Zika Virus Mosquitoes Unlikely Above 6,500 Feet

On March 11, 2016, the U.S. Centers for Disease Control and Prevention (CDC) updated its Zika-related travel advice to exclude destinations located above 6,500 feet (2,000 meters). This revision is based on findings that Aedes mosquitoes, the primary vectors for the Zika virus, are rare at these higher elevations.

The updated guidance provides greater geographic precision for travelers, particularly those at higher risk. For 37 travel destinations where the CDC had previously issued level 2 travel alerts—indicating that visitors should practice enhanced precautions—the agency added elevation maps. These maps identify specific areas above 6,500 feet that are exempt from level 2 precautions, including those for pregnant women.

Data Supporting Elevation Thresholds

The decision to revise the travel notices followed a spatial analysis conducted by CDC scientists. The research focused on the probability of occurrence of Aedes aegypti mosquitoes and historical data regarding the transmission of the dengue virus, which is spread by the same mosquito species as Zika.

According to a report published in the Morbidity and Mortality Weekly Report (MMWR) on March 18, 2016, the CDC examined data from 16 countries. The analysis included reports of Aedes aegypti from 1960 through 2014 and human dengue cases from 1960 through 2012.

The findings revealed that the mosquito species was largely absent in locations above 6,500 feet. The data showed that only 1.1% of reported dengue cases occurred above that elevation. Because of this strong correlation, the CDC determined that the risk for mosquito-borne exposure to the Zika virus is considered minimal at elevations above 2,000 meters.

These findings support revising the Zika travel notice to reflect enhanced geographic precision regarding the likelihood of Zika virus presence at certain elevations

Morbidity and Mortality Weekly Report (MMWR)

Risks to Pregnant Women and Fetal Health

The refinement of these travel alerts is especially critical for pregnant women. Zika is a flavivirus primarily transmitted by Aedes species mosquitoes, specifically Ae. Aegypti and Ae. Albopictus. While the virus may cause mild symptoms in most adults, it poses significant risks during pregnancy.

Infection during pregnancy can be passed from a woman to her fetus, which may result in adverse reproductive outcomes and birth defects, including microcephaly. Because of these risks, the CDC advises that pregnant women avoid travel to elevations below 2,000 meters in countries where there is active Zika virus transmission.

As of March 11, 2016, the CDC maintained that the decision to delay or cancel travel is a personal and complex one. Travelers are encouraged to consider their destination and their ability to protect themselves from mosquito bites, as there is currently no vaccine to prevent Zika infection.

Regional Trends and Vector Control

The update comes amid ongoing challenges in controlling the virus across the Americas and the Caribbean. The Zika virus was first reported in Brazil in May 2015 and spread rapidly thereafter.

The Pan American Health Organization (PAHO) reported on March 10, 2016, that Zika infections appeared to be increasing in Panama and Venezuela, as well as throughout the Americas in general. In response to these trends, PAHO detailed efforts to ramp up mosquito-control operations to limit the spread of the virus.

The CDC’s use of elevation data allows for a more nuanced approach to travel warnings. By distinguishing between low-lying areas and high-altitude regions, the agency can provide more accurate risk assessments for travelers visiting countries that may have active outbreaks in some regions but minimal risk in others.

Travelers are advised to check the most current risk maps and recommendations provided by the CDC before making travel plans to ensure they have the most up-to-date information on mosquito-borne risks in their specific destination.