Pandemics of animal origin: The past carries many lessons, say scientists

This colorized transmission electron microscope image shows SARS-CoV-2 -the virus that causes COVID-19 —isolated from a patient in the United States. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, being crown-like. Credit: NIAID-RML, CC BY 2.0
This colorized transmission electron microscope image shows SARS-CoV-2 — the virus that causes Covid-19 — isolated from a patient in the United States. Covid-19 underscores the devastating, long-lasting societal and economic consequences of emerging infectious diseases, according to researchers. Image: NIAID-RML, CC BY 2.0

Concerns over the threat of human infectious diseases of animal origin have grown, according to researchers, who stress that better international collaboration will be critical in improving the response to the next pandemic.

Pandemics do not respect international borders, Guha Dharmarajan and his fellow researchers cautioned in a paper published recently in the journal Zoonoses.

Emerging infectious diseases are one of the greatest public health challenges. About three-quarters of these diseases are of animal origin.

The research team carried out a systematic review to identify the main characteristics of past epidemics of animal origins and predict areas with high future disease emergence risk. They looked at what past epidemics can teach us about preventing the next pandemic.

Countermeasures against future pandemics of animal origin must focus on several key mechanisms, they wrote.

First, the eco-epidemiological contexts favoring spillover events must be clearly established, they said. Second, pathogen surveillance must be scaled up, particularly around species and regions that have a high disease emergence risk.

Third, spillover risk must be reduced through proactive strategies to interrupt animal-to-human transmission chains.

Major diseases of animal-origin that affect human health. The bat-borne Hendra virus in Australia, which can infect humans via horses, is described in (F).
Major diseases of animal origin that affect human health. The bat-borne Hendra virus in Australia, which can infect humans via horses, is described in (F). The graphic shows a timeline of the emergence of diseases of animal origin which are considered to be a threat to global health or which require urgent research as identified by the World Health Organization, including: (a) Crimean-Congo hemorrhagic fever; (b) Dengue fever; (c) Marburg virus disease; (d) Lassa fever; (e) Rift Valley fever; (f) Hendra virus disease; (g) Highly Pathogenic Asian Avian Influenza A subtype H5N1; (h) Nipah virus disease; (i) HIV/AIDS; (j) Zika fever; (k) Ebola virus disease; (l) Sudden Acute Respiratory Syndrome (SARS); (m) Influenza A virus subtype H1N1; (n) Middle East Respiratory Syndrome (MERS); (o) Coronavirus Disease 2019 (COVID-19). For each disease, the year of initial identification (round symbols on the timeline) or declaration of a public health emergency of international concern (square symbols on the timeline) is shown. The range of each disease is also given as a map highlighting areas where transmission is reported (red areas) and the location from where the pathogen was first reported (blue symbol). Also, depicted are the major routes of transmission in the zoonotic source population (green arrows), primary zoonotic event (red arrows) and mode of maintenance in the human population (green arrows). Diseases include those that are strictly zoonotic and maintained in the human population only through transmission from a vertebrate animal host (for example, Rift Valley fever and Hendra virus disease), diseases that are primarily maintained by zoonotic spillover but which can also be transmitted directly between humans (for example, Ebola/Marburg virus diseases and MERS), and diseases of animal-origin which show very efficient human-to-human transmission (e.g., HIV infection, H5N1/H1N1 influenza). Several diseases are suspected to be of zoonotic origin but the vertebrate animal reservoir remains unconfirmed (e.g., Ebola virus disease, SARS and COVID-19). Image: Dharmarajan et al. DOI: 10.15212/ZOONOSES-2021-0028

The review team said improved source identification and real-time spatial tracking of diseases are crucial to decrease epidemic potential and prevent epidemics from becoming pandemics.

The authors noted that human history has been punctuated by many pandemics, including the bubonic plague (14th century), the Spanish flu (20th century), HIV/AIDS (the 20th and 21st centuries), and coronavirus disease 2019 (Covid-19).

“Covid-19 has underscored the devastating, long-lasting societal and economic consequences of emerging infectious diseases,” they wrote.

“Particularly alarmingly, the risk of novel disease emergence in human populations is increasing because of the confluence of numerous drivers of global environmental change, including those associated with climate, land use, and the agricultural industry.

“This risk is aggravated by the ever-increasing resistance to antimicrobial drugs and insecticides used for disease vector control and the growing potential for the rapid spread of diseases with increased global transport.”

Concerns over the threat of human infectious diseases of animal origin have grown, they said. “These pathogens vary considerably in terms of the conditions favoring spillover events and the resultant public health consequences.”

Spillover events are inherently unpredictable, they said. The development of a pandemic episode is highly associated with a sequential probability of pathogen-human encounters, infection, and transmission.

These probabilities are essentially determined by several factors that modulate human exposure to novel pathogens. Among these are agricultural intensification, changes in land use, and increased contact between humans and domestic animals.

“Indeed, agricultural drivers have been estimated to be associated with approximately 25% of all diseases and 50% of zoonotic diseases that have emerged in human populations,” they said. Understanding such drivers in affecting disease spillover risk is important.

Other future countermeasures include scaling up surveillance at the human-animal interface and reducing spillover frequency.

The current approach to pandemics of animal origin tends to be reactive, they noted. A more proactive approach to preventing future pandemics would be to shift focus toward interrupting animal-human spillover and subsequent transmission chains.

Efforts should also go into decreasing epidemic potential.

“Although the identification of the animal source of a pathogen is critical to prevent future spillover events, it often is a highly challenging undertaking.”

For example, more than 40 years after the discovery of the Ebola virus, the natural reservoir remains unknown, although strong evidence supports the involvement of bats.

They noted that live animal markets selling domesticated species have been identified as sources of many pathogens in humans.

“However, the risks of novel zoonoses are particularly high in live animal markets that sell wildlife, because these markets bring humans and wild animals into proximity, often under conditions of poor hygiene.

“Given the real risks of novel zoonoses entering human populations through live animal markets selling wild animals, governments worldwide must decrease the nutritional dependency of local populations on meat procured from wild animals and ban the trade of wildlife species.”

The regulation of live animal markets must also be improved to decrease the risks of zoonotic disease transmission by segregating live animals of different species from one another and from humans, improving slaughter standards, and improving sanitation and hygiene.

“Until these biosecurity measures are in place, improved surveillance of potentially high-risk locations, such as live animal markets, will be critical.”

Future preventive measures need to be improved, they said.

“A particularly urgent need exists for global governments to recognize the value of biodiversity protection in pandemic prevention.

“Consideration of environmental determinants, climate changes and related risks to human health, and ecosystem integrity and relevant management systems will be critical.

“In proactively addressing future emergencies, a critical need exists to understand that human health is inextricably associated with animal health, as well as ecosystem structure and function.

“Thus, cross-sector collaboration must be strengthened, particularly as it relates to emergency preparedness and response, including the harmonized translation of policy guidance into action.”

They continued: “As the risk of zoonotic disease emergence increases globally, an urgent need remains for strategies to prevent future zoonotic pandemics.

“Such strategies will require an interdisciplinary research agenda, as well as robust intra- and international collaboration at the interface of science, policy and society.”

The review team comprised Dharmarajan, Ruiyun Li, Emmanuel Chanda, Katharine Dean, Rodolfo Dirzo, Kjetill Jakobsen, Imroze Khan, Herwig Leirs, Zheng-Li Shi, Nathan Wolfe, Ruifu Yang, and Nils Chr. Stenseth, from a range of institutions in India, Norway, the Republic of Congo, the United States, Belgium and China.

Guha Dharmarajan, Ruiyun Li and Emmanuel Chanda et al. The Animal Origin of Major Human Infectious Diseases: What Can Past Epidemics Teach Us About Preventing the Next Pandemic?. Zoonoses. Vol. 2(1). DOI: 10.15212/ZOONOSES-2021-0028

The review, published under a Creative Commons License, can be read here.

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