Coronaviruses are enveloped spherical viruses of the size of 120-160 nm. Their genetic material is encoded within a single strand of RNA of positive sense (+RNA). Club- or petal-shaped spikes covering coronaviruses are the inherent features of theirs, giving them the resemblance to the solar corona in the microscopic imaging - hence the "corona-" prefix. Given both the genome size and the level of their genetic complexity, coronaviruses are the largest viruses with an RNA strand identified to date.

Initially, the family of Coronaviridae had been subject to the division into three groups, as based on serological methods. After molecular techniques were introduced, a new classification of the family was carried out and currently there exist Alpha-, Beta-, Gamma- and Deltacoronaviruses, as such distinguished.

Alphacoronaviruses include numerous animal pathogens (e.g. feline infectious peritonitis, swine epidemic viral diarrhoea), as well as two coronaviruses affecting humans (HCoV-229E and HCoV-NL63). Betacoronaviruses include such viruses affecting animals as Mouse Hepatitis Virus (MHV), equine and canine coronaviruses, as well as three coronaviruses affecting humans (HCoV-0C43, HCoV-HKU1 and SARS-CoV). Gamma- and Deltacoronaviruses include pathogens infecting birds.

Highly-magnified, digitally-colorized transmission electron micrograph (TEM) highlighting the particle envelope of a single, spherical-shaped Middle East Respiratory Syndrome Coronavirus (MERS-CoV) virion through the process of immunolabeling the envelope proteins using Rabbit HCoV-EMC/2012 primary antibody, and Goat anti-Rabbit 10nm gold particles. Please see the Flickr link below for additional NIAID photomicrographs of the MERS-CoV. Image credits: National Institute of Allergy and Infectious Diseases (NIAID).

Coronaviruses may infect a considerable lot of vertebrate species, usually to cause illnesses affecting their respiratory system or intestines. From the point of view of veterinary medicine, the coronaviruses attacking cats, dogs, swine and domestic cattle appear to be the most relevant.

The feline coronavirus has two biotypes - Feline Enteric Coronavirus (FeCoV) and FIPV. The former one is very common in cats. The infection with it occurs asymptomatically, or, it may cause diarrhoea. After the infection, a cat may become the carrier of the virus, while the contact with this pathogen is usually not very threatening. However, there exists a certain variation of this virus - FIPV - that demonstrates an ability to replicate itself in macrophages. It causes a serious and - usually fatal - disease in cats named Feline Infectious Peritonitis (FIP).

In the case of dogs, there are two coronavirus diseases: CCoV virus - causing the infections affecting their digestive systems - and Canine Respiratory Coronavirus (CRCoV): a virus discovered in 2003 which causes the infections of canine respiratory system. It is currently assumed that 88% and 33% of the entire population of domestic dogs demonstrate seropositivity with regard to CCoV and CRCoV, respectively (as having had contact with the pathogen). Statistically and subsequently, 33% and 9% of seropositive dogs are the carriers of CCoV and CRCoV.

Digitally-colorized transmission electron micrograph (TEM) of spherical-shaped Middle East Respiratory Syndrome Coronavirus (MERS-CoV) virion. Image credits: National Institute of Allergy and Infectious Diseases (NIAID).

The etiology of coronavirus diseases in swine appears more complex. The most frequently recognised coronavirus pathogens include Porcine Epidemic Diarrhoea Virus (PEDV), Transmissible Gastroenteritis Coronavirus (TGEV), Porcine Respiratory Coronavirus infection (PRCV), Hepatitis E Virus - a hemagglutinating virus causing encephalitis and meningitis (HEV, an emetic and devastating disease), PorCoV/HKU15 - probably a new coronavirus causing diarrhoea in piglets, first reported to the World Organisation for Animal Health in April, 2014 as a so-called "emerging disease" (with its disease outbreaks affecting the majority of US states) and Porcine Deltacoronavirus (PDCoV, SDCV), manifesting itself with the symptoms similar to PEDV virus, first found in China in 2012 (first outbreaks were discovered in the USA and Canada in the year of 2014). Coronavirus infections are particularly threatening to piglets, with the death rate reaching even 100% (especially in the case of PEDV, PDCoV and TGEV) in affected populations. One of the most recent reports on the serious consequences of coronavirus infections in swine comes from the USA and the year of 2013, when the first outbreak of epidemic diarrhoea in swine caused by PEDV virus was recorded on the American continent. At the farm where the disease was first diagnosed, the losses among the suckling piglets reached 100%. Throughout the months to follow, the disease was recorded at more than 1100 American farms located in the territory of 19 different states. In total, there were more than 5 million suckling pigs that died due to PEDV virus in the USA throughout 2013.

Before the beginning of the 21st century, coronaviruses had usually been regarded as pathogens not being harmful to humans that would instead be responsible for causing relatively mild colds. The image underwent a dramatic change in the years of 2003 and 2012, when the often - and potentially - fatal coronaviruses as SARS and MERS were discovered. The epidemiology of these diseases is under constant supervision of the World Health Organization. It is worth knowing that both diseases are of zoonotic nature - in the case of MERS, that is currently present in the Middle East, camels constitute an intermediary reservoir for the virus.

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