Coronaviruses are RNA viruses that infect a wide range of mammalian and avian species, and many are capable of transmitting between species. CoV-229E and OC43 coronaviruses jumped from bats to an intermediate species to humans many years ago (the jump for OC43 to humans was thought to occur around 1898), and these viruses now cause the common cold in humans. Other coronaviruses that have spilled over to humans more recently have caused more serious disease; including SARS-CoV, which emerged in humans in 2002, MERS-CoV in 2012, and SARS-CoV-2 in 2019. The original hosts for all three of these viruses were bats. The intermediate hosts for SARS-CoV were thought to be racoon dogs as the virus was noted in these animals in live game animal markets in Guangdong province (China) shortly after Severe Acute Respiratory Syndrome (SARS) disease broke out in 2003. Additionally, blood samples from around 20% of humans working in these markets had antibodies to a SARS-like virus despite having no history of the disease; therefore, the virus must have been present in the market for some time until one variant evolved enough to pass efficiently into humans. Shortly after this finding, the decision was made to ban wild game animal markets in Guangdong; this strategy almost certainly prevented the re-emergence of the virus from this reservoir, and the SARS outbreak in 2003 was contained in just a few months.
The intermediate hosts for MERS-CoV (causing Middle East Respiratory Syndrome [MERS] disease) were dromedary camels, and zoonotic transmission via camels is still ongoing. All major outbreaks of MERS have been in the Middle East; however, Peiris pointed out that no zoonotic MERS disease has yet occurred in Africa, despite 77% of the global dromedary population being located there (only 4% of dromedary camels are in the Middle East). The virus is known to be present in Africa, as around a third of camel abattoir workers in Africa have evidence of past infection based on their T-cell responses. A study has shown that the virus from African individuals replicates less efficiently in the human lung and bronchus than the virus taken from Saudi Arabian individuals—which might contribute to less severe disease. However, MERS could potentially become a problem in Africa as there is no surveillance for the disease there.
The intermediate host for SARS-CoV-2 (the viral cause of the current COVID-19 pandemic) is as yet unknown. The earliest cases in 2019 were all geographically clustered around the seafood market in Hunan province (China), which also sold wild game animals. Wild game animals are potential reservoirs of emerging infectious diseases and zoonotic risk; in a sample of 1941 game animals in China, 102 mammalian-infecting viruses were identified; 65 of these viruses were new and 21 were considered potentially high-risk to domestic animals and humans. Peiris highlighted the threats that we currently face, including with regard to the succession of variants of concern for SARS-CoV-2 (alpha, beta, delta, and omicron). Coronaviruses have a high mutation rate, and these mutations allow for plasticity and adaptation, and subsequent emergence of variants. The three main drivers of variant emergence are growing population immunity (either through vaccination or natural infection) forcing the virus to develop to evade immunity; viral persistence in immunocompromised individuals leading to emergence within-patient variant; and viral spill back from humans to other animals, where mutations occur leading to partial immune escape. SARS-CoV-2 has spilled back from humans to white-tailed deer and mink, and there is evidence that it has mutated into mink. Additionally, over the past 10 years other coronaviruses have spilled over to humans, including porcine delta CoV and canine CoV.
Finally, Peiris discussed options for better pandemic avoidance and response in the future. Both SARS-CoV and SARS-CoV-2 belong to the subgenus sarbecovirus, and strategies to deal with variants and future coronavirus pandemics include the possible development of a pan-sarbecovirus vaccine. Additionally, existing platform technologies should be able to develop vaccines that can be rapidly transferrable to new pathogens. However, Peiris commented that vaccines cannot be the sole strategy for future pandemics, as by the time a vaccine is rolled out the first wave of disease has already happened. Measures such as reducing or banning the trade of live wild animals and infection control measures, such as improved ventilation in public spaces to reduce transmission, are important generic strategies for lowering the risk of zoonotic pandemic emergence. Increasing the capacity for enhanced disease detection and surveillance is also crucial. Peiris emphasized that spillovers of viruses from animals to humans are occurring because of human activities, such as population growth, deforestation and climate change, and the wild animal trade, and that we will see spillovers with more frequency if risk-reduction measures at the animal –human interface are not applied. It is important to link human health, animal health, and environmental health as one.
The European Congress of Clinical Microbiology & Infectious Diseases 2022. April 23-26, 2022; Lisbon, Portugal. https://www.eccmid.org/
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