Ten years of Plasmodium malariae infections in an International Health Reference Center in Barcelona: temporal trends and insights

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Abstract
Plasmodium malariae remains one of the most neglected human malaria parasites. Historically considered of low prevalence and limited clinical significance, emerging evidence suggests its true burden may be underestimated due to diagnostic challenges and the parasite's ability to persist at low densities. A retrospective observational study was conducted at the Vall d'Hebron-Drassanes International Health Unit (Barcelona) from June 2015 to May 2025. Malaria diagnosis was performed using microscopy or quantitative PCR (qPCR), with the diagnostic method selected according to clinical presentation and epidemiological risk. Clinical and epidemiological data were collected and analyzed by descriptive statistics. Among 6,293 individuals screened for malaria, 404 (6.4%; CI 95%: 5.8-7.1) were positive for Plasmodium spp., of whom 55 (13.6%; CI 95%: 10.6-17.3) were infected with P. malariae. Most infections (52/55; 94.5%) were detected in asymptomatic migrants from sub-Saharan Africa during routine screening, with very low parasite densities (≤ 80 parasites/µL). Notably, 13/55 patients had been living in Spain for more than 12 months prior to diagnosis, supporting long-term persistence of low-level parasitemia. Furthermore, one symptomatic traveler, experienced a clinical relapse after artemisinin-based combination therapy. This study describes the trend in the diagnosis of P. malariae at a referral center in a non-endemic European setting. The high proportion of asymptomatic chronic carriers underscores a silent parasite reservoir that may hinder malaria elimination efforts. Increased awareness and improved diagnostic strategies are needed to better characterize and control this overlooked species.

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