Chikungunya: New CDC Website Predicts Spread in United States

Veronica Hackethal, MD

October 10, 2014

On October 6, the Centers for Disease Control and Prevention (CDC) unveiled a new website called nowcast that could help in the fight against the spread of chikungunya virus and provide clinicians with information for differential diagnosis, according to a CDC news release.

"We developed this tool to assess how likely chikungunya-infected travelers are to arrive in various cities and how likely it is that local transmission may occur," Michael Johansson, PhD, said in the news release. Dr Johansson is a CDC biologist who worked to develop the model on which the website is based. "Anticipating risk is essential for developing and targeting prevention and control recommendations. Our ultimate goal is to help health departments worldwide make informed decisions about how to reduce the impact of chikungunya in their communities."

The current outbreak of chikungunya virus, which started in the Caribbean in December 2013 and has caused nearly 750,000 reported cases in the Caribbean and Central, South, and North America, according to the CDC.

Thus far, imported cases from infected travelers arriving in the United States number around 1200. As of September 30, there have been 11 locally transmitted cases in Florida.

Most experts believe that cases of chikungunya will mirror the pattern of dengue and that a large outbreak in the United States is unlikely. Because chikungunya is spread by daytime biting mosquitoes, experts recommend prevention of mosquito bites with appropriate clothing and repellent.

The nowcast website provides estimates of areas in which chikungunya virus activity has most likely occurred in the last month and comes up with monthly estimates of when and where chikungunya is most likely to appear next, as well as where infected travelers are most likely to arrive. It looks at variables such as the pattern of existing chikungunya cases, airline flight patterns, and climate.

Results are displayed in graphs and global maps and will be updated monthly by the CDC. Estimates are expressed as a range of probabilities. For example, a probability of 0.2 would suggest a 20% chance for the initiations of local transmission. A probability of nearly 1.0 would suggest an almost 100% chance of this scenario. Areas with probabilities of 10% or less do not appear on the website.

The nowcast website is based on a mathematical model developed by the CDC and published online August 11 in PLOS One. In the first 4 months of 2014, the model was able to accurately pinpoint 8 of 10 locations predicted to be likely areas of chikungunya transmission. Estimates since then have been similarly accurate.

The CDC nowcast model cannot predict:

  • severity of local outbreaks;

  • introduction of chikungunya from outside the Americas, such as Asia;

  • spread of other mosquito-borne viruses, such as dengue or West Nile virus; and

  • the effect of mosquito prevention activities.

"If physicians are paying attention to the information, the website could help them start to consider chikungunya virus in a differential diagnosis," Dawn Wesson, PhD, told Medscape Medical News when contacted for an outside opinion. Dr Wesson's research focuses on vector-borne diseases. She is an associate professor at the Tulane University School of Public Health and Tropical Medicine, New Orleans, Louisiana.

"Beyond numbers of imported cases coming in to a particular area, other factors that contribute to local transmission are the strain of chikungunya virus introduced, whether it is transmitted more efficiently by Aedes aegypti or by A Albopictus, and the predominant mosquito species," Dr Wesson commented.

A albopictus, although widespread in the Eastern United States, is not very efficient at spreading chikungunya virus, Dr Wesson explained.

"Large populations of Aedes aegypti, the most efficient of the two mosquito vectors, are only present in more southern parts of several states, including Florida, Louisiana, and Texas," Dr Wesson added, "Smaller populations are also present in Arizona and California. Because of these issues, many of the more temperate areas may not have the mosquito populations to support transmission, even though they have large numbers of imported cases."

Nowcast estimates seem to bear this out. For September, the model indicated an elevated risk for the arrival of infected travelers from other parts of the Americas into the United States, Europe, and South American countries south of the equator. There was a high probability of local chikungunya transmission in the southern United States, but there was a "great deal of uncertainty" about local spread in other areas. Likewise, the risk for local transmission was low for "most" countries in South America south of the equator, according to the press release.

Dr Wesson has disclosed no relevant financial relationships.

PLOS One. Published online August 11, 2014. Full text

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