Researchers say yellow fever in Brazil means a chance of the disease coming to the US

The country’s top infectious diseases researcher warned Wednesday that the spread of yellow fever in Brazil could have consequences for the US.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the US could see small localized spread of the dangerous virus, similar to Florida’s 2016 Zika outbreak, if an unusually large yellow fever epidemic in rural Brazil takes root in either or both of that country’s two large metropolises, Sao Paolo and Rio de Janeiro. US territories — for instance Puerto Rico and the US Virgin Islands — might also face local transmission, Fauci and a colleague wrote in a perspective published in the New England Journal of Medicine.

“In an era of frequent international travel, any marked increase in domestic cases in Brazil raises the possibility of travel-related cases and local transmission in regions where yellow fever is not endemic,” they noted.

In an interview with STAT, Fauci stressed that he thinks it is unlikely this will happen, but doctors need to be aware of the risk so they will spot cases if they start to occur in Brazil’s major cities, or if travelers bring the virus back to the US.

“You have to have it on the radar screen,” he said. “If you don’t think of a diagnosis, you’ll never make the diagnosis.”

Read more: Massive yellow fever vaccination campaign begins in Congo

The last epidemic of yellow fever occurred in the US in 1905. But in earlier times, epidemics regularly happened in the United States, with large outbreaks in major cities — New York, Philadelphia and Boston — in the late 1700s resulting in more than 10,000 deaths. Later, the virus restricted itself to the south; New Orleans had outbreaks through the 1800s.

In Brazil and other places where yellow fever is still endemic, there are two patterns of transmission. Sylvatic, or jungle yellow fever, is spread from infected forest-dwelling mosquitoes to primates. Occasionally people will be bitten and infected, but humans play an incidental role in this transmission cycle. At times, however, mosquitoes that live in cities will become infected, triggering urban yellow fever outbreaks. One such epidemic occurred last year in Angola and the Democratic Republic of Congo. In those outbreaks, the virus is spread by Aedes mosquitoes, which are ubiquitous in cities in the parts of the world where they are found.

Aedes mosquitoes also spread the Zika virus, which has raced through Latin America during the past couple of years. But yellow fever, which is related to the Zika virus, is a far more dangerous foe. Deaths from Zika are rare, but not so yellow fever. Most people do recover, but between 15 percent to 20 percent of patients have a more severe illness and between 20 percent and 60 percent of the people who develop this more severe illness may die.

So far the Brazilian outbreak has been a sylvatic one. But concern has arisen because the states involved are located close to Sao Paolo and Rio. Those cities have populations of more than 12 million and more than six million people respectively. People in these cities aren’t routinely vaccinated against yellow fever, Fauci noted — a fact that could allow for spread if the virus is introduced.

“That’s the thing that’s the big if,” Fauci said. “If it does make a transition from sylvatic to urban, let’s go. All hands on deck here.”