Bats (order Chiroptera) are the second largest order of mammals (1411 species). They are the only mammals capable of flying actively, which allowed them to be globally distributed. In general, flight has led to high refinement of order Chiroptera, and orientation through echolocation, which allows them navigate in reduced visibility environments. In-flight body temperature rises, which speeds up their metabolism and puts their bodies in a constant state of high fever. The temperature varies from 38 to 41 °C. This type of energy production and consumption is huge and intolerable for other mammals. High energy potential leads to the release of huge amount of free radicals, which damage DNA molecules and kill the cells of mammals. However, this does not happen to bats and scientists have found out why. They have a mutation which accelerates the cell’s ability to find and repair damaged DNA molecules. In addition, their cellular mechanism practically does not allow the damaged cells to multiply, as is the case with all other mammals, and cancer is rare.
In all mammals, the immune system triggers a group of signaling proteins called interferons. When a virus enters a cell, interferons produce signals to warn the other cells to activate their antiviral mechanism and prevent the virus from further multiplying. In bats, interferons are constantly activated and there is no mechanism to switch them off and thus they constantly control the viruses. Therefore, extreme energy production, high body temperature, mechanisms for the rapid recovery of damaged DNA, and the specific immune system make bats a suitable reservoir of different types of RNA viruses. These viruses are adapted to the dynamic cellular life of their hosts as well as to bats. RNA viruses from bats cannot be transmitted directly to humans because their fine-tuning requires other organisms (intermediate hosts) in which the cells viruses adapt to the final host. Similar examples from the recent past are SARS-CoV coronaviruses and the MERS-CoV virus. In 2002, SARS-CoV caused Severe Acute Respiratory Syndrome (SARS) and has affected about 30 countries, of which, for 800 people, the outcome was lethal. It is established that similar coronaviruses are carried from bats but the infection to people did not happen directly. The source of the infection is civets, a species of rare predator that is sold freely to markets in China. The virus has long been spread by the intermediate host in Chinese markets.
In 2012, the Middle East Respiratory Syndrome (MERS) appeared. The intermediate hosts are camels, who have been carrying the coronavirus for about 30 years. MERS caused an epidemic in the Arabian Peninsula, about 2000 people were infected, and the death rate was 50%. The scenario for the SARS-CoV-2 virus, which causes the COVID-19 syndrome, is similar. The virus genome is most similar to the CoV RaTG13 coronavirus known from the intermediate horseshoe bat (Rhinolophus affinis), which inhabits the Yunnan province, China and Southeast Asia. The virus binding receptor at the intermediate horseshoe bat and other horseshoe bats cannot directly bind to the human ACE2 receptor located on the outside of the lung cells. There was a tuning time (mutation) period in the intermediate host that took years to separate SARS-CoV-2.
Globally, mammalian organisms, including humans, are full of different corona-viruses. They are divided into specific groups, and interspecific transmission is extremely rare, since the virus must adapt to the specific cellular receptor of the final host. Therefore, direct transmission from bat to human with SARS-CoV-2 is excluded.
Bats are the natural “biopesticides” on Earth. Globally, one bat consumes a huge amount of the known pests on forests and farmlands such as the insect pests on coniferous plantations, cereals and fruit orchards. Bats regulate the numbers of the Greater Wax Moth; whose larvae are parasites on beehives.
Many bat species control the mosquito populations, which are the main reservoir and vector of Zika, Yellow fever and Dengue. One bat colony can eat about 100 tons of insects per night.
The danger does not come from bats or other animals, but from the human beings. People penetrate into previously virgin places, destroy natural habitats at unimaginable speeds, and move quickly from one to another point of the world. All this allows pathogens to overcome the interspecies barriers that previously prevented them from emerge and spill-over uncontrollably.
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