title: Disease progression in Plasmodium knowlesi malaria is linked to variation in invasion gene family members.
abstract: Emerging pathogens undermine initiatives to control the global health impact of infectious diseases. Zoonotic malaria is no exception. Plasmodium knowlesi, a malaria parasite of Southeast Asian macaques, has entered the human population. P. knowlesi, like Plasmodium falciparum, can reach high parasitaemia in human infections, and the World Health Organization guidelines for severe malaria list hyperparasitaemia among the measures of severe malaria in both infections. Not all patients with P. knowlesi infections develop hyperparasitaemia, and it is important to determine why. Between isolate variability in erythrocyte invasion, efficiency seems key. Here we investigate the idea that particular alleles of two P. knowlesi erythrocyte invasion genes, P. knowlesi normocyte binding protein Pknbpxa and Pknbpxb, influence parasitaemia and human disease progression. Pknbpxa and Pknbpxb reference DNA sequences were generated from five geographically and temporally distinct P. knowlesi patient isolates. Polymorphic regions of each gene (approximately 800 bp) were identified by haplotyping 147 patient isolates at each locus. Parasitaemia in the study cohort was associated with markers of disease severity including liver and renal dysfunction, haemoglobin, platelets and lactate, (r = ≥ 0.34, p =  <0.0001 for all). Seventy-five and 51 Pknbpxa and Pknbpxb haplotypes were resolved in 138 (94%) and 134 (92%) patient isolates respectively. The haplotypes formed twelve Pknbpxa and two Pknbpxb allelic groups. Patients infected with parasites with particular Pknbpxa and Pknbpxb alleles within the groups had significantly higher parasitaemia and other markers of disease severity. Our study strongly suggests that P. knowlesi invasion gene variants contribute to parasite virulence. We focused on two invasion genes, and we anticipate that additional virulent loci will be identified in pathogen genome-wide studies. The multiple sustained entries of this diverse pathogen into the human population must give cause for concern to malaria elimination strategists in the Southeast Asian region.
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia.
authors.forename: Atique M
authors.initials: AM
authors.lastname: Ahmed
authors.affiliation: School of Medicine, University of St Andrews, St Andrews, United Kingdom.
authors.forename: Miguel M
authors.initials: MM
authors.lastname: Pinheiro
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia.
authors.forename: Paul C
authors.initials: PC
authors.lastname: Divis
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia.
authors.forename: Angela
authors.initials: A
authors.lastname: Siner
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia; Faculty of Resource Science and Technology, University Malaysia Sarawak, Kuching, Sarawak, Malaysia.
authors.forename: Ramlah
authors.initials: R
authors.lastname: Zainudin
authors.affiliation: Sibu Hospital, Sibu, Sarawak, Malaysia.
authors.forename: Ing Tien
authors.initials: IT
authors.lastname: Wong
authors.affiliation: Sarikei Hospital, Sarikei, Sarawak, Malaysia.
authors.forename: Chan Woon
authors.initials: CW
authors.lastname: Lu
authors.affiliation: Division of Clinical Sciences, St. George's, University of London, London, United Kingdom.
authors.forename: Sarina K
authors.initials: SK
authors.lastname: Singh-Khaira
authors.affiliation: School of Medicine, University of St Andrews, St Andrews, United Kingdom.
authors.forename: Scott B
authors.initials: SB
authors.lastname: Millar
authors.affiliation: Clinical Blood Sciences, St. George's, University of London, London, United Kingdom.
authors.forename: Sean
authors.initials: S
authors.lastname: Lynch
authors.affiliation: Institute of Medical Microbiology and Hygiene, University of Tübingen, Tübingen, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Willmann
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia.
authors.forename: Balbir
authors.initials: B
authors.lastname: Singh
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia; Division of Clinical Sciences, St. George's, University of London, London, United Kingdom.
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Krishna
authors.affiliation: Malaria Research Centre, University Malaysia Sarawak, Kuching, Sarawak, Malaysia; School of Medicine, University of St Andrews, St Andrews, United Kingdom; Division of Clinical Sciences, St. George's, University of London, London, United Kingdom.
authors.forename: Janet
authors.initials: J
authors.lastname: Cox-Singh
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018450:Disease Progression; D005260:Female; D006801:Humans; D008288:Malaria / Q000453:epidemiology / Q000469:parasitology / Q000635:transmission; D008297:Male; D008875:Middle Aged; D018512:Parasitemia / Q000235:genetics / Q000469:parasitology; D016790:Plasmodium knowlesi / Q000235:genetics / Q000472:pathogenicity; D015800:Protozoan Proteins / Q000235:genetics*; D055815:Young Adult
chemical_list: D015800:Protozoan Proteins