ABSTRACT
The respiratory tract is protected by a thin layer of mucus that is constantly produced by goblet (mucous) cells, serous cells, submucosal glands, and transepithelial fluid exchange along conducting airways (134). The mucus layer forms a mechanical and immune barrier. The design of the upper airways, in particular the nose, allows trapping of larger particles onto the mucosa. The mucociliary apparatus creates a constant flow of mucus directed caudally from the nose to the pharynx, where it is eventually swallowed. As a result, horses do not normally show evidence of a nasal discharge. Mucus secretion is influenced by environmental factors and the submucosal vascular network under the control of the autonomic nervous system. For example, when breathing dry cold air, the nose adds significantly more water to the air, which results in more condensation and dripping of clear secretions from the nose.
