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Airborne Spread of SARS

By Eric Toner, M.D., April 6, 2005

Two recent studies, one from Hong Kong and the other from Toronto, provide evidence for airborne (small droplet aerosol) transmission of Severe Acute Respiratory Syndrome (SARS) in the hospital setting. While small droplet airborne transmission of the SARS-related coronavirus (SARS-CoV) has been generally accepted in the setting of aircraft and in the Amoy Gardens superspreading event, many experts have favored large droplet transmission as being the primary route of spread in the hospital setting. Large droplets do not travel more than 3-6 feet, and their spread can be prevented by use of a simple mask. Aerosols can stay suspended for prolonged periods, travel long distances, and require a much higher level of precaution, including the use of N-95 (or higher) masks, HEPA filters, and negative pressure rooms.

The Toronto study [JID 2005:191 (1 May)] found evidence of SARS-CoV in air samples taken from the room of a SARS patient who was not undergoing a procedure considered “high risk” for generating aerosols. While this study demonstrated airborne dissemination of the SARS CoV, because there were no secondary cases, actual transmission of the disease was not proved. In the Hong Kong Study [CID 2005:40 (1 May)], a temporal-spatial analysis of the large nosocomial outbreak (>100 cases) at the Prince of Wales Hospital suggests true airborne transmission of the illness.

The implications of these studies for hospital infection control are significant. They support the use of a high level of respiratory protection and isolation for SARS patients. Furthermore, it would be interesting to see these same research techniques applied to the study of the spread of other respiratory diseases.

12 Healthcare Workers Killed in the Largest Ever Outbreak of Marburg

By Eric Toner, M.D.

The largest ever outbreak of Marburg Hemorrhagic Fever has been ongoing in the Uige region of northern Angola since October. As of April 2, the death toll stood at 150, with a reported mortality rate of 92%. Included in this total are 12 healthcare workers, including at least 2 volunteer foreign doctors. The 92% death rate is much higher than has ever been reported before from this disease, suggesting that the outbreak is larger than it appears and that there may be other milder cases that have gone unreported thus far. To date the outbreak has been confined to one rural area, but it has not yet been brought under control.

Marburg is a zoonotic filovirus related to Ebola. Its animal reservoir is unknown, although it has been found in monkeys. Like Ebola, it causes high fever and rash followed by a DIC-like coagulopathy. It appears to be spread primarily through contact with bodily fluids, although limited respiratory spread is possible. Although Marburg is on the Category A list of potential biological weapons, there is no reason to suspect that this outbreak is intentional.

This outbreak illustrates the danger presented by filoviruses, especially to healthcare workers. It further reminds us of the ongoing danger of new, highly lethal, highly contagious diseases that are emerging from previously remote parts of the world. Such a disease can be transported by aircraft anywhere in the world in less than its incubation period.

By Eric Toner, M.D.