Respiratory disease in performance horses

The anatomical structures involved are known as the respiratory tract and can be divided into upper (URT) and lower respiratory tract (LRT). The URT consists of the nostrils, nasal passages, pharynx (throat), larynx (voice box) and trachea (windpipe). Structures within the chest are known as the LRT. The trachea branches forming smaller and smaller tubes to take air to all parts of the lungs where gas exchange occurs.

The volume of air breathed in and out at rest needs to increase up to fiftyfold during heavy exercise to supply the oxygen required by the muscles. The respiratory tract of the horse has many adaptations to make this possible, but what are the signs that something is wrong?

Signs of respiratory tract disease include abnormal respiratory noise, discharge from one or both nostrils, coughing, poor performance and slow recovery after exercise.

Obstructions tend to cause turbulence in the airflow, which may be heard at rest or by the rider during exercise. Different problems result in characteristic noises. For example, dorsal displacement of the soft palate (DDSP) tends to cause a gurgling noise along with the horse pulling up suddenly; whereas horses with laryngeal weakness are known as “roarers” due to the characteristic noise produced. 

Nasal discharge from one nostril (check that it is always the same nostril) usually suggests a problem of the nasal passage on the same side. Common causes include tooth root abscesses, sinusitis, or ethmoid haematoma (a type of tumour). If nasal discharge is coming from both nostrils, it usually means the problem originates lower down the tract, i.e. between the throat and the lungs.

The type of discharge also gives an indication of the cause of the problem. Thick yellow discharge is often associated with bacterial infection, although infection may not be the primary cause. Bloody discharge can be caused by trauma, ethmoid haematomas or exercise-induced pulmonary haemorrhage (EIPH “bleeders”). Watery or white discharge may be seen in horses with inflammatory airway diseases.  

Coughing has many possible causes. Probably the most common is a reaction to dust or other allergens such as pollen. Never feed dusty or mouldy feed or hay. Viral infections including EHV-1 and EHV-4 (equine herpes virus) can be a factor.   See Anita’s article on the next page for more information.

A thorough history and clinical exam will help to narrow down the possible causes. If the problem is intermittent, a diary can be helpful to isolate causative factors. A rebreathing bag may be used by your vet. Unfortunately, we can’t ask the horse to take deep breaths so a large bag is placed over the horse’s nose so that he rebreathes the same air. The build-up of carbon dioxide will make the horse take deep breaths and subtle abnormal lung sounds may become apparent.

URT problems are usually best further investigated by endoscopy – a flexible camera is passed along the respiratory tract via the nostrils and many abnormalities can be visualised this way, or trails of discharge can be followed back to find the origin. Samples may also be collected from the airways, for laboratory analysis. The types of cells present can give more clues towards a diagnosis.

In some cases, it may be necessary to observe the horse at work, and endoscopy on a treadmill may allow visualisation of a problem that only occurs during exercise. Treadmill endoscopy is available at Massey. 

Radiography may be useful, particularly to visualise the internal structures of the head such as sinuses and tooth roots.

Treatment

The key to successful treatment of respiratory conditions is getting an accurate diagnosis. Different problems require very different solutions ranging from minor management changes such as soaking hay, instigating a vaccination programme for EHV, to surgery for some of the mechanical URT problems.