Epidemiology of indigenous Plasmodium knowlesi infection in Sarawak, 2011-2019.

Objectives: To characterise the state-wide epidemiology of indigenous knowlesi malaria in Sarawak from 2011 to 2019.

Methods: Longitudinal retrospective study was conducted based on Sarawak knowlesi malaria surveillance data recorded from 2011-2019. Only indigenous cases were included and information extracted for analysis comprised age, sex, occupation, ethnicity, case severity, hospital admission and parasite density.

Results: Over the 9 years, 8473 indigenous knowlesi malaria cases were recorded. Age group 40-49 years, males, plantation workers and Iban communities recorded the highest percentage of cases in each demographic variable. Most of the cases were uncomplicated (n = 7292; 86.1%) and 89.6% (n = 7589) of the total cases were reported with ≤20 000 parasites/μl of blood. Age group and ethnic group are associated with the severity of knowlesi malaria in Sarawak. Multivariable logistic regression indicated that the age group 60+ years had the highest odds of developing severe knowlesi malaria compared with other age groups (AOR 2.48; 95% CI 1.22, 5.02; p = 0.012). Bidayuh patients were more likely to develop severe knowlesi malaria than Ibans, the largest ethnic group among knowlesi malaria patients (AOR 1.97; 95% CI 1.31, 2.97; p = 0.001).

Conclusions: Identification of risk groups is important for the implementation of prevention programs and treatments targeting at specific group to combat knowlesi malaria effectively.