Coronavirus (continued)
Coronaviruses and spillover events
The term "species barrier" encompasses the concept that
natural mechanisms prevent the crossing of pathogens from
one species to another. The term spillover refers to the trans-
mission of a pathogen from a host species to a recipient species
(for example, from a vertebrate animal to a human). Research
conducted before 2002 demonstrated that mammalian coro-
naviruses could cross species barriers by infecting birds. Bovine
coronaviruses also could infect turkeys, but not chickens and
pigs. However, the 2002 spillover of SARS-CoV-1 jumping from
wild animals to humans surprised the medical community.
The first cases of SARS in 2002 were observed in a wild-ani-
mal trader, restaurant chefs, and exotic-animal food handlers,
who butchered and sold the meat at live markets in Guang-
dong province in China. In China, the meat of wild animals is
considered more natural and nutritious compared to farmed
meat. Freshly prepared meat is preferred. Live animals, typically
kept in stacked open-air cages, may be bought from hundreds
of small markets. The stressed animals may be infected and
maintained in cramped unsanitary conditions, allowing corona-
viruses (and other pathogens) to spread more easily.
During the early SARS outbreak, scientists used molecular
probes to detect certain regions of the coronavirus genome
in the domestic and wild animals being sold at the live mar-
kets. Moreover, blood samples were drawn from workers with
exposure to the animals, and serological surveys detected the
presence of coronavirus antibodies. The animal traders and
food handlers had significantly higher antibody titers against
coronaviruses compared to the general population. Thus, the
animals sold at the live markets were the likely source of SARS-
CoV-1. The first animals found to carry SARS-CoV-1 were palm
civets and raccoon dogs. After culling of the masked civets at
the live markets, SARS cases ceased. Further investigations de-
termined that Chinese horseshoe bats (Rhinolophus) were the
primary host or natural animal host (reservoir) of SARS-CoV-1.
The current knowledge of primary and intermediate hosts for
human coronaviruses is summarized in Fig. 4.
+
ward
'
s
science
Fig. 3: Classification of coronaviruses. Human coronaviruses are shown in bold type. Highly pathogenic coronaviruses in humans are shown in red type. Human coronaviruses belong to two
different genera: Betacoronavirus and Alphacoronavirus. (Credit: Teri Shors)
