Severe Acute Respiratory Syndrome (SARS)
The most recent common ancestor (MRCA) of all coronaviruses is
estimated to have existed as recently as 8000 BCE, although some models place the
common ancestor as far back as 55 million years or more, implying long
term coevolution with bat and avian species. The most recent common
ancestor of the alpha coronavirus line has been placed at about 2400 BCE, of
the beta coronavirus line at 3300 BCE, of the gamma coronavirus line at 2800
BCE, and of the delta coronavirus line at about 3000 BCE. Bats and birds,
as warm-blooded flying vertebrates, are an ideal natural reservoir for
the coronavirus gene pool (with bats the reservoir for alpha coronaviruses and beta coronavirus – and birds the
reservoir for gamma coronaviruses and delta coronaviruses). The large number
and global range of bat and avian species that host viruses has enabled
extensive evolution and dissemination of coronaviruses.
Many human coronaviruses have their
origin in bats. The human coronavirus NL63 shared a common ancestor with a
bat coronavirus (ARCoV.2) between 1190 and 1449 CE. The human coronavirus 229E
shared a common ancestor with a bat coronavirus (GhanaGrp1 Bt CoV) between 1686
and 1800 CE. More recently, alpaca coronavirus
and human coronavirus 229E diverged sometime before 1960. MERS-CoV emerged in
humans from bats through the intermediate host of camels. MERS-CoV,
although related to several bat coronavirus species, appears to have diverged
from these several centuries ago. The most closely related bat coronavirus
and SARS-CoV diverged in 1986. The ancestors of SARS-CoV first infected
leaf-nose bats of the genus Hipposideridae; subsequently, they spread to horseshoe bats in the species Rhinolophidae,
then to Asian palm civets, and finally to humans.
Unlike other beta coronaviruses, bovine coronavirus of the species Beta
coronavirus and subgenus Embecovirus is
thought to have originated in rodents and
not in bats. In the 1790s, equine coronavirus diverged from the bovine
coronavirus after a cross-species jump. Later in the 1890s, human
coronavirus OC43 diverged from bovine coronavirus after another cross-species
spillover event. It is speculated that the flu pandemic of 1890 may have been caused by this
spillover event, and not by the influenza virus,
because of the related timing, neurological symptoms, and unknown causative
agent of the pandemic. Besides causing respiratory infections, human
coronavirus OC43 is also suspected of playing a role in neurological diseases. In the 1950s, the human coronavirus
OC43 began to diverge into its present genotypes. Phylogentically,
mouse hepatitis virus (Murine coronavirus), which infects the mouse's liver
and central nervous
system, is related to
human coronavirus OC43 and bovine coronavirus. Human coronavirus HKU1, like the
aforementioned viruses, also has its origins in rodents
·
INFECTION IN HUMANS
Coronaviruses vary
significantly in risk factor. Some can kill more than 30% of those infected,
such as MERS-CoV, and some are relatively harmless, such as the
common cold. Coronaviruses can cause colds with major symptoms, such as fever, and a sore
throat from
swollen adenoids. Coronaviruses can cause pneumonia (either direct viral
pneumonia or
secondary bacterial pneumonia) and bronchitis (either direct viral bronchitis or
secondary bacterial bronchitis). The human coronavirus discovered in
2003, SARS-CoV, which causes severe acute respiratory
syndrome (SARS), has a
unique pathogenesis because it causes both upper and lower respiratory tract
infections.
·
Six species of human coronaviruses are known, with one
species subdivided into two different strains, making seven strains of human
coronaviruses altogether.
- Human
coronavirus OC43 (HCoV-OC43), β-CoV
- Human
coronavirus HKU1 (HCoV-HKU1), β-CoV
- Human
coronavirus 229E (HCoV-229E), α-CoV
- Human
coronavirus NL63 (HCoV-NL63), α-CoV
·
Three human coronaviruses
produce symptoms that are potentially severe:
- Middle East
respiratory syndrome-related coronavirus (MERS-CoV), β-CoV
- Severe
acute respiratory syndrome coronavirus (SARS-CoV), β-CoV
- Severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2), β-CoV
·
Common cold
·
Main article: Common
cold
· The human coronaviruses HCoV-OC43, HCoV-HKU1, HCoV-229E, and HCoV-NL63 continually circulate in the human population and produce the generally mild symptoms of the common cold in adults and children worldwide. These coronaviruses cause about 15% of common colds. While 40 to 50% of colds are caused by rhinoviruses. The four mild coronaviruses have a seasonal incidence occurring in the winter months in temperate climates. There is no preponderance in any season in tropical climates.
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