- SARS Spread In Buildings By Airborne Droplets
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- By Karla Gale
Reuters Health
4-22-4
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- (Reuters) -- Virus-laden aerosols appear to be responsible
for the large community outbreak of severe acute respiratory syndrome (SARS)
that occurred in Hong Kong in 2003, according to results of study of detailed
airflow-dynamics. Although this type of transmission is probably not common,
"airborne spread of a concentrated source of virus can infect many
persons within a short period of time," lead investigator Dr Ignatius
T S Yu told Reuters Health. In such circumstances: "Avoidance of close
contacts alone may not be adequate," he added. "The prevention
of aerosolization of the virus source should take priority," especially
in hospitals. "One should think about containing and/or removing the
aerosols through engineering means, and personal protective gear will have
to be stepped up."
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- The SARS outbreak that Dr Yu and his team investigated
affected more than 300 residents of Amoy Gardens, a private housing complex,
they explain in the 22 Apr 2004 issue of the New England Journal of Medicine
[N Engl J Med 2004;
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- 1710-2 http://content.nejm.org/cgi/content/abstract/350/17/173
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- 1731-9 http://content.nejm.org/cgi/content/short/350/17/1710.
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- Hypotheses to explain the spread include direct person-to-person
contact, transmission through the sewage disposal system, use of communal
facilities, and even roof rats running through sewage and water pipes.
The epidemic curve and spatial distribution of the cases, however, are
consistent with virus-laden aerosols spread from a single source, the report
indicates.
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- Dr Yu, at the Chinese University of Hong Kong, and colleagues
obtained detailed site plans of the drainage system of the housing complex,
along with meteorological data. They modeled the effects of turbulence
on airflow and the dispersal of pollutants. Among the initial 187 cases
in the housing complex, the index patient and 99 others lived in one building.
The epidemic started on 21 Mar 2003 and peaked on 24 Mar 2003, with onset
of symptoms occurring between 24 and 26 Mar 2003 in most patients. The
risk of contracting the infection was 5 times higher in units on middle
and upper floors than on lower floors.
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- An environmental health team found that a bathroom exhaust
fan could have distributed contaminated aerosols into an airshaft connected
to other units in the building. It appears that wind then distributed the
aerosols from open doors and windows to other buildings.
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- The index patient had extremely high concentrations of
SARS coronavirus in feces and urine compared with respiratory secretions,
the authors note. Because this patient had used a toilet in the building
during a bout of diarrhea, the authors suggest that "huge numbers
of aerosols [were generated] when toilets were flushed."
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- Similar events would require "a large concentrated
dose of the virus source, a mechanism of dispersing the source into fine
aerosols that can be suspended in air for a long period of time, and the
appropriate weather and wind conditions and topography so that the virus
could be spread to a large number of people in a short period of time,"
Dr Yu said. In such an event, "many persons could be affected within
a very short period of time [and] transmission [could be] much more efficient
than close contact transmission," he added. The clinical implications
of airborne transmission are particularly important for infection control
in hospitals and in public indoor settings such as airplanes and schools,"
Dr Chad J Roy and Donald K Milton write in a related editorial. Dr Roy
at the US Army Medical Research Institute of Infectious Diseases in Fort
Detrick, Maryland, and Dr Milton at Harvard School of Public Health in
Boston recommend aggressive air-sanitation measures, such as upper-room
germicidal ultraviolet irradiation.
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- Dr Yu and his colleagues are now planning field experiments
using tracer particles or gas under similar weather conditions. "Our
team of medical epidemiologists and ventilation engineers will continue
to collaborate to study other diseases and health conditions, infectious
or non-infectious, related to airflows and ventilations."
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- -- ProMED-mail
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- [A significant feature of the Amoy Gardens incident would
appear to have been the high concentration of SARS coronavirus present
in the feces and urine of the index case, relative to the amount present
in respiratory secretions. If this is a general phenomenon, it would support
other observations that handwashing was among the most important factors
in containing the spread of infection within hospitals. - Mod.CP]
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- Patricia A. Doyle, PhD Please visit my "Emerging
Diseases" message board at: http://www.clickitnews.com/ubbthreads/postlist.php?Cat=&Board=emergingdiseases
Zhan le Devlesa tai sastimasa Go with God and in Good Health
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