RAO in horses - What equine asthma really is, and how stable management can help

Coughing in horses is often dismissed as a minor issue. Sometimes, it's the first sign of one of the most common chronic diseases.

When coughing is not a minor issue

It's often heard in the stable. A short cough when saddling up. A clearing of the throat when hay is shaken out. "He's always done that, it's not a big deal" – a phrase heard in many stable aisles.

Sometimes it really is harmless. And sometimes it's the first, subtle hint of one of the most common chronic diseases in horses: RAO – Recurrent Airway Obstruction.

What is RAO – and why is it now called equine asthma?

RAO is a chronic, non-infectious (not caused by bacteria or viruses) inflammation of the lower airways in horses. Historically, the disease was known by various names: COB (Chronic Obstructive Bronchitis), COPD (Chronic Obstructive Pulmonary Disease), or simply "heaves".

Today, veterinary medicine groups all these terms under a new umbrella term: Equine Asthma. This collective term distinguishes two levels of severity:

IAD (Inflammatory Airway Disease): the milder to moderate form, primarily affecting younger horses.
RAO (Recurrent Airway Obstruction): the severe form, often referred to as "heaves," which usually occurs in middle-aged and older horses.

The disease shows striking parallels to human asthma – which is why affected horses even serve as a model in research for studying human COPD diseases.

How widespread is RAO really?

The numbers are impressive – and should make every horse owner take notice.
Severe equine asthma (the severe form, RAO) is estimated to affect 10 to 15 percent of all horses kept in conventional stable housing in temperate climates. For older horses, this number increases even further: More than 50 percent of 12- to 14-year-old horses already show signs of chronic obstructive bronchitis.

This means that RAO is not a rare exception. It is one of the most common chronic diseases in the entire horse population.

What happens in the horse's body

To understand why RAO develops, it helps to look at the horse's origins.
With their large lung capacity, horses are made for a life outdoors – for the open steppe, for constant movement, for clean air. Stable housing presents a completely different environment for their sensitive airways than the one for which they were evolutionarily shaped.

When a horse susceptible to RAO comes into contact with allergens (substances that trigger an exaggerated immune response) such as dust, mold spores, or pollen, the immune system overreacts. This leads to a massive influx of neutrophils (a type of white blood cell that normally fights pathogens) into the airways.

The result is two problems simultaneously: bronchospasm (a painful constriction of the small airways) and increased mucus production. Both together significantly narrow the airways – the horse literally has more difficulty breathing.

Two specific triggers have been particularly studied: Aspergillus fumigatus and Faenia rectivirgula – two types of mold that can be found in hay and straw and are proven to cause bronchial hypersensitivity.

The most common triggers

The list of triggers is long, but it can be summarized into a few main factors:

Dust from hay and straw – the largest and best-researched source. Every time a horse eats hay or stands in its stall, it inhales millions of particles: dust, mold spores, bacteria.

Ammonia – formed during the breakdown of urine and feces. Concentrations as low as about 10 ppm (parts per million, a unit of measurement for gas concentration in the air) can irritate the sensitive respiratory mucous membranes.

Mold from feed – especially with poorly dried or overly moist hay.

Pollen – in a special variant called SP-RAO (Summer Pasture associated RAO), where symptoms paradoxically occur in the pasture instead of the stable.

Genetic predisposition – some horses are inherently more sensitive than others, even under identical housing conditions.

Recognizing symptoms – from mild cough to heaves

RAO usually develops insidiously, in episodes, with symptom-free phases between them. This can make early detection difficult.

Early signs:

• Short cough at the beginning of a training session
• Slightly increased respiratory rate after exertion
• Occasional coughing when eating hay

Advanced signs:

• Coughing even at rest, even in the stall
• Nasal discharge
• Flared nostrils
• Increased sweating even with light exertion
• Noticeably reduced performance

Severe signs (heaves):

• Difficulty breathing even when standing
• Visible "heave line" – a line along the abdominal muscles caused by increased effort during exhalation
• Significantly reduced performance

Important: complete recovery in the medical sense is often no longer possible in chronic cases – structural changes in the lungs can be irreversible. Early detection is therefore even more crucial.

Diagnosis – how the vet determines RAO

Diagnosis is never made by mere observation alone. A thorough veterinary examination typically includes:

• A detailed clinical examination
• Blood gas analysis
• Endoscopy of the airways (an examination with a small camera) followed by collection of mucus directly from the trachea for analysis

Stable management – the most important measure of all

Here comes perhaps the most important finding in all RAO research: Treatment does not begin with medication. It begins with the environment.

Soak or steam hay. Simply sprinkling hay with water is not enough – only thoroughly submerged or steamed hay (at a minimum of 100°C) is sufficiently dust-free. Important to know: Wet hay increases microorganisms faster, so it should be fed promptly.

Use low-dust bedding. Straw is significantly dustier than finer alternatives. Shavings, pellets, or fine mineral bedding absorb moisture better and further reduce dust exposure.

Muck out consistently. Wet spots should be completely removed daily – not just covered. Standing urine is the main source of ammonia formation.

Ensure good ventilation. Even in winter – ammonia is heavier than air and collects on the ground, where the horse directly inhales it.

Remove the horse from the stable during mucking out and bedding. The greatest dust exposure occurs during these activities. After about 30 minutes, most of the dust has settled again.

Do not shake or store hay in the stable. This also applies to neighboring stalls: if you have a horse with respiratory problems, you should try to ensure that no additional dust is stirred up in the surrounding area.

Where zeolite and natural support have their place

Important first: Zeolite does not cure RAO and is not a substitute for veterinary treatment. The scientifically proven basis of any support remains stable management – addressing the causes, not just the symptom.

However, zeolite can be a useful supplement in the following ways:

As bedding, fine-pored zeolite binds ammonia directly at the source and keeps the stall floor drier – an important building block for reducing ammonia levels in the air.

Fed internally, zeolite can help bind mycotoxins (mold toxins) ingested through contaminated feed – before they burden the body systemically.

In stable hygiene, a combination of dry, low-ammonia bedding and a stable microbiome created by AM+PLUS microorganisms can help prevent the development of conditions that promote putrefaction and mold in the stable.

We are deliberately reserved here: There are currently no specific, independent studies proving that zeolite directly treats or cures RAO. What it can do is be a sensible addition within the framework of a well-thought-out overall concept – alongside hay steaming, good ventilation, and consistent stable management.

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On our knowledge page, you'll learn how the mineral is formed, how it works, and why quality is crucial. → What is Zeolite?

FAQ – Frequently Asked Questions

❓ Is RAO curable?
No, not entirely in the medical sense – especially if the disease is already advanced and structural changes have occurred in the lungs. However, RAO is reversible in the sense that symptoms can significantly decrease or disappear completely if the triggering factors are consistently removed from the environment.

❓ Can my horse with RAO still be ridden?
In many cases, yes – depending on the severity and the veterinary assessment. It is important to consult individually with the veterinarian, as exertion should be avoided during acute episodes.

❓ What is the best bedding for RAO?
Low-dust alternatives such as fine shavings, pellets, or fine-pored mineral bedding are less dusty than classic straw and also bind moisture and ammonia better.

❓ Do I have to completely eliminate hay for RAO?
Not necessarily – but the hay should be consistently soaked or steamed to drastically reduce dust content. Alternatively, haylage or hay cobs can be a less dusty option.

❓ How quickly do improvements in stable management take effect?
This varies individually. Some horses show noticeable improvement after just a few days of consistent dust reduction, while for others – especially in advanced cases – it takes longer or the improvement remains limited.

❓ Can zeolite cure RAO?
No. Zeolite is not a treatment for RAO and should never be seen as a substitute for veterinary care or consistent stable management. As bedding, however, it can contribute to a clear reduction in ammonia exposure. And as a feed additive, zeolite supports the reduction of pollutants and molds.

Read more

👉 HORSEKRAFT Zeolite bedding – against ammonia & thrush →
👉 AM+PLUS for horses – microorganisms for skin & coat →
👉 Zeolite for horses – the big Q&A guide →