Author: Emily Cockerill, BSc (Hons) Equine Science, RVN.

Emily embarked into a career in veterinary nursing following the completion of an Equine Science degree. Having worked for equine and small animal general practice, she now works in a large referral hospital as a wards nurse. She has particular interests in ECC and hospitalised canine behaviour.

Check out the recording of the BVNA webinar presented by Emily on 20^th June 2025 for more information: Heat Stroke – Not just hot days! Recognising and managing hyperthermia in canine patients.

What is Heat Stroke?

Hyperthermia is a progressive life-threatening condition that is characterised by a non-pyrogenic rise in core body temperature. The canine thermoregulatory centre resides in the anterior hypothalamus – activation of which causes compensatory responses such as panting, sweating and shade seeking behaviours to aid thermoregulation to maintain a consistent core body temperature. Hyperthermia may occur due to exposure to high temperatures, over-exertion, pain/stress response and/or increased muscle activity – these factors overwhelm the thermoregulatory centre and there is an imbalance in heat-generating and heat-loss mechanisms ^[1,2].

Hyperthermia differs from pyrexia, and it is important is distinguish which the patient is suffering from before beginning treatment. Pyrexia is the increased core body temperature due to resetting of the hypothalamic thermoregulatory centre to a higher temperature. Infectious disease, immune-mediated disease, neoplasia or drug reactions cause the body to release pyrogens, and these raise the thermoregulatory set point. Pyrexia is generally accompanied with other signs of illness, such as vomiting, shivering, diarrhoea, lethargy or hyporexia. Often, the most pertinent sign that you have a patient who is truly hyperthermic rather than pyrexic is the presence of panting.

Physiology of Hyperthermia

Firstly, there is an increase in cardiac output and decrease in systemic vascular resistance due to peripheral vasodilation; this aims to shift blood to the periphery for cooling. There is then dilation of the splanchnic vasculature, leading to venous pooling and a decrease in circulating plasma volume. This causes hypotension and a decrease in cardiac output. The patient then may enter circulatory shock – there are three subcategories of circulatory shock, and heat shock patients can experience all three – cardiogenic, hypovolaemic and distributive ^[2].

There is then the risk of thermal induced cellular damage and multiorgan dysfunction. Cellular thermal injury can lead to DIC (Disseminated Intravascular Coagulation), hypoxic injury to organs, acute renal failure, damage to platelets causing thrombocytopenia. Neuronal necrosis can also lead to neurological dysfunction such as marked changes in mentation and seizures ^[2].

Risk Factors

There are a variety of factors that can increase the risk of hyperthermia (Table 1).

Table 1 Risk factors of hyperthermia ^[3]

Risk factors	Examples
Brachycephalic dogs	More likely to become hyperthermic due to their poor ineffective evaporative ability (stenotic nares, elongated soft palate and hypoplastic trachea) and their tendency to develop laryngeal oedema
Overweight dogs	It is harder for them to cool via radiation or conduction.
Exertion	Either exercise on a hot day, or prolonged exercise on a day of any
Temperature	Body temperature rises more quickly than cooling methods work, plus it becomes harder to cool via evaporation.
Heat pad use	In animals unable to move away
Seizuring/muscle activity	Creates heat energy that overwhelms the cooling mechanisms. Also, these animals are generally unwell and dehydration can make heat stroke more likely.
Cardiac/respiratory conditions	Such as asthma and laryngeal paralysis – these prevent or hinder panting.
Humidity	Even on cooler days, if there is a high level of relative humidity, heat stroke can occur. Higher levels of air moisture mean cooling methods such as panting and sweating become less effective.
Double coated dogs	Are at higher risk of heat stroke as they struggle with heat dissipation owing to their thicker coats.

Clinical Signs

• Panting (tongue may be flattened)
• Seeking shade or water
• Congested tongue and/or mucous membranes
• Tachycardia
• Hypersalivation
• Vomiting and diarrhoea
• Agitation – to lethargy
• Weakness or collapse
• Epistasis
• Seizures

Current Research

Parnes et al., ^[4] found partial water immersion cooled dogs quicker than isopropyl alcohol application. In addition, alcohol application increased dog’s heart rates more than water immersion. Isopropyl alcohol also creates a burn risk if defibrillation is needed.

Carter at al.,^[5] evidenced that cold-water immersion can be used to effectively and safely cool dogs with exertional hyperthermia. Progressive temperature increases in many dogs, even after exercise has finished, supports “cool first, transport second”.

Hall et al., ^[6] found that around 21% of dogs suffering from heat stroke were cooled before transportation. Just under 24% of dogs were cooled using one of the two recommended cooling methods (cold water immersion or active evaporative cooling with application of cold water). Most common method – application of wet towels, which is not effective.

In horses – Takahashi et al.,^[7] found that horses need aggressive cooling to prevent exertional heat illness, particularly in hot and humid conditions. Continually showering with tap water was the most effective way of decreasing the horse’s core temperature

First Aid Management ^[4,5&8]

• **Cold water immersion**
• If immersion not possible (dog’s temperament, size, unconsciousness, unable to safely maintain own airway etc) then continual application of cold water
• Ensure cold water is hitting skin rather than running off coat
• Cold fan to assist cooling via convection
• Do not use wet towels on top of patient
• Do not use isopropyl alcohol in place of water
• Consider clipping patient if double coated
• Cold water enemas via rectal catheter – 10ml/kg cold water or cold sterile saline over 10 minutes
• Cold water bladder lavage via urinary catheter – 10ml/kg cold, sterile saline over 5-10 minutes
• Prepare airway intervention equipment
• Preparation of intravenous fluid therapy

Post Event Monitoring

If an owner has cooled before transportation, you may have a normothermic or hypothermic dog with severe internal damage. Therefore, post event monitoring is very important. DIC can take hours to days to appear after initial insult therefore close monitoring for clinical signs is vital. Some dogs may suffer heat stroke associated acute kidney injury (AKI), so monitoring kidney function after heat stroke means this can be detected and treated promptly. Acute respiratory distress syndrome may also occur owing to pulmonary oedema following heat stroke, therefore observing respiratory signs is helpful ^[8].

Summary 

Firstly, it is important to establish whether the patient is pyrexic or hyperthermic in these cases. If speaking to the owner over the phone and hyperthermia is suggested then advise they begin active cooling before transporting the patient; this may not reduce the patient’s temperature, but it could help to stop it from raising further. Once the patient is at the practice, begin (or continue) rapid, active cooling if the temperature still requires it. It is important to regularly check the patient’s temperature to prevent hypothermia – recommendations for when to stop vary in literature but is generally considered to be around 39.5 – 39.7 degrees Celsius. Close post-event monitoring is very important to detect and manage any secondary conditions, such as DIC or AKI.

References

1. Bruchim, Y (2018). Management of Heat Stroke in the Dog. World Small Animal Veterinary Association Congress Proceedings. Available from this link.
2. Romanucci, M., & Salda, L. D. (2013). Pathophysiology and pathological findings of heatstroke in dogs. Veterinary Medicine: Research and Reports, 1-9.
3. Hall, E. J., Carter, A. J., & O’Neill, D. G. (2020). Incidence and risk factors for heat-related illness (heatstroke) in UK dogs under primary veterinary care in 2016. Scientific reports, 10(1), 9128.
4. Parnes, S. C., Mallikarjun, A., Ramos, M. T., Stone, T. A., & Otto, C. M. (2023). A randomized cross-over study comparing cooling methods for exercise-induced hyperthermia in working dogs in training. Animals, 13(23), 3673.
5. Carter, A.J., Hall, E.J., Bradbury, J., Beard, S., Gilbert, S., Barfield, D. and O’Neill, D.G., 2024. Post-exercise management of exertional hyperthermia in dogs participating in dog sport (canicross) events in the UK. Journal of Thermal Biology, 121, p.103827.
6. Hall, E.J., Carter, A.J., Bradbury, J., Beard, S., Gilbert, S., Barfield, D. and O’Neill, D.G., 2023. Cooling Methods Used to Manage Heat-Related Illness in Dogs Presented to Primary Care Veterinary Practices during 2016–2018 in the UK. Veterinary Sciences, 10(7), p.465.
7. Takahashi Y, Ohmura H, Mukai K, Shiose T, Takahashi T. A Comparison of five cooling methods in hot and humid environments in Thoroughbred horses. J Equine Vet. 2020;91:103130–103130. doi: 10.1016/j.jevs.2020.103130.
8. Caldas, G. G., da Silva, D. O. B., & Junior, D. B. (2022). Heat stroke in dogs: Literature review. Veterinární medicína, 67(7), 354