It has a mortality rate of at least 40 per cent and no antidote, yet the preferred treatment for heat stroke in dogs has been virtually unchanged since the condition was first recognised. By Tracey Porter
Rising global temperatures are the cause of an ever-greater number of threats. But few cause more distress to veterinary emergency and critical care specialist Dr Mark Haworth than heat-related illnesses (HRIs), in particular, the potentially fatal condition of heat stroke in dogs.
Having dealt with the fall out of life-threatening illnesses in small animals for the past 15 years, Dr Haworth says gaining a better understanding of why domestic animals develop heat stroke can help to refine future prevention strategies.
He believes better education about the impact of heat-related illness (HRI) among dogs may also assist veterinary professionals when advising owners on breed selection.
There are two forms of HRI: non-exertional due to high environmental temperatures and exertional resulting from strenuous exercise. “The latter has actually been documented in other countries as more prevalent, but these occur during the warmer months so they go hand in hand,” he says.
While few figures are kept about the incidence of heat stroke in dogs in Australia, Dr Haworth says anecdotal evidence suggests the numbers vary from state to state largely because environmental conditions and humidity play a big role in the development of the condition.
A research paper published in the UK last year reviewed the veterinary records of over 900,000 dogs with the aim of determining the most common triggers of heat-related illness.
The study found that while the general consensus is that HRI is typically due to dogs being left in hot cars, this was incorrect.
Instead, exercise was the most common trigger of heat stroke in dogs (in 74.2 per cent of events), followed by environmental (12.9 per cent). Vehicular confinement was responsible for just 5.2 per cent of diagnosed cases.
Dr Haworth says heat stroke is about heat gain versus heat loss. Dogs lose the majority of heat by convection and radiation to the environment. But as environmental temperatures increase during the spring and summer months, evaporation then becomes more important.
This is because dogs perform evaporative cooling by panting. Their bodies also send blood to the periphery to lose heat to the environment from the body’s core.
Dr Haworth says this can further narrow the airway—“think big floppy tongues on hot days”—and increase resistance to breathing, turbulence and airway inflammation.
The extra work of breathing this creates also generates heat and increases oxygen consumption, he says.
“This vicious cycle impedes the loss of heat, and heat stroke may ensue unless an alternate cooling mechanism can be employed.”
Those most at risk
Dr Haworth says this is why brachycephalic (flat-faced) dogs such as pugs and French bulldogs are more susceptible to suffering heat stroke than other breeds.
Other risk factors include dogs who are old, overweight or who have dense hair coats. Those that already have airway diseases such as laryngeal paralysis, collapsing trachea or cardiovascular disease are also more likely to experience HRI.
Because the veterinary diagnosis of HRI is heavily dependent upon an accurate history of the events leading up to the animal’s presentation, Dr Haworth says it is vital that veterinary professionals recognise the specific risk factors for HRI in dogs to help support earlier recognition, diagnosis and appropriate management.
Dr Haworth says every organ in the body is impacted by heat stroke as the “thermal insult” takes place.
Typically, animals with heat stroke will present with elevated temperatures but this isn’t always the case. “Classically elevated temperatures are above 40°C. Excessive noisy panting is usually witnessed as these animals attempt to lose heat, so you can often hear the patient before you see them.”
Dr Haworth says research has shown most dogs with heat stroke may collapse or present in vasodilatory shock while displaying signs of injected mucous membranes, rapid capillary refill time, bounding pulses, very high heart rates, lethargy and even seizures. Dogs may also have bleeding which may be noted when clipping the fur from the skin. Vomiting and diarrhoea are also common, he says.
Heat stroke is notoriously difficult to treat. As no antidote exists, supportive therapy and close monitoring is key. In the initial stabilisation period, oxygen, sedation, and cooling the animal is paramount, Dr Haworth says. “This can be efficiently achieved by continuous drenching using tap water combined with fans to accelerate convection and evaporation. This should be done by owners for around 10 minutes. In the clinic we will cool the animal until rectal temperatures reach 39.5°C and then cease the cooling to avoid overshooting to hypothermia. Rapid intravascular volume expansion with isotonic fluids is usually accompanied in the initial phase to combat decreased effective circulating volume. After the initial stabilisation period, fluid balance may be tricky, as there’s likely ongoing losses from the gastrointestinal tract which may be complicated by acute kidney injury (AKI) and oliguric or anuric renal failure. Fluid overload then becomes a risk to the patient.”
Dr Haworth says a patient with active bleeding and prolonged clotting times will require fresh frozen plasma for clotting factors or fresh whole blood for platelets if they are severely low.
Getting some nutrients in can start the healing process for the body, however, as more organ systems shut down the likelihood of mortality increases, he says.
Globally, the mortality rate sits at between 40-64 per cent.
Dr Haworth says while treatment has not changed much over time, treatments for AKI—such as continuous renal replacement therapy—are now becoming possible. Access to this technology for veterinary patients, however, is expensive and has limited availability.
Reducing the incidence of heat stroke really comes down to educating owners, he says. “Surgical correction of stenotic nares and elongated soft palates early on, and management toward a lean body condition will play a massive role in reducing heat stroke and giving these dogs a greater quality of life.”