Why is SARS-CoV-2 so successful for outbreak? An ecotoxicological perspective

– Synanthropic primary hosts carrying viruses may burden new dangers for humanity. The SARS-CoV-2 virus causing the COVID-19 pandemic presently infected near 2.1 million humans. Virus characteristics resulted in a successful outbreak due to various reasons: ( i ) patient are infectious before experiencing symptoms; ( ii ) carrying hosts might be symptomless, disease outcome in ill patients depends on underlying conditions, age and sex; ( iii ) intermediate hosts, acting as reservoirs living close contact with humans (e.g., livestock, pets); ( iv ) possible virus mutations from animal/person to person/person transmission; ( v ) the virus can spread through the air; ( vi ) the half-life of the virus is long, reaching some days, thus feces and street dust increase the hazard, and contact spread is also turned into critical.

Zoonotic diseases are caused by any pathogens that jumped from animals to human. Viral zoonosis is frequently mentioned. Pathogens of African origin include (a) the human immunodeficiency viruses (HIV) are believed to have originated from chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) as sources of HIV-1, and from sooty mangabey (Cercocebus atys) as a source of HIV-2; (b) the West Nile virus (WNV) originated from widely different bird species and transmitted by blood-feeding mosquitoes (Aedes and Culex); (c) the Zika virus (ZIKV) originated from monkeys (e.g., rhesus macaque, Macaca mulatta) also transmitted by mosquitoes.
Severe acute respiratory syndrome (SARS) is caused by coronaviruses that are known to be of Asian and Middle Eastern origins. Coronaviruses share a characteristic morphological feature of having numerous glycoprotein projections, giving them a crown-like appearance (Figure 1). The cell receptor for them is the angiotensin-converting enzyme II, found in vertebrates. This is a possible reason for the apparent very divergent (from bats, pangolins, snakes to civets, camels, ferrets, cats and human) and bizarre primaryintermediate-dead-end/symptomless host chains (Table 1).
The Middle East respiratory syndrome (MERS-CoV) was described in September 2012 from Saudi Arabia (Zaki et al., 2012) with high mortality rate for humans (WHO: 858 deaths, >30% mortality rate mainly in the Middle East). The Egyptian tomb bat (Taphozous perforates) was suggested as a primary host (Memish et al., 2013), but Nycteris and Pipistrellus species were also suspected as reservoirs (Annan et al., 2012). Dromedary camel (Camelus dromedarius) was found as an intermediate host (Reusken et al., 2013). other wildlife animals were also sold" (Ji et al., 2020). Fruit bats are used for food purposes, although prohibited to be hunted in China (Mickleburgh et al., 2009;Mildenstein et al., 2016). The first person/person transmission affecting a US citizen has been reported (Ghinai et al., 2020). Snakes (Zhang C. et al., 2020;Ji et al., 2020) (Wan et al., 2020). The Rhinolophus sinicus bat has also been included to the list. This virus replicates efficiently in ferrets and cats. It is transmitted in cats via respiratory droplets (Shi et al., 2020). Ocular implication cannot be ignored, either (Seah & Agrawal, 2020).
SARS-CoV-2 is being found to be the most successful pathogen for pandemic ( Figure 2). Presently, the incredibly sad record is -April 16, 2020 -~2.1 million infected, including ~142 thousands of deaths, according to ECDC, Europe.
In 2015, Bill Gates gave a remarkable presentation in the "TED Talks" series (Gates, 2015), where he brilliantly and at the same time astoundingly summarized the characteristics of an extraordinarily successful microorganism which can cause a serious outbreak. Thus, the Ebola virus caused >11 thousand deaths in Africa near Ebola River (West Africa, 2013-2016. This was a wake-up call for humanity. The Ebola virus had strong effects (mortality rate is ~50%) on humans before the patient started to spread the infection. Outbreaks are of even higher challenge, where the victims turn to be infectious before a serious stage of illness developed. The situation where the virus does not cause any symptoms among a high percent of humans -making them a symptomless host -is a disastrous scene in the epidemiological sense. This is the worst-case situation for a local outbreak turning into a pandemic. Ebola has not spread through the air and so the outbreak has not reached the urban areas, where could have further spread by international travel. The health priority system needs to have a strong epidemiological basis, as well as innovation for vaccines and diagnostics -sends Gates his message to us. Later the emergence of SARS-Cov-2 further expanded these warnings.
Coronavirus disease 2019 (COVID-19) spreads through the air, but also with feces dust. The virus can also infect one of our most popular pets, cats -causing only lethargy on them. Coughing helps to spread the virus. Although large and small infectious droplets settle from the air within 1-2 meters from the source, their droplet nuclei, still infectious, may remain airborne for 1-3 hours and travel to 6-40 meters (Hashmi, 2013;Bourouiba, 2020). This is the reason why masks of FFP3 respiratory protection class are so important. In closed spaces such as public transportation (metro, bus or tram) it is easy to be exposed to a virus-smog. The next feature, how long the infectious lifetime of SARS-CoV-2 is.
The virus appears to be lastingly viable on surfaces of plastic (half-life 7 hrs up to ~80 hrs), stainless steel (half-life 6 hrs up to ~70 hrs), cardboard (half-life 4 hrs up to ~40 hrs) (van Doremalen et al., 2020). An open question, how long the half-time on street dust is. As a safety measure, disinfection on street surfaces is frequent today, despite the lack of evidence proving its efficacy against the spread of the outbreak. Figure 2. Sketch for a successful outbreak SARS-CoV-2 is constantly mutated (Andresen et al., 2020), presently eight tribes are known (Weise, 2020). Instability of the viral RNA and coat-protein does not facilitate the development of diagnostics and vaccination. Intermediate and symptomless hosts may serve as reservoirs and enable further mutations. Operational safety of rare animal markets, quite popular today, is highly questionable. Travelling and food transportation is particularly risky for the pandemic.

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