Coronavirus (continued)
Many wild animals have been surveyed since SARS-CoV-1 was
isolated in 2003. In addition to coronaviruses, bats continue to
harbor some of the most diverse and highly pathogenic viruses,
such as Ebola, Nipah, Hendra, and rabies viruses. Bats are found
everywhere on Earth, with the exception of Antarctica, and ac-
count for 20% of all mammals. They play an important role in ag-
riculture by pollinating fruit trees and controlling insect popula-
tions (for example, mosquitoes). In Asia, more than 50 species of
bats are hunted and eaten by low-income populations. Bats also
are used in traditional medicine, and their guano (excrement) is
harvested to use as fertilizer for jackfruit, eggplant, papaya, and
other economically important crops.
Bats are capable of sustained flying speeds up to 160 km per
hour (99 miles per hour) for hours at a time, achieving long dis-
tances that aid virus transmission. Because they possess remark-
ably strong antiviral immune responses and limited self-damaging
inflammatory responses, bats rarely exhibit disease symptoms.
Bats are being increasingly recognized as flying germ factories for
zoonotic diseases that become lethal when they jump to humans.
Undoubtedly, the conditions at some geographic locations will be
ideal for spillover and a potential epidemic.
Coronaviruses are masters of
RNA recombination
What determines whether a coronavirus will jump to hu-
mans? Interestingly, the mutation rate of SARS-CoV-1 has been
estimated to be consistent with that of other viruses that have
RNA genomes. The viral RNA genome has a high-fidelity rate,
undergoing 0.17 mutations per genome per day or 2 mutations
per human passage (few errors occur during viral replication).
Coronaviruses are masters of RNA recombination, which is a
genetic mechanism whereby two different viral RNA genomes
hybridize and exchange regions of RNA at precise locations.
Coronavirus genomes undergo RNA recombination at high
frequencies during replication. In doing so, the integrity of the
RNA is preserved (replacing any mistakes made during rep-
lication). In addition, coronaviruses can pick up novel genes,
enabling them to adapt to a new host. RNA recombination can
occur between the same and different coronavirus genomes
(and between host RNA, which is less common).
The RNA region that codes for the S protein is located in the
most variable region of the coronavirus genome. RNA recom-
bination events resulting in S-protein mutations are typical
examples of how viral genomic changes allow a coronavirus to
adapt and attach to a host receptor, enabling the coronavirus
to infect a different species. In addition, changes in other loca-
tions of the coronavirus genome can increase its adaptability to
new hosts, or the genome may incorporate host/cellular RNA
into its genome that codes for an accessory factor that permits
its replication in a different species. Thus, it is more apparent
than ever that emerging coronaviruses are a serious threat.
Fig. 4: Primary and intermediate hosts of human coronaviruses showing cross-species transmission. No concrete evidence exists about the intermediate host (or hosts) of HCoV NL63 and
HCoV HKU1. The years of isolation or emergences of each viral strain are also indicated. (Credit: Teri Shors)
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