Public Health Response to the First Locally Acquired Malaria Outbreaks in the US in 20 Years.

Importance: In 2023, the US reported 10 locally acquired mosquito-transmitted malaria cases of 4 genetic lineages in 4 states, the first such outbreaks detected in 20 years and the largest in 35 years. Objective: To present the investigations, interventions, and challenges in the public health response to the malaria outbreaks and provide recommendations for future outbreaks. Design, Setting, and Participants: This qualitative study was an interdisciplinary public health response to the locally acquired malaria outbreaks in May to December 2023 and included case investigations, enhanced case finding, polymerase chain reaction analysis of captured Anopheles spp mosquitoes for Plasmodium spp parasites, and novel targeted amplicon sequencing of Plasmodium spp in patient blood samples. Public health interventions included incident command activation, clinician outreach, community awareness, and vector control. Patient data were acquired through public health surveillance as part of National Notifiable Disease Surveillance. Exposure: Plasmodium vivax-infected and Plasmodium falciparum-infected Anopheles spp mosquitoes. Main Outcomes and Measures: Confirmed malaria infection via blood film microscopy and polymerase chain reaction, presence of Plasmodium spp in Anopheles spp mosquitoes, and genetic markers associated with an endemic region of origin and parasite strain relatedness via targeted amplicon sequencing. Results: The study included 10 patients (mean [SD] age of 39.5 [15.0] years; 7 male [70%]) from Florida, Texas, Maryland, and Arkansas with locally acquired mosquito-transmitted malaria and 783 Anopheles spp mosquitoes across 4 states. No patient had a recent history of international travel or blood-borne exposures. Outbreak cases had epidemiologic links within but not across state lines. P vivax was detected in 3 Anopheles crucians in Florida. Sequencing data showed that all Florida P vivax cases shared the same Plasmodium strain. The Texas and Arkansas P vivax cases were genetically distinct from each other and from Florida's cases. All 9 P vivax strains had genetic signatures that were consistent with Central and South American origin. Maryland's P falciparum parasites were consistent with African origin. The outbreaks were contained. Conclusions and Relevance: In this qualitative study of locally transmitted malaria, outbreaks remained contained to individual counties, with Florida's P vivax cases linked to a single strain distinct from those in other states. Sustained Plasmodium spp transmission is unlikely in the US, though increases in global travel and migration, population, temperatures, and persistence of Anopheles spp vectors may increase risk for locally acquired malaria. Clinicians should prescribe chemoprophylaxis for patients traveling to endemic regions, ensure timely diagnosis and treatment, and facilitate public health reporting. Researching US Anopheles spp ecology and control methods while accelerating efforts to reduce malaria globally could mitigate future risk.