Objectives: Human Plasmodium knowlesi infections have been reported from several South-East Asian countries, excluding India, but its drug susceptibility profile in mixed-infection cases remains unknown.
Methods: The chloroquine resistance transporter (CRT) and dihydrofolate reductase (DHFR) genes of P. knowlesi and other Plasmodium species were sequenced from clinical isolates obtained from malaria patients living in the Andaman and Nicobar Islands, India. The merozoite surface protein-1 and 18S rRNA genes of P. knowlesi were also sequenced from these isolates.
Results: Among 445 samples analysed, only 53 of them had P. knowlesi-specific gene sequences. While 3 of the 53 cases (5.66%) had P. knowlesi monoinfection, the rest were coinfected with Plasmodium falciparum (86.79%, n = 46) or Plasmodium vivax (7.55%, n = 4), but none with Plasmodium malariae or Plasmodium ovale. There was discordance in the drug resistance-associated mutations among the coinfecting Plasmodium species. This is because the P. knowlesi isolates contained wild-type sequences, while P. falciparum isolates had mutations in the CRT and DHFR marker genes associated with a higher level of chloroquine and antifolate drug resistance, respectively. The mutation pattern indicates that the same patient, having a mixed infection, may be harbouring the drug-susceptible P. knowlesi parasite and a highly drug-resistant P. falciparum parasite.
Conclusions: A larger human population in South-East Asia may be at risk of P. knowlesi infection than reported so far. The different drug susceptibility genotypes of P. knowlesi from its coinfecting Plasmodium species in mixed infections adds a new dimension to the malaria control programme, requiring formulation of an appropriate drug policy.