An unreported spike in cases from 2017 is revealed through international travellers, a technique that could help with early detection in other epidemics.

By studying cases of people infected by Zika virus while travelling abroad, researchers have uncovered a previously unreported outbreak in Cuba.

Nathan Grubaugh at Yale School of Public Health in New Haven, Connecticut, and his collaborators examined a total of 153 cases of people who had visited Cuba from Florida or Europe between 2016 and 2018. The team estimates that Cuba experienced an outbreak in which somewhere between 1,000 and 20,000 people became infected, most of them in 2017 — a period when the Caribbean country reported no Zika cases to the relevant international agencies. Elsewhere in the Americas, cases had peaked a year before, but the Cuban outbreak might have been delayed by the country’s aggressive mosquito-control efforts, the authors suggest.

By sequencing the Zika genome from 14 people, the team revealed that contagion had spread to Cuba mainly from other Caribbean countries. The authors say that their method could be extended to other infectious diseases, such as influenza, to unveil large outbreaks in countries where detecting or reporting of local cases is difficult.