Worms, insects, lizards: they all can develop a fever
Most of us, especially those with children, know how alarming and worrying elevated body temperature, in other words fever, can be. We use a thermometer to find out whether a child is feverish, but traditionally Trobriand Islanders used head lice which they’d put on the heads of their children. These ectoparasites do not like high temperatures and therefore their disappearance from the heads of a child would indicate to the parent that the child has developed a fever and something is wrong with the child’s health; after all a healthy child entertains head lice. Fever is probably the oldest and best known manifestation of an infection and ill health, but its function is still debated and it is not generally accepted whether it’s a harmful or trivial, temporary side effect, or whether it is actually beneficial.
It might come as a surprise to hear that insects, crayfish and even worms like leeches can develop a fever following an infection. The question, of course, arises how one would know that these ectothermic animals, commonly referred to as cold-blooded, become feverish. Since their body temperatures are controlled from the outside, hence the term ectothermic, a feverish individual will seek an environment with a temperature that is higher than that which an uninfected, healthy individual would select. Observations based on fevers in a range of quite unrelated ectothermic animal species led to the conclusion that the fever control system of both invertebrate and vertebrate organisms was basically similar.
Strangely, however, pathogenic micro-organisms that are most susceptible to higher temperatures like Neisseria gonorrhoea rarely cause high fever, whereas those micro-organisms which do induce highly elevated body temperatures, like for instance the malaria parasite, are unaffected by these. Studies by a certain Dr M Kluger of the Michigan Medical School on cold-blooded lizards have unequivocally shown that animals, which were injected with a pathogen and then had a chance to select a higher than normal environmental temperature, were significantly more successful in repelling the disease than lizards, which were not given the opportunity to raise their body temperature.
The fact, however, that an increase in temperature concomitantly lowers the serum iron content of the blood and iron represents an essential co-factor for the growth of any bacteria, suggests that perhaps not the elevated temperature by itself, but the lack of iron available to the pathogen, is the reason why the feverish lizards survive. This theory was put to the test in a mammal, namely the New Zealand rabbit, and found to hold true. At the febrile temperature of 41°C, the growth of the pathogenic bacteria was inhibited by low but not by high blood iron concentrations. An explanation for the thousands of years old enigma of fever and its evolutionary significance, thus, seems to have been found at last – thanks in part to Dr Kluger’s lizards and New Zealand rabbits.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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