Deforestation, corona, and bats: on the emergence of a contagious epidemic.

The COVID-19 pandemic (also colloquially described as "coronavirus pandemic," "corona crisis") is a Breakout of the new Respiratory disease COVID-19 (or "Covid-19", for Corona virus disease 2019). The first infections outside the People's Republic of China were reported as early as January 2020, but it is only since March 11, 2020 that the World Health Organization (WHO) has officially assumed a pandemic (the first since pandemic H1N1 in 2009/10).

"In 1997, as clouds of smoke hung over Indonesia's rainforests because an area about the size of Pennsylvania was burned to make way for agriculture - with the fires further exacerbated by drought - trees choked in the haze could no longer produce fruit. Because of this, the resident bats found no other way to forage, flying elsewhere and carrying a deadly disease with them," begins the article by freelance science journalist Katarina Zimmer, which was published on National Geographic on Nov. 22, 2019.

Zimmer writes that bats then hid in Malaysian orchid trees. Pigs suddenly became ill. Possibly they ate fallen, withered fruits on which the bats had previously nibbled. Malaysian farmers would have tasted of these fruits.In 1999, 265 people were found to have dangerous brain inflammation, with 105 suffering death from the so-called Nipah virus.

Several researchers point out that the Nipah virus is just one of many contagious diseases from regions where massive deforestation has occurred for decades. Various scientifically proven studies assume a causal link between deforestation and a complex cascade of events that ultimately transport disease-causing viruses to every corner of the world.

Mosquito bites and man-made "multipliers".   

In a complex analysis of satellite and health data recently published in the journal Proceedings of the National Academy of Sciences, Erin Mordecai of MacDonald and Stanford Universities reported a significant impact of Amazon Basin deforestation on malaria transmission - consistent with some previous research.

Although these are high estimates, the dimensions are illuminating: Between 2003 and 2015, a 10 percent annual increase in forest loss was recorded. Malaria cases increased by 3 percent annually during that time. In one year of the study, for example, an additional 618 square kilometers (1,600 square miles) of cleared forest - the equivalent of nearly 300,000 football fields - was associated with an additional 10,000 malaria cases.

This effect had been most pronounced in the forest interior. Here, the forest areas were still intact, with mosquitoes colonizing a moist habitat at their edges.

"Because tree-lined mosquito glades are disappearing, contagions in urban areas could increase."

Other studies, such as that of epidmiologist Amy Vittor at the Emerging Pathogens Institute at the University of Florida, show: Along the deforested forest edges, an ideal habitat is formed for the breeding of the mosquito Anopheles darlingi, the most important vector of malaria in the Amazon.

Through careful research in the Peruvian Amazon, Vittor found higher numbers of larvae in warm, partially shaded pools that formed next to roads and debris cut into the forest. "Those were the places where Anopheles darlingi really liked to hang out," the researcher recalls.

Given the ongoing fire in the Amazon in 2019, these results did not bode well. Data released in November 2019 showed that an area 12 times the size of New York City was destroyed in the Amazon in 2019. However, rainwater cannot be regenerated from trees and plants in such mowed-down places.

Conclusion: bats as vectors of CoV disease?   

The paper published in April 2018 titled "Bats, Coronaviruses, and Deforestation: Toward the Emergence of Novel Infectious Diseases?" (Source: US National Library of Medicine / National Institutes of Health) by Aneta Afelt, Roger Frutos and Christian Devaux establishes a clear link between bats, forest clearing, and coronaviruses. Due to evolving land use, bat populations are settling in areas closer to human habitation (Reuter et al., 2016).

However, the researchers differentiate between humans and animals:

"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 an exception. Furthermore, perhaps with the exception of Australian bat lyssavirus (ABLV) and Duvanhage virus, there is no clear case of direct transmission of virus from bats to humans (Tignor et al., 1977; Hanna et al., 2000; Paweska et al., 2006)."

In fact, experts believe that bats are more beneficial than many assume. For example, bats help pollinate fruit trees (Whittnaker et al., 1992; Kelm et al., 2008) and they would also help contain insect populations (Leelapaibul et al., 2005; Kalka et al., 2008). However, nowadays, about 56 species of bats are hunted and consumed by low-income populations in Asia (Mildenstein et al., 2016). They are also used in traditional medicine (Walker, 2005; Ashwell and Walston, 2008) and on farms to produce guano fertilizer (Chhay, 2012; Thi et al., 2014).

However, the analyses of the researchers mentioned above emphasize that there would have been several CoV transmission events between bats, civets, and humans even before the 2002 SARS outbreak (Zheng et al., 2004). For example, their publication states, "The biological problem of virus emergence has not fundamentally changed, but the likelihood of the risk occurring is increasing due to environmental stresses and changes."

Anthropization and occurrence of diseases

At Anthropization or anthropization (translated from English) is understood in geography and ecology as the transformation of open spaces, landscapes, and natural environments by human actions. Anthropic erosion, for example, describes the process of human activity that degrades terrain and soil.

Rural areas are characterized by a great diversity of landscapes that include houses, barns, fields, orchards, and forests of varying densities. It is commonly believed that deforestation and anthropization can lead to the disappearance of species. However, this is not always true when new environments provide acceptable habitat for a large number of animals and favor, for example, a higher diversity of bats.

In addition, house lights attract large numbers of insects at night, which are easy prey for insectivorous bats. Houses and barns provide shelter for cave-dwelling bats, while orchards and fields attract frugivorous bats. This attractive effect of anthropized environments on bats with diverse biological needs leads to higher concentrations and biodiversity of bat roosts (Han et al., 2015; Plowright et al., 2015; Reuter et al., 2016; Lacroix et al., 2017a,b; Walsh et al., 2017; Afelt et al., 2018).

This increases the risk of virus transmission through direct contact, pet infections, or contamination through urine or feces. This is because bats likewise produce viruses adjacent to human dwellings (Plowright et al., 2015; Afelt et al., 2018). Because CoVs (see infobox "Novel Bat CoV Disease.") are primarily pathogens of animal diseases, there is a risk of disease occurrence in both domestic animal diseases and human diseases.

Animal-borne viruses: a historic enemy with a bright future

The novel coronavirus is known as SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2). The respiratory disease caused by the virus is called COVID-19 (Coronavirus disease 2019).

Recently, major infections of humans by bat viruses have been documented, although they may have occurred earlier in history. Coronaviruses (CoVs abbreviated) have mostly been associated with animal diseases, with livestock and domestic animals acting as intermediate carriers for transmission to humans.

Approximately 4.4 percent of rats sold at three live markets in the Mekong Delta region of Vietnam and 22 percent of bats sampled at three bat farms carried CoV, representing a high level of animal contamination (Berto et al., 2017). Prior to the emergence of SARS-CoV and MERS-CoV in humans, the four known human CoVs (HCoV-HKU1, HCoV-229E, HCoV-NL63, and HCoV-OC43) were reported to be endemic and responsible for mild to moderate respiratory illness for more than three decades.

There is evidence that alpha-CoVs from the bat Hipposideros caffer ruber share common ancestors with human HCoV-229E (Pfefferle et al., 2009) and that a related virus infected captive alpacas (Vicugna pacos), while another related virus infected camels (Corman et al., 2016).

In addition, HCoV-NL63 found in 9.3 percent of samples from individuals hospitalized for respiratory illness. Thus, there are sequence similarities between bat (Perimyotis subflavus) CoV ARCoV.2 and infected humans, while HCoV-NL63 can replicate in cell lines from the lungs of tricolored bats (Huynh et al., 2012).

MERS-CoV is closely related to both bat CoV HKU4 (in Tylonycteris bats) and bat CoV HKU5 (in Pipistrellus bats). Collectively, these data illustrate the complex dynamics of CoV circulation between bats and wild or domestic animals (cattle, pigs) prior to interbreeding with humans.

However, bats may not necessarily be involved in primary infections of humans. However, write Aneta Afelt, Roger Frutos and Christian Devaux in their 2018 study titled "Bats, Coronaviruses, and Deforestation: Toward the Emergence of Novel Infectious Diseases?"

"The situation is different with the emergence of a novel pathogen within the immunologically active human population. In such a case, the risk of large epidemics accompanied by high mortality is very high. Once adapted to humans, CoVs can evolve to develop a more efficient intra-species mode of transmission. During the SARS outbreaks in Taiwan and Toronto, certain individuals were very efficient at transmitting SARS CoV and were referred to as "superspreaders" (McDonald et al., 2004).

A total of 83.2 percent of transmission events were epidemiologically linked to five "superspreaders," all of whom had pneumonia diagnosed at the first medical consultation.

Resumée (on the spread of coronaviruses).

Because the increasing impact of human activities on ecosystems is unlikely to abate, there is a need to strengthen CoV surveillance in wildlife, cattle, domestic animals, and humans to better understand the dynamics of interspecies transmission and improve risk assessment, early warning, and intervention (Devaux, 2012).

Unfortunately, the problem of bat-borne viruses is not limited to CoVs. Of the 60 viral species reported to be associated with bats, 59 are RNA viruses that may be responsible for the emergence and reemergence of infectious diseases in humans (Brook and Dobson, 2015).

However, is the risk for disease occurrence now directly related in the distribution of bat species? Several examples can be found in other virus families. Hendra virus was detected in 1994 after the deaths of 30 horses and 1 male in Hendra, Australia. The most likely mode of human contamination was aerosols from sick horses initially contaminated by urine or amniotic fluid from Pteropus bats (Weatherman et al., 2017). Nipah virus is another example of the combined effects of deforestation and attraction to anthropized environments. Pteropus bats affected by deforestation settled in barns, where they transmitted the virus to pigs, which in turn infected humans (Chadha et al., 2006).

"It remains obvious that the risk of new viruses emerging is very high."

Anthony and colleagues have estimated that at least 3,204 CoVs are circulating in bats in 2018 (Anthony et al., 2017). Regardless of the accuracy of this prediction, it remains evident that the risk of new viruses emerging from bats is likely to be very high. Now, since the Asian continent in particular - along with South America - is one of the regions of the world where population growth is highest, while deforestation rates are extreme, it fulfills all the conditions - see also sanitary conditions - to become the site of infectious disease emergence or re-emergence.

The "One Health" concept recognizes that human health is linked to animal health and the environment. However, the world's population faces so many problems in terms of increasing urban population, decreasing agricultural land and poorly managed urbanization in many places.

Infobox:

Novel bat CoV disease.

Coronaviruses (CoV) have been associated with various animal diseases for a long time, for example, birds show infectious corona infections, but there are also respiratory infection in cattle (BRD-BCoV), calf diarrhea, in pigs and dogs SDCV, PEDV, SECD, in felines, in animals of the feline family intestinal disease or infectious peritonitis (Saif, 2014).

People have always had waves of influenza and common colds. However, SARS appeared in China in 2002 and spread exponentially to 29 other countries with a 10 percent mortality rate. More recently, the MERS-CoV outbreak in Saudi Arabia in 2012 had a mortality rate of 38 percent. The occurrence of these two events involving highly pathogenic CoVs sheds light on the threat that coronaviruses pose to humans.

Bats harbor many viruses (Calisher et al., 2006), particularly coronaviruses, which make up 31 percent of their virome (Chen et al., 2014). In addition, bats show remarkable resistance to viruses (Omatsu et al., 2007; Storm et al., 2018). The risk of emergence of a novel bat CoV disease is therefore foreseeable. (mm)

Notice:

Researchers already pointed out the "superspreaders" years ago  

It will certainly be critical to pay special attention to "superspreaders" that are very efficient at transmitting CoVs through exposure to respiratory droplets, for example, says the study "Bats, Coronaviruses, and Deforestation: Toward the Emergence of Novel Infectious Diseases?" published in April 2018 in the U.S. National Library of Medicine / National Institutes of Health. A section from this reads:

"Disease emergence is a random process, and it is therefore impossible to predict the scenarios and dynamics of emerging infectious diseases. The attractive effect of anthropized environments on bats is an important risk factor for the occurrence of novel bat-borne diseases in both humans and animals. Given the proportion of CoVs described in bats, 31 percent, the risk of emerging CoV-associated diseases should be seriously considered in the future."

If the priority is to discover therapeutic options and vaccines (Graham et al., 2013; Zumla et al., 2016), it is even more important to work on educating and sensitizing people to the risks associated with the anthropized environment.

To the reference list

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright of the texts published above:

Michael Merz, ENVIRONMENTAL PERSPECTIVES