Experts stress vigilance over emerging, unpredictable diseases

See related article posted today: "US urged to launch major campaign against emerging diseases"

Oct 29, 2008 – WASHINGTON, DC (CIDRAP News) – Health authorities must remain alert to new and renascent disease threats, experts warned this week at an international medical meeting.

In a series of briefings and presentations of data, speakers at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) and the 46th annual meeting of the Infectious Diseases Society of America (IDSA) ticked off a long list of bacterial, viral, and fungal pathogens whose recent behavior has taken scientists and public health authorities by surprise.

The concern expressed in their reports was echoed by a report released today by the nonprofit Trust for America's Health (see cross-reference link above).

There was abundant evidence at the meeting of pathogens' ability at taking advantage of changes in the human environment, from increased travel and trade to the growth of mega-cities to climate change.

The chikungunya experience
Dr. Harold Townson of the Liverpool School of Tropical Medicine outlined the unexpected behavior of chikungunya, a mosquito-borne virus that causes debilitating joint pain that lasts for months or years. The virus was identified in Africa in 1952, and moved to India and Asia along trade routes.

In 2005, though, a minor mutation allowed it to cause a massive outbreak on the French Indian Ocean island of Reunion; it sickened 260,000 people, one third of the island's inhabitants. It spread to France, causing about 900 cases there and 37 in the United States, through a chain of transmission that started with French soldiers sent to assist with the outbreak and European tourists returning home.

And in 2007, it caused 250 cases of illness in Italy—not as a result of the French outbreak, but in a separate introduction via a visitor from Kerala, India. On analysis, the Italian strain proved to be a different strain from the Reunion outbreak, but it possessed the same mutation, an adaptation that allowed more copies of the virus to move into mosquito salivary glands and deliver a larger dose to victims.

With no effective treatment and no vaccine, Townson said, the only feasible strategy for blocking chikungunya is mosquito control. But it must be practiced routinely, a painstaking, expensive undertaking that is beyond the resources of many areas in the developing world.

"Chikungunya poses a major public health problem for authorities throughout the world, because by the time you discover it, it is too late for environmental control measures to be effective," he said Monday.

Though the appearance of chikungunya in Italy has been taken as an indicator of global warming, the mosquito responsible for spreading the virus was already present in the area thanks to global trade, Townson said. But he cautioned that climate change may yet play a role in the spread of mosquito-borne diseases—not because of warming temperatures, but because increased rainfall may make drier areas more hospitable to the bugs.

Mosquito control and dengue
The low-tech but often unachievable measures of spraying pesticides and eliminating standing water are essential to controlling the mosquito-borne disease dengue and its most serious form, dengue hemorrhagic fever, said Dr. Duane Gubler of the Asia-Pacific Institute of Tropical Medicine and Infectious Diseases in Singapore.

The disease has been amplified by the ferocious growth of Asian mega-cities, where new construction provides an almost endless series of pools and puddles for mosquitoes to breed, and has expanded its range around the world.

Ironically, excellence in mosquito control created dengue's current foothold in the Americas, Gubler said. The disease was eradicated in South America in the 1950s as a byproduct of campaigns to eliminate yellow fever. Those campaigns were so successful that they were stopped after several decades, and dengue resurged, going from zero cases in Central and South America in 1959 to more than 600,000 in 2007 and becoming endemic in 28 countries.

"The take-home message is: Yes, the United States is at risk," Gubler said Tuesday. But in a bit of good fortune, he said, dengue's most efficient vector, the mosquito Aedes aegyptii, has been pushed out of most of its US range by another foreign import, the Asian tiger mosquito Aedes albopictus.

"Albopictus is a very good host for dengue but a very inefficient vector," he said. "So though we are at risk of transmission in the US, it is not likely we are going to have a major epidemic of the disease here."

Keeping an eye on hot spots
Assessing new disease threats requires investment in new tools, said Dr. Ian Lipkin of Columbia University's Mailman School of Public Health in New York City. Lipkin directs a World Health Organization collaborating center on diagnostics for emerging infections and recently received a philanthropic grant aimed at early detection.

Using rapid-throughput technology, researchers in his lab have identified 75 previously unknown infectious agents within a year. Most recently, they identified a never-before-seen arenavirus as the cause of an ongoing hemorrhagic fever cluster in South Africa—extremely rapid work, as the first case was hospitalized only Sep 12.

And to be as prepared for new threats as possible, it is necessary to get even further forward of the epidemic curve, by keeping an eye on the "hot spots" where concentrations of humans and animals are likely to birth new diseases, Dr. Jeremy Farrar said Monday. Farrar, the director of the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Ho Chi Minh City, has been at the forefront of treating human victims of avian influenza H5N1.

As examples of the unpredictability of pathogens' behavior in such hot spots, he offered the experience of Streptococcus suis, a bacterium commonly found in pigs in Southeast Asia. S suis is a frequent cause of mild meningitis in humans in Asian pig-growing areas, but in 2005 it changed its behavior, roaring into an acute infection that killed more than 30 people.

"Learning the lessons of what caused a normal commensal [type of] bacteria to suddenly expand into a large outbreak with 30% mortality has implications for emerging diseases generally," Farrar said Monday.

Emerging drug resistance
Still, the most important emerging threat may not be any one disease, but the growing resistance of many diseases to the drugs used against them, he said. He pointed to evidence presented at the conference of a malaria cluster in Cambodia that is resistant to artesunate, a botanically based compound that is the centerpiece of malaria treatment in much of the developing world.

"The most important emerging disease of all is drug resistance, in which Asia leads the world," he said. "The emergence of drug resistance, which is somewhat insidious and which does not get the headlines it deserves, is going to challenge us more in the 21st century, I suspect, than any individual disease."

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