An Estimated 5700 Zika Cases Went Unreported in 2017 Cuban Outbreak

Using travel data and genomic sequencing, investigators have discovered that an unreported Zika virus outbreak occurred in Cuba in 2017.

Although Zika virus was first identified decades ago in the Zika Forest of Uganda, the virus was relatively unknown before 2015 when a very public outbreak sparked in Brazil. Following additional outbreaks in other South American, Central American, and Caribbean countries along with associated cases of microcephaly in infants, the World Health Organization (WHO) declared a Public Health Emergency of International Concern in February 2016. Although the case counts waned over that year, a new report published in the journal Cell says that a large outbreak of Zika virus was just beginning in Cuba, one which went unreported until investigators began to connect the dots.

Mosquito exposure and lack of access to adequate health care can result in undetected cases of Zika virus and other illnesses, the investigators reported. “Pockets of virus transmission that occur in countries with inadequate reporting can therefore facilitate ‘hidden’ outbreaks, increasing the risk of infected travelers causing outbreaks in new regions of the world,” the authors wrote. “Thus, underreported or unrecognized local outbreaks may prolong epidemics and hinder global efforts aimed at halting virus spread.”

The study team hypothesized that Zika outbreaks could still be occurring in the Americas without being detected by public health agencies. To find locations with continuing and unreported Zika virus outbreaks, the investigators used data on international travel-associated Zika cases from the Florida Department of Health and the European Centre for Disease Prevention and Control (ECDC) to examine to identify transmission not captured by reporting. In addition, the study team used clinical sequencing of Zika virus from infected travelers.

They discovered that in 2017 there was a spike in Zika cases in travelers who had been to the Caribbean, and found that between June 2017 and October 2018, 98% of travel-associated Zika cases reported in Florida and Europe came from Cuba. In addition, the investigators found that 76% of the Zika cases associated with travel from Cuba were diagnosed in 2017. Although Cuba reported 187 laboratory-confirmed Zika cases in 2016, it reported no local Zika cases to the Pan American Health Organization or other international public health agencies in 2017 and 2018.

In an interview with Contagion^®, Scott F. Michael, PhD, professor of biological sciences at Florida Gulf Coast University, and an author on the study, explained how his team’s work highlights the ways Zika virus may still be silently spreading. “One of the most important aspects of our study is that by using a combination of molecular epidemiology and travel information, we can reconstruct outbreaks without local reporting, regardless of the reason,” he said. “Using a similar approach, unreported disease outbreaks could be monitored from any location with significant travel. As travel is the major route of outbreak spread, this approach might be useful in many situations.”

By creating a model based on existing data on the relationship between local and travel-associated Zika infection rates, the study team determined that the Cuba outbreak likely had 5707 unreported Zika cases, comparable to large outbreaks in other Caribbean countries. More than 99% of those cases occurred in 2017, although the study team called it unusual that the Cuban outbreak had a 1-year delay compared with other Caribbean outbreaks. “Although we don’t have direct data, we speculate that mosquito control efforts may have had enough impact to hold off establishment of local transmission for a year,” Michael said.

As Zika outbreaks continue to occur in countries such as Angola, India, Cabo Verde, Vietnam, and Thailand, the study team says their findings shed light on the importance of case reporting and could be used to detect additional “hidden” outbreaks.