Do fever and anapyrexia exist? Analysis of set point-based definitions

Area of Science:

• Physiology
• Thermoregulation
• Homeostasis

Background:

• Fever and anapyrexia are traditionally defined by body temperature changes relative to a set point (SP).
• Existing SP-based definitions for thermoregulatory responses have faced criticism but remain unchallenged.
• Understanding the precise mechanisms of fever and anapyrexia is crucial for physiological research.

Purpose of the Study:

• To critically evaluate the validity of set-point (SP)-based definitions of fever and anapyrexia.
• To test whether experimental data align with the theoretical underpinnings of these thermoregulatory responses.
• To propose alternative frameworks for defining thermoregulatory states.

Main Methods:

• Comparison of SP-based definitions with experimental data on thermoeffector activation thresholds.
• Analysis of body temperature (T(b)) dependence on ambient temperature (T(a)) in fever and anapyrexia.
• Evaluation of the consistency of experimental findings with single SP shift models versus effector threshold dissociation.

Main Results:

• Experimental data partially support SP increase for fever but are incompatible with a decreased SP for anapyrexia.
• Anapyrexia mechanisms align with effector threshold dissociation, not a reduced SP.
• Body temperature in anapyrexia cases showed strong dependence on ambient temperature, contradicting SP-based definitions.

Conclusions:

• The SP-based definition of fever has limited experimental support.
• The phenomenon termed anapyrexia, as defined by a decreased SP, does not exist.
• Alternative definitions for thermoregulatory responses, focusing on active and passive process balance, are proposed.