Corona Viruses Are the Largest

Corona viruses are the largest positive strand RNA viruses with a diameter of nearly 10 nanometers -- nm and are the largest genomes of any RNA virus. Both humans and animals get infected with corona viruses causing respiratory and enteric diseases. The corona viruses together with the toroviruses and anteriviruses are part of a group known as nidovirales that produce a series of mRNA with a common end.

Corona viruses are "responsible for causing one-third of 'common cold' and the recent outbreak of Severe Acute Respiratory Syndrome - SARS." (Board of Trustees of the University of South Carolina, 2007) During common cold in humans, the major site of the virus replication is the epithelial cells of the respiratory tract. The virus has an incubation period of three days. The virus spread is limited by the immune response of majority of the patients, but this immunity does not last very long.

Corona viruses are unstable and the infection spreads through nasal secretions like the aerosols caused by sneezes. Even though corona viruses' infections are normally local, they have an affinity to spread in humans with infections in middle ear, in some pneumonia in immuno-supressed patients and in myocarditis, but, in animals, systemic infections can be much more severe. Majority of the conona viruses infections go undiagnosed and the disease is self-limiting. Diseases can be diagnosed through immuno-electron microscope and serology.

"No anti-virals for routine coronaviruses infections are there, however OTC remedies to alleviate symptoms prove helpful" (Board of Trustees of the University of South Carolina, 2007) Corona viruses are responsible for a broad range of infections in humans as also animals, but normally are classified into two categories with respiratory or enteric tropisms. Mouse Hepatitis Virus -- MHV and rat corona viruses are the ones most often seen in the laboratory animal settings.

Whereas a lot of methods are available for the diagnosis of corona virus infection, serologic tests, mainly ELISA and IFA are the most normally used. MHV is best administered on a preventive basis. Removal of MHV from population needs termination of breeding and stopping of immature mice into the population. (Compton; Barthold; Smith, 1993) Corona Viruses: genome structure and categories: Corona viruses were isolated for the first time from chickens in 1937. Following the discovery of Rhinoviruses during the 1950s, half of all the causes of cold were attributed to known agents.

During 1965, Tyrell and Bynoe used the cultures of human ciliated embryonal trachea to propagate the first human corona virus -- HCoV in vitro. Presently, there are an estimated 15 species which fall under this category that infect not just humans, but cattle, pigs, rodents, cats, dogs and birds. Besides some severe veterinary pathogens particularly chickens are available. SARS which is a category of "viral pneumonia with symptoms that includes fever, a dry cough, dyspnea- shortness of breath, headache, and hypoxaemia i.e.

low blood oxygen concentration." ("Corona Viruses: Introduction," 2007) Corona virus infection occurs worldwide and is very common. Infection mostly occurs during winter with children being affected the most. The number of corona viruses serotypes and the intensity of antigenic variation in not known. "Repeated infections seem to happen throughout life, referring multiple serotypes where four types are known and antigenic variation, therefore the chances for immunization seem to be miserable." ("Corona Viruses: Introduction," 2007) The diagnosis tests for corona virus infection come under two categories.

(i) Serological testing: This test for anti-corona viruses comprises of fluorescent antibody and enzyme-associated imunosorbent assays -- ELISA that sense antibodies against the virus generated in response to the infection. Even though some patients have detectable corona virus antibody in the space of 14 days of start of illness, definitive interpretation of negative corona virus antibody can happen solely in case of specimens that are received after 21 days of starting of fever.

("Corona Viruses: Introduction," 2007) ii) Molecular Testing: This type of testing comprises of reverse transcriptase-polymerase chain reaction or RT-PCR tests that is particularly for the RNA from this new corona virus. This is able to detect presence of infection within the initial 10 days following the start of fever in some patients infected with SARS, however the "time period of detectable viraemia and virus shedding is not known, therefore RT-PCR tests which were undertaken very lately could generate adverse results." ("Corona Viruses: Introduction," 2007) However commercial diagnostic tests are presently available.

Corona viruses with almost sequence resemblance to the surface spike protein of human SARS isolates have been isolated in Guangdong, China, from really healthy masked palm civets, a mammal resembling closely to cat and related to mongoose. It is believed that humans got the infection as they raised and killed the animals instead of consumption of raw meat of the animal. Significant experience of development of corona virus vaccines for veterinary purpose is there. On average, inactivated corona virus vaccines encourage poor protection.

"The spike protein can trigger immunity; however the internal nucleoprotein has also been stated to trigger protective immunity." ("Corona Viruses: Introduction," 2007) A graphic of genome structure of corona viruses and replication is shown in Exhibit -I. ("Corona Viruses: Introduction," 2007) The genome of corona viruses consists of structural and non-structural genes, inclusive of several so-called accessory genes. Every group 1b corona viruses encode a singular accessory protein between the spike and envelope genes, save for human corona viruses --HCoV 229E.

The virons are 80-150 nm in diameters and possess a unique morphology, having elongated petal shaped spikes which render the virus a projection of crown from which the Latin word Corona comes. Corona viruses are classified into three major groups based in phylegeneticn and serological relationships. "Group 1 and 2 comprises of different mammalian corona viruses while bird viruses constitute group 3.

Every corona viruses use a common gene organization in which the replicase gene includes the 5'-two thirds of the genome and is composed of two overlapping open reading frames -- ORFs, ORF1a and ORF1b." (Dijkman; Jebbink; Wilbrink, et. al, 2006) The functions of corona virus accessory proteins are normally not understood, nevertheless reverse genetic analyses of Mouse Hepatitis Virus -- MHV and Feline Infectious Peritonitis -- FIPV indicate that they are not needed for virus multiplication.

Besides, deletion of MHV and FIPV accessory genes outcome in attenuation in their respective hosts, illustrating that accessory genes symbolize pathogenicity factors. The group 1 corona viruses can be categorized into two genetic subgroups i.e. 1a and 1b. Those belonging to the "group 1a comprises canine coronaviris, FIPV, transmissible gastroenteritis virus -- TGEV, and ferret enteric corona virus." (Dijkman; Jebbink; Wilbrink, et. al, 2006) Within Group 1b are included porcine epidemic diarrhea virus -- PEDV, human corona virus NL63 or HCoV-NL63 and human corona virus 229E or HCoV-229E.

(Dijkman; Jebbink; Wilbrink, et. al, 2006) Carona Viruses: Recent findings: Latest findings about corona virus infection in poultry and avian population reveals that the number of avian species in which corona viruses have been spotted have gone up twice in the space of a couple of years. Whereas the corona viruses in these species have all been in corona virus Group 3, as for the well-known corona viruses of the domestic fowl, turkey and pheasant, experimental proof is found to recommend that birds are not restricted to infection with Group 3 corona viruses.

Experiments with several of the Group 1 corona viruses which by far have been from mammals have indicated that they are not restricted to multiplying or causing disease in a single host. It has been found that SARS-corona virus possesses a wide host range. Distinctly there is a capability for the "materialization of new corona viruses' diseases in domestic birds, from avian as well as mammalian sources." (Cavanagh, 2005) Besides, other latest studies suggest that bats.