Covid-19: Where From? Why Now?
Yves here. There’s still a lot of speculation on how Covid-19 originated. Our medical sleuth Ignacio has a look.
By Ignacio Moreno Echanove, an epidemiologist
I recently read an article (1, in Spanish) that made the case that Covid-19 could somehow be a pandemic of our own creation. Not that humans created the agent but human practices facilitated the transfer and spread of the disease. So far, we have just learnt, the epidemics could have been originated in wild animal farms or markets in China though we still don’t know the precise location and the intermediary species possibly involved. It is assumed, and a lot of known molecular data so indicates, that this virus has an origin in bats (reviewed in 2) but it’s ability to infect humans was probably acquired in an intermediate host –probably a host with similarities in the virus receptor with humans– and then tuned in a hypothetical human subject 0 of the epidemics into the virus now is infecting so many and adapting even more in the process. Just like SARS 1.0 did in civets before jumping to humans in 2002.
One can rightly ask why two relatively rare events like SARS 1.0 (2002), and SARS 2.0 (2019) have come to occur in a relatively short span. We can add avian origin H5N1 Flu in 2003, also originated in South Eastern Asia.
If you don’t believe in mere coincidences, I agree with you. Something must be facilitating this. But there are differences. First, H5N1 Flu has avian origin while both SARS come originally from bats. Second, flu is associated with farming while both SARS are associated with wild animal trafficking. As for the first difference, avian and mammal coronavirus diverged about 300 million years ago according to molecular clock studies and none of the known human coronavirus can be associated with known avian coronavirus while many flu strains are thought to be originated in birds. This still leaves the former question unanswered.
The first article linked says that some of the animal and human epidemics we have been seeing lately would have a relation with industrial farming. Indeed, farming has almost certainly been the origin of most infectious human diseases since as long as 7.000 years ago. Moreover, intensive industrial farming practices may be behind several outbursts of novel animal and human diseases as these may result in rapid amplification of diseases (3).
(Quote from ref. 3): There is a near consensus among experts that overuse of antibiotics, crowded and unsanitary livestock conditions, unnatural feed diets, and a lack of diversification are responsible for some serious global health risks.
I am OK with this and it may explain the 5 bacterial and viral diseases listed in the article. But it has been shown that H5N1 flu was originated in a family farm so it is not always a case of centralized industrial production in megafarms (4). Moreover, such megafarms are in many cases better isolated from wild sources than traditional farms and might offer less opportunities for species jumping from the wild as in SARS.
If SARS has a relationship with wild animal farming and markets, what has then this to do with farming?
The original article goes on to say that an increase in wild animal trafficking in China could be related with a sharp drop in meat production in China in 2019. An outbreak of African Swine Fever disease starting in 2018 caused a large production decrease in China during 2019 (5). Pork accounts for more than 60% of total meat production in China (broiler meat excluded). According to the National Statistics Bureau of China (NSBCh) pork meat production fell in China by 21% in 2019 with a total loss of about 11.5 million tons of meat (6). An increase in lamb and beef meat production could compensate only for a small part of this. The NSBCh does not yet provide for meat price changes in 2019 but this is a solid suggestion that wild animal trafficking could have increased partly to compensate for shortages of pork meat. At least in international markets this caused a spike in frozen pig meat (7). This looks like a solid line of research to explore.
Interestingly, I found a link that makes the opposite argument: industrial farming, and not wild animal farms are to blame (8). According to this analysis, a SADS (pig coronavirus) epidemic a few years ago in pork farms in China suggests that industrial farming and not familiar farms, are to blame.
(Quote from 8 Many of the animals on this list are industrially farmed in China, even wild animals like civets and pangolins are intensively farmed for their use in Chinese medicines. Suspicions that wild animal farms may have been behind the Covid-19 outbreak have already led the Chinese government to shut down 20,000 wild animal farms across the country.
But hardly any attention has been given to some other animals on this list, which more clearly meet the “high population density” criteria. Pigs would be one obvious candidate from this list, for several reasons. For one, pigs and humans have very similar immune systems, making it easy for viruses to cross between the two species, as happened with the Nipah virus outbreak in Malaysia in 1998… [emphasis mine]
I would argue that high density is to blame for amplification of diseases but this doesn’t say anything about the origin. As I wrote before megafarms may be better isolated from the wild than the familiar farms they defend in the article. Moreover, given the large number of wild animal farms that Chinese authorities closed, the chances of a fortuitous jump from bats, civets, racoons, pangolins etc. to humans seem greatly higher on these than on megafarms. So far, no coronavirus jump has been demonstrated from pigs to humans whereas civets were demonstrated for SARS 1.0. My opinion is that it would be a big mistake to overlook their potential role in this and possible future outbreaks.
Besides, the fact that Chinese authorities closed these farms is very telling. It is important that many mammal species are screened to find a CoV which is closest to SARS CoV 2 to prevent new outbreaks. A recent article said, on the basis of ACE2 receptor similarities, that reptiles might be discarded for search but Bovidae (cows…) or Cricetidae (rodent subgroup) should be included in the search (9), but similarities in ACE 2 do not prove much about the origin of SARS CoV 2.
I find it annoying that after a couple of months no an exhaustive work has been published on this topic. After some noises on snakes and pangolins and now Bovidae or Cricetidae we are still in the dark. Are Chinese authorities retaining information that could somehow discredit them? According to (10):
Just weeks before the outbreak, China’s State Forestry and Grassland Administration (SFGA) was still actively encouraging citizens to get into farming wildlife such as civet cats – a species pinpointed as a carrier of Sars, a disease similar to Covid-19. The SFGA regulates both farming and trade in terrestrial wildlife, and quotas of wildlife products – such as pangolin scales – allowed to be used by the Chinese medicine industry.
I think The Guardian may have got it right.
- Causalidad de la pandemia, cualidad de la catástrofe. https://www.msn.com/es-es/news/tecnologia/causalidad-de-la-pandemia-cualidad-de-la-cat-c3-a1strofe/ar-BB11SABK
- Zoonotic origins of human coronaviruses. https://www.ncbi.nlm.nih.gov/pubmed/32226286
- A Review on “Bird Flu: A Virus of Our Own Hatching” by D.J. Sencer. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3375804/
- 5 modern diseases grown by factory farming. https://theweek.com/articles/457135/5-modern-diseases-grown-by-factory-farming
- China’s 2019 pork output plunges to 16-year low as disease culls herd. https://www.reuters.com/article/us-china-economy-output-pork-idUSKBN1ZG08H
- National Bureau of Statistics, China. Annual data. http://data.stats.gov.cn/english/easyquery.htm?cn=C01
- FAO: Food Outlook BIANNUAL REPORT ON GLOBAL FOOD MARKETS. http://www.fao.org/3/CA6911EN/CA6911EN.pdf
- New research suggests industrial livestock, not wet markets, might be origin of Covid-19. https://www.grain.org/en/article/6437-new-research-suggests-industrial-livestock-not-wet-markets-might-be-origin-of-covid-19
- SARS-CoV-2 spike protein favors ACE2 from Bovidae and Cricetidae. https://onlinelibrary.wiley.com/doi/epdf/10.1002/jmv.25817
- Coronavirus closures reveals vast scale of China’s secretive wildlife farm industry.https://www.theguardian.com/environment/2020/feb/25/coronavirus-closures-reveal-vast-scale-of-chinas-secretive-wildlife-farm-industry