Nasolacrimal duct occlusion (NLDO) is a blocked tear duct, which is a common eye condition in horses. NLDO can develop for a number of reasons which include:

• Dacryoliths or cast formation, within the lacrimal sac
• Trauma
• Inflammatory disease
• Tooth root disease
• Sinus disease: Sinus disease often occurs in conjunction with, and in other instances may contribute to the development of NLD obstruction.
• Involutional stenosis
• Tumor
• Congenital

NLDO usually occurs during the warmer months and presents as chronic discharge visible from one or both of the horse's eyes. Sometimes hair loss and mild swelling of the eyelid is present due to the excessive discharge, depending on how long the teat duct has been blocked. The condition isn't serious, but can attract flying insects which can lead to a secondary bacterial infection.