COVID-19-Zoonosis or Emerging Infectious Disease?

Abstract

The World Health Organization defines a zoonosis as any infection naturally transmissible from vertebrate animals to humans. The pandemic of Coronavirus disease (COVID-19) caused by SARS-CoV-2 has been classified as a zoonotic disease, however, no animal reservoir has yet been found, so this classification is premature. We propose that COVID-19 should instead be classified an "emerging infectious disease (EID) of probable animal origin." To explore if COVID-19 infection fits our proposed re-categorization vs. the contemporary definitions of zoonoses, we reviewed current evidence of infection origin and transmission routes of SARS-CoV-2 virus and described this in the context of known zoonoses, EIDs and "spill-over" events. Although the initial one hundred COVID-19 patients were presumably exposed to the virus at a seafood Market in China, and despite the fact that 33 of 585 swab samples collected from surfaces and cages in the market tested positive for SARS-CoV-2, no virus was isolated directly from animals and no animal reservoir was detected. Elsewhere, SARS-CoV-2 has been detected in animals including domesticated cats, dogs, and ferrets, as well as captive-managed mink, lions, tigers, deer, and mice confirming zooanthroponosis. Other than circumstantial evidence of zoonotic cases in mink farms in the Netherlands, no cases of natural transmission from wild or domesticated animals have been confirmed. More than 40 million human COVID-19 infections reported appear to be exclusively through human-human transmission. SARS-CoV-2 virus and COVID-19 do not meet the WHO definition of zoonoses. We suggest SARS-CoV-2 should be re-classified as an EID of probable animal origin.

Keywords: COVID-19; SARS-CoV-2; emerging infectious disease (EID); spillover; zoonoses.

Figure 1

Pathway diagram for pathogen spill-over to humans from animals describes three distinct processes. (1) Zoonoses: Pathogens that are transmitted from an animal reservoir directly or indirectly (e.g., foodborne, vector-borne, etc.) to humans causing disease; (2) Emerging Infectious Diseases: Pathogens that cause an emergent infectious disease in humans and persist in human populations irrespective of an animal reservoir. Genetic origins may show links to non-human animals, but these diseases undergo a more complex process of evolution not necessarily dependent on a specific animal reservoir, and usually evolve to be independent of animals, (3) Zooanthroponosis: “Reverse zoonosis” whereby humans transmit infection to animals. Infected animals, may or may not then go on to circulate the pathogen or establish a disease within conspecific population. A disease can fall into more than one category as exemplified in the figure by Nipah, MERS, and SARS-CoV-2. Spill-over origin for zoonoses and emerging pathogens (i.e., animals) is given proportion by the size of the circles. The relative infection frequency of spill-over from domestic animals and wildlife is quantified in the percentages shown. Proportion of animals was determined from a review of the literature which found that only 4% of global mammalian biomass is wild, of which >50% is estimated to be marine mammals, with livestock making up ~60% of the remainder (39). Percent of emerging pathogens coming from wildlife was derived from the assessment that ~60% of EIDs are zoonotic, with 71.8% of those found to have wildlife genetic origins (6). Percent zoonoses derived from domestic vs. wild species is a rough estimate based on available data from a variety of publications and considers the following: zoonoses have been estimated to cause 2.5 billion cases of human illness, 2.4 billion of which are caused by thirteen diseases, all of which bear relevance to agriculture (40); zoonotic viral richness is strongly correlated with mammalian species diversity and abundance with domestic species found to harbor, on average, 19.3 zoonotic viruses compared to an average of 0.23 harbored by wildlife, suggesting wildlife harbor >0.5% of viral diversity (43). In addition, an analysis of mammalian species with the greatest number of viruses shared with humans found the top eight species to be domestic (43). Our estimate that 99% of zoonotic diseases spill over from domestic animals is considered reliable for direct zoonotic transmission and indirect foodborne infection, though less reliable for vector borne diseases.