Sunday, March 13, 2016

MMWR: Revision to CDC's Zika Travel Notices: Little Risk At Elevations Above 2,000 Meters

Credit CDC Travel Notices

# 11,138

The CDC's travel advice to pregnant women to defer travel to countries where the Zika virus is circulating have been blanket `whole country' notices, even when the Zika virus has only been found active in limited regions of an affected nation.

The Aedes mosquito - currently considered the primary vector of the Zika virus -  is rarely found above elevations of 2,000 meters, and that has prompted the CDC to revise their travel advice. 

On Friday the CDC updated 37 destination-specific Zika travel notices, with maps for those with regions with elevations above 6,500 feet. They also published a Questions and Answers: Zika risk at high elevations, which includes this advice:

What do you recommend for pregnant women who decide to travel to high elevations in an area with Zika?

The risk of getting Zika from a mosquito at elevations above 6,500 feet (2,000 meters) is minimal. However, traveling through an area of low elevation or stopping, even briefly, in a low elevation area on the way to higher elevation increases the risk of getting Zika from a mosquito, and pregnant woman should strictly follow steps to prevent mosquito bites while in these areas. Pregnant women traveling to these areas should use condoms or not have sex with men who may have traveled to lower elevations in an area with Zika. Pregnant women should also be alert to changes in their travel plans that may take them to elevations below 6,500 feet.

Providing rationale for this policy change, the CDC published an Early Release MMWR (excerpts below), that examines the scientific evidence.

Of course, `minimal risk' isn't `zero risk', but this information should provide some reassurance to pregnant women who intend to either travel to - or currently live in - high elevations of Zika affected countries.

As we learn more about the Zika virus and its vectors, this advice - like all CDC guidance - is subject to future revision.

Format:Martin Cetron, MD1 

Since May 2015, when Zika virus, a flavivirus transmitted primarily by Aedes aegypti mosquitoes, was reported in Brazil, the virus has rapidly spread across the Region of the Americas and the Caribbean. The association between maternal Zika virus infection and adverse fetal and reproductive outcomes, including microcephaly, prompted CDC to issue a Level 2 alert travel notice* for the 37 countries and U.S. territories (at the national and territorial level) that have reported recent Zika virus transmission as of March 11, 2016. In addition to mosquito bite precautions for all travelers, CDC advises that pregnant women postpone travel to affected countries and U.S. territories. 

Within a nation’s borders, ecologic characteristics, which determine the distribution of mosquito vectors, can vary considerably. CDC conducted a spatial analysis, focusing on the probability of occurrence of Ae. aegypti, to support the demarcation for subnational travel alerts. 

Based on results of this analysis, travel that is limited to elevations higher than 2,000 m (6,562 ft) above sea level is considered to have minimal (approximately 1%) likelihood for mosquito-borne Zika virus transmission, even within countries reporting active transmission. Women who are pregnant should avoid travel to elevations  less than 2,000 m in countries with active Zika virus transmission.  
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As a postscript, I would mention that yesterday Crof carried a report entitled  Bolivia: Zika mosquito adapting to life at 2,600 meters above sea level, which quoted the concerns of a Bolivian epidemiologist.

Without data, it is very difficult to evaluate the significance of this report, or whether it applies beyond Bolivia.

A study in 2012 the American Journal of Tropical Medicine and Hygiene called The Dengue Virus Mosquito Vector Aedes aegypti at High Elevation in México warned that climate change could alter the elevational range of the Aedes mosquito, but found the Aedes mosquito was commonly encountered up to 1,700 m and present but rare from 1,700 to 2,130 m.

Friday's MMWR report indicated that Aedes mosquitoes were rare above 2,000 meters, but did not claim they were completely absent.   

A finding which is consistent with the AJTMH report above.  Unless and until we start seeing evidence of Zika, Dengue, or Chikungunya transmission at these altitudes, the risks still appear to be low.

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