Nasal discharge in horses can arise for many different reasons, some of these being a lot more serious than others. Sometimes a horse which has been inside its stable for a day or more can develop a nasal discharge after exercise. However if the horse is having any trouble breathing, has feed coming out of its nostrils, is breathing noisily or appears very ill, a veterinarian should be contacted immediately.
There are many different types of nasal discharge in horses, but the two which have a yellow color are called mucoid and purulent. Mucoid discharge is very similar to mucous in texture, whereas purulent discharge is very thick and more like pus, and tends more towards green-yellow in color. Whether the discharge comes from one nostril (unilateral) or both (bilateral) is an important distinction, as these arise from different areas in the horse's body. It can start and stop suddenly or last for several days, and it can be the only symptom or one of many. Discharge with an odor is often linked to bacterial infections in the sinuses or teeth.
Mucoid discharge is often present mixed with a watery, clear fluid known as serous discharge. This can simply be caused by wind or dust irritating the nasal passage, although in these cases the discharge is mainly clear. It could also be indicative of allergies, an upper respiratory tract viral infection, or the beginnings of a more serious illness. If the discharge is caused by an allergy, it is normally accompanied by discharge from the eyes. Viral infections or other illnesses will be accompanied by other symptoms such as fever and loss of appetite.
Purulent discharge is caused by a bacterial infection in the respiratory tract. If it is coming from just one nostril, it can be a sign of a sinus infection, a bacterial infection in the guttural pouch (part of the respiratory tract which is found only in horses) or an infection in the nasal passage. Purulent discharge coming from both nostrils can again be a sign of a bacterial infection in the guttural pouch. It may also be due to an upper respiratory tract infection called strangles, or an illness in the lower respiratory tract such as pneumonia.
To diagnose the cause of nasal discharge, your veterinarian will give the horse a thorough physical examination and take details of the horse's history. Important factors include: any other symptoms the horse may be suffering, whether other horses in the same barn are showing similar signs, if the horse has recently been to a show, or if any new horses have recently come to the stable. The age of the horse and its vaccination history are other things the vet will need to know. Lung examinations, blood samples, X rays, nasal swabs, airway endoscopy and ultrasound may all be utilized in the diagnosis of the horse's condition.