Coronas are not (just) beers.

Severe acute respiratory syndrome (SARS), as its name suggests, is a respiratory illness Coronaviridaewhich largely targets the upper respiratory tract, with its defining symptom being a fever with a temperature of 38°C (100.4°F) or higher. Other symptoms include flu-like symptoms such as chills, muscle aches, headaches, fatigue and sore throat. As the illness progresses, patients may then develop a dry cough and/or shortness of breath, followed by pneumonia, respiratory failure, and acute respiratory distress, potentially leading to death. The incubation period of the virus is about 2-13 days, with a mean of 5 days.

SARS is caused by the virus SARS-coronavirus (SARS-CoV). This virus was responsible for the SARS epidemic of November 2002 to July 2003. At the end of epidemic, there were a total of 8273 cases, and 775 deaths. The case fatality rate – that is, the number of deaths as a ratio of the number of cases – has been stated as being somewhere between 9.6%-11%, depending on the definition of case fatality rate used. Although the case fatality rate was somewhat high (the case fatality rate for the recent 2009 influenza pandemic was about 0.03%), the number of deaths was likely mitigated by the fact that many of the people infected by SARS-CoV had access to very good supportive/palliative healthcare, helping their immune systems to fight off the virus.

An electron micrograph of a SARS-CoV virion. Image by CDC/ C. D. Humphrey and T. G. Ksiazek.

An electron micrograph of a SARS-CoV virion. Note the distinctive “corona”, which gives the family Coronaviridae its name. Image by CDC/ C. D. Humphrey and T. G. Ksiazek.

SARS-CoV is a coronavirus, so named for the protein spikes that surround this spherical virus, which looks like a corona. Although SARS-CoV was initially suspected to have originated in masked palm civets – which were commonly sold in markets in China, where some of the first cases of SARS was reported – it was finally traced back via phylogenetics to the Chinese horseshoe bat. Both the human and civet cat SARS-CoV were found to have descended from bat coronaviruses.

Although there are no cures or vaccines currently available for SARS-CoV, there is a vaccine in development by MassBiologics, cooperating with researchers at NIH and the CDC. The 2002-2003 epidemic was successfully controlled by quarantining those infected, and suspected to be infected by the virus, and is a viable alternative solution in countries where such measures can be effectively implemented. The determination of the incubation period of the virus was a vital part of this counter-measure, as it enabled health officials to determine how long had to elapse before people who had come into contact with SARS patients could be declared free from SARS.

A similar coronavirus surfaced in Saudi Arabia in April 2012. Known as Middle East respiratory syndrome coronavirus (MERS-CoV), there have been 94 confirmed cases, 16 probable cases and 47 dead, with a case fatality rate of approximately 50%, as of August 2013. Like SARS, this is a viral respiratory illness; its symptoms include fever, cough and shortness of breath, which may be followed by kidney failure and respiratory distress, potentially leading to death. People who have weaker immune systems are more likely to die from Middle East respiratory syndrome (MERS). Thus far, all cases have been linked to the Middle Eastern area.

A negatively-stained electron micrograph of MERS-CoV,  revealing its ultrastructural morphology. Image by CDC/ Cynthia Goldsmith; Azaibi Tamin, Ph.D.

A negatively-stained electron micrograph of MERS-CoV, revealing its ultrastructural morphology. Image by CDC/ Cynthia Goldsmith; Azaibi Tamin, Ph.D.

MERS-CoV can be transmitted from person to person if there has been close contact without protective equipment e.g. gloves and masks, but in general, it appears that there has not been a sustained spread of MERS in people. There are three main epidemiological patterns to the transmission of MERS-CoV – 1) sporadic cases, probably after coming into contact with an animal carrying MERS-CoV. 2) Family clusters and 3) healthcare workers are similar, as they are both sets of people who would have come into close contact with an infected person while the person is contagious. The incubation period of MERS-CoV seems to be from 2-14 days; thus, isolation protocols similar to those as carried out in the SARS 2002-2003 epidemic can be applied to people who are suspected to be infected with MERS-CoV. There is also no known cure or vaccine for MERS-CoV. The only treatment available is supportive/palliative care, ensuring that the patient manages to live while their immune system fights off the virus.

Not much is known about MERS-CoV; it seems to be a genetic match to a coronavirus found in Egyptian tomb bats, but recently there has been a report that camels may be the actual animal reservoir for this strain of coronavirus. It also appears that a large amount of virus is actually needed to cause an infection, as only about 20% of our respiratory epithelial cells – that is to say, about 20% of our lungs and airways – are carrying the protein receptor DPP4, which allows MERS-CoV to enter, and hence infect, them.

On an slightly more unsettling note, it seems that there has been at least one case of asymptomatic MERS – that is to say, a person was infected, and then produced anti-MERS-CoV antibodies, without ever showing signs of MERS. If asymptomatic MERS can be confirmed, and people with asymptomatic MERS can transmit the virus to other people, this could potentially explain the sporadic cases of MERS that seem to pop up out of nowhere. Of course, this would also lower the case fatality rate of MERS-CoV. There are also further concerns about the potential for a serious epidemic as there will be an increase in the number of people going to Mecca in Saudi Arabia for the Muslim Haj. While a vaccine could potentially be developed, it might be that the MERS epidemic would burn itself out the same way the SARS epidemic did, thus rendering the development of the vaccine unnecessary and unprofitable.

 

References:

Summary of probable SARS cases with onset of illness from 1 November 2002 to 31 July 2003

Tracking SARS back to its source

S. K. P. Lau, P. C. Y. Woo, K. S. M. Li, Y. Huang, H-W. Tsoi, B. H. L Wong, S. S. Y. Wong, S-Y. Leung, K-H. Chan and K-Y. Yuen. Severe acute respiratory syndrome coronavirus-like virus in Chinese horseshoe bats PNAS Vol. 102 No. 39 14040–14045, doi: 10.1073/pnas.0506735102

Special Online Edition: The SARS Epidemic

CDC | MERS

A. Rambaut MERS-Coronavirus Molecular Epidemiology and Genetic Analysis – Origin and Evolution

C. BEM Reusken, B. L. Haagmans, M. A. Müller, C. Gutierrez, G-J. Godeke, B. Meyer, D. Muth, V. S. Raj, L. Smits-De Vries, V. M. Corman, J-F. Drexler, S. L. Smits, Y. E. El Tahir, R. De Sousa, J. van Beek, N. Nowotny, K. van Maanen, E. Hidalgo-Hermoso, B-J. Bosch, P. Rottier, A. Osterhaus, C. Gortázar-Schmidt, C. Drosten, M. PG Koopmans Middle East respiratory syndrome coronavirus neutralising serum antibodies in dromedary camels: a comparative serological study The Lancet Infectious Diseases, Volume 13, Issue 10, 859 – 866 doi:10.1016/S1473-3099(13)70164-6

R. Roos Saudis, WHO Report 8 Silent MERS Cases

D. Butler Receptor for new coronavirus found