Longitudinal study of Plasmodium falciparum and Plasmodium vivax in a Karen population in Thailand.

BACKGROUND: Clinical case treatment of malaria infections where Plasmodium falciparum and Plasmodium vivax are sympatric has achieved effective reductions in P. falciparum prevalence and incidence rates, but has been less successful for P. vivax. The high transmissibility of P. vivax and its capacity to relapse have been suggested to make it a harder parasite species to control. METHODS: A clinical malaria case treatment programme was carried out over a decade in a Karen community composed of seven hamlets on the Thai-Myanmar border. RESULTS: From 1994 to 2004, prevalence rates of both P. falciparum and P. vivax decreased by 70-90% in six of the seven study hamlets, but were unchanged in one hamlet. Overall, incidence rates decreased by 72% and 76% for P. falciparum and P. vivax respectively over the period 1999-2004. The age-incidence and prevalence curves suggested that P. vivax was more transmissible than P. falciparum despite a greater overall burden of infection with P. falciparum. Male gender was associated with increased risk of clinical presentation with either parasite species. Children (< 15 years old) had an increased risk of presenting with P. vivax but not P. falciparum. CONCLUSION: There was a considerable reduction in incidence rates of both P. vivax and P. falciparum over a decade following implementation of a case treatment programme. The concern that intervention methods would inadvertently favour one species over another, or even lead to an increase in one parasite species, does not appear to be fulfilled in this case.

Very high carriage of gametocytes in asymptomatic low-density Plasmodium falciparum and P. vivax infections in western Thailand.

BACKGROUND: Low-density asymptomatic infections of Plasmodium spp. are common in low endemicity areas worldwide, but outside Africa, their contribution to malaria transmission is poorly understood. Community-based studies with highly sensitive molecular diagnostics are needed to quantify the asymptomatic reservoir of Plasmodium falciparum and P. vivax infections in Thai communities. METHODS: A cross-sectional survey of 4309 participants was conducted in three endemic areas in Kanchanaburi and Ratchaburi provinces of Thailand in 2012. The presence of P. falciparum and P. vivax parasites was determined using 18S rRNA qPCR. Gametocytes were also detected by pfs25 / pvs25 qRT-PCRs. RESULTS: A total of 133 individuals were found infected with P. vivax (3.09%), 37 with P. falciparum (0.86%), and 11 with mixed P. vivax/ P. falciparum (0.26%). The clear majority of both P. vivax (91.7%) and P. falciparum (89.8%) infections were not accompanied by any febrile symptoms. Infections with either species were most common in adolescent and adult males. Recent travel to Myanmar was highly associated with P. falciparum (OR = 9.0, P = 0.001) but not P. vivax infections (P = 0.13). A large number of P. vivax (71.5%) and P. falciparum (72.0%) infections were gametocyte positive by pvs25/pfs25 qRT-PCR. Detection of gametocyte-specific pvs25 and pfs25 transcripts was strongly dependent on parasite density. pvs25 transcript numbers, a measure of gametocyte density, were also highly correlated with parasite density (r 2 = 0.82, P < 0.001). CONCLUSIONS: Asymptomatic infections with Plasmodium spp. were common in western Thai communities in 2012. The high prevalence of gametocytes indicates that these infections may contribute substantially to the maintenance of local malaria transmission.