About 600 million years ago, when the cooking that gave rise to the first animal forms was taking place, fever appeared. Some living beings discovered that, when they were sick, spending time in a warmer place could relieve them. Today it is known that this rise in temperature improves metabolism, optimizes immune function and suppresses the growth of pathogens. The strategy was so successful that cold-blooded animals, such as fish or lizards, have preserved it for millions of years, some seeking warm water and others spending more time in the sun. When warm-blooded animals appeared, such as humans, this defense system appeared again, but in this case without the need to seek heat, because these species can regulate their internal temperature to achieve it.
This Monday, the magazine PNAS by a team of researchers from East China Normal University in Shanghai and the Center for Marine Science and Technology in Qingdao, who have used Nile tilapia to understand why fever helps us when we suffer from an infection . And it confirms that this survival strategy emerged many millions of years before the appearance of warm-blooded animals.
In a laboratory experiment, researchers observed that tilapia that were infected with the bacteria Edwardsiella piscicida They moved for five days to the tank area where they lived with a higher temperature. This behavior, with which a cold-blooded fish produces the effects of fever, triggered a series of changes in its body that helped it fight the infection. As if the heat had been a sudden touch, the production of enzymes that destroy T lymphocytes, which coordinate the adaptive response of the immune system, slowed down, allowing them to live longer, and the spleen lymphocytes became more toxic.
When the tilapia felt weak and looked for warm water, as happens to humans when they have a fever, it lost its hunger and became lethargic. Thus, the fish stopped eating and, as happens, its body activated cellular autophagy, a recycling system that discards elements and releases energy to better respond to the infection.
The study of tilapia also shows how new elements that arrive in the toolbox of successful living beings are preserved for hundreds of millions of years, surviving countless species extinctions, and are combined with other innovations. When the fever arose, there was no living being that had adaptive immunity, the sophisticated defense system with which vertebrates defend themselves today. The rise in fever served to enhance innate immunity, which, unlike what T lymphocytes do, does not specifically attack the pathogen that is causing problems. This part of the immune system was also analyzed in tilapia. When the fish approaches warm water, the transcription of nitric oxide, a powerful antimicrobial, is accelerated and proteins are produced that trigger an inflammatory response. In addition, high temperatures make it difficult for some viruses and bacteria to reproduce.
About 450 million years ago, the first animals with adaptive immunity appeared, a series of mechanisms that identify the specific invader. Although the response is slower, it is also more effective and generates a memory that allows it to better repel subsequent attacks from the same organism. The tilapia study published in PNAS shows how, although this defense mechanism appeared 150 million years after the fever, elements of adaptive immunity such as T lymphocytes integrated the signals of the increase in temperature to be able to perform their work better in case of infection.
The authors of the work recall that the ability to raise body temperature, by raising the temperature, as warm-blooded animals do, or by moving to warmer places, as cold-blooded animals do, plays an essential role in improving immunity and fighting infections, something that improves the chances of survival. In laboratory experiments, it has been seen that reducing fever with pharmacological methods or by preventing movement increases the mortality of infected animals.
