Loading...
Artemisinin combination regimens are a key component of malaria-control programs. Recent reports confirm reduced parasite susceptibility to artemisinin in western Cambodia (JW Infect Dis Apr 4 2012). To determine whether such resistance has spread to or emerged in other areas, researchers performed a longitudinal study involving patients with uncomplicated hyperparasitemic falciparum malaria (≥4% infected red blood cells) seen at clinics along the Thailand–Myanmar border, where treatment response has been diminishing.
Patients received 7 days of oral artesunate, usually combined with mefloquine, doxycycline, or clindamycin. Parasite counts were measured every 6 hours until blood smears were negative. Between 2001 and 2010, 3202 patients were studied.
The mean parasite clearance half-life increased from 2.6 hours in 2001 to 3.7 hours in 2010 (vs. 5.5 hours for 116 patients from western Cambodia in 2007–2010). The proportion of patients with clearance half-lives >6.2 hours increased from <1% in 2001 to 20% in 2010. Genotyping in 1583 infections revealed 148 unique 93-locus parasite genotypes; patients harboring parasites with the same genotype had similar clearance times. No genotypes in Thailand were identical to those in western Cambodia. Slow clearance attributable to parasite genetics increased from 32% in 2001–2004 to 58% in 2007–2010.
Phyo AP et al. Emergence of artemisinin-resistant malaria on the western border of Thailand: A longitudinal study. Lancet 2012 Apr 5; [e-pub ahead of print]. (http://dx.doi.org/10.1016/S0140-6736(12)60484-X)
Uhlemann A-C and Fidock DA. Loss of malarial susceptibility to artemisinin in Thailand. Lancet 2012 Apr 5; [e-pub ahead of print]. (http://dx.doi.org/10.1016/S0140-6736(12)60488-7)
Comment
These findings show that heritable resistance in parasites is present and increasing in western Thailand — an area contiguous with Myanmar, which has a large burden of malaria and weak public health infrastructure. Resistance in Thailand is believed to have developed locally, rather than by importation of parasites from Cambodia. Under selective pressure of continued exposure to artemisinin-containing regimens, the resistant parasites will persist and be preferentially transmitted. As noted by the authors and editorialists, identifying the primary genetic determinant of such resistance is among the many priorities in trying to track, treat, and contain resistant parasites.