In view of the recently suggested association of severe disease in P. vivax with multidrug resistance [14] , expression levels of two genes suspected in having a pivotal role in mediating clinical resistance to chloroquine, pvcrt-o and pvmdr1, were determined. Significantly, the patient with severe symptoms had a 1.6-fold increase in pvmdr1 levels and a 21.9-fold increase in pvcrt-o levels compared to the other patient who presented to the same hospital with P. vivax infection and mild symptoms (Figure 2b ). This finding was validated by analysing parasite material obtained previously from two other patients from the Brazilian Amazon who also had mild symptoms (Additional file 2).