Notes from the Field: Increases in Imported Malaria Cases — Three Southern U.S. Border Jurisdictions, 2023

Introduction

Malaria is a severe and potentially fatal mosquitoborne disease caused by infection with Plasmodium spp. parasites. Although malaria is no longer endemic in the United States, imported infections are reported annually; the primary risk group has been U.S. residents traveling to areas where malaria is endemic (1). In 2023, sporadic locally acquired mosquito-transmitted malaria cases were reported in several U.S. states (2,3). This report describes increases in imported malaria cases in 2023 compared with 2022 in three public health jurisdictions along the U.S. southern border.

Investigation and Outcomes

During January–December 2023, a total of 68 imported malaria cases were identified from reportable disease surveillance systems in Pima, Arizona (18), San Diego, California (27), and El Paso, Texas (23), compared with 28 cases in 2022 (three in Pima, 12 in San Diego, and 13 in El Paso) (Table). Because malaria case counts were higher than expected, enhanced case investigations were initiated. Malaria cases were defined according to CDC case definitions.* To describe imported malaria cases in these three jurisdictions, this report summarized patient travel and illness characteristics by U.S. residence status. New arrivals were non–U.S.-born persons who had arrived in the United States within the preceding 6 months and were classified into the following three subgroups: 1) newly arrived refugees (i.e., officially admitted to the United States as part of the U.S. Refugee Admissions Program), 2) other new arrivals (including asylum seekers and other migrants), and 3) persons whose immigration status was unknown. Among jurisdictions, differences were identified in epidemiologic investigation protocols for patients without a local address and whether they were included in local surveillance case counts. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^†

Among the 68 imported malaria cases identified in 2023, 15 (22%) occurred among U.S. residents, two (3%) among newly arrived refugees, 49 (72%) among other newly arrived migrants (i.e., asylum seekers and other migrants), and two (3%) among travelers with unknown immigration status. The local public health jurisdictions attempted an interview with 61 (90%) patients. Among the 68 patients with malaria, 33 (49%) met residence criteria for inclusion in local surveillance case counts (i.e., the 15 U.S. residents, two newly arrived refugees, and 16 [33%] of the 49 other newly arrived migrants). The U.S. residents and refugees traveled directly from another country with endemic malaria to the United States. Among the 49 other newly arrived migrants, 46 (94%) had traveled through one or more countries with endemic malaria, including the country of origin (complex travel). The median travel duration was 29 days (range = 8–85 days), and 36 (73%) persons reported having traversed land borders. Overall, 63 (91%) patients with malaria were hospitalized; no deaths were reported. Nearly one third (21; 31%) of patients with malaria experienced severe disease (1), of which Plasmodium vivax was reported among 11 (52%), P. falciparum among six (29%), and another or unknown Plasmodium spp. parasite among four patients. Severe malaria was more common among other newly arrived migrants (18 of 49; 37%) than among U.S. residents (one of 15; 7%).

Preliminary Conclusions and Actions

Imported malaria in three U.S. southern border jurisdictions increased in 2023, particularly among new arrivals to the United States with recent, complex transit through at least one country with endemic malaria. During the same period, entry of asylum seekers and other migrants into the United States across the southern land border increased.^§ In light of the different jurisdictional protocols used in case investigations, implementation of classifications and consistent investigation and reporting protocols for non-U.S. residents could facilitate better characterization of malaria incidence among new arrival subgroups in different jurisdictions.^¶

New arrivals to the United States with complex travel through areas with endemic malaria are potentially at higher risk for malaria and, for reasons not fully understood, for more severe illness. Health care professionals should obtain a complete travel history, consider malaria among symptomatic patients with recent travel through areas where malaria is endemic, and initiate prompt testing and, if indicated, treatment.** Outreach and education about malaria directed to local health care professionals and to new arrivals with recent travel in areas with endemic malaria are crucial because prompt care seeking, diagnosis, and treatment of malaria will reduce morbidity in this population.