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Conference Summary
Emerging Infections: What
Have We Learned from SARS?
Alison P. Galvani*
*University of California, Berkeley, California, USA
Suggested citation
for this article:
Galvani AP. Emerging infections: what have we learned from SARS? [conference
summary] Emerg Infect Dis [serial on the Internet]. 2004 Jul [date cited].
Available from: http://www.cdc.gov/ncidod/EID/vol10no7/04-0166.htm
Given the current size and mobility of the human population, emerging
diseases pose a continuing threat to global health. This threat became
reality with the outbreak of severe acute respiratory syndrome (SARS).
The emergence of a disease requires two steps: introduction into the human
population and perpetuated transmission. Although preventing the introduction
of a new disease is ideal, containing a zoonosis is a necessity. The lessons
that we have learned from SARS were the topic of a meeting of The Royal
Society on January 13, 2004, in London, England.
Zoonoses are responsible for most emerging infectious diseases, including
infections caused by Ebola virus, West Nile virus, monkeypox, hantavirus,
HIV, and new subtypes of influenza A. In the case of SARS coronavirus
(SARS-CoV), serologic evidence indicates that the virus was spread through
interspecies transmission from wild game markets in Guangdong, China (Malik
Peiris, University of Hong Kong). This finding led to bans in the wild
meat trade from Nan Shan Zhong (Guangzhou Respiratory Disease Research
Institute) similar to the ban on eating nervous system tissue from cows
that was implemented after new variant Creutzfeldt-Jakob disease emerged
in Britain.
Ecologic changes, concomitant with increasing contact between humans
and animal disease reservoirs, contribute to zoonoses. The emergence of
SARS was facilitated by increased contact between people and animal disease
reservoirs as the wild meat industry expanded recently. Global warming
will likely contribute to the spread of dengue beyond tropical regions
(Tony McMichael, National Centre for Epidemiology and Population Health,
Canberra, Australia). Habitat fragmentation by deforestation may increase
the contact between people and reservoir species. For example, hemorrhagic
fever virus has been linked to deforestation in South America.
Containing an emerging disease depends on rapidly designing and implementing
a control strategy appropriate to the epidemiology of the disease. Interdisciplinary
and international collaboration occurred with unprecedented rapidity during
the SARS outbreak. The network of laboratories in 17 countries organized
by the World Health Organization (WHO) coordinated information sharing
(David Heymann, WHO) and was instrumental in rapidly identifying the etiologic
agent of SARS (1) and in fulfilling Koch's postulates
(2) (Albert Osterhaus, Erasmus University, Rotterdam).
As is typical of an emerging disease, no vaccines or drugs to combat
SARS existed, making quarantine, patient isolation, travel restrictions,
and contact precautions the only means of limiting transmission. Mathematic
models provided a framework for evaluating alternative control measures
and making predictions about the course of the epidemic (3,4).
Previously, similar models had guided public health policy, for example,
in halting an outbreak of hoof and mouth disease in the United Kingdom
in 2001 (5,6). One of the complications in setting parameters
in an emerging disease model is the difficulty in estimating epidemiologic
limits from the initially small sample sizes. Thus, openly sharing data
and analysis of key model parameters are vital.
The model must be appropriate to the nature of the disease and the accuracy
of the parameter estimates (7). Stochasticity inherent
in transmission dynamics will be particularly pronounced when infection
prevalence is low. Population heterogeneity and the network structure
of human interactions will affect the spread of an emerging disease. In
the 2003 SARS outbreak, healthcare workers were at particular risk (8)
and acted as bridges carrying the infection from the hospital and causing
community wide epidemics. High-risk "core groups" have been
a major focus of HIV/AIDS models for years (9), but the
movement of SARS patients into the core (i.e., the hospital) adds a further
complication (3).
The two waves of SARS clusters in Toronto (Robert Maunder, Mount Sinai
Hospital, Toronto) highlight the need for surveillance even after an outbreak
appears extinguished. Management of the SARS epidemic also demonstrated
that public service infrastructure, which affords the greatest chance
of success (3), is essential to the rapid containment
of an outbreak. In areas most affected, contact tracing was important
(10). In Guangdong, police departments tracked down contacts
of infected persons, who were then followed up for 10 days after exposure.
Evaluating the surge capacity of public health services and hospitals
is one way to assess the preparedness of a medical system.
The case-fatality rate is a key determinant of the public health impact
of an emerging disease and was high for SARS at approximately 15% (11).
The relationship between infectiousness and onset of symptoms is also
important. Patient isolation has greater potential as a control strategy
if the illness can be diagnosed before the person becomes infectious (Roy
Anderson, Imperial College London). In contrast, persons infected with
influenza virus are highly infectious before they become symptomatic.
The rapidity of pathogen turnover means that evolution in pathogen populations
can occur on a time scale that is epidemiologically relevant. Indeed,
SARS-CoV evolved during the course of the SARS outbreak in China (12).
Similarly, influenza is perpetuated in the human population by the evolution
of new antigenic variants every year (Robin Bush, University of California,
Irvine) (13). Even if the transmissibility of an emerging
disease is initially below the threshold necessary to sustain it in a
population, the potential for the organism's evolution to higher levels
may exist (14,15). Thus, one should not become complacent
about diseases that are repeatedly introduced through zoonosis, but teeter
on the edge of sustainability within the human population.
The success with which WHO coordinated the global collaboration in containing
SARS galvanized the World Health Assembly to grant WHO greater authority
to verify outbreaks, conduct investigations of outbreak severity, and
evaluate the adequacy of control measures. The outcome of this new authority
will depend on integrating the expertise of public health officials, medical
doctors, and epidemiologists worldwide with guidance from disease transmission
models. The SARS outbreak demonstrated that an epidemic in one part of
the world is not just an individual nation's problem but a global problem.
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