Multiple Episodes of P. vivax Malaria in the Amazon Basin: Relapses or Frequent Infection

The predominant species of malaria in South America is P. vivax, which has been difficult to control and eliminate because of the prevalence of relapse cases. Repetitive malaria episodes are likely a result of failed primaquine treatment, host differences in primaquine metabolism and/or frequent reinfection. In the region of Loreto, P. vivax cases relapse approximately 10% of the time, even with primaquine treatment. If a patient receives a subclinical dosage of primaquine, then the relapse rate is even higher. To compare potential differences in exposure, we carried out spatial analysis of malaria cases in the village Padrecocha (population 1,200), located 6 km from Iquitos City, the largest in the Peruvian Amazon Basin. Malaria cases presenting at the local health center from 2010 – 2015 were defined as relapse cases if they experienced more than one episode of P. vivax in a 12 month period, whereas those with a single episode during the study period were classified as controls. From 2011 to 2014, we identified 56 people with a single infection and 48 relapse cases. Of these individuals, forty-three individuals had two episodes of malaria and 5 individuals had three episodes. Normal and relapse cases had similar age and gender distributions as well as parasite loads (p=0.161). In contrast, the spatial distributions for cases versus controls were significantly different with relapse cases living closer to water bodies (p < 0.001) and further from the health post (p-value=0.166) than controls. Nearest neighbor analysis showed more clustering of people experiencing single infections in the center of the city compared to relapse cases that were more disbursed and farther from the city center (p<0.001). Further analysis is needed to determine what is leading to the relapse of these patients. Clustering in frequent malaria patients could be caused by an increased risk for infection, lack of access to medical resources, or increasing poverty with increased distance from the community’s economic center.