Asymptomatic and submicroscopic Plasmodium vivax infection: a study previous to malaria-free certification in Argentina

BACKGROUND: Argentina was certified malaria-free by the WHO in 2019 after eight years with no instances of local transmission. Epidemiological surveillance was focused on detecting Plasmodium vivax in areas where the last cases had been reported. METHODS: During the national malaria surveillance program (2013-2018), several human blood samples were collected across neighborhoods using traditional microscopy and molecular techniques (2013-2017) to detect any silent circulation of P. vivax. In 2022, post-certification, 92 blood samples preserved in Whatman FTA paper were analyzed for the first time to detect the presence of Plasmodium parasites by amplifying and sequencing a fragment of the Plasmodium cytochrome b gene. RESULTS: A submicroscopic, asymptomatic P. vivax infection was identified in a 66-year-old individual from La Bendición neighborhood, Salvador Mazza, Salta province (northwestern Argentina). The individual had no history of travel to any P. vivax-endemic region. This case was the first detected infection among the samples collected from various localities along Argentina's borders with Bolivia (northwest) and Brazil (northeast). DISCUSSION: This finding highlights the risk of silent circulation of P. vivax in areas previously assumed to be malaria-free and raises concerns regarding the timing of the certification. This is the first molecularly confirmed submicroscopic infection of P. vivax reported prior to malaria-free certification in Argentina and the Southern Cone region. CONCLUSIONS: Currently, there is no active epidemiological or entomological surveillance in the area where the case was recorded. It is imperative to strengthen the surveillance system to maintain the country's malaria-free status within the national health system agenda and address the risks of silent malaria circulation and re-emergence. Furthermore, to comply with the national malaria program guidelines and WHO requirements to maintain malaria-free certification, the development and implementation of highly sensitive diagnostic tools to detect asymptomatic cases is crucial.