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Several coronaviruses can cause fatal systemic diseases in animals, including feline infectious peritonitis virus (FIPV), hemagglutinating encephalomyelitis virus (HEV) of swine, and some strains of avian infectious bronchitis virus (IBV) and mouse hepatitis virus (MHV). These coronaviruses can replicate in liver, lung, kidney, gut, spleen, brain, spinal cord, retina, and other tissues (Holmes). Coronaviruses cause economically important diseases in domestic animals.

Human coronaviruses (HCoVs) were previously only associated with mild diseases. They are found in both group 1 (HCoV-229E) and group 2 (HCoV-OC43) and are a major cause of normally mild respiratory illnesses (Makela). They can occasionally cause serious infections of the lower respiratory tract in children and adults and necrotizing enterocolitis in newborns (McIntosh, El-Sahly, Folz, Sizun). The known human coronaviruses are able to survive on environmental surfaces for up to 3 hours (Sizun). Coronaviruses may be transmitted from person-to-person by droplets, hand contamination, fomites, and small particle aerosols (Ijaz).

SARS-related CoV seems to be the first coronavirus that regularly causes severe disease in humans.

SARS Co-V

Genome Sequence

In April 2003, a Canadian group of researchers from the Michael Smith Genome Sciences Centre in Vancouver, British Columbia, and the National Microbiology Laboratory in Winnipeg, Manitoba, were the first to complete the genome sequencing of the new coronavirus (Marra), followed two days later by colleagues from the CDC (Rota).

The genome sequence data of SARS Co-V reveal that the novel agent does not belong to any of the known groups of coronaviruses, including two human coronaviruses, HCoV-OC43 and HCoV-229E (Drosten, Peiris, Marra, Rota), to which it is only moderately related. The SARS-CoV genome appears to be equidistant from those of all known coronaviruses. Its closest relatives are the murine, bovine, porcine, and human coronaviruses in group 2 and avian coronavirus IBV in group 1. For links to the most recent sequence data and publi

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cations, see the NCBI web page http://www.ncbi.nlm.nih.gov/genomes/SARS/SARS.html.

It has been proposed that the new virus defines a fourth lineage of coronavirus (Group 4, Marra). The sequence analysis of SARS-CoV seems to be consistent with the hypothesis that it is an animal virus for which the normal host is still unknown and that has recently either developed the ability to productively infect humans or has been able to cross the species barrier (Ludwig). The genome shows that SARSCoV is neither a mutant of a known coronavirus, nor a recombinant between known coronaviruses.

As the virus passes through human beings, SARS-CoV is apparently maintaining its consensus genotype and seems thus well-adapted to the human host (Ruan). However, genetic analysis is able to distinguish between different strains of SARS-CoV, which is of great value for epidemiological studies and may also have clinical implications (Tsui).

Morphology

Negative-stain transmission electron microscopy of patient samples and of cell culture supernatants reveals pleomorphic, enveloped coro- navirus-like particles with diameters of between 60 and 130 nm. (Ksiazek, Peiris).

Examination of infected cells by thin-section electron microscopy shows coronavirus-like particles within cytoplasmic membrane-bound vacuoles and the cisternae of the rough endoplasmic reticulum. Extracellular particles accumulate in large clusters, and are frequently seen lining the surface of the plasma membrane (MMWR 2003; 52: 241248).

Organization

The SARS-CoV genome contains five major open reading frames (ORFs) that encode the replicase polyprotein; the spike (S), envelope (E), and membrane (M) glycoproteins; and the nucleocapsid protein

(N).

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SARS Co-V 35

The main function of the S protein is to bind to species-specific host cell receptors and to trigger a fusion event between the viral envelope and a cellular membrane. Much of the species specificity of the initial infection depends upon specific receptor interactions. In addition, the spike protein has been shown to be a virulence factor in many different coronaviruses. Finally, the S protein is the principal viral antigen that elicits neutralizing antibody on behalf of the host.

The M protein is the major component of the virion envelope. It is the major determinant of virion morphogenesis, selecting S protein for incorporation into virions during viral assembly. There is evidence that suggests that the M protein also selects the genome for incorporation into the virion.

One remarkable feature about coronavirus RNA synthesis is the very high rate of RNA-RNA recombination.

Detection

SARS Co-V has been detected in multiple specimens including extracts of lung and kidney tissue by virus isolation or PCR; bronchoalveolar lavage specimens by virus isolation, electron microscopy and PCR; and sputum or upper respiratory tract swab, aspirate, or wash specimens by PCR (Ksiazek, Drosten).

High concentrations of viral RNA of up to 100 million molecules per milliliter were found in sputum (Drosten). SARS-associated coronavirus RNA was detected in nasopharyngeal aspirates by RT-PCR in 32% at initial presentation (mean 3.2 days after onset of illness) and in 68% at day 14 (Peiris 2003b). In stool samples, viral RNA was detected in 97% of patients two weeks after the onset of illness. 42% of urine samples were positive for viral RNA (Peiris 2003b).

Viral RNA was also detected at extremely low concentrations in plasma during the acute phase and in feces during the late convalescent phase, suggesting that the virus may be shed in feces for prolonged periods of time (Drosten).

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Stability and Resistance

Work is on-going to evaluate the stability of SARS-CoV and its resistance against various environmental factors and disinfectants.

Preliminary results, obtained by members of the WHO multicenter collaborative network on SARS diagnosis (see: http://www.who.int/csr/sars/survival_2003_05_04/en/index.html), show that the virus is stable in feces and urine at room temperature for at least 1-2 days. The stability seems to be higher in stools from patients with diarrhea (the pH of which is higher than that of normal stool).

In supernatants of infected cell cultures, there is only a minimal reduction in the concentration of the virus after 21 days at 4°C and – 80°C. After 48 hours at room temperature, the concentration of the virus is reduced by one log only, indicating that the virus is more stable than the other known human coronaviruses under these conditions. However, heating to 56°C inactivates SARS-CoV relatively quickly. Furthermore, the agent loses its infectivity after exposure to different commonly-used disinfectants and fixatives.

Natural Host

Research teams in Hong Kong and Shenzhen detected several coronaviruses that were closely related genetically to the SARS coronavirus in animals taken from a southern Chinese market that was selling wild animals for human consumption. They found the virus in masked palm civets (Paguma larvata) as well as some other species. All six of the civets included in the study were found to harbor SARS coronavirus, which was isolated in cell culture or detected by a PCR molecular technique. Serum from these animals also inhibited the growth of SARS coronavirus isolated from humans. Vice versa, human serum from SARS patients inhibited the growth of SARS isolates from these animals. Sequencing of viruses isolated from these animals demonstrated that, with the exception of a small additional sequence, the viruses are identical to the human SARS virus (Cyranoski; Enserink 2003a).

The study provides a first indication that the SARS virus exists outside a human host. However, at present, no evidence exists to suggest

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