Retrospective clinical case series study in 2017 identifies Plasmodium knowlesi as most frequent Plasmodium species in returning travellers from Thailand to Germany

Non-human primate malaria in travellers

Emerging zoonotic malaria caused by Plasmodium knowlesi, Plasmodium cynomolgi and Plasmodium inui occurred in humans amid a decline of malaria prevalence in Thailand during the past decade. Diagnosis of non-human primate malaria in ecotourists is challenging. Molecular diagnosis seems to be mandatory such as species-specific polymerase chain reaction (PCR) and targeted amplicon deep sequencing.

Knowlesi malaria: Human risk factors, clinical spectrum, and pathophysiology

Plasmodium knowlesi is endemic across Southeast Asia, and is the commonest cause of zoonotic malaria. The spectrum of clinical disease from P. knowlesi infection ranges from asymptomatic infection, through to severe malaria and death. Over 90% of clinical disease occurs in adults, mostly living in forest edge areas undergoing intensive land use change. With a 24-h asexual life cycle in humans, high parasite counts are possible, but most clinical cases of knowlesi malaria are uncomplicated with low parasitaemia. In co-endemic areas, median parasitaemia in knowlesi malaria is lower than that seen in vivax and falciparum malaria, suggesting a lower fever threshold. Severe malaria occurs in 6-9% of symptomatic adults. Manifestations of severe malaria from P. knowlesi are similar to those seen with falciparum malaria, with the notable absence of coma. Age, parasitaemia, cardiovascular comorbidities and delayed diagnosis are risk factors for severe disease and death, which are only seen in adults. Thrombocytopenia is near-universal in adults, likely related to platelet-red cell binding and clearance. Mechanisms underlying the microvascular sludging seen in fatal disease in non-natural primate hosts and the microvascular accumulation of parasites in fatal human disease are not clear. Marked reductions in deformability of both infected and uninfected red blood cells are associated with disease severity in both humans and other non-natural primate hosts, likely contributing to impaired microvascular perfusion and organ dysfunction. Endothelial activation, endothelial dysfunction, glycocalyx degradation and haemolysis are also associated with, and likely contribute to, severe disease and organ dysfunction, particularly acute kidney injury.

Case of Plasmodium knowlesi Malaria in Poland Linked to Travel in Southeast Asia

We report a case of Plasmodium knowlesi malaria imported to central Europe from Southeast Asia. Laboratory suspicion of P. knowlesi infection was based on the presence of atypical developmental forms of the parasite in Giemsa-stained microscopic smears. We confirmed and documented the clinical diagnosis by molecular biology techniques.