Viruses that transmit from animals to humans are new pathogens that pose significant dangers, as the immune system has never encountered them before, while they continuously mutate to adapt to their new hosts.
The increasing emergence of animal-origin viruses is largely due to livestock farming and human encroachment into natural environments, which can lead to illnesses ranging from mild to severe, and even fatal. Following Covid-19, new pathogens such as the Marburg virus in Africa and the Langya virus in China have emerged. Some diseases that were once localized to specific areas are now spreading to multiple countries, such as monkeypox. The common factor among these diseases is that they all originate from animals.
Scientists estimate that 60% of known infectious diseases and 75% of newly emerging infectious diseases have animal origins. The pathogens can be bacteria, viruses, or parasites, transmitted to humans through contact in agricultural or wild environments, as well as through food and water. They can cause a wide range of symptoms, from mild to severe, and can even be fatal.
International reports since 2012 indicate that 56 zoonotic diseases have caused 2.5 billion infections and 2.7 million deaths globally each year. These diseases include Q fever, dengue fever, avian influenza, Ebola, and anthrax.
Before Covid-19, respiratory diseases similar to influenza, contracted from animals, had significant consequences over the past century. The Spanish flu resulted in 50 million deaths in 1918, while the Hong Kong flu killed 700,000 people in 1968.
The question arises: why are zoonotic diseases so dangerous? Part of the reason lies in the immune system and natural selection. The specific animals transmitting the viruses also play a crucial role.
The Impact of Animal Viruses on the Immune System
One reason animal viruses are so dangerous is their novelty. The human immune system has never encountered these “uninvited guests” and does not know how to respond.
According to researchers, most viruses that enter the body are successfully eliminated by the immune system or are passed through the digestive system and expelled. However, some animal pathogens can replicate within human hosts.
The initial phase of animal viruses multiplying in the human body is critical. At this stage, the virus can mutate and evolve, adapting and improving its chances of survival in the new host.
When this happens, the immune system must respond and keep up with the virus’s evolution. However, the human immune barrier typically requires some time (often several days or longer) to activate. Meanwhile, the virus may have already recombined, potentially evading immunity.
A researcher searching for signs of malaria, Zika, and other pathogens in a bat in Uganda. (Photo: National Geographic)
The Evolutionary Arms Race
In other words, animal viruses and the human immune system are engaged in an evolutionary arms race. Only one of the two opponents can win, or both may reach a stalemate.
“The hypothesis is that as viruses evolve within a host, they gradually become less dangerous. They need to ensure their own transmissibility, so they do not want to kill the host too quickly before finding a new one,” explains Christopher Coleman, an associate professor of Infectious Immunology at the University of Nottingham.
When viruses fully adapt to their host (whether human or animal), they can become harmless. Professor Coleman cited examples of animal coronaviruses, such as avian bronchitis virus in chickens and feline infectious peritonitis virus in cats. The mortality rate in animals can be nearly 100%, but they cannot spread or cause disease in humans.
“On the other hand, some viruses evolving in animals can be more transmissible and deadly when they enter human populations,” Coleman added.
The reason is that the immune systems of animals are very different from those of humans. They possess protective mechanisms that humans do not have.
In fact, highly pathogenic viruses such as nCoV, SARS, MERS, and Ebola all originate from bats. These animals can carry dangerous pathogens without suffering from the diseases themselves.
A 2020 study by Cara Brook, a researcher at the University of California, Berkeley, indicated that the unique immune response in bats allows them to maintain high viral loads without becoming ill.
“Some bat species have an antiviral immune response known as the ‘Interferon pathway,’ which is always in an activated state,” Brook and colleagues explained in their report.
In most other mammals, an excessive immune response can lead to harmful inflammation. However, bats have adapted and possess anti-inflammatory properties to protect their bodies from such damage.
Increasing Risks of Disease Transmission
The United Nations estimates that the global population will reach 9.7 billion by 2050. This means an increasing demand for food, which in turn heightens susceptibility to zoonotic diseases transmitted through food, according to The Lancet.
Pathogens in livestock production chains cause recurring outbreaks. Viruses can sometimes be found in dairy products, as well as in meat by-products used for flavoring.
Farming practices in some parts of the world, such as cohabiting species, feeding, and slaughtering, combined with inadequate quarantine measures, increase the likelihood of disease transmission from livestock to pets, creating new viral strains in the future.
