How did viruses from animals to humans evolve?

Viruses from animals appear more and more due to livestock activities, invasion of humans into the natural environment, and can cause mild and severe illness to death. After Covid-19, new pathogens such as Marburg virus in Africa, Langya virus in China appeared one after another. Some diseases that were once thought to be endemic in specific areas have now spread to many countries, such as monkeypox. What they all have in common is that they all come from animals.

Scientists estimate that 60% of known infectious diseases and 75% of emerging infectious diseases are of animal origin. Pathogens can be bacteria, viruses or parasites, which are spread to humans by contact in agricultural or wild environments, through food and water. They cause a variety of symptoms, ranging from mild to severe, even fatal.

An international report from 2012 shows that 56 zoonotic diseases cause 2.5 billion infections and 2.7 million deaths globally each year. Those diseases include Q fever, dengue fever, bird flu, Ebola and anthrax.

Before Covid-19, flu-like respiratory diseases, contracted by animals, had left many consequences over the past century. The Spanish flu caused 50 million deaths in 1918, the Hong Kong flu killed 700,000 in 1968.

The question is why are zoonotic diseases so dangerous? Part of that is due to the immune system and natural selection. The specific animal that transmits the virus also plays an important role.

Effects of animal viruses on the immune system

One reason animal viruses are so dangerous is their novelty. The human immune system has never been in contact, does not know how to react to these "uninvited guests".

According to researchers, most viruses that enter the body are successfully destroyed by the immune system, or pass through the digestive system and excreted. However, some zoonotic pathogens can replicate in human hosts.

The initial stage of animal virus replication in the human body is very important. At this point, the virus can mutate and evolve, adapting and improving to survive in the new host.

When this happens, the immune system needs to react and keep up with the evolution of the virus. However, a person's protective barrier takes a while (usually a few days or so) to activate. Meanwhile, the virus may have recombined or even escaped immunity.

A researcher looks for signs of malaria, Zika and other pathogens in a bat in Uganda.

Evolution race

In other words, animal viruses and human immune systems entered a race. Only one of the two opponents can win, or both are at a stalemate.

"The hypothesis is that as viruses grow in their host, they gradually become less dangerous. They need to ensure their own transmissibility, so they don't want to kill the host quickly before finding a second host. new," explains Christopher Coleman, an associate professor of Infectious Immunology at the University of Nottingham.

When viruses fully adapt to their host (human or animal), they can become harmless. Associate Professor Coleman gave examples of animal viruses in the corona family such as bronchitis virus in chickens, infectious peritonitis virus in cats. The mortality rate in animals is almost 100%, but they cannot spread or cause disease in humans.

"On the other hand, some viruses that have evolved in animals are more transmissible and lethal when they enter human populations," Coleman added.

The reason is that the immune system of animals is very different from that of humans. They have a special defense mechanism that humans do not have.

In fact, very harmful viruses like nCoV, SARS, MERS and Ebola all originated in bats. This animal can survive completely normally when carrying dangerous pathogens.

A 2020 study led by Cara Brook, an expert at the University of California Berkeley, shows that bats' unique immunity helps them maintain high viral loads without getting sick.

"Some bats have an antiviral immune response known as the 'Interferon pathway', which is always in an activated state," Brook and colleagues explain in the report.

In most other mammals, an overactive immune response causes harmful inflammation. However, bats have adapted and have anti-inflammatory properties to protect the body from these harmful effects.

Increased risk of disease transmission

The United Nations estimates that the global population will grow to 9.7 billion people by 2050. This means an increasing demand for food, which in turn increases susceptibility to food-borne zoonoses. products, according to the Lancet.

Pathogens in the livestock production chain cause repeated outbreaks. Viruses are sometimes found in dairy products, as well as meat by-products used for flavoring.

Cultivation practices in some parts of the world such as confining species, feeding and slaughtering, in addition to inadequate quarantine, increase the possibility of disease transmission from livestock to livestock, creating new strains of viruses in the Future.