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Could Yellow Fever Return to the United States?

Peter Hotez and Kristy Murray from Baylor College of Medicine highlight the potential for yellow fever to return to the southern cities of the United States

Endemic regions of yellow fever in Africa (Image Credit: CDC)
Endemic regions of yellow fever in Africa (Image Credit: CDC)

In the summer-fall of 1878 an epidemic of yellow fever destroyed the city of Memphis, Tennessee.  Likely introduced into the Caribbean by trade from the West Coast of Africa and later brought up the Mississippi River by a steamer ship (the Emily B. Souder) with sick and dying sailors, yellow fever killed an estimated 5,000 Memphis residents, almost one-third of its population who did not flee the city that August [1].   According to Molly Caldwell Crosby in her detailed account, the summer-fall 1878 yellow fever epidemic in the Mississippi Valley was possibly “the worst urban disaster in American history” [1].

Among the factors responsible for the 1878 tragedy were an unusually warm winter and spring that year, which helped Aedes aegypti mosquitoes  to flourish in the Mississippi Valley, together with a lack of adequate urban drainage and a functioning sewer system, and a susceptible (non-immunized) population – the yellow fever vaccine would not be developed for another 50 years.

Today, the world’s yellow fever-endemic areas are restricted to Sub-Saharan Africa (figure image) and tropical regions of South America, but there are a few red flags suggesting the possibility that the “yellow jack” (a historical term used to once describe yellow fever) could return to the US.  The Ae. aegypti mosquito can now be found in many areas of the southern United States.  This is an area where US poverty rates are at their highest, along with its fellow travelers poor urban housing and neglected foci of standing water.  The area has also experienced unusually warm winters and springs over the past few years.  Indeed, dengue fever, another arbovirus infection transmitted by Ae. aegypti mosquitoes, was recently shown to have emerged in Houston, Texas in 2003.  Although Max Theiler received the Nobel Prize for developing the yellow fever vaccine in 1951, vaccination rates in the US are practically non-existent except among travelers to endemic areas.

There are several examples of US vulnerability to yellow fever, including our home city of Houston, which has recently emerged as a true gateway city and globalization hub. For instance, today, Houston hosts the world’s largest number of Nigerian expats (who provide important and skilled expertise for our city’s oil and energy industry), and there are direct flights to and from Lagos, the largest Nigerian city.  A recent study from the Division of Global Migration and Quarantine of the CDC (Centers for Disease Control and Prevention) found that US travelers to Nigeria are especially likely to decline vaccination, despite the fact that its urban areas are at especially high risk for yellow fever outbreaks. The culmination of travelers returning to Houston from endemic areas, subtropical climate, high prevalence of Ae. aegypti mosquitoes, and areas of dense housing overlapped with poverty place Houston at risk for yellow fever emergence.

We need to seriously evaluate the risks of the major southern cities of the US, including Houston, but also New Orleans, Tampa, and Miami for their vulnerability to Aedes-transmitted arbovirus infections, such as yellow fever. As we have pointed out, cities such as Houston have emerged as important endemic zones for neglected tropical diseases. While we are aware that US urban areas may not be as vulnerable to yellow fever as Memphis was more than a century ago, there is still an important risk that needs to be considered as part of our national emergency preparedness, particularly in light of an emerging dengue problem (i.e., another Ae. Aegypti mosquito transmitted virus infection) in Houston and other southern coastal US areas.


  1. Crosby MC  (2006) The American Plague: The Untold Story of Yellow Fever, the Epidemic that Shaped our History  Berkley Books, New York, pp 74-75.
  1. Having spent the better part of 30 years in Houston employed with a mission of mosquito suppression, I came away to retirement with a few observations regarding local mosquito vectors and the diseases they promoted.

    1. That the citizens could raise more mosquitoes, than could be economically killed or prevented.

    2. It was seldom evident that efforts at killing them made much of a dent in existing populations.

    3. That mysteriously, outbreaks of malaria, yellow fever and dengue, never got out of hand with plenty enough vectors around to spread an outbreak.

    4. That these diseases apparently had the sufficient numbers of susceptible victims at hand, as Saint Louis Encephalitis did, and were not accomplishing a comparative result.

    5. That occasional reports malaria, yellow fever, and dengue cases did appear in Houston, but never became a focus that spread.

    I suspected that de facto quarantine of morbid victims and medical treatment for them was close enough to universal to interrupt any significant amount transmission.

    I noted that while yellow fever, dengue, and malaria were apparently more dependent upon human hosts, that Saint Louis Encephalitis could not be human hosted, and that there were plentiful bird hosts. Hosting was no problem in any of the cases. One difference, there were no wild hosts for malaria.

    It seemed that the difference between what was observed the case in Houston and other U.S. cities against foreign examples with significant problems was comparative standards of housing and medical care. Poverty levels in the U.S., that in many other countries would have resulted in unscreened units of housing, were far more likely to be screened. Behavioral factors influenced by indoor entertainment availability, (TV) likely played a part as well. Greater incidence of air conditioned housing also seemed likely to be involved. Behavioral factors coupled with standards of housing, and disease host requirements worked together to discourage transmission.

    Driving the streets of Houston in poorer sections where un-air conditioned housing was more likely produced observations of families, and neighbors standing and sitting outdoors socializing during evening twilight, where radiative cooling was accomplished more rapidly than indoors. This was prime time for feeding Culex quinquefasciatus, so it was not too surprising SLE had a greater success rate than the other common mosquito borne diseases. Since I have left that scene, the more widely adapted West Nile virus has successfully immigrated with a likely larger variety and quantity of hosts and victims. I recall public health officials gritting their teeth over the removal of vector defenses at international terminals back in the 1960s.

    Dengue, yellow fever, and malaria transmission is accomplished by vectors having somewhat a different host and required vector feeding pattern from SLE, so that they are not yet prevalent under conditions seen here. It is not surprising to me that these diseases did have a historically greater prevalence, when living conditions in the U.S. had a greater resemblance to that of third world countries now. The thought has occurred to me that conditions in the U.S. can change, so that economic, political, and sociological factors, that still support a sufficient standard of living, could deteriorate to the necessary point for successful transmission. I worry that a population grown bored in the comforts of success, well demonstrated in the past as possible, will seek relief via change for change’s sake. Economic success is the goose that lays the eggs that defend our shores, that promote our health, that entertain us, that makes possible all that one could hope for from a nation’s material well being. And that goose requires the healthy operation of a constitution in the application of which its ownership refrains from disruptive behaviors and decisions, that tend promote their suppressing, starving, or killing each other or that goose.

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