Which temperature dies faster hot or cold

Hot, cold, humid? What viruses need to survive

The hope is that everything will get better in the spring. When things go well, this new coronavirus, also known as SARS-CoV-2, behaves like the influenza virus. Then spring with its rising temperatures would put an end to the pathogens - and the spook would be over. The coronavirus season would have swept over us and ebbed just as much as the flu wave every year when winter draws to an end.

So much for hope. However, virologists like Thomas Pietschmann can "strictly and honestly not yet say whether SARS-CoV-2 behaves as desired, because we do not yet know the virus."

Pietschmann is a molecular virologist and conducts research at the Center for Experimental and Clinical Infection Research, called Twincore, in Hanover. He deals with so-called RNA viruses - the hepatitis C virus is one of them. And SARS-CoV-2 also belongs to this group.

Virus? Unknown!

"The special thing about this virus is that humans are confronted with it for the first time. From the data we have from China, it can be concluded that the virus only passed from animals to humans once and from there has spread, "says Pietschmann.

In other words: unlike influenza viruses, with which almost everyone has had contact at some point, our immune system is not prepared for an attack with corona pathogens.

In addition, the external conditions in the northern hemisphere are currently almost perfect for the rapid spread of the virus. On the one hand there is the temperature. Respiratory viruses, i.e. those that spread via the airways, have an easy job when it is cool. "Viruses are more stable at low temperatures. Similar to food that can be kept in the refrigerator the longest," explains Pietschmann.

Cool and dry, please!

The warmer it gets, the more difficult the conditions are for many viruses. "The coronavirus is surrounded by a lipid layer, i.e. a layer of fat," says Pietschmann. This is not particularly heat-resistant, so that the virus quickly breaks when the temperature rises. "Other viruses, such as the norovirus, are much more stable because they mainly consist of proteins and genetic material."

For other pathogens, the temperature also only plays a subordinate role. Dengue viruses are found mainly in tropical and subtropical regions. This is less because dengue pathogens like it warm; rather, the hot regions are the preferred place of residence for the mosquitoes that transmit the virus. "In this case, it is not the temperature that plays the main role in the spread of the pathogen, but the animal that transmits the virus," says Pietschmann.

The humidity also has a major influence on the transmittability of respiratory viruses. Once the pathogens have been conveyed out of the airways with a strong sneeze, they are literally hanging in the air. "On cold and mostly dry winter days, the small droplets with the virus float longer in the air than when the air humidity is high," says Pietschmann.

In this way, the pathogens can spread rapidly. However, at first they do it quietly and secretly. A few weeks can pass from the first contact with the pathogen to the first symptoms of the disease. The length of this so-called incubation period depends on the properties and biology of the virus.

Double X chromosome and estrogens

Fever, pain, and chills are typical symptoms of a viral illness and a sign that the body is fighting the invaders. How successful this fight is depends not only on the age and health of the infected person, but also on their gender. In the case of the coronavirus, too, the figures show that women have significantly better cards than men: The mortality rate for men is 2.8 percent, which is significantly higher than that of women at 1.7 percent.

"On the one hand, this has genetic reasons," says Pietschmann, "because some immune-relevant genes, for example genes that are responsible for recognizing pathogens, are coded on the X chromosome." Because women have two X chromosomes and men only have one, the female sex has an advantage here.

The female sex hormone estrogen also helps women to defend themselves against the viral disease. "Some immunologically relevant genes also have binding sites for estrogens where these genes are switched on. This means that these genes are also controlled by the hormones," explains Pietschmann.

Perhaps the corona season will actually end with the beginning of spring in the northern hemisphere. According to the World Health Organization (WHO), however, there are more than 20 cases of SARS-CoV-2 in Australia, as well as one case in Brazil - in the southern hemisphere. Winter is yet to come.

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    Author: Hannah Lesch