The common cold comes and goes with the seasons, and as winter approaches many horse owners are stuffing their barn coat pockets full of tissues to combat the perpetual runny nose that accompanies the ailment. Not unlike us, although not necessarily more commonly in the winter months, our horses can also come down with ailments that cause runny noses–typically with drainage from both nostrils. The causes of runny noses in horses are plentiful, but if a horse has a nasal discharge coming down only one nostril, it most likely due to some form of sinus disease.

"Inflammation of the sinus, called sinusitis, is by far the most common cause for a unilateral (one-sided) nasal discharge in horses," explained Paddy Dixon, MVB, PhD, MRCVS, professor of equine surgery in the University of Edinburgh’s Division of Veterinary Clinical Sciences during his presentations at the 12th Congress of The World Equine Veterinary Association, held Nov. 2-6 in Hyderabad, India.

According to Dixon, some of the potential causes for the unilateral, white to green, thick mucus discharging from one or more of the seven paired sinuses in a horse include:

  • Primary sinusitis (no known etiology and the most common manifestation);
  • Dental disease (the tooth roots project into the maxillary sinus);
  • Cysts in the sinuses;
  • Neoplasia (tumors) of the sinuses (e.g., squamous cell carcinoma);
  • Fungal infection of sinuses (e.g., Aspergillus spp.);
  • Hemorrhagic polyps (e.g., progressive ethmoid hematomas) in sinuses; and
  • Trauma.

"Using an endoscope to directly and careful