Although human infections of Plasmodium knowlesi have been found throughout Southeast Asia, most cases originated from Malaysian Borneo. In Thailand, P. knowlesi malaria was considered extremely rare. However, during October 2017-September 2018, there was a surge in the number of reported P. knowlesi cases. Here, a series of six cases of P. knowlesi malaria found during this period in Songkhla and Narathiwat provinces of southern Thailand are presented. All cases were confirmed by polymerase chain reaction. The unprecedented case number in the affected area is a warning sign of an increasing P. knowlesi burden in the south of Thailand.
Plasmodium knowlesi, a zoonotic malaria species, has become a significant public health concern in Southeast Asia. In regions such as Malaysia and southern Thailand, P knowlesi incidence has risen, even as other human malaria parasites are nearing elimination. Similar to its close relative Plasmodium vivax, P knowlesi relies on the Duffy antigen receptor for chemokine (DARC) as a key receptor for erythrocyte invasion. Only Duffy-positive individuals are thought to be susceptible to clinical infection. Here, we demonstrate that P knowlesi possesses greater invasion plasticity than previously recognized. This parasite can bypass the need for DARC, as shown by its in vitro adaptation to invade and replicate within Duffy-negative (Fy-) erythrocytes. This adaptation is stable and independent of DARC binding, enabling the adapted parasite line to be maintained in Fy- erythrocytes and to resist inhibition by α-DARC antibodies. Genomic analysis identified a genomic recombination event between the parasite's dbpα and dbpγ genes, resulting in a new chimeric gene dbpαγ. Using CRISPR-Cas9 targeted reversion, we could demonstrate that dbpαγ is essential for invasion of Fy- erythrocytes. These findings shed new light on the invasion plasticity of P knowlesi, with implications for the parasite's potential spread beyond Southeast Asia and for understanding the complex host-cell specificity and atypical invasion pathways seen in P vivax.