Silent Circulation of <em>Plasmodium vivax</em>: First Asymptomatic Malaria Case Following Malaria-Free Certification in Argentina

For eight years prior to Argentina&#039;s malaria-free certification in 2019, there were no instances of local transmission. Epidemiological surveillance focused on detecting Plasmodium vivax in areas where the last cases had been reported. During the national malaria surveillance program (2016-2018) in Salvador Mazza (Salta Province, northwestern Argentina), several neighborhoods were randomly selected for the collection of human blood samples to detect any silent circulation of P. vivax. Diagnosis of malaria parasites relied on traditional microscopy and molecular detection using blood collected on filter paper, by amplifying and sequencing a portion of the Plasmodium cytochrome b gene. An autochthonous case of P. vivax was identified in an asymptomatic 64-year-old individual in La Bendición neighborhood, Salvador Mazza. The individual had never traveled to any P. vivax-endemic region. This case is the first detected among 92 samples collected from various localities along Argentina&#039;s borders with Bolivia (northwest) and Brazil (northeast). This finding highlights the possibility of silent circulation of P. vivax in areas previously assumed to be malaria-free and raises concerns regarding the timing of the certification, prompting a reevaluation of the current situation. The extent of P. vivax circulation among asymptomatic individuals remains largely unknown. This is the first molecularly confirmed asymptomatic case of P. vivax reported following malaria-free certification in Argentina and the Southern Cone region. Currently, there is no active epidemiological or entomological surveillance in the area where the case was recorded. The permeable nature of the border facilitates disease transmission, and the lack of information about the movement of asymptomatic individuals is particularly concerning. It is imperative that the surveillance system responds effectively to maintain the status as a malaria-free country, prioritizing this within the national health system agenda due to the potential implications of silent malaria circulation and the risk of re-emergence and re-establishment.