Sinus Disease in Horses: What Persistent One-Sided Nasal Discharge Means

By Dr Duncan Houston

If your horse has ongoing discharge from one nostril, especially if it smells or keeps coming back, this is not something that resolves on its own.

Sinus disease is one of the most common causes of chronic nasal discharge in horses, and it is frequently missed early because it can look like a simple respiratory issue.

The key difference is persistence.
If it is still there after basic treatment, there is usually an underlying cause that needs to be addressed.

Quick Answer

Persistent, one-sided nasal discharge in horses is most commonly caused by sinus disease, often linked to dental infection, sinus infection, or less commonly cysts or masses. Antibiotics alone are rarely enough. Diagnosis requires imaging and examination, and treatment often involves flushing, dental extraction, or surgery.

Why Sinus Disease Is Often Missed

Early signs can look mild:

• light discharge

• no obvious pain

• normal appetite

This leads to:

• delayed investigation

• repeated antibiotic use

• worsening underlying disease

In practice, the turning point is when discharge becomes:

• persistent

• one-sided

• foul-smelling

That is when sinus disease moves to the top of the list.

What Is Happening Inside the Sinus

The horse’s skull contains multiple interconnected sinus cavities.

These spaces:

• are air-filled

• drain through narrow openings

• sit close to the roots of the upper cheek teeth

Problems arise when:

• drainage is blocked

• infection develops

• material accumulates

Once this happens, the sinus becomes a closed space that cannot clear itself effectively.

The Most Important Clinical Sign

One-Sided Nasal Discharge

This is the key pattern.

If discharge is:

• coming from one nostril

• persistent beyond 1 to 2 weeks

• thick, coloured, or foul-smelling

This is highly suggestive of sinus involvement.

Bilateral discharge is more commonly associated with respiratory infections.
Unilateral discharge points to something local.

How Serious Is It?

Low Risk

• short-term discharge

• both nostrils involved

• mild signs

What this means: likely upper respiratory issue

Moderate

• discharge lasting more than 1 to 2 weeks

• becoming thicker or discoloured

What this means: sinus disease should be investigated

High Risk

• one-sided discharge

• foul smell

• no response to antibiotics

What this means: underlying cause likely present

Severe

• facial swelling

• reduced appetite

• difficulty chewing

• blood in discharge

What this means: advanced disease or structural involvement

Common Causes of Sinus Disease

Dental Infection

This is one of the most common causes.

The roots of upper cheek teeth extend close to or into the sinus.

When infected:

• bacteria spread into the sinus

• foul discharge develops

This often requires tooth removal, not just medication.

Primary Sinus Infection

Can occur on its own but often:

• follows respiratory infection

• becomes trapped due to poor drainage

These cases often require flushing.

Sinus Cysts

Fluid-filled structures that:

• block drainage

• cause swelling

• distort the face in some cases

Ethmoidal Hematomas

Non-cancerous growths that:

• bleed intermittently

• cause nasal discharge

Tumours

Less common but important to rule out when:

• signs persist

• swelling or deformity is present

Why Antibiotics Often Fail

This is one of the most important clinical points.

The sinus is:

• poorly drained

• difficult for antibiotics to penetrate effectively

If material is trapped:

• infection persists

• symptoms return

In practice, this is why repeated antibiotic courses often fail.

How Vets Diagnose Sinus Disease

Diagnosis is based on combining findings.

Endoscopy

• visualises nasal passages

• helps rule out other conditions

Imaging

• radiographs

• CT scans where available

These show:

• fluid levels

• tooth root involvement

• masses or structural changes

Dental Examination

Essential in most cases:

• checks for decay or infection

• identifies source of sinus contamination

What Treatment Actually Involves

Treatment depends on the cause, but often includes:

Sinus Flushing

Creating access to:

• remove pus and debris

• restore drainage

This is often repeated over several days.

Tooth Extraction

Required when:

• dental infection is present

Without removing the source, the sinus will not resolve.

Surgical Removal

Used for:

• cysts

• masses

• structural abnormalities

What To Do Right Now

If your horse has nasal discharge:

• note whether it is one-sided or both

• monitor how long it has been present

• check for smell or colour change

• avoid repeated antibiotic use without diagnosis

If discharge persists beyond 1 to 2 weeks, especially on one side, seek veterinary assessment.

When Is This an Emergency?

Urgent assessment is needed if:

• swelling develops on the face

• the horse stops eating normally

• discharge becomes bloody

• pain is obvious

These signs suggest more advanced disease.

Common Mistakes

• assuming all nasal discharge is respiratory

• repeating antibiotics without investigation

• ignoring one-sided discharge

• delaying dental examination

• waiting too long before imaging

In practice, early investigation leads to much simpler treatment.

Long-Term Outlook

With correct diagnosis and treatment:

• most horses recover well

• performance returns to normal

The key factor is identifying and removing the underlying cause.

Frequently Asked Questions

Is one-sided nasal discharge always serious?
Not always, but it should always be investigated if it persists.

Can sinus infections resolve on their own?
Rarely. Most require intervention due to poor drainage.

Is dental disease a common cause?
Yes. It is one of the most frequent underlying causes.

Is sinus surgery safe?
Yes. It is commonly performed and generally well tolerated.

How long does recovery take?
Often days to weeks depending on severity and treatment required.

Final Thoughts

Sinus disease in horses is not complicated once you recognise the pattern.

The key indicators are:

• one-sided discharge

• persistence

• lack of response to simple treatment

These signs almost always point to an underlying cause that needs to be addressed, not masked.

Early action makes treatment simpler and recovery faster.

If you are unsure whether your horse’s nasal discharge is something minor or a sign of deeper sinus disease, ASK A VET™ can help you assess the situation and guide your next steps with clear, practical advice.