More than 100 years have passed since the mosquito-borne yellow fever virus killed tens of thousands of people in epidemics that raged through the American South and Texas.
Now scientists write in October 19 issue of the New England Journal of Medicine warn that yellow fever could re-emerge in southern states, thanks to climate change creating suitable environments for disease-carrying Aedes aegypti mosquitoes.
There's still time to prepare for possible yellow fever outbreaks, but the U.S. must take action as soon as possible to prevent them, they say Richard WamaiNortheastern professor of cultures, societies and global studies, and Neil Maniarddirector of Northeastern's Master of Public Health program.
“It is inevitable that yellow fever and other vector-borne diseases will continue their course here in this country, including from South to North as temperatures shift upwards,” says Wamai.
“That will happen without a doubt. The question is, can we implement better controls?'
Fighting yellow fever will take a combined approach, including vaccination, surveillance and controlling mosquito populations, known as vector control, Wamai says.
“We shouldn't assume that just because we haven't had an outbreak for more than 100 years of a disease, we might not have additional outbreaks in the future, because climate change is proving otherwise now,” says Maniar.
“The warmer months last longer. We have increasing rainfall, higher humidity levels,” says Maniar. “We're going to see a longer time of year where we have these favorable conditions for mosquito-borne diseases.”
Why are scientists worried?
The New England Journal of Medicine report says mosquito-borne infections “have begun to accelerate in the American South,” with Florida and Texas experiencing dengue, chikungunya and Zika outbreaks over the past decade.
All of these diseases are carried by the same aedes mosquito that also transmits yellow fever, part of the reason researchers in the New England Journal of Medicine say “there are new reasons to worry about a possible resurgence.”
Yellow fever has a deadly legacy in the US, having been responsible in the 1800s for the deaths of tens of thousands of people in southern US coastal cities and towns on the Mississippi River.
“An 1853 epidemic in New Orleans may have killed 11,000 people, or nearly 10% of the population. And in a five-month period in 1878, Mississippi Valley cities, especially Memphis and Vicksburg, saw about 120,000 people get sick and 20,000 die,” the article states.
Even northern cities have been affected. The American Society for Microbiology says that a record heat wave in Philadelphia in 1793 caused an outbreak of yellow fever that decimated the population within months.
More recently, yellow fever re-emerged in Venezuela in 2019, and Brazil “experienced an epidemic nearly three times larger than had been recorded in the previous 36 years” from 2016-2019, according to the New England Journal of Medicine article, which blames overheating. climate that promotes diseases carried by the Aedes aegypti mosquito.
“I would be concerned about both yellow fever and some other mosquito-borne diseases in terms of the risk of an increase in the number of cases here in the U.S. or an increase in the risk of potential outbreaks in certain areas of the country,” Maniar says.
What is yellow fever?
Yellow fever came to the Western Hemisphere from Africa during the forced transport of enslaved people, Wamai says. The World Health Organization reports that as of this year, the disease is endemic in areas of 34 African countries and 13 countries in Central and South America.
There is no human-to-human transmission of the disease, which is caused by the bite of an infected female mosquito, Wamai says. But outbreaks often appear in densely populated areas to which mosquitoes are attracted.
The The Centers for Disease Control says symptoms can appear quickly and include chills, fever, severe headache, back pain, body aches, nausea, fatigue and weakness.
Most people recover, but the death rate for those with severe disease can be as high as 60 percent, Wamai says. “People are vomiting, bleeding. Kidney function is deteriorating.”
The “yellow” refers to the jaundice that afflicts some people with the disease, Wamai says.
He says the most recent global estimate of the burden of yellow fever reports that in 2018 there were 109,000 critical infections and 51,000 deaths in Africa and South America, a fatality rate of 46.8%.
There is no targeted treatment, but there is a vaccine
“There are no cures for yellow fever,” says Wamai. Patients receive supportive care to manage their symptoms, including rest, fluids, and the use of medications that reduce pain and fever.
“The solution is a vaccine,” which has proven to be very effective and only improves over time, Wamai says.
“When someone gets a vaccine, they are protected for life,” says Wamai.
The other advantage of vaccination is that the immunized person is no longer a potential competent carrier of yellow fever and cannot transmit the virus through mosquito bites, he says.
Whether the U.S. has enough yellow fever vaccine to immunize people in the event of an outbreak is questionable, Wamai says.
One solution could be to offer a “fractional vaccine,” such as one-fifth of a normal dose, that might only last a year, he says.
That could buy enough time to vaccinate far more people than the number of full doses available to overcome an outbreak while vaccine production ramps up, Wamai says.
He says Northeastern students who have traveled to Kenya to work in his clinic to treat visceral leishmaniasis have been given fractional doses of yellow fever vaccine when Boston hospital travel clinics have run out.
“The bigger issue is: what if? What if yellow fever breaks out here in the US? And if we start seeing more cases, then what?' asks Maniar.
“Are we prepared for this? And in areas of the world where yellow fever is endemic, (can we) make sure there is enough vaccine to control yellow fever in those areas?'' says Maniar.
Yellow fever 'cannot be eradicated'
Warmer temperatures, rainfall and humidity help mosquitoes thrive, as does poverty, Wamai says.
People housed in areas with poor sanitation and sanitation and in houses with improperly installed doors and windows live in conditions ripe for the emergence of mosquito-borne diseases, he says.
There is a need for consistent surveillance of mosquito populations, as well as measures to control their growth, Wamai says. The CDC says mosquito control measures include eliminating mosquito larval habitats and applying larvicides to kill mosquito larvae.
Nonhuman primates are reservoirs for yellow fever “so there's no way to eradicate it,” Wamai says. “You can't kill all the animals.”
“The way to make sure it doesn't come back is to maintain surveillance, maintain vector control and ensure vaccination readiness and big campaigns to vaccinate people,” he says.
Cynthia McCormick Hibbert is a reporter for Northeastern Global News. Email her at c.hibbert@northeastern.edu or connect with her on X/Twitter @HibbertCynthia.
