Pyrexia, Hyperthermia, Fever

What part of the brain functions as the thermoregulatory center (“thermostat“)?

Anterior Hypothalamus

How do animals cool down/initiate heat loss?

• Panting

• Vasodilation

• Postural changes (IMAGE)

• Precipitation (paws)

• Increased grooming (cats)

• Seek cool environments

What is the difference between Hyperthermia and Fever/Pyrexia?

• Hyperthermia

  • Body temperature that is in excess of the normal range

• Fever and Pyrexia

  • Synonymous terms

  • A type of hyperthermia where the “set point“ in the hypothalamus has been reset

  • The body is now trying to maintain a body temperature higher than normal values

What causes the anterior hypothalamus to be reset to a higher temperature?

• Pyrogens

  • Exogenous Pyrogens

    • Foreign substances induce the immune system to release endogenous pyrogens in an attempt to kill the invader

  • Endogenous Pyrogens

    • Cytokines released by the host cell to induce fever

What is the most important cytokine involved in inducing fever?

Interleukin-1 (IL-1)

What is the primary cell involved with Endogenous Pyrogens?

Macrophages

How do endogenous pyrogens affect the Anterior Hypothalamus?

1. They travel in the blood to the AH (brain)

2. Bind to vascular endothelial cells

3. Stimulate the release of prostaglandins (PGE2)

4. Set point is reset at a higher temperature

T/F: NSAIDs are good at treating hyperthermia

False, they are good at treating Pyrexia/fever

Why can NSAIDs be used to treat Pyrexia?

They block the release of Prostaglandins (PGE2) in the AH, preventing the internal temperature from being reset

What is Heat Stroke? Can NSAIDs treat it?

• Inadequate heat dispersion

  • The body cannot handle the excess temperature

• It’s not fever so NSAIDs are ineffective

How is heat stroke treated? How is exercise hyperthermia treated?

• Total Body Cooling

• IV fluids

These diseases have similar issues just slightly different causes

What is one way a patient can have exercise hyperthermia without performing a strenuous activity (hunting, jogging, working)?

Seizures/tremors

What is Pathologic Hyperthermia? Can it be treated with NSAIDs?

• Lesions in the AH that disrupt the thermoregulatory centre

  • Ex: Hydrocephalus

• No

T/F: Hypermetabolic disorders can cause severe hyperthermia

False, they typically can only cause mild hyperthermia

Name 2 metabolic conditions (hyper metabolic disorders) that could cause hyperthermia

1. Hyperthyroidism

   1. Thyroid hormone is responsible for body temperature, development, metabolism

2. Pheochromocytoma

   1. Tumor of adrenal medulla

   2. Causes excess eppinepherine

What are the steps you should take in order to differentiate fever from inadequate heat dispersion (hyperthermia)?

1. Just because the pet’s temperature is increased does not mean that it has a fever

   1. Look for evidence of attempted heat dispersion

      1. Panting, postural changes

   2. If present, then this is not a fever or the fever is breaking

When the fever is less than 106°F (41°C), how should you treat it?

1. The fever is not life-threatening at this stage and likely has benefits

   1. Inhibit viral replication, increased leukocyte function

2. Identify and treat the underlying disease, not the fever

When the fever is between 106-109°F (<41°C), how should you treat it?

• Consider treatment, the fever may be doing more harm than good

• Use NSAIDs

• IV fluids

When the fever is >109°F (43°C), how should you treat it?

• This is a life-threatening hyperthermia

  • Loss of cell membrane integrity, enzyme denaturation, DIC, and organ damage

  • Guarded prognosis

• Treatment depends on the cause

  • If pyrexia

    • NSAIDs, IV fluids, cool patient down ASAP

  • If heat stroke

    • Total body cooling

      • Cool water bath (ICE COLD IS BAD IT CAUSES VASOCONSTRICTION)

      • Wet towels (change constantly)

      • Fans

      • Wet paws with alcohol

    • IV fluids

    • Cold water enema

What is one thing that you have to be very conscious of when treating a patient with heat stroke?

• Overcooling

  • You want to correct the hyperthermia without causing hypothermia

  • Stop cooling the patient when they get under 104°F (40°C)

    • Let the patient return to normal temperature itself, 104°F is not a dangerous fever

What are the steps involved in diagnosing a fever of unknown origin (FUO)?

1. Complete history and physical exam

2. Bloodwork

   1. CBC, Chem, UA, urine culture

3. Diagnostic imaging

   1. Radiographs, ultrasound, echocardiogram

4. Specific infectious disease testing

   1. Viral (FeLV, FIV), bacterial (lepto), protozoal, rickettsial, fungal

5. Specialized diagnostic tests

   1. Arthrocentesis (Immune-mediated polyarthritis), CSF tap, CT/MRI