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.

Background Long-term in vitro culture of blood stage Plasmodium parasites invariably leads to asynchronous parasite development. The most often used technique to synchronize Plasmodium falciparum culture is sorbitol treatment, which differentially induces osmotic lysis of trophozoite- and schizont-infected red blood cells due to presence of the new permeation pathways in the membranes of these cells. However, sorbitol treatment does not work well when used to synchronize the culture-adapted Plasmodium knowlesi A1-H.1 line. Methods A number of common solutes were tested in lieu of sorbitol for synchronization of P. knowlesi A1-H.1 ring stage. Results Guanidine hydrochloride was found to selectively lyse trophozoite- and schizont-infected red blood cells, yielding highly synchronous and viable rings. Conclusions A method for synchronization of P. knowlesi in human red blood cells was developed. Requiring only common laboratory reagents, this method is simple and should be applicable to most laboratory settings.