Section: Clinical Methods & Interventions

Dog Heat Related Illness

Heat related illness (HRI) in dogs represents a continuum of clinical syndromes ranging from mild heat stress to life-threatening heatstroke. As global ambient temperatures rise and the popularity of brachycephalic breeds increases, the veterinary profession has witnessed a significant surge in HRI cases. Recent epidemiological work from the VetCompass programme has demonstrated that HRI is not merely a problem of dogs left in hot cars; exertional heatstroke, particularly in working and sporting dogs, is a greater threat to canine health in temperate climates like the United Kingdom [14]. This pillar article synthesises the latest evidence, clinical grading tools, and management protocols to equip veterinary professionals and dedicated pet owners with a comprehensive understanding of canine HRI.

Quick Q&A

Question: What is the most important first-aid step for a dog showing signs of heat related illness?

Answer: The most critical step is to immediately initiate active cooling using methods such as dousing the dog with tap water (not ice water) and moving them to a shaded, ventilated area. Avoid using ice or very cold water as this can cause peripheral vasoconstriction, trapping heat internally. Seek emergency veterinary care without delay.

Pathophysiology of Canine Heat Related Illness

Dogs have limited capacity for thermoregulation compared to humans. They possess few eccrine sweat glands, relying primarily on evaporative cooling through panting and limited sweating from paw pads. When environmental heat load exceeds the dog's ability to dissipate heat, core body temperature rises. Once the core temperature exceeds approximately 40.6°C (105°F), a cascade of systemic pathophysiology ensues.

At the cellular level, heat stress denatures proteins, disrupts mitochondrial function, and triggers the release of pro-inflammatory cytokines. This leads to a systemic inflammatory response syndrome (SIRS), endothelial injury, and disseminated intravascular coagulation (DIC). Multi-organ dysfunction typically involves the central nervous system (cerebral oedema), kidneys (acute kidney injury), liver (hepatic necrosis), and gastrointestinal tract (mucosal barrier breakdown leading to endotoxaemia). The severity of HRI is directly proportional to the magnitude and duration of hyperthermia.

Clinical Grading: The VetCompass Tool

One of the most significant advances in the clinical management of canine HRI has been the development and validation of the VetCompass clinical grading tool. Originally proposed by Hall et al. in 2021, this tool provides a standardised framework for categorising HRI severity [13]. The tool was subsequently revised and validated using a large cohort of dogs presenting to UK emergency veterinary practices in 2022 [1, 4].

The grading system classifies HRI into four stages:

  • Grade 1 (Mild): Dogs present with mild hyperthermia (rectal temperature 39.0-40.5°C), excessive panting, and mild lethargy. No evidence of organ dysfunction.
  • Grade 2 (Moderate): Temperature 40.5-42.0°C with more pronounced tachypnoea, hypersalivation, and mild neurological signs such as ataxia or dull mentation.
  • Grade 3 (Severe): Temperature >42.0°C with severe neurological compromise (stupor, seizures, coma), vomiting/diarrhoea, and evidence of coagulopathy.
  • Grade 4 (Critical): Life-threatening multi-organ failure, refractory hypotension, DIC, and cardiorespiratory arrest.

This tool allows clinicians to rapidly stratify patients, guide treatment intensity, and provide prognostic information to owners. In the 2022 UK study, Grade 3 and 4 cases carried a mortality rate of approximately 50% [4].

Epidemiology and Risk Factors

Incidence and Regional Variations

The incidence of HRI in dogs varies significantly by geography and climate. In New South Wales, Australia, a longitudinal study spanning 1997 to 2017 found that daily ambient temperatures and the frequency of extreme heat days were strongly associated with increased risk of HRI mortality [2, 6]. In the UK, the incidence of HRI under primary veterinary care in 2016 was estimated at 0.98% (approximately 1 in 100 dogs) [15]. However, the 2022 emergency care study reported a higher incidence, reflecting both increased awareness and more severe environmental conditions [4].

Breed and Conformation

Brachycephalic breeds are disproportionately affected by HRI. The anatomical constraints of brachycephalic obstructive airway syndrome (BOAS) severely impair evaporative cooling capacity. Clark (2022) conducted a systematic review confirming that brachycephalic dogs, particularly English Bulldogs, French Bulldogs, and Pugs, are at significantly increased risk [8]. Hall et al. (2022) identified that brachycephalic breeds had 2.5 times greater odds of developing severe or fatal HRI compared to mesocephalic breeds [9].

Other high-risk breeds include:

  • Chow Chows (thick coat, inefficient panting)
  • Golden Retrievers and Labrador Retrievers (high exercise drive)
  • Siberian Huskies and Alaskan Malamutes (adapted for cold climates)

Exertional vs. Environmental Heatstroke

A critical distinction exists between exertional and environmental (classic) heatstroke. Exertional HRI occurs during or immediately after physical activity, even in moderate ambient temperatures. Hall et al. (2020) found that exertional heatstroke accounted for a larger proportion of UK cases than classic environmental heatstroke [14]. This is particularly relevant for dogs participating in canine sports such as canicross, agility, and flyball. A 2024 study on post-exercise management of canicross dogs in the UK revealed that many handlers lacked adequate cooling protocols, placing dogs at risk [5].

Environmental HRI typically results from confinement in hot environments (e.g., cars, poorly ventilated kennels) or exposure to extreme heat waves. Moon et al. (2021) documented that pet dogs in the southern United States experience significant environmental heat exposure, with rural dogs having fewer opportunities for indoor cooling [12].

Body Condition and Age

Obesity is a well-established risk factor for HRI. Adipose tissue acts as a thermal insulator and increases metabolic heat production. Older dogs and those with underlying cardiorespiratory disease are also at increased risk due to reduced thermoregulatory reserve.

Clinical Presentation and Diagnosis

History and Signalment

A thorough history should include:

  • Duration and intensity of heat exposure or exercise
  • Access to shade and water
  • Breed, age, weight, and body condition score
  • Pre-existing medical conditions (particularly cardiac, respiratory, or endocrine)
  • Current medications (e.g., diuretics, phenothiazines)

Physical Examination Findings

Key clinical signs include:

  • Hyperthermia: Rectal temperature >39.2°C (102.5°F) is abnormal; temperatures >40.6°C (105°F) are considered critical.
  • Respiratory: Tachypnoea, panting, stertor (in brachycephalic dogs), and in severe cases, respiratory distress.
  • Cardiovascular: Tachycardia, weak pulses, prolonged capillary refill time, and hypotension.
  • Neurological: Altered mentation (depression, stupor, coma), ataxia, seizures, and mydriasis.
  • Gastrointestinal: Vomiting, diarrhoea (often haemorrhagic), and hypersalivation.

Diagnostic Testing

The minimum database for suspected HRI includes:

  • Haematology: Haemoconcentration, thrombocytopenia, leukocytosis or leukopenia.
  • Biochemistry: Elevated liver enzymes (ALT, AST), azotaemia (BUN, creatinine), hyperphosphataemia, and hyperkalaemia.
  • Coagulation Profile: Prolonged PT and aPTT, elevated D-dimers, and thrombocytopenia indicate DIC.
  • Blood Gas Analysis: Metabolic acidosis with respiratory compensation.
  • Urinalysis: Proteinuria, haemoglobinuria, and casts.

Advanced imaging (thoracic radiographs, abdominal ultrasound) may be indicated to assess for aspiration pneumonia, pancreatitis, or other complications.

Emergency Management and Treatment

Immediate Cooling

The cornerstone of HRI management is rapid, controlled cooling. The goal is to reduce core temperature to 39.5°C (103°F) within 30-60 minutes. Overcooling (hypothermia) must be avoided as it can precipitate cardiac arrhythmias and worsen coagulopathy.

Recommended cooling methods:

  • Dousing with tap water (15-20°C) is the most effective and safest method [7].
  • Placing the dog in front of a fan to enhance evaporative cooling.
  • Applying cool, wet towels to the groin, axillae, and neck (replace frequently).
  • Intravenous fluid therapy with cooled crystalloids (if vascular access is available).

Contraindicated methods:

  • Ice water immersion (causes peripheral vasoconstriction and shivering).
  • Alcohol wipes (risk of toxicity and rapid cooling).
  • Gastric or peritoneal lavage with cold water (invasive and unproven benefit).

A UK-based study by Hall et al. (2023) reviewed cooling methods used in primary care practices and found that while most veterinarians used water dousing, there was significant variability in technique [7]. Standardised protocols are urgently needed.

Fluid Resuscitation

Aggressive intravenous fluid therapy is essential to correct hypovolaemia and support organ perfusion. Isotonic crystalloids (e.g., lactated Ringer's solution, Normosol-R) at shock doses (60-90 mL/kg in dogs) should be administered in boluses, titrated to effect. Colloids may be considered in cases of refractory hypotension.

Advanced Therapies

For severe cases with multi-organ failure, advanced interventions may be necessary:

  • Extracorporeal therapy (ECT): Tracy et al. (2022) reported successful use of haemodialysis in a dog with severe heatstroke, demonstrating that ECT can effectively remove inflammatory mediators and support renal function [11].
  • Plasma transfusion: For DIC with active bleeding.
  • Vasopressors: For refractory hypotension (e.g., norepinephrine, vasopressin).
  • Anticonvulsants: Diazepam or levetiracetam for seizure control.

Monitoring

Continuous monitoring of core temperature, heart rate, respiratory rate, blood pressure, and urine output is mandatory. Serial blood work (every 4-6 hours) should assess organ function and coagulation status.

Prognosis and Outcomes

Prognosis is highly dependent on the grade of HRI at presentation. In the VetCompass validation study, Grade 1 cases had an excellent prognosis (mortality <5%), while Grade 4 cases carried a mortality rate exceeding 75% [1, 4]. Factors associated with poorer outcomes include:

  • Core temperature >42°C at presentation
  • Severe neurological signs (coma, seizures)
  • DIC
  • Acute kidney injury requiring dialysis
  • Brachycephalic conformation

Long-term sequelae may include chronic kidney disease, neurological deficits (e.g., cerebellar ataxia), and persistent thermoregulatory dysfunction.

Prevention and Owner Education

Prevention is the most effective strategy against HRI. Veterinary professionals should counsel owners on the following:

Environmental Management

  • Never leave a dog in a parked car, even for a few minutes, even with windows cracked.
  • Provide constant access to shade and fresh, cool water.
  • Avoid outdoor exercise during peak heat hours (10 AM to 4 PM).
  • Use cooling mats, vests, and fans for high-risk dogs.

Breed-Specific Considerations

Owners of brachycephalic breeds should be advised that their dogs have a significantly reduced capacity for thermoregulation. The AVA (Australian Veterinary Association) and AVMA both recommend that brachycephalic dogs should not be exercised in temperatures exceeding 25°C (77°F). Weight management and BOAS surgery (e.g., nares resection, soft palate resection) can improve airway function and reduce HRI risk.

Working and Sporting Dogs

Detection dogs working in hot climates, such as those studied by Slotta-Bachmayr et al. (2024), require careful monitoring of thermoregulation and hydration status [3]. Handlers should implement mandatory rest breaks, shade provision, and pre- and post-exercise temperature checks. The use of cooling vests and electrolyte supplementation may be beneficial.

Heat Wave Preparedness

During extreme heat events, owners should be proactive. Tripovich et al. (2025) demonstrated that mortality risk increases significantly on days with extreme ambient temperatures [2]. Veterinary clinics should consider sending heat alerts to clients and offering cooling station services.

Special Populations

Puppies and Geriatric Dogs

Puppies have a higher surface area-to-volume ratio and less efficient thermoregulation. Geriatric dogs often have concurrent diseases (e.g., heart failure, renal disease) that impair their ability to cope with heat stress.

Dogs with Pre-existing Conditions

Dogs with laryngeal paralysis, tracheal collapse, or cardiac disease are at heightened risk. Medications such as furosemide (which causes dehydration) and phenothiazines (which impair thermoregulation) should be used with caution during hot weather.

Conclusion

Dog heat related illness is a preventable yet increasingly common veterinary emergency. The development of the VetCompass clinical grading tool has revolutionised the ability to stratify patients and guide treatment. As global temperatures continue to rise, the veterinary profession must prioritise owner education, breed-specific risk communication, and the implementation of standardised cooling protocols. For the individual clinician, rapid recognition, aggressive cooling, and multi-organ support remain the pillars of successful management. For the profession, ongoing surveillance and research, such as that conducted by the VetCompass programme, will be essential to mitigate the impact of this life-threatening condition.

References

  1. Beard S, Hall EJ, Bradbury J, et al. Evaluation and validation of a revised VetCompass clinical grading tool for heat-related illness in dogs. Sci Rep. 2026;16:41513723.
  2. Tripovich JS, Gopi K, Morgan GG, et al. Feeling the heat: associations among daily ambient temperatures, extreme heat days and risk mortality in Australian dogs from New South Wales (1997-2017). Aust Vet J. 2025;103(4):e40568805.
  3. Slotta-Bachmayr L, Oyugi A, Mutoro N, et al. Detection Dogs Working in Hot Climates: The Influence on Thermoregulation and Fecal Consistency. Animals (Basel). 2024;14(18):e39272240.
  4. Beard S, Hall EJ, Bradbury J, et al. Epidemiology of heat-related illness in dogs under UK emergency veterinary care in 2022. Vet Rec. 2024;194(12):e38783549.
  5. Carter AJ, Hall EJ, Bradbury J, et al. Post-exercise management of exertional hyperthermia in dogs participating in dog sport (canicross) events in the UK. J Therm Biol. 2024;120:e38518416.
  6. Tripovich JS, Wilson B, McGreevy PD, et al. Incidence and risk factors of heat-related illness in dogs from New South Wales, Australia (1997-2017). Aust Vet J. 2023;101(12):e37899616.
  7. Hall EJ, Carter AJ, Bradbury J, et al. Cooling Methods Used to Manage Heat-Related Illness in Dogs Presented to Primary Care Veterinary Practices during 2016-2018 in the UK. Vet Sci. 2023;10(7):e37505869.
  8. Clark AE. Heatstroke and brachycephalic dogs - is there an increased risk? Vet Evid. 2022;7(3):e42004676.
  9. Hall EJ, Carter AJ, Chico G, et al. Risk Factors for Severe and Fatal Heat-Related Illness in UK Dogs-A VetCompass Study. Vet Sci. 2022;9(6):e35622759.
  10. Hall EJ, Radford AD, Carter AJ. Surveillance of heat-related illness in small animals presenting to veterinary practices in the UK between 2013 and 2018. Open Vet J. 2022;12(1):e35342739.
  11. Tracy A, Lynch A, Messenger K, et al. Use of extracorporeal therapy in a dog with heatstroke. J Vet Emerg Crit Care (San Antonio). 2022;32(2):e34904781.
  12. Moon KE, Wang S, Bryant K, et al. Environmental Heat Exposure Among Pet Dogs in Rural and Urban Settings in the Southern United States. Front Vet Sci. 2021;8:e34676256.
  13. Hall EJ, Carter AJ, Bradbury J, et al. Proposing the VetCompass clinical grading tool for heat-related illness in dogs. Sci Rep. 2021;11(1):e33767275.
  14. Hall EJ, Carter AJ, O'Neill DG. Dogs Don't Die Just in Hot Cars-Exertional Heat-Related Illness (Heatstroke) Is a Greater Threat to UK Dogs. Animals (Basel). 2020;10(8):e32751913.
  15. Hall EJ, Carter AJ, O'Neill DG. Incidence and risk factors for heat-related illness (heatstroke) in UK dogs under primary veterinary care in 2016. Sci Rep. 2020;10(1):e32555323.