International Travelers Act as ‘Sentinels’ for Zika Transmission.

A School of Public Health analysis of Zika cases identified and treated by doctors participating in a global health surveillance network has underscored the importance of the role of travelers as “sentinels” of Zika virus circulation.

In a case series review published in the Annals of Internal Medicine, SPH researchers also highlighted the “critical need for high-quality, widely available, standardized diagnostic tests for Zika virus infection.”

Davidson Hamer, professor of global health and director of the travel clinic at Boston Medical Center, led the team analyzing 93 cases of Zika virus infection that were identified and tracked via GeoSentinel, a global surveillance network comprising 63 specialized travel and tropical medicine clinics in 30 countries.

No commercial diagnostic test for Zika virus existed when the Zika cases were tracked, compounding the difficulty in initially determining whether a traveler’s rash, fever, joint pain, and other nonspecific flu-like symptoms were actually caused by Zika infection.

The analysis found that travel to and from South America, Central America, and the Caribbean—places with highly publicized Zika outbreaks—was highest for the United States, but most cases identified by GeoSentinel were European travelers.

Researchers said the discrepancy may have been due to better access to testing in large European health centers and “differences in the proportion of ill travelers captured by GeoSentinel sites in Europe relative to North America.” In the US, diagnostic testing was limited to specific categories of patients such as pregnant women and patients with Guillain-Barré syndrome (GBS), an immune system disorder that can cause muscle weakness lasting for years.

Cases identified in the US could only be tested through the Centers for Disease Control and Prevention, resulting in few US patients tested and delays in their results. Hamer and the team also recommended that clinicians warn travelers visiting areas with ongoing Zika outbreaks of the risk for GBS and pregnancy complications, which include severe birth defects such as microcephaly and fetal death.

All the patients tracked via GeoSentinel were thought to have contracted the disease through bites from infected Aedes mosquitos. But researchers warn that recent evidence has documented alternative routes of Zika virus transmission, including sexual contact, blood transfusion, organ transplantation, laboratory exposure, and potentially breast milk, tears, and saliva.

Researchers said the non-vector methods of Zika transmission, in particular transmission through unprotected sex, “increase the potential for virus spread, and the complexity of the public health challenge to prevent infections.”

—Michael Saunders