SCIENCE SUPPLEMENTS – Equine Respiratory Disease: Common Causes PART 2

Dr David Marlin and Dr Kirstie Pickles

Last week we looked at the frequency of equine respiratory disease in the horse population and common presenting clinical signs , if you missed this you can read it HERE.

This week we will examine common causes of respiratory disease in horses. These can be broadly divided up into the categories below. These are not necessarily stand-alone categories however as some diseases, e.g. equine asthma, may span more than one of these classifications (allergic and irritant).

Infectious: respiratory disease caused by infectious agents

Fig 1 Purulent nasal discharge in a horse with strangles.
  • Viruses e.g. equine influenza virus, equine rhinovirus, equine herpesvirus types 1 & 
  • Bacteria e.g. Streptococcus equi (Fig. 1; strangles), Streptococcus zooepidemicus, Streptococcus pneumoniae, Pasteurella species, Actinobacillus species
  • Mycoplasma (small organisms closely related to bacteria) e.g. Mycoplasma equirhinus
  • Fungi e.g. Aspergillus species infection of the guttural pouch

Allergic: respiratory disease caused by allergic reactions to inhaled materials (allergens)

  • Pollen – from grasses, trees, shrubs, crops etc
  • Mould/fungi – present in hay and straw, particularly in poorly saved hay and straw
  • Dust/forage mites – ubiquitous in the equine stable environment

Irritant: respiratory disease caused by inhaled material that is irritant (but not allergic)

  • Ammonia – produced by bacteria in the bedding from the protein urea in urine
  • Bacterial toxins – ubiquitous in the stable environment and present in high levels in deep litter beds and poor-quality hay and straw
  • Dust
  • Cold air – this is similar to “ski asthma” in people
  • Environmental pollutants such as PM10 (particulate matter made up of soot, smoke, metals, nitrates, sulphates, dust water and rubber) that is small enough to reach the terminal airways), ozone, nitrogen dioxide and sulphur dioxide


  • Lung bleeding during exercise (exercise-induced pulmonary haemorrhage, EIPH)

    Fig. 2: Endoscopic picture (top) of a stick in the right main stem bronchus and below the stick following removal.
  • Pulmonary fibrosis
  • Foreign body e.g. inhalation of a stick (Fig. 2)
  • Neoplasia (cancer Fig. 3)
  • Pulmonary oedema (water on the lungs)
  • Pulmonary embolism or thrombosis (air bubbles and blood clots that trap in the lungs)

Infectious disease is the most common respiratory disorder in young horses (less than 2 years old), with bacterial infection more common than viral. Young horses, like children, initially suffer a lot of mild respiratory disease when mixing with other youngsters, although in children this is mainly caused by viruses. Whilst severely affected horses may be treated with antibiotics, it is not possible to ‘protect’ a horse from this phase of development by continuously treating them with antibiotics. It has been shown that this is a necessary part of the development of the immune system and may actually be protective against certain immune mediated disease such as asthma and leukaemia. For most sport horses this is not a problem as this phase usually occurs between 1-3 years of age, but it can be a significant problem for two-year-old racehorses.

Fig. 3: Endoscopic picture of a tumour growing in a main bronchus (white arrow) and associated purulent discharge (black arrow).

In young horses (2-7 years old) inflammatory airway disease (IAD), which has recently been reclassified as mild asthma, is the most common respiratory disease. This is a subclinical disease that causes poor performance, but no obvious clinical signs at rest, which is very common in young racehorses. Mild equine asthma, like the more severe form, is an inflammatory response to the environment in which the horse is kept, however allergy does not appear to play a role in this mild form. These horses do not necessarily progress to developing severe asthma and can recover and never have further respiratory disease.

In older horses (approximately 7 years and older) severe equine asthma is the most common respiratory disease seen. Previous names for this disease have been recurrent airway obstruction (RAO), ‘heaves’ and chronic obstructive pulmonary disease (COPD). Clinical signs such as increased respiratory effort, cough and nasal discharge are present and usually obvious at rest. Severe equine asthma is believed to be caused by an allergy to airborne particles (allergens) in the environment. Typically, clinical signs are linked to the stable environment (and therefore winter) but some horses have clinical signs at pasture which is referred to as ‘summer onset equine asthma’ (previously referred to as summer pasture associated obstructive pulmonary disease [SPAOPD]). The horse’s respiratory tract is exposed to a myriad of airborne allergens and dust particles in a stable. Even a completely healthy horse (or horse owner!) can develop airway inflammation if exposed to a very dusty environment. Most evidence points to fungal spores, present in hay and straw, as being the principle allergens. When a susceptible horse breathes in the allergen, the respiratory system overreacts and the small airways go into spasm and narrow. The airways also become inflamed and produce increased amounts of mucus and inflammatory cells. Once this occurs, the horse’s lungs become hyperreactive to other irritants such as dust, ammonia from urine, and even cold air. However, the disease is reversible and, if the allergen(s) is removed, the airway will return to normal and clinical signs will disappear (‘disease remission’).

NEXT WEEK: Diagnosis of common equine respiratory diseases

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