Atypical Plasmodium knowlesi Infection in a Patient with Autoimmune Haemolytic Anaemia Returning from Thailand: A Case Report

Imported malaria in non-immune travellers remains a potentially life-threatening infectious disease, and Plasmodium knowlesi is increasingly recognised as a cause of malaria acquired in Southeast Asia. We report the unusual case of a 64-year-old man with warm autoimmune haemolytic anaemia (AIHA) on long-term low-dose prednisolone therapy who developed high fever, chills, fatigue, arthralgia, myalgia, and jaundice five days after returning from Thailand. The initial clinical and laboratory presentation was interpreted as an exacerbation of AIHA, and high-dose corticosteroids were administered. A malaria rapid diagnostic test (RDT) was negative; however, subsequent blood-film microscopy demonstrated Plasmodium spp., and polymerase chain reaction confirmed P. knowlesi infection. Antimalarial therapy with artemether/lumefantrine was started eight days after symptom onset. Parasite density subsequently decreased from 4720 parasites/µL to 1307 and 96 parasites/µL over two days; after three days, only two parasites were detected in the entire thick blood film, accompanied by decreasing bilirubin and lactate dehydrogenase levels and an increase in haemoglobin. This case highlights that P. knowlesi malaria may be diagnostically obscured by pre-existing haemolytic disease, negative RDT results, and atypical laboratory findings. Confirmatory microscopy and molecular testing should be pursued in febrile travellers returning from Southeast Asia, even when initial malaria screening is negative.