Symptoms and Signs – Host Defenses or Pathogen Effects

Should a fever be treated? To answer that question, we should consider whether fever has a function. There are (at least) three possibilities for the function of fever. The first is that fever has no function – it may be a byproduct or an epiphenomenon of some other process. The second possibility is that it has function, but not for the patient. Fevers might benefit pathogens, promoting pathogen replication and transmission. The third possibility is that fever is part of a host defense.  Fever helps patients get rid of infection or potentiate immune function.

The host defense hypothesis of fever predicts that higher temperatures help eliminate infections. This proposition has been tested – in iguanas!

Iguanas are a great test animal for this because their temperature can be manipulated and changed by altering their environment. Matthew Kluger – a physiologist who did some of his groundbreaking work on fever right here in Albuquerque – injected iguanas with bacteria and then placed the animals in a heated enclosures at different temperature. Half the animals housed at 93 degrees died, and all the iguanas kept at 108 degrees survived!

Fever is accompanied by endogenous pyrogens – chemicals produced by the body that change the temperature set point of warm-blooded animals. These molecules seem to be important in generating inflammatory response that combats infection. So….should we treat iguanas or humans with tylenol or aspirin? Iguanas are more likely to die when given aspirin-like salicylates. The data is more uncertain in humans. What do you think?

Fever aside, consider whether diarrhea or sneezing benefits the patient or the pathogen…

Kluger’s review of fever can be found here: The Adaptive Value of Fever

Please read that one, and go to eReserve to download the Signs and Symptoms in Infectious Disease, Chapter 3 in Why We Get Sick. Those are the only two required readings for next session. Blair Wolf PhD will be presenting that topic on 8/15/08.

An additional reading is here: EVReview

Good luck!

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