Background: Plasmodium vivax is a predominant species of malaria in parts of South America and there is increasing resistance to drugs to treat infections by P. vivax. The existence of latent hypnozoites further complicates the ability to classify recurrent infections as treatment failures due to relapse, recrudescence of hyponozoites or re-infections. Antigen loci are putatively under natural selection and may not be an optimal molecular marker to define parasite haplotypes in paired samples. Putatively neutral microsatellite loci, however, offer an assessment of neutral haplotypes. The objective here was to assess the utility of neutral microsatellite loci to reconcile cases of recurrent parasitaemia in Amazonian P. vivax populations in Peru. Methods. Patient blood samples were collected from three locations in or around Iquitos in the Peruvian Amazon. Five putatively neutral microsatellite loci were characterized from 445 samples to ascertain the within and amongst population variation. A total of 30 day 0 and day of recurrent parasitaemia samples were characterized at microsatellite loci and five polymorphic antigen loci for haplotype classification. Results: The genetic diversity at microsatellite loci was consistent with neutral levels of variation measured in other South American P. vivax populations. Results between antigen and microsatellite loci for the 30 day 0 and day of recurrent parasitaemia samples were the same for 80% of the pairs. The majority of non-concordant results were the result of differing alleles at microsatellite loci. This analysis estimates that 90% of the paired samples with the same microsatellite haplotype are unlikely to be due to a new infection. Conclusions: A population-level approach was used to yield a better estimate of the probability of a new infection versus relapse or recrudescence of homologous hypnozoites; hypnozoite activation was common for this cohort. Population studies are critical with the evaluation of genetic markers to assess P. vivax biology and epidemiology. The additional demonstration of microsatellite loci as neutral markers capable of distinguishing the origin of the recurrent parasites (new infection or originating from the patient) lends support to their use in assessment of treatment outcomes.
Bibliographical noteFunding Information:
The authors would like to acknowledge the contribution of the patients to this study, the microscopy performed by Delores Rimarachin, Gerson Guedes and Greys Braga, and the map prepared for publication by Hugo Valdivia. The authors greatly appreciate comments by G Christain Baldeviano and Willy Lescano on this manuscript. This work was developed and sponsored jointly by the Instituto Nacional de Salud of the Peruvian Ministry of Health and the US Naval Medical Research Unit Six (NAMRU-6). Funding for this project was received from the Global Emerging Infections Surveillance and Response System of the US Department of Defense (AFHSC/GEIS). This work was supported and funded by work unit number 847705.82000.256B.B0016. Additional financial support from the CDC Drug Resistance Working Group and support from the Atlanta Research and Education Foundation (Atlanta, GA, USA) are appreciated.
- Antigen locus
- Microsatellite markers
- Plasmodium vivax