Bats, Coronaviruses, and Deforestation: Toward the Emergence of Novel Infectious Diseases?

摘要

Introduction Coronaviruses (CoV) were for a long time associated with several major veterinary diseases such as avian infectious coronavirus, calf diarrhea, winter dysentery, respiratory infections (BRD-BCoV) in cattle, SDCV, PEDV, SECD in swine and dog, intestinal disease or Feline Infectious Peritonitis (Saif, 2014 ), and the human mild and common cold. However, SARS emerged in 2002 in China and spread across 29 other countries with a 10% death rate. More recently, the MERS-CoV outbreak in Saudi Arabia in 2012 displayed a death rate of 38%. The emergence of these two events of highly pathogenic CoVs shed light on the threat posed by coronaviruses to humans. Bats are hosting many viruses (Calisher et al., 2006 ) and in particular coronaviruses, which represent 31% of their virome (Chen et al., 2014 ). Furthermore, bats display a remarkable resistance to viruses (Omatsu et al., 2007 ; Storm et al., 2018 ). The risk of emergence of a novel bat-CoV disease can therefore be envisioned. Of bats and men Although human blood has been found in the diet of D. ecaudata bats in Brazil (Ito et al., 2016 ), indicating that bats can feed on humans, this is exceptional. Furthermore, with perhaps the exception of Australian Bat Lyssavirus (ABLV) and Duvanhage virus, there is no clear case of direct transmission of the virus from bats to humans (Tignor et al., 1977 ; Hanna et al., 2000 ; Paweska et al., 2006 ). Usually, bats are beneficial to humans by playing a major role in agriculture since they pollinate fruit trees (Whittnaker et al., 1992 ; Kelm et al., 2008 ) and help controlling populations of insects (Leelapaibul et al., 2005 ; Kalka et al., 2008 ). Today, in Asia, 56 species of bats are hunted and consumed by low-income populations (Mildenstein et al., 2016 ). They are also used in traditional medicine (Walker, 2005 ; Ashwell and Walston, 2008 ) and on farms for the production of guano (Chhay, 2012 ; Thi et al., 2014 ). Bioinformatic analysis suggested that there were already several CoV transmission events between bats, civets and humans before the 2002 SARS outbreak (Zheng et al., 2004 ). The biological problem of viral emergence has not fundamentally changed, however the probability of occurrence of the risk is increasing owing to environmental change and higher environmental pressure. Anthropization and the accidental nature of disease emergence The “One Health” concept recognizes that human health is connected to animal health and to the environment. Southeast Asia (SEA) is the region in the world that has suffered the greatest rate of deforestation with a loss of 30% of forest surface over the last 40 years (Figure 1A ). In Thailand, agricultural lands amounted to 23% in 1960 of total land area vs. 40% since 1985 ~(1) Similar trends were observed in other Southeast Asian countries~(1). In Cambodia, agricultural surfaces doubled from 15% in the 1980s up to 30% in 2000. An even higher increase was observed in Vietnam with an increase from 20% in 1990 to 35% nowadays. In Indonesia, the growth rate rose from 21% in the 1980s to 31.5% nowadays. Deforestation is currently linked to increased agricultural surfaces and poorly-managed urban growth (Figures 1B–D ). Human population in SEA increased by 130 million between 2001 and 2011 and is expected to rise by almost 250 million by 2030 ~(2) This demographic growth generates pressure on land use, agricultural land and deforestation, with the most common activities being farming, logging, and hunting. For instance, in Sumatra (Figure 1D ), an area deforested over the last 13 years was turned into a dynamically growing suburban zone with intensive farming. Owing to evolving land-use, bat populations are setting up in areas closer to human dwellings (Reuter et al., 2016 ). Anthropized rural environments are characterized by a wide diversity of landscapes comprising houses, barns, fields, orchards, and woods of differing density. The common belief is that deforestation and anthropization will lead to the disappearance of species. This is not always true and anthropized environments can provide an acceptable habitat for a large range of bat species, generating thus a higher diversity of bats and in turn of bat-borne viruses next to human dwellings (Plowright et al., 2015 ; Afelt et al., 2018 ). Anthropization generates a highly diverse environment in the vicinity of human populations, characterized by differing forest densities. Unlike natural environments which are highly selective, these altered landscapes are acceptable by a wide range of bat species, usually not encountered together. They can find there anthropized environmental niches compatible with their roosting and hunting needs (Walsh et al., 2017 ; Afelt et al., 2018 ). Furthermore, house lights attract a large number of insects at night, offering easy prey for insectivorous bats. Houses and barns offer shelter for cave-dwelling bats while orchards and fields attract frugivorous bats. This attractive effect o