The nasal discharge in a horse accompanied by unilateral Horner's syndrome may indicate which condition?

The presence of nasal discharge in a horse alongside unilateral Horner's syndrome is indicative of guttural pouch mycosis. This condition involves a fungal infection that primarily affects the guttural pouch, a unique anatomical structure in equines that houses critical neurovascular structures. When the fungal infection occurs, it can lead to anatomical disruptions, potentially affecting the adjacent sympathetic nerves and resulting in Horner's syndrome, which is characterized by drooping of the eyelid, constricted pupil, and sunken eyeball on the affected side.

Unilateral nasal discharge can also occur due to other conditions, but the specific combination of nasal discharge and Horner's syndrome suggests a more serious underlying issue, such as guttural pouch mycosis. This distinction is vital, as it may necessitate specific diagnostic evaluations, including endoscopy or imaging, to confirm the presence of fungal lesions in the guttural pouch. This scenario highlights the significance of recognizing the relationship between neurological signs and respiratory symptoms in equine medicine.