Pediatric Mixed Plasmodium vivax–P. falciparum Infection with Disparate Parasitemias: Diagnostic and Surveillance Challenges

<b>Background and Clinical Significance:</b> Malaria remains a significant public health issue in Latin America, where <i>Plasmodium vivax</i> predominates but <i>P. falciparum</i> continues to circulate. Mixed-species infections are uncommon and can pose diagnostic challenges, particularly when parasite densities differ markedly, increasing the risk of underdetecting <i>P. falciparum</i> with conventional methods. <b>Case report:</b> We report a 9-year-old boy from an endemic area with a six-day febrile syndrome. Thick smear and peripheral blood film microscopy, complemented by rapid diagnostic tests for pan-<i>Plasmodium</i> and HRP2 antigens, confirmed a mixed infection with <i>P. vivax</i> (5500 parasites/µL) and <i>P. falciparum</i> (562 parasites/µL). The patient was hemodynamically stable, without severe malaria criteria, and laboratory values were within normal limits. Following confirmation of normal glucose-6-phosphate dehydrogenase activity, treatment with artemether-lumefantrine was initiated, followed by primaquine for hypnozoite eradication. Clinical evolution was favorable, with progressive defervescence, treatment tolerance, and documented parasite clearance. <b>Conclusions:</b> This case illustrates the risk of underestimating <i>P. falciparum</i> in mixed infections with disparate parasitemias and highlights the value of integrated diagnostic approaches in resource-limited endemic settings. It also underscores surveillance limitations that can misclassify mixed infections, potentially affecting epidemiological estimates and treatment strategies. Timely recognition and comprehensive diagnostic evaluation are essential to ensure appropriate antimalarial therapy, prevent complications, and inform public health interventions in regions where both species coexist.