Heatstroke in Exotic Companion Mammals

Key Points

  • Heatstroke is characterized by neurologic dysfunction paired with hyperthermia.
  • History is usually the most helpful factor in diagnosing heatstroke, however owners do not always realize when the environment is too warm for their pet chinchilla or when ventilation is inadequate for their rabbit.
  • Always consider heatstroke when body temperature exceeds 104° to 105°F (40° to 40.6°C) without evidence of inflammation.
  • Heat-induced illness can cause dysfunction of all major organ systems.
  • Treatment is aimed at reducing core body temperature while supporting organ function with oxygen and fluid resuscitation.
  • Do not immerse your patient in cold water or ice baths since this will cause severe peripheral vasoconstriction, thereby inhibiting the patient’s ability to dissipate heat.
  • Do not cool your patient to the point of hypothermia, as this will worsen prognosis.

Understanding heatstroke

Heatstroke is the most severe form of heat-related illnesses. In this life-threatening condition, the body is unable to dissipate heat load at a rate that accommodates excessive heat levels (Fig 1).

ferret Roy Maloon

Figure 1. Heatstroke is a life threatening condition. Photo credit: Roy Maloon via Flickr Creative Commons.

Heatstroke is characterized by neurologic dysfunction paired with hyperthermia (Box 1). Although variations in tissue perfusion can lead to decreased, normal, or increased rectal temperature, measurements typically exceed 104° to 105°F (40° to 40.6°C).

Box 1. Clinical signs of hyperthermia

  • Lethargy, depression (early)

  • Weakness (early)

  • Ataxia (early)

  • Hyperemic mucous membranes

  • Brief capillary refill time

  • Poor peripheral perfusion, weak pulses

  • Compensatory sinus tachycardia

  • Cardiac arrhythmias

  • Panting, tachypnea

  • Respiratory distress


  • Hypersalivation

  • Diarrhea, hematochezia, melena

  • Petechia

  • Oliguria, anuria

  • Muscle tremors

  • Collapse

  • Loss of consciousness

  • Seizure activity

  • Coma

  • Cardiopulmonary arrest

Heatstroke can quickly develop with exposure to high ambient temperatures, particularly when certain risk factors related to species, husbandry, or body condition (Box 2) are present.

Box 2. Factors that increase the risk of heatstroke


Signalment

  • Extremely young or old age

  • Chinchillas (Chinchilla laniger) are native to the cool mountainous regions of South America and do not tolerate temperatures above 80°F (27°C) well

  • Rabbits (Oryctolagus cuniculus) are also susceptible to heat stress, particularly breeds with a thick coat

  • The ferret (Mustela putorius furo) can also overheat rapidly because of its long, lean shape



Environment

  • Humid climate

  • Ambient temperature exceeding 85°F (29°C) (see signalment above)



Husbandry

  • Closed environment with poor ventilation

  • Housed outdoors without shade

  • Lack of drinking water



Medical history

  • Obesity

  • Dehydration

  • Underlying disease, concurrent stress

  • Cardiovascular disease

  • Neuromuscular disease

  • Previous history of heat-related illness

History is usually the most helpful factor in diagnosing heatstroke, however owners do not always realize when the environment is too warm or when ventilation is inadequate. Always consider heatstroke in your differential diagnosis list when body temperature exceeds 104° to 105°F (40° to 40.6°C) without evidence of inflammation. Be sure to rule out other conditions such as central nervous system emergencies.

Hypotension or shock may develop in up to 30% of patients suffering from heatstroke. The most serious complication is multisystem organ dysfunction (Box 3). Dysfunction of all major organ systems can occur due to denaturation of cellular proteins and enzyme activities, inappropriate shunting of blood, low blood pressure and decreased oxygen delivery, as well as lactic acidosis. Severe abnormalities in electrolytes and pH can lead to cerebral edema and death.

Box 3. Heatstroke can lead to dysfunction of all major organ systems
Affected OrganPossible pathologic changes


Cardiovascular

  • Cardiac dysrhythmias

  • Hypovolemia

  • Myocardial ischemia and necrosis



Central nervous system

  • Neuronal damage

  • Parenchymal hemorrhage

  • Cerebral edema



Gastrointestinal

  • Mucosal ischemia and ulceration

  • Bacterial translocation

  • Endotoxemia

  • Pancreatic injury



Hepatobiliary

  • Hepatocellular necrosis



Renal

  • Acute renal failure



Hemolymphatic/immune

  • Hemoconcentration

  • Thrombocytopenia

  • Disseminated intravascular coagulation (DIC)



Musculoskeletal

  • Rhabdomyolysis

Key points of urgent care

Begin treatment immediately once heatstroke is suspected. Intensive care is aimed at reducing body temperature while supporting organ function. A variety of techniques can be used to lower core body temperature. Administration of intravenous (IV) or intraosseous (IO) fluids is a popular internal cooling technique that also serves to support organ function (see below). Surface cooling techniques are also commonly employed (Box 5). Peritoneal dialysis, gastric lavage and rectal administration of cool, isotonic fluids have all been described in dogs and cats, however keep in mind that the rabbit’s rectal wall is relatively thin, delicate, and more easily traumatized.

Box 5.Surfacing cooling techniques
Cooling method
DefinitionMethod
ConductionIncreases the temperature gradient between the skin and the external environmentApply cold packs over large vessels in the axilla, groin and neck
EvaporationIncreases the water vapor pressure gradient between the skin and the environment

  • Spray or immerse the skin with tepid water 77° to 86°F (25° to 30°C)

  • Apply alcohol to the ears, foot pads, axilla and groin

ConvectionIncreases the velocity of air impacting the skinPass ambient air across the body with a fan

Cool the animal gradually, while closely monitoring rectal or esophageal temperature. Discontinue cooling methods as body temperature approaches 103°F (39.4°C).

 

DO NOT…

  • Cool the patient to the point of hypothermia as this will worsen prognosis.
  • Immerse your patient in cold water or ice baths since this will cause severe peripheral vasoconstriction, thereby inhibiting the patient’s ability to dissipate heat.
  • Administer antipyretics in heatstroke. These non-steroidal anti-inflammatory agents are indicated for management of true fever.

While cooling the patient, it is also important to support organ function. Administer supplemental oxygen via oxygen cage, face mask or nasal catheter until it has been determined that the patient can maintain adequate arterial oxygenation. Administer IV or IO fluids to replenish volume and correct acid-base/electrolyte abnormalities.

 

Case management

  1. Monitor the patient closely. Intensive care is often needed for a minimum of 24 to 48 hours. It is crucial to minimize stress in all hospitalized small mammals, however closely monitor heart rate, blood pressure, urine output, and of course body temperature. After an episode of heat stress, core temperature may be unstable for a few days.Monitor ferrets for early signs of concurrent gastritis such as reduced appetite or teeth grinding, and promptly begin treatment. Prophylactic treatment against Helicobacter gastritis should at least include administration of histamine (H2) receptor blockers.
  2. Continue supportive care. Provide intravenous or intraosseous fluids and encourage cage rest.
  3. Obtain a minimum database.
    • Collect a blood sample in order to run a complete blood count, biochemistry panel, and urinalysis to rule out underlying disease processes. Blood work may also help to identify potential sequelae such as disseminated intravascular coagulation (DIC).
    • Survey chest and abdominal radiographs can identify underlying cardiopulmonary disease or identify predisposing factors.
    • Also perform a complete neurologic exam including mentation and cranial nerve function to establish a baseline.
  4. Begin broad-spectrum antibiotics if bloody diarrhea develops. Gastrointestinal bleeding will predispose the patient to bacterial translocation and overgrowth of opportunistic pathogens.See Antimicrobial Therapy and Dysbiosis and Gastrointestinal Problems in Rabbits for more information.
  5. Administer anti-inflammatory agents?Non-steroidal anti-inflammatory agents such as meloxicam (0.3 mg/kg PO q24h) are sometimes recommended to minimize endotoxemia. Corticosteroids have not been shown to be helpful and rabbits are also extremely sensitive to the immunosuppressive effects of these drugs.
  6. Administer benzodiazepines as needed. Take steps to reduce the production of heat associated with shivering or agitation (midazolam 0.25-1.0 mg/kg IM). Also use benzodiazepines to control seizure activity. Use diazepam (1-2 mg/kg IV) to control status epilepticus in the rabbit.
  7. Treat complications:  A variety of complications may arise such as renal failure or cerebral edema. Manage cerebral edema using cat/dog protocols.

Prognosis

The prognosis for heatstroke is often guarded to grave in small mammals, and many affected animals do not survive. Prognosis can vary with the prior health status of your patient as well the degree and duration of heat stress. The prognosis also worsens when multiple organ dysfunction or severe central nervous system disturbance is documented (Box 6).

Box 6. Factors that can worsen prognosis

  • Ventricular arrhythmia

  • Hypoglycemia

  • Hypoalbuminea

  • Bilirubinemia

  • Elevated creatine values

  • Decreased cholesterol

Prevention

Since the prognosis for heatstroke is poor, it is much better to prevent rather than treat this condition. Small mammals, such as ferrets, chinchillas, and rabbits, should be housed in well-ventilated enclosures at temperatures ideally between 65° to 75°F (18° to 24°C). An adequate supply of cool, drinking water should always be available. Provide hutch rabbits with adequate shade.

Warn owners that an episode of heatstroke can predispose pets to future problems with heat stress.

 

Looking for additional information?

Check out “Heat Stroke” by Kara Burns, LVT, VTS (Nutrition) on the Veterinary Team Brief website at http://www.veterinaryteambrief.com/article/heat-stroke.

 

References