How fast will the debilitating chikungunya virus spread through the U.S.?

Southern states are most at-risk, new tool says

CDC last week unveiled its Nowcast website, a tool that can assess the probability of the chikungunya virus spreading through different regions of the country and provide physicians with data for differential diagnosis.

The tool was developed by CDC researchers as a way to "assess how likely chikungunya-infected travelers are to arrive in various cities and how likely it is that local transmission may occur," according to CDC biologist Michael Johansson, who helped develop the model on which the website is based.

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Background on chikungunya

The chikungunya virus was first identified in Tanzania in 1952, but the virus has swept through southern and eastern Africa in recent decades, according to the World Health Organization (WHO). The name "chikungunya" derives from the Kimakonde word for "to become contorted," which refers to the fever and intense muscle and joint pain that the virus inflicts on its victims for weeks and sometimes years.

Although the virus is rarely fatal, individuals who contract the virus "probably wish they would die" because it is so painful, says Robert Novak, a global health professor at the University of South Florida in Tampa.

There is currently no vaccine or cure for those affected, other than attempting to alleviate symptoms. And although the virus often runs its course in about a week, pain can linger for months or years.

Chikungunya first arrived in the Western hemisphere in December 2013. Since then, the virus has spread to 17 countries or territories in the Caribbean, North American, and South America, and nearly 750,000 cases have been reported. It has been linked to at least 14 deaths in the Caribbean, although those individuals likely had other health issues.

Debilitating 'chikungunya' virus reaches Florida

In July, two Florida residents became the first two American residents to domestically contract chikungunya. 

As of Sept. 30, there have been nearly 1,200 imported cases from infected travelers arriving in the United States and 11 locally transmitted cases in Florida.

Details of 'Nowcast'

Nowcast will provide estimates of areas in which chikungunya virus activity has potentially occurred during the past month. It also will generate monthly estimates of when and where the virus is likely to appear next based on variables like existing cases, airline flight patterns, and climate.

 However, Nowcast cannot be used to predict:
  • The severity of local outbreaks;
  • Introduction of the virus from outside the Western hemisphere;
  • The spread of other mosquito-borne viruses, such as dengue or West Nile; and
  • The impact of mosquito prevention activities.

According to Medscape, the tool's estimates are expressed as probabilities. For instance, a probability of 0.2 would indicate a 20% chance for potential local transmission, while a probability of 1.0 would mean an almost 100% chance. Areas with probabilities of 10% or less are not listed on the website.

During the first four months of 2014, the model on which Nowcast is based was able to accurately detect eight of 10 locations predicted to be likely areas of chikungunya transmission.

 

According to the September model, there was an elevated risk for the arrival of infected travelers from other parts of the Americas into the United States, Europe, and South American countries below the equator. In addition, the model reported that there was a high probability of local chikungunya transmission in the southern United States.

However, there was a "great deal of uncertainty" about local spread in other areas. Similarly, the model predicted a low risk of local chikungunya spread for "most" countries south of the equator, according to the press release.

What researchers are saying

CDC in a statement said, "Anticipating risk is essential for developing and targeting prevention and control recommendations," adding, "Our ultimate goal is to help health departments worldwide make informed decisions about how to reduce the impact of chikungunya in their communities."

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 Dawn Wesson, an associate professor at the Tulane University School of Public Health and Tropical Medicine, says the tool "could help [researchers] start to consider chikungunya virus in a differential diagnosis" (Infection Control Today, 10/6; Hackethal, Medscape Medical News, 10/10).


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