A Perfect Viral Storm Brewing?

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by Eric Ellman
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Eighteen months ago researchers from Cornell University revealed a model that described conditions for an epidemic of Chikungunya virus in New York City1.   Combining climate data and eco-epidemiological parameters, they assigned a 38% probability of an outbreak given the arrival of a single infected traveler that summer.  Considering that the disease had never been reported in the New World the paper might have seemed alarmist.  But while New York’s summer of 2013 was Chikungunya-free, recent news suggests it’s only a reprieve.  Six months after they’d conjectured it happening, the western hemisphere’s first Chikungunya epidemic is well underway, 1800 miles south-southwest of New York centered on the Caribbean island of St. Martin.

By New Years Day The Center for Infectious Disease Research and Policy confirmed 66 cases of St. Martins Islanders with the virus whose symptoms and manner of contagion both resemble dengue fever. 2  Like dengue (sometimes called “break bone” fever), Chikungunya is spread by mosquitos inclined to reproduce near human settlements.  Aedes aegypti is the mosquito historically implicated in the spread of both diseases, a denizen of the tropics seldom found north of Florida and the U.S.-Mexico Border.  What concerns health officials is that the virus has undergone a mutation that makes another and much hardier mosquito, Aedes albopictus, the Asian tiger mosquito, a better vector for Chikungunya.3  Asian tiger mosquitos are already endemic from Texas through the mid-Atlantic and as far north as New York.  Come next summer should the Big Apple, or any of the other Eastern Seaboard cities where outbreaks were projected occur, it will be a striking example of the predictive power that comes from coupling computer models that parameterize the epidemiology of vector-borne disease with projections for climate change.

With a high transmission rate, and the disease already endemic far from its point of origin in Africa, it seems only a matter of time before Chikungunya reaches the U.S.  Authors of the Cornell study suggest that arrival of just one infected person during New York’s summer of mosquito activity could spark an epidemic that would sicken one in 5,000 people.  For more southerly cities with temperatures that allow mosquitos to be active year-round, the prospects may be worse.

Not the first arbovirus to hit New York City

An east coast outbreak of a previously unknown disease might seem like déjà vu to Durland Fish. Sixteen years ago as professor of infectious disease at Fordham University he was the first scientist on the scene when West Nile Virus (WNV) announced its presence via a massive die-off of birds at the Bronx Zoo the summer of 1999. 

With confirmation by the Center for Disease Control that WNV had arrived in the western hemisphere, the nation’s public health infrastructure responded.  Within a year, a WNV component was added to the Center for Disease Control’s Epidemiology Laboratory Capacity program (begun by concerned legislators after a landmark report on emerging tropical diseases was published five years previously). Originally focused on the New York metropolitan area, the fund provided $7MM to combat the spread of West Nile Virus.  Spending increased as the virus spread throughout the continental United States. To date, $230MM has been spent on trainings, surveillance and early detection.   But comparatively little has been spent to better understand the habits of the insects that cause the disease, says Durland Fish, now Director of the Yale Institute of Biospheric Studies Center for EcoEpidemiology. 

“Very little is known about the breeding habits of albopictus in the Northeast,” says Dr. Fish, who for years has advocated for more study of mosquitos and ticks which are carriers of thousands of diseases that are candidates for new epidemics in an age of international commerce and global warming.

With the means for detecting arrival and spread of West Nile Virus already in place, how much better prepared are health authorities for the threat of Chikungunya?  And how will their response incorporate the lessons learned from our experience with West Nile?

Dr. Roger S. Nasci, Chief of the Arboviral Diseases Branch
Division of Vector-Borne Diseases at the National Center for Emerging and Zoonotic Infectious Diseases, writes that “the increased arbovirus surveillance capacity that followed the introduction of WNV, accompanied with the recognition of Chikungunya as a threat and advance preparation, particularly the guidelines for preparing for CHIK introduction that we developed with PAHO puts the U.S. and the region in a better position to detect cases and respond appropriately.”

We hope so.  The symptoms of West Nile Virus often don’t manifest and are comparatively mild compared with Chikungunya where severe arthritis may linger for years.  And the disease’s arrival among a previously unexposed population can be explosive.  The strain that emerged in the Reunion Island outbreak of 2004 affected 260,000 of the island’s 770,000 people. 

Three weeks after it was first reported in the Caribbean, Chikungunya has now reached 5 islands and infected over 300 people.  Asian tiger mosquitos, able to withstand cooler temperatures and with eggs that remain viable even after brief freezing, could triple their range in the United States by the end of the century5.  U.S. public health officials are forewarned, though without better tools to predict and control the mosquito responsible for its spread they could be better forearmed.

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1http://www.news.cornell.edu/stories/2012/12/chikungunya-outbreaks-us-are…

2. http://www.cidrap.umn.edu/news-perspective/2013/12/caribbean-chikungunya-outbreak-grows-poses-threat-us

3 Chikungunya virus adapts to tiger mosquito viaevolutionary convergence: a sign of things to come?  Xavier de Lamballerie1*, Eric Leroy2, Rémi N Charrel1, Konstantin Ttsetsarkin3, Stephen Higgs3 and Ernest A Gould1

4http://cid.oxfordjournals.org/content/44/11/1401.full

5http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060874