Mass blood survey for malaria: pooling and real-time PCR combined with expert microscopy in north-west Thailand

Background: Asymptomatic carriage of Plasmodium falciparum and Plasmodium vivax is common in both low-and high-transmission settings and represents an important reservoir of infection that needs to be targeted if malaria elimination is to succeed. Methods: Mass blood examinations (475 individuals) were conducted in two villages in Mae Hong Son, an area of endemic but low-transmission malaria in the north-west of Thailand. The microscopist at the local malaria clinic did not detect any infections. Pools of four samples were screened by real-time PCR; individual members of all of the positive pools were then re-examined by expert microscopy and by a second species-specific PCR reaction. Results: Eight subjects were found to be positive by both PCR and expert microscopy and one was found to be positive by PCR alone. The slides contained asexual stage parasites of P. vivax, P. falciparum and Plasmodium malariae, but no gametocytes. The local clinic was notified within two to eight days of the survey. Conclusion: A combination of pooling, real-time PCR and expert microscopy provides a feasible approach to identifying and treating asymptomatic malaria infections in a timely manner.