specific prevention strategy has been identified given the sporadic, poorly understood nature of GPM. General measures include:
- Good stable hygiene — minimise fungal spore load from hay, bedding, and damp organic matter
- Investigate any episode of spontaneous epistaxis promptly — do not wait for a second or third episode before seeking veterinary attention
When to Call the Vet
Guttural pouch mycosis is a veterinary emergency. Call your vet immediately if:
- Your horse has any episode of spontaneous nosebleed, particularly from one nostril only
- There are signs of cranial nerve dysfunction — facial asymmetry, difficulty swallowing, drooping ear or lip
- A horse is showing Horner’s syndrome — ptosis, miosis, and facial sweating on one side
- Your horse has been previously diagnosed with GPM and begins bleeding again
Do not delay seeking veterinary help for any unprovoked nosebleed — the first episode may be small, but subsequent haemorrhages can be fatal within minutes.
Prognosis
The prognosis is guarded. Horses with pure epistaxis and no neurological involvement that undergo prompt arterial occlusion have a reasonable chance of recovery. Cases with significant cranial nerve damage have a more guarded prognosis for full return to function. Horses that experience massive haemorrhage before treatment can be initiated frequently do not survive. Early diagnosis and referral to a specialist equine hospital significantly improves outcomes.
Long-term recurrence of fungal infection following successful treatment can occur and warrants continued monitoring.
Frequently Asked Questions
Is a nosebleed in a horse always serious?
Not always, but it should always be investigated promptly. Minor, bilateral nosebleeds following intensive exercise (exercise-induced pulmonary haemorrhage, or EIPH) are common in racehorses and are not immediately life-threatening. However, spontaneous unilateral nosebleed in a resting horse has a more concerning differential list that includes guttural pouch mycosis, and warrants urgent endoscopic examination of the guttural pouches.
What are the guttural pouches, and why do horses have them?
The guttural pouches are air-filled outpouchings of the Eustachian tubes found exclusively in horses and closely related equids. Their precise physiological function remains debated — one hypothesis is that they act as a cooling mechanism for arterial blood supplying the brain during intense exercise. Each pouch communicates with the nasopharynx via a slit-like opening, which is why endoscopic examination through the nasal passage can access them.
Can guttural pouch mycosis be treated medically without surgery?
Medical treatment alone (antifungal drugs, local applications) can clear the fungal infection but does not adequately protect against fatal haemorrhage from eroded arteries. Surgical arterial occlusion is required in cases involving the carotid arteries. Some very mild cases without major vascular involvement may be managed medically, but this decision must be made by an experienced equine surgeon following thorough imaging assessment.
How long does recovery from guttural pouch mycosis take?
Recovery timelines vary considerably. Following successful arterial occlusion and antifungal treatment, the fungal plaque typically resolves over several weeks to months. Neurological deficits — particularly dysphagia and laryngeal paralysis — may take many months to improve and some may be permanent. Horses with significant neurological damage may require 6–12 months of rehabilitation management.