Viral Traffic on the Move
We now know a great deal about the factors that allow novel infections to originate and spread. Major outbreaks during the past decade, including those of hantavirus pulmonary syndrome, Ebola, hemolytic uremic syndrome, West Nile, and (currently) severe acute respiratory syndrome (SARS), all followed the same pattern.
In developing the conceptual framework of emerging infections, including the idea of viral or microbial traffic, I hoped to help us better prepare for unexpected threats: infections whose very existence, as in the case of HIV/AIDS, may be unknown before they erupt in the human population. The emphasis has been on identifying key commonalities among the various diseases, on understanding their ecological roots (many are infections of other species that are given opportunities to be introduced into the human population), and on developing generic public health capabilities, such as enhanced surveillance and response, to deal with the common factors. Although the initial focus was on viruses, it was clear that the same principles applied to infections caused by other types of microbes as well.
Since the late 1980s, there has been a welcome flurry of activity. In 1989, John LaMontagne and I organized the National Institutes of Health (NIH) conference on emerging viruses. In the 1990s, the Institute of Medicine of the National Academies convened a Committee on Emerging Microbial Threats to Health, which issued its report in 1992. The Centers for Disease Control and Prevention (CDC) developed a strategic plan for emerging infectious diseases; launched a new journal, Emerging Infectious Diseases; and began holding regular meetings. The Program for Monitoring Emerging Diseases (ProMED), which was founded in 1993, was aimed at developing effective plans for global infectious disease surveillance and making highly fragmented surveillance activities more seamless and coordinated. One important spinoff was ProMED-mail, an open email listserv started in 1994 for reporting outbreaks and discussing emerging diseases. ProMED-mail (now run by the International Society for Infectious Diseases) currently serves about 30,000 subscribers worldwide. Also in the 1990s, the World Health Organization (WHO) began to develop new programs to enhance global surveillance.
All of these developments have led to better detection of outbreaks and to a clearer recognition of the remaining problems. But much work still needs to be done.
Public health surveillance remains the key to recognition and rapid response. An effective early warning system requires three elements on the front lines: clinical recognition, epidemiologic investigation, and laboratory diagnosis. A current challenge is to better integrate these components, because the system remains fragmented and responsibilities are often broadly diffused. Moreover, each of the components could be greatly improved.
Early warning begins with clinical recognition: There has to be a way to identify patients with disease syndromes. This usually involves the proverbial astute clinician who notices something unusual and reports it. One way to improve the odds of detection is to ask clinicians to look for and report certain types of syndromes, such as flu-like illnesses. As the SARS outbreak demonstrates, however, communication is key. The alert must be reported swiftly and effectively, and it must set in motion a timely response. In the case of SARS, it appears that some doctors in south China and Hong Kong knew they were witnessing unusual outbreaks but did not warn others. If they had, SARS might have been contained early on. Although public communication has generally improved, it remains among the weakest links in an outbreak. Another need is to develop standards to ensure that data systems can share information.
Next comes epidemiologic investigation, the medical detective work that helps identify the source of infection and the mode of transmission. Most of this work involves tracking down cases and interviewing patients. Even one case of a highly unusual disease, such as inhalation anthrax or Ebola, should be sufficient to trigger an aggressive investigation. But adequate resources often are not available.
Laboratory diagnosis is the third component of the triad. Because many diseases of concern are zoonotic (transmissible from animals to humans), diagnosis can draw on the resources and expertise of the veterinary community. Developing technologies for molecular diagnosis will increasingly make it possible to identify even previously unknown pathogens. Nevertheless, laboratory capacity still needs much improvement.
Bioterrorism and public health
One of the biggest changes since 1990 is the degree to which bioterrorism has become a public health priority. Although there had long been concern about vulnerability to biowarfare and bioterrorism, the anthrax episode in the fall of 2001 made it clear that the concern is no longer theoretical. Until very recently, the important role of public health at the frontlines of bioterrorism preparedness was unrecognized. This was so even though the CDC’s Epidemic Intelligence Service, home of the famed “Disease Detectives” and one of the most important training programs for epidemiologic surveillance and investigation, had been started by Alexander Langmuir in 1950 specifically to develop defenses against bioterrorist attack. Although concern about emerging infections has helped stimulate funding for the chronically underappreciated public health system, the threat of bioterrorism motivated the first real infusion of new money into public health in decades.
Many of the capabilities needed to defend against bioterrorism are the same as those needed to combat natural emerging infections. In both instances, the problem is an unexpected outbreak of infectious disease, of which the first indication is likely to be sick people in emergency rooms or clinics. Indeed, as with the anthrax attacks, the public health and medical responses may be under way before the true nature of the outbreak is recognized. Public health and the interface with the health care system are therefore key elements in any effective response to bioterrorism.
Whether the biggest threat is natural or engineered, much remains to be done. Efforts to strengthen surveillance and response worldwide and to improve communication must be accelerated and sustained. Further, we have only scratched the surface in terms of understanding the ecology of infectious diseases and developing strategies for regulating microbial traffic. We need tools for better predictive epidemiologic modeling when a new infection first appears and for better analysis of the factors that transfer pathogens across species. One encouraging development is the program in the ecology of infectious diseases that was started a few years ago by the National Science Foundation in cooperation with NIH. The creation of microbial impact assessments, which I proposed in my 1990 article, is now even more feasible because of new technologies such as polymerase chain reaction.
SARS is a good yardstick of our progress during the past 13 years. The syndrome was unusual because novel infections that spread from person to person are relatively rare. Once cases were finally reported, the public health response was vigorous. WHO warned health care providers, researchers rapidly identified a candidate virus, and prototype diagnostic tests quickly became available. The vast reach of the Internet was instrumental in sharing information and coordinating activities worldwide. Despite these advances, SARS had already spread to many countries. In fact, had the disease been as transmissible as influenza, it would have invaded virtually every country in the world by the time the public health response had begun. So what SARS tells us is that although we have come a long way since 1990, we still have a long way to go.