Tropical Medicine in the USA

Likely a consequence of climate change, conditions once categorized as “tropical” medicine can actually occur anywhere the right conditions are met. Thus it’s no longer necessary to live in the tropics to see local cases of malaria, dengue, or Vibrio vulnificus. RIDOH tracks the incidence and prevalence of these conditions and others. 


Malaria actually used to be endemic in the U.S. but was eliminated in the mid-1950s. Since then, malaria has typically only been seen in travelers returning to the U.S. from endemic areas, amounting to around 2,000 cases per year. But in 2023, for the first time since its elimination, there were locally acquired cases of both Plasmodium vivax (Florida, Texas) and P. falciparum (Maryland). Anopheles mosquitoes, which transmit malaria, are present predominantly in the southeastern U.S.

Recognizing malaria is particularly challenging in non-endemic countries. Affected patients have fever and a flu-like illness. Malaria’s characteristic fever cycles occur every other day for P. vivax and P. ovale, and every third day for P. malariae; cycles in P. falciparum are irregular and lack any distinct periodicity. Importantly, classic fever patterns are usually not seen early in the course of malaria, and therefore the absence of fever cycles cannot be used to rule it out. Severe malaria, which most often occurs from P. falciparum, can include a variety of symptoms including encephalopathy, severe anemia, coagulopathy, and multi-organ failure. 


Dengue is another rising threat with cases in the Americas increasing from 1.5 million in the 1980s to 17.5 million in 2010-19 to a record 4.5 million cases in 2023 alone, including in places that had never seen transmission before. Florida had 176 locally acquired cases of dengue last year, while Puerto Rico had more than 1,000. Aedes mosquitoes, which transmit dengue, are present predominantly across the southern U.S. 

Clinically apparent dengue is more common among adults, whereas children are usually asymptomatic. Symptomatic dengue presents with fever, frontal headache (often with retro-orbital pain), nausea/vomiting, body aches, and sometimes a macular rash. The development of petechiae after applying a BP cuff (aka positive tourniquet test) is a characteristic sign and buzzword on board exams. Severe dengue, which manifests as a systemic vascular leak syndrome characterized by bleeding, shock, and organ failure, occurs in around 1 in 20 symptomatic patients. 


Vibrio vulnificus is a gram-negative rod that most often causes severe diarrhea and, less often, wound infections or septicemia (presenting as shock with severe cellulitis or necrotizing fasciitis, which is how it most often appears on board exams). Overall, Vibrio species cause about 80,000 illnesses in the U.S. every year, and while V. vulnificus infections are a smaller subset at about 200 cases a year, they carry a 20% mortality rate. Transmission occurs via consumption of undercooked seafood or wound exposure to seawater. 

The past 10 years have seen eight times as many V. vulnificus infections as the U.S. had in the 1980s, with the disease’s geographic range moving northward around 48 km/year along the eastern seaboard, including cases in Rhode Island and the rest of New England. 

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