Abstract
Coronaviruses
(CoVs), enveloped positive-sense RNA viruses, are characterized by
club-like spikes that project from their surface, an unusually large RNA
genome, and a unique replication strategy. Coronaviruses cause a
variety of diseases in mammals and birds ranging from enteritis in cows
and pigs and upper respiratory disease chickens to potentially lethal
human respiratory infections. Here we provide a brief introduction to
coronaviruses discussing their replication and pathogenicity, and
current prevention and treatment strategies. We will also discuss the
outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome
Coronavirus (SARS-CoV) and the recently identified Middle Eastern
Respiratory Syndrome Coronavirus (MERS-CoV).
Keywords: Nidovirales, Coronavirus, positive-sense RNA viruses, SARS-CoV, MERS-CoV
Animal Coronaviruses
Coronaviruses
cause a large variety of diseases in animals, and their ability to
cause severe disease in livestock and companion animals such as pigs,
cows, chickens, dogs and cats led to significant research on these
viruses in the last half of the 20th century. For instance,
Transmissible Gastroenteritis Virus (TGEV) and Porcine Epidemic Diarrhea
Virus (PEDV) cause severe gastroenteritis in young piglets, leading to
significant morbidity, mortality, and ultimately economic losses. PEDV
recently emerged in North America for the first time, causing
significant losses of young piglets. Porcine hemagglutinating
encephalomyelitis virus (PHEV) mostly leads to enteric infection but has
the ability to infect the nervous system, causing encephalitis,
vomiting and wasting in pigs. Feline enteric coronavirus (FCoV) causes a
mild or asymptomatic infection in domestic cats, but during persistent
infection, mutation transforms the virus into a highly virulent strain
of FCoV (Feline Infectious Peritonitis Virus, FIPV), that leads to
development of a lethal disease called feline infectious peritonitis
(FIP). FIP has wet and dry forms, with similarities to the human
disease, sarcoidosis. FIPV is macrophage tropic and it is believed that
it causes aberrant cytokine and/or chemokine expression and lymphocyte
depletion, resulting in lethal disease [63].
However additional research is needed to confirm this hypothesis.
Bovine CoV, Rat CoV, and Infectious Bronchitis Virus (IBV) cause mild to
severe respiratory tract infections in cattle, rats, and chickens,
respectively. Bovine CoV causes significant losses in the cattle
industry and also has spread to infect a variety of ruminants, including
elk, deer and camels. In addition to severe respiratory disease, the
virus causes diarrhoea (‘winter dysentery’ and ‘shipping fever’), all
leading to weight loss, dehydration, decreased milk production, and
depression [63].
Some strains of IBV, a γ-coronavirus, also affect the uro-genital tract
of chickens causing renal disease. IBV significantly diminishes egg
production and weight gain, causing substantial losses in the chicken
industry each year [63]. More recently, a novel coronavirus named SW1 was identified in a deceased Beluga whale [64].
Large numbers of virus particles were identified in the liver of the
deceased whale with respiratory disease and acute liver failure.
Although, electron microscopic images were not sufficient to identify
the virus as a coronavirus, sequencing of the liver tissue clearly
identified the virus as a coronavirus. It was subsequently determined to
be a γ-coronavirus based on phylogenetic analysis but it has not yet
been verified experimentally that this virus is actually a causative
agent of disease in whales. In addition, there has been intense interest
in identifying novel bat CoVs, since these are the likely ultimate
source for SARS-CoV and MERS-CoV, and hundreds of novel bat
coronaviruses have been identified over the past decade [65].
Finally, another novel group of nidoviruses, Mesoniviridae, were recently identified as the first nidoviruses to exclusively infect insect hosts [66,67].
These viruses are highly divergent from other nidoviruses but are most
closely related to the roniviruses. In size, they are ∼20 kb, falling in
between large and small nidoviruses. Interestingly, these viruses do
not encode for an endoribonuclease, which is present in all other
nidoviruses. These attributes suggest these viruses are the prototype of
a new nidovirus family and may be a missing link in the transition from
small to large nidoviruses.
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