Sinusitis is an infection of one or all sinus cavities, usually caused by an upper respiratory tract infection (primary sinusitis). Other causes of sinusitis include dental disease, cysts, tumours, fungal infections, and even head trauma.

The horse’s paranasal sinuses are linked air-filled chambers which connect with the nasal cavity and are lined with a mucous membrane which produces fluid and mucus. While it would make sense for the drainage point for this fluid to be located on the floor of the sinuses, it is actually positioned towards the upper area, causing the sinuses to be overwhelmed during increased mucus production. As nasal breathers, horses require the ability to breathe in large quantities of air and if this function is compromised due to excessive mucus or inflammation, performance and general health will suffer.

Symptoms include:

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