Aspergillosis in Horses: Fungal Respiratory and Sinonasal Infection

Aspergillosis is a fungal infection caused by Aspergillus species, molds that are common in the environment. In horses, this infection is uncommon but important because it can involve the nasal passages and sinuses, the guttural pouch, or more rarely the lungs. The exact form matters, because the risks, testing, and treatment plan can look very different from one horse to another.

In sinonasal disease, fungal growth may cause chronic nasal irritation, discharge, and sometimes facial discomfort. In guttural pouch mycosis, fungal plaques can develop near major blood vessels and cranial nerves. That form is especially serious because it can lead to sudden bleeding from the nose, swallowing problems, or nerve-related signs. Pulmonary aspergillosis is rare in horses and is more often reported in horses with severe underlying illness or immune compromise.

For pet parents, the key point is that aspergillosis is not the same as a routine bacterial respiratory infection. If your horse has persistent one-sided nasal discharge, repeated nosebleeds, or trouble swallowing, your vet may recommend a more targeted workup to look for fungal disease.

Aspergillus fungi are widespread in hay, bedding, dust, soil, and other organic material. Most horses inhale fungal spores regularly without becoming sick. Disease usually develops when spores are able to colonize damaged tissue or when the horse has another health problem that makes infection easier to establish.

In horses, aspergillosis has been associated with guttural pouch mycosis, sinonasal infection, and rarely pulmonary disease. For lung involvement, veterinary references note that severe intestinal inflammation or other causes of reduced immune defense may play a role. In practical terms, your vet may look for recent serious illness, prolonged stress, poor body condition, heavy environmental mold exposure, or other factors that could have weakened normal defenses.

This does not mean a pet parent caused the problem. Many horses with fungal disease do not have an obvious trigger. Still, moldy hay, damp bedding, dusty storage areas, and delayed evaluation of chronic nasal signs can all make it harder to control respiratory health.

Diagnosis starts with a careful exam and a clear history of the signs you have noticed at home. Your vet will want to know whether the discharge is one-sided or two-sided, whether there has been any bleeding, and whether your horse has trouble swallowing, coughing, fever, or reduced performance. Because fungal disease can mimic bacterial sinusitis, dental disease, and other upper airway problems, testing is usually needed.

A common next step is endoscopy of the upper airway and guttural pouches. This allows your vet to directly look for fungal plaques, bleeding sites, discharge, or nerve-related changes. Depending on the suspected location, your vet may also recommend skull radiographs, sinus imaging, or advanced imaging such as CT at a referral hospital.

To confirm the cause, your vet may collect samples for cytology, biopsy, fungal culture, and sometimes molecular testing. Culture alone is not always enough because Aspergillus can be present in the environment, so results are interpreted along with the horse's signs, endoscopic findings, and tissue evaluation. Bloodwork may also be used to assess overall health and help guide anesthesia, sedation, or supportive care decisions.

You cannot remove Aspergillus from the environment completely, but you can lower exposure and support your horse's normal airway defenses. Focus on clean, dry hay and bedding, good barn ventilation, and prompt removal of moldy feed or damp organic material. Store hay where it stays dry, and avoid feeding visibly moldy or dusty forage.

Prevention also means taking chronic respiratory or nasal signs seriously. A horse with ongoing one-sided discharge, repeated mild nosebleeds, or swallowing changes should be examined sooner rather than later. Early evaluation gives your vet a better chance to identify fungal plaques or sinus disease before complications develop.

If your horse is recovering from a major illness, especially severe gastrointestinal disease, follow your vet's aftercare plan closely. Good nutrition, hydration, dental care, and management of other underlying conditions all help support the immune system. These steps cannot guarantee prevention, but they can reduce risk and improve the odds of catching a problem early.