In recent years, extensive wildfires and smoke exposure have impacted horse operations around North America. Evacuation and preventive protocols are often challenging and put a great deal of strain on livestock and their human caretakers.

Equally as concerning is the risk of on-site fires. With long periods of drought recurring across North America year after year, building fires are an increasing threat to the health and safety of many equine facilities.

Fires pose considerable danger to horses, even from a significant distance. In particular, horses can suffer from varying degrees of poisoning and injury associated with smoke inhalation.

Symptoms of smoke inhalation injury in horses vary depending on the severity of exposure, the specific chemical composition of the smoke, and the presence of pre-existing respiratory conditions. The prognosis varies from good to extremely poor depending on the duration of exposure, underlying conditions, and severity of symptoms.

Prepare yourself for emergencies by learning why smoke is a toxic hazard for your equines and what steps you can take to protect your herd.

Equine Air Quality Index Tool | Mad Barn USA

Fire Hazards for Horses

Every year, horses around the world perish or are injured due to barn fires. Uncontrolled fires also spread through vegetation, often exacerbated by dry conditions, high temperatures, and strong winds.

When material combusts, it undergoes a rapid oxidative reaction with oxygen from the air that releases light, heat, and the by-product smoke. [1]

Smoke is a complex and evolving mixture of hot air, carbon gasses, solid particulates, fumes, and vapors emitted as substances combust.

The chemical composition of smoke is highly variable depending on the properties of the burning material and the fire environment. Fires with ample oxygen availability burn hotter and produce less smoke than those in oxygen-poor spaces. [1]

Components of Smoke

Most of the properties of smoke are hazardous to horses in some way. Common components of smoke include: [1][5][6][7]

  • Carbon dioxide (CO2): a common by-product of wildfires and is not highly toxic. However, if CO2 displaces most or all of the oxygen in an enclosed space, it can lead to suffocation.
  • Carbon monoxide (CO): a toxic gas that can result in severe poisoning if inhaled. Lower-temperature fires produce higher concentrations of CO.
  • Hydrogen cyanide (NCH): a highly poisonous chemical compound formed when materials high in nitrogen burn.
  • Particulate matter (PM): a mixture of solid and liquid air droplets, which vary in diameter from 10 to less than 2.5 micrometers; Particulate material in fire smoke is the result of incomplete combustion; when inhaled, PM can settle in the lungs, interfering with their overall health and function.

In addition, the following compounds commonly found in smoke are all severely irritating to the respiratory tract: [3][4]

  • Ammonia
  • Hydrocarbons
  • Nitrogen oxides
  • Hydrogen sulfide

Types of Fires

Fires can be divided into two main categories: structural fires and wildfires, depending on the fuel source. Both categories of fires can be ignited naturally or by humans, and both pose a threat to horses. Further, wildfires can spark structural fires and vice-versa.

Structural Fires

Structural fires are fires that involve the combustion of building components. Smoke from these fires is rich in carbon monoxide, a colorless, odorless gas that displaces oxygen from the horse’s red blood cells, leading to suffocation. [4][8]

Burning plastics and other nitrogen-containing materials (e.g., wool, nylon, and silk) commonly found in horse apparel, tack, and other equipment release hydrogen cyanide, which is highly toxic. [4][8]

Thermal breakdown of synthetic materials like those often stored in barns where horses are kept can produce irritating fumes like ammonia and sulfur dioxide, causing severe respiratory tract damage. [4][8]

Barn Fires

While all structural fires can cause death and severe injury to horses and humans alike, barn fires are of particular concern in the equine community. These fires constitute a severe threat to farm animals, with hundreds of thousands of deaths reported each year. Between 2013 and 2023, over six million deaths due to barn fires were reported. [2][9]

In the upper Midwest and northeastern states, most barn fires occur in winter, representing 37% of annual fires. The leading cause of barn fires in the United States is malfunctioning electrical heating devices such as heat lamps and space heaters. [9]

Other common causes include: [9]

  • Electrical malfunctions (other than heating equipment): 25%
  • Machinery: 17%
  • Other causes (weather-related fires, wildfires, vehicle crashes, spilled gas, fireworks): 14%
  • Arson: 4%

Wildfires

Wildfires are unplanned fires that burn vegetation, including trees, grasses, and brush. While destructive, wildfires are a natural occurrence in many ecosystems. They play a vital role by consuming dead vegetation, clearing debris, and stimulating new plant growth. [10][11]

Wildfire smoke is a complex mixture of chemical compounds, including carbon dioxide, water vapor, nitrogen oxides, and particulate matter. Wildfire smoke is particularly high in fine particulate matter, which is the primary health risk to horses and humans. [4][10]

Smaller than 2.5 micrometers in diameter, these tiny particles can bypass the upper respiratory tract and reach the lower airways (alveoli) in both animals and humans. [4][10] A sufficiently high exposure to fine particulate matter, whether by a large single dose or chronic low dose, can impair overall health and function in the lungs.

Smoke Inhalation Injury

Horses exposed to smoke can suffer from a wide range of injuries affecting multiple organ systems, from thermal burns to secondary pulmonary infections. A general term for adverse health effects related to smoke exposure is smoke inhalation injury.  [12][13]

Symptoms of smoke inhalation injury in horses vary and are correlated to several factors, including:

  • Chemical composition of smoke
  • Fire environment
  • Duration of exposure

Smoke can cause damage to the respiratory system through a combination of several mechanisms of action, including

  • Internal thermal burns
  • Chemical burns
  • Oxygen deprivation
  • Foreign matter interference

Thermal Burns

Depending on how close smoke is to the fire source, it may have an extremely high temperature – high enough to burn the delicate lining of the horse’s upper respiratory tract (URT) upon inhalation.

Damage to the URT can lead to fluid buildup in the lungs (edema), potentially obstructing the airways. Additionally, heat damages the cilia, tiny hair-like structures that help propel mucus and debris out of the respiratory tract. [4][14]

Impairment of this clearance mechanism allows foreign particles and pathogens to lodge in the airways, further compromising respiratory function in affected horses. [4]

Chemical Burns

Depending on its composition, smoke can contain severe chemical irritants, including ammonia and sulfur dioxide. Ammonia (NH3) is a highly water-soluble irritant rapidly absorbed by the upper respiratory tract.

In addition to causing initial pain, inflammation, and irritation, once inhaled these chemicals damage the horse’s mucous membranes and cilia. As with thermal burns, damage to these structures exposes the horse to a higher risk of developing secondary upper respiratory airway infections. [14][15]

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Oxygen Deprivation

Smoke can contain several asphyxiants, gasses that cause hypoxia (insufficient oxygen). These gasses can be divided into simple asphyxiants that displace oxygen in the air, and chemical asphyxiants that interfere with the cellular oxygen transport mechanism. [14][16]

Chemical asphyxiants are found in higher concentration in smoke caused by structural fires; they include: [14][16]

  • Carbon monoxide: once inhaled, CO binds to hemoglobin in the bloodstream, displacing oxygen with which hemoglobin has a lower affinity (200-250 times lower than CO).
  • Hydrogen cyanide: rapidly inhibits mitochondrial function, which stops aerobic metabolism.
  • Hydrogen sulfide: binds to enzymes that are involved in cellular respiration, impairing function. At high concentrations H2S depresses the brain’s respiratory center.

Simple asphyxiants limit oxygen availability. The most common simple asphyxiant in smoke is carbon dioxide. CO2 is formed by the complete oxidation of hydrogen and oxygen and is a common component of wildfire smoke. [16]

Foreign Matter Interference

In addition to toxic chemicals, smoke contains carbon particulate matter (PM). These small droplets range from 10 to less than 2.5μm in diameter. Wildfires tend to produce smoke that contains the smallest kind of PM, known as fine particulate matter. [4][5] [14]

PMs smaller than 2.5 μm are able to bypass the horse’s natural filtration system and enter the lungs, where they cause inflammation and liquid buildup, leading to pulmonary edema.

Particulate matter can also cause eye irritation and impact the immune system, leaving affected horses more vulnerable to secondary bacterial infections. [4]

Symptoms

The clinical signs associated with smoke inhalation injury in horses vary depending on the severity of exposure, the specific chemical components of the smoke, and the presence of pre-existing respiratory conditions.

Symptoms are subdivided into early, intermediate, and late phases, in accordance with the timing of onset. [13][14]

Clinical features of the early phase occur within 24 hours from exposure and may include: [14]

  • Singed hair on the face and muzzle
  • Soot-stained nasal discharge
  • Rapid, shallow breathing (tachypnea)
  • Difficulty breathing (dyspnea)
  • Coughing
  • Drooling
  • Weakness
  • Pale mucous membranes
  • Rapid and weak pulse
  • Lethargy
  • Lack of responsiveness
  • Coma or death

Symptoms of the intermediate phase occur within one to five days from smoke inhalation and may include: [14]

  • Severe respiratory distress
  • Shock
  • Airway obstruction
  • Fever

In some cases, symptoms during the intermediate phase are due to multiple organ failure.  Depending on which system is affected, symptoms may include: [14][17]

  • Lethargy
  • Loss of appetite
  • Colic
  • Fever
  • Seizures
  • Collapse
  • Incoordination
  • Coma

Symptoms of the late phase, which occurs within five to seven days from exposure, are caused by bronchopneumonia and include [14][17]:

  • Rapid pulse (tachycardia)
  • Fever
  • Rapid breathing (tachypnea)
  • Nasal discharge
  • Bad breath
  • Abnormal lung sounds

Diagnosis

A comprehensive diagnostic approach is crucial for confirming smoke inhalation injury, assessing its severity, and identifying potential complications in horses, including: [14]

  • History of exposure
  • Physical examination
  • Blood tests
  • Blood gas analysis
  • Pulse oximetry
  • Electrocardiogram

Diagnostic imaging is a particularly useful tool in diagnosing and treating smoke inhalation injury in horses. Chest X-rays are used to identify pulmonary edema, inflammation, pneumonia, or airway obstruction. Due to the progressive nature of smoke injury, serial X-rays are often used to track the condition’s progression and response to treatment. [14]

In addition, upper airway endoscopy is used to identify soot deposits, airway inflammation, and tissue death in the lining of the respiratory tract. [14]

Treatment

The first step in treatment is immediately removing the horse from smoke exposure. Further treatment options depend on the severity of symptoms and progression of the condition, options include: [14]

Prognosis

Prognosis is extremely variable and depends on multiple factors such as: [12][13][14]

  • Severity of exposure: the duration of exposure to smoke directly impacts the extent of lung damage
  • Smoke composition: the specific chemical composition of smoke influences the type and severity of damage
  • Severity of symptoms: horses that develop severe symptoms and secondary bacterial infections have lower chances of survival and higher chances of complications
  • Pre-existing conditions: horses suffering from pre-existing respiratory conditions carry a poorer prognosis compared to healthy horses with the same levels of exposure
  • Timing of treatment: prompt medical intervention significantly improves the chances of full recovery

Prevention

Horse owners and barn mangers can take steps to protect their horses from smoke inhalation injury. By implementing these practices and staying up to date with current environmental conditions, caretakers can mitigate the risk of fire hazards and related injuries.

Prevention of Barn Fires

Barn fires are a highly preventable tragedy that impact hundreds of thousands of animals, including horses, each year. The vast majority (85%) of barn farms are the result of human error or electrical malfunctions. [9]

The good news is there are a number of operational and structural preventive strategies, including: [2][9][18][19][20]

  • Conduct annual inspections of the premises with the fire department and an electrical specialist
  • Replace malfunctioning electrical equipment promptly
  • Install smoke and heat detection systems
  • Install sprinkler systems
  • Keep and maintain fire extinguishers and fire blankets on the premises
  • Choose fire-resistant materials when building or renovating

In addition, proper storage and disposal of manure is critical as it is highly combustible.

Prevention and Mitigation of Wildfires

Wildfires are much less preventable in comparison to structural fires, as they are naturally occurring, uncontrolled, and often massive. Further, the expanse of annual wildfires is increasing in size due to climate change, with the average wildfire in North America burning over 20,000 acres. [21]

While not preventable, adverse health effects of wildfires can be mitigated by: [22]

  • Checking the Air Quality Index daily during wildfire season and using it to direct daily equine management choices
  • Limiting training and exercise when smoke is present to minimize irritation and accumulation of particulate matter in the lungs
  • Soaking hay to reduce further inhalation of dust particulates

Horses with chronic respiratory symptoms may benefit from symptomatic or prophylactic use of bronchodilators (equine inhalers). Speak to your veterinarian if living near wildfires seems to cause chronic symptoms in your horse.

Summary

Fire poses a significant threat to horses and humans alike. The majority of severe morbidity and mortality associated with fire are linked to smoke inhalation.

  • Smoke is a complex mixture of gasses and chemical particulates
  • When inhaled smoke can damage many different organ systems
  • Barn fires pose the most critical health concern to horses and other farm animals
  • Smoke inhalation injury causes damage through several mechanisms of action, including thermal injury, chemical burns, asphyxiants, and particulate matter

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References

  1. Friedman, R. Principles of Fire Protection Chemistry and Physics. Jones and Bartlett Publishers, 2009.
  2. Gimenez, R. M. et al. A Review of Strategies to Prevent and Respond to Barn Fires Affecting the Horse Industry. AAEP. 2008.
  3. Alarie, Y. Toxicity of Fire Smoke. Critical Reviews in Toxicology. 2002.
  4. Cope, R. B. Smoke Inhalation Injury in Animals. MSD Veterinary Manual. 2021.
  5. Particulate Matter (PM) Basics. United States Environmental Protection Agency. 2023.
  6. Bengtsson, P.E & Le, T.K.C. Soot particles – the remnants of incomplete combustion. Combustion Physics, Lund University. 2023.
  7. Martin, J. W. et al. Soot Inception: Carbonaceous Nanoparticle Formation in Flames. Progress in Energy and Combustion Science. 2022.
  8. Wildland Fire Gear: Wildfire Categories and Choosing PPE. Lakeland Industries. 2024.
  9. Barn Fires: A Deadly Threat to Farm Animals. Animal Welfare Institute. 2022.
  10. Wildfire Smoke: A Guide for Public Health Officials. United States Environmental Protection Agency. 2019.
  11. Benefits of Fire. National Park Service U.S. Department of the Interior. 2020.
  12. Lavoie, J.-P., & Hinchcliff, K. W. Blackwell’s Five-Minute Veterinary Consult Equine. Second Edition. Wiley-Blackwell. 2009.
  13. Wilson, D. A., Ed. Clinical Veterinary Advisor: The Horse. St. Louis, Mo: Elsevier Saunders. 2012.
  14. Hovda, L. R. Blackwell’s Five-Minute Veterinary Consult Clinical Companion Equine Toxicology. Wiley Blackwell. 2022.
  15. Southwood, L. L., & Wilkins, P. Equine Emergency and Critical Care Medicine. Boca Raton: CRC Press, Taylor & Francis Group. 2015.
  16. Tan, K.-H., & Wang, T.-L. Asphyxiants: Simple and Chemical. Ann Disaster Med. 2005.
  17. Lascola, K. M. Pleuropneumonia in Horses. MSD Veterinary Manual. 2023.
  18. Adam, E. N. How to Cope With Barn Fires. AAEP. 2012.
  19. Eppich, W. Barn Fire Protection. Fire Safety in Barns. Accessed Apr. 23, 2024.
  20. The Danger of Smoke. Fire Safety in Barns. 2014.
  21. Dahl, K. In a Changing Climate, What Does a “Normal” Year of Wildfires Look Like?. The Equation. 2023.
  22. Guidelines for Horses Exposed to Wildfire Smoke. UC Davis School of Veterinary Medicine. 2017.