Disease monitors 'looking in the wrong places'
Health leaders need global strategy for spotting disease threats.
The world's health watchdogs are looking in the wrong places for the next dangerous epidemics, according to an analysis of global trends in emerging disease outbreaks over the past few decades.
The study gives a fresh perspective on global disease by tracking the history, from 1940 to 2004, of the emergence and spread of 335 infectious diseases. The extensive work helps to quantify the effect of well-known risk factors, such as population density, on the probability of a disease taking hold in a given area.
Although the data haven't yet been used to map out specific future hotspots for disease, they do suggest that watchdog groups should invest more in monitoring regions such as tropical Africa, Latin America and Asia. These areas have the greatest threats of newly emerging epidemics, say the survey's authors, but they have traditionally received the least surveillance.
A globally coordinated strategy is required to spot and stop outbreaks before they can spread across the world, argues Kate Jones of the Institute of Zoology in London, one of the researchers behind the new report.
"We need to think more broadly, with a global vision," she says. "Everyone will be affected [by new disease outbreaks]. We are all on the same planet — there's nowhere to hide."
Lessons from the past
Jones and her colleagues, led by Peter Daszak of the Consortium for Conservation Medicine in New York, tracked the infectious diseases over a 64-year span. They included many diseases that have gone on to cause worldwide misery, such as drug-resistant tuberculosis, chloroquine-resistant malaria and HIV. The survey paints a picture in which new diseases arise as a result of dramatic increases in human population density, international trade and travel, and changes to agricultural practices.
Such changes have caused a dramatic increase in the rate at which new diseases have emerged since the 1940s, the researchers say. During the 1980s alone, the worst decade in their data set, almost 100 new pathogens emerged, they report in Nature1.
That is probably due to the ravages of the most notorious pathogen to appear in the 1980s — HIV — the researchers note.
HIV, like 60% of the diseases studied by the researchers, is a zoonotic pathogen — it leapt from animals to humans. Of these zoonotic diseases, 72% came from wildlife as opposed to domesticated animals, the researchers found. Recent examples include Nipah virus in Malaysia, and the infamous SARS outbreak in Guangdong, China, which practically shut down international travel in Southeast Asia in 2002.
Disease surveillance, and rapid responses such as the one to the SARS outbreak, will be vital in containing future outbreaks, says Mark Woolhouse, an epidemiologist at the University of Edinburgh, UK. "The first role is a watching brief," he says. "As SARS showed, the quicker you catch them, the less the damage." Containment in that instance was largely successful: of 774 reported cases in the 2002 outbreak, three-quarters were restricted to Chinese territory.
Spotting new diseases is a different skill to the monitoring of established diseases such as malaria, Woolhouse points out. "We do have a good understanding of the global disease burden," he says. "But this is not about the existing disease burden — it's looking specifically at novel disease events."
Knowing where to direct surveillance will require more work, Jones says.
Most disease surveillance efforts focus on developed regions such as Europe, North America, Australia and parts of Asia, the researchers say. This is partly because these countries can afford better surveillance, but also because these areas were disease hotspots during the twentieth century. The United States, western Europe, Japan and Australia all had growing population densities and antibiotic use during this time, which may have driven the emergence of new diseases.
Areas such as Africa, Latin America and Southeast Asia are undergoing similar social changes today. Jones and her colleagues suspect that the biggest threat will come from low-latitude countries, particularly where people mix freely with domesticated animals such as poultry.
Jones suggests that her results be merged with climate models and demographic data to build a more precise picture of where diseases are most likely to strike in the future. "What will happen in the future, we don't know," she says. "We need to look at how the environment will change, and make a forecast."
- Jones, K. E. et al. Nature 451, 990-993 (2008).