The ongoing COVID-19 pandemic has devastated the global communities, but scientists have been warning for years that a pandemic like this is destined to happen - it is just a matter of time. While much of the focus in the world today is on the immediate problem, COVID-19, which is rightly so in order to save lives, we as a society will have to take a long and deep look at the root cause of the problem and seriously do something about it to prevent future pandemics.
Animal pathogens are the sources for the majority of emerging human infections, and likely the root cause for future pandemics and other emerging infections as well. According to the U.S. Center for Disease Control and Prevention, approximately 3 out of every 4 emerging human infectious diseases are zoonotic and came from animals. Take recent emerging viral diseases for examples, SARS, MERS, H1N1 pandemic flu, H5N1 bird flu, Ebola, and now COVID-19 are actually all animal viruses that jump species to infect humans.
The viruses causing both SARS and MERS originated from bats, and the novel coronavirus causing COVID-19 likely originated from bats as well. For SARS, the virus transmitted from bats to civet cat, and the infected civets then spread the virus to humans. In the case of MERS, the virus transmitted from bat to dromedary camels, which then spread to humans. For COVID-19, it is unknown at this point about the intermediate animal reservoir that spread the virus to humans. The H1N1 pandemic flu is a quadruple reassortant virus, with virus gene segments originated from pigs, birds, humans. The Ebola virus originated from fruit bats, and spread to humans through close contacts with infected bats, non-human primates and other animals.
Many less-known viruses also spread from various animal species to humans causing important and serious diseases. For example, animal strains of hepatitis E virus in pig, rat, deer, rabbit and other species infect humans causing cluster or sporadic cases of acute and chronic hepatitis E via direct contact with infected animals or consumption of undercooked animal meats. Nipah virus transmits to humans from infected pigs or fruit bats causing encephalitis and respiratory diseases with a 50-75% high mortality rate. Hantavirus pulmonary syndrome, a deadly respiratory disease, is caused by hantavirus that is transmitted from rodents to humans. Hendra virus, which causes meningitis or encephalitis in humans, naturally infects fruit bats. The virus can spread from fruit bats to horses, when can then transmit to humans through close contacts with infected horses.
Many more viruses from various animal species are known to be zoonotic and infect humans. In addition to virus, other animal pathogens such as bacteria and parasites can also cross species barriers and infect humans. Unless we effectively eliminate or control the sources of these zoonotic animal pathogens, new human infections will continue to emerge from animals.
Beside those animal pathogens that are known to infect humans, there also exist an extremely large number of other animal pathogens with unknown zoonotic potential, particularly viruses, in every domestic and wildlife animal species. For example, bats are known to harbor more than 200 viruses, and many of them are zoonotic and also infect humans. Take an agricultural animal, pig, for another example, about 20 new viruses were discovered from pigs in the past 30 years, and some of them are known to infect humans. The coronavirus that causes COVID-19 is believed to originate from bats. However, there exist at least more than 50 other recognized coronaviruses that are known to infect a large number of domestic and wildlife animal species such as cat, dog, swine, cattle, birds, rodents, etc.
Unfortunately, the sad reality is that, due to the lack of resources, scientists know very little about the biology and disease-causing potential for most of these so-called "animal viruses", despite the fact that they are the sources for the majority of emerging human infections. Considering the immense selective pressures for many of these animal viruses to constantly evolve and mutate, novel variants of animal viruses with potentially altered host tropism to humans will surely emerge in the future. Future pandemics most likely will emerge from these so-called animal pathogens.
Factors promoting spillover or cross-species infection
The majority of emerging infectious diseases in humans are of animal origins, largely due to increasingly close interactions among humans, domestic animals and wildlife. Believe or not, we the humans are to blame for many of the emerging human infections. Human activities such as climate change, deforestation, urbanization, intensive animal farming practice, backyard farming, animal poaching, and bushmeat consumption all bring animal pathogens closer to human habitats, leading to spillover or cross-species infections in humans.
Climate changes can lead to ecological change of pathogen-carrying vectors such as mosquitoes and ticks, and thus promoting the transmission of vector-borne infectious diseases such as West Nile, Mayaro, and Chikungunya. Deforestation reduces the habitats of wildlife animals and therefore increase the chances of contacts between humans and wild animals, leading to spillover infection of humans by animal pathogens. Intensive animal farming practice favors rapid spread and genetic mutations of animal pathogens, which may yield novel strains that acquire the ability to infect humans. Backyard farming in some countries bring the animal pathogens closer to humans causing zoonotic infection between humans and other animals. Animal poaching, wildlife animal markets, and bushmeat consumption offer a breeding ground for animal pathogens to jump species and infect humans.
Addressing the root cause to prevent future pandemics
The most cost-effective solution to prevent future pandemics and other emerging human infectious diseases is to seriously tackle the root cause of the problem by investing resources to control or eliminate these so-called animal pathogens in their own animal hosts, before they jump species to infect humans. Unfortunately, the current resources across the world are heavily aliquoted towards studying human pathogens. Consequently, the large number of animal pathogens from wildlife, agricultural and domestic animal species have long been neglected and grossly understudied, and for most of the animal pathogens we know very little about them, except for the fact that most of the emerging human infectious diseases came from animals.
The current COVID-19 pandemic provides a sobering reminder that policy-makers and legislature across the globe will have to take a long and deep look at the root cause of all these emerging human infection, and start to invest resources to address the root cause, the so-called animal pathogens. Similarly, we, the society as a whole, must work together across the globe and seriously address the many human activities that are known to facilitate and promote cross-species infection of humans by animal pathogens. Collectively, we will have to modify our social, cultural, and industrial habits in order to prevent future pandemics.
Member, U.S. National Academy of Sciences
University Distinguished Professor, Virginia Polytechnic Institute and State University, USA