Numerical Distributions of Parasite Densities During Asymptomatic Malaria.

BACKGROUND: Asymptomatic parasitemia is common even in areas of low seasonal malaria transmission, but the true proportion of the population infected has not been estimated previously because of the limited sensitivity of available detection methods. METHODS: Cross-sectional malaria surveys were conducted in areas of low seasonal transmission along the border between eastern Myanmar and northwestern Thailand and in western Cambodia. DNA was quantitated by an ultrasensitive polymerase chain reaction (uPCR) assay (limit of accurate detection, 22 parasites/mL) to characterize parasite density distributions for Plasmodium falciparum and Plasmodium vivax, and the proportions of undetected infections were imputed. RESULTS: The prevalence of asymptomatic malaria as determined by uPCR was 27.5% (1303 of 4740 people tested). Both P. vivax and P. falciparum density distributions were unimodal and log normal, with modal values well within the quantifiable range. The estimated proportions of all parasitemic individuals identified by uPCR were >70% among individuals infected with P. falciparum and >85% among those infected with P. vivax. Overall, 83% of infections were predicted to be P. vivax infections, 13% were predicted to be P. falciparum infections, and 4% were predicted to be mixed infections. Geometric mean parasite densities were similar; 5601 P. vivax parasites/mL and 5158 P. falciparum parasites/mL. CONCLUSIONS: This uPCR method identified most infected individuals in malaria-endemic areas. Malaria parasitemia persists in humans at levels that optimize the probability of generating transmissible gametocyte densities without causing illness.

The epidemiology of subclinical malaria infections in South-East Asia: findings from cross-sectional surveys in Thailand-Myanmar border areas, Cambodia, and Vietnam.

BACKGROUND: The importance of the submicroscopic reservoir of Plasmodium infections for malaria elimination depends on its size, which is generally considered small in low transmission settings. The precise estimation of this reservoir requires more sensitive parasite detection methods. The prevalence of asymptomatic, sub-microscopic malaria was assessed by a sensitive, high blood volume quantitative real-time polymerase chain reaction method in three countries of the Greater Mekong Sub-region. METHODS: Cross-sectional surveys were conducted in three villages in western Cambodia, four villages along the Thailand-Myanmar border and four villages in southwest Vietnam. Malaria parasitaemia was assessed by Plasmodium falciparum/pan malaria rapid diagnostic tests (RDTs), microscopy and a high volume ultra-sensitive real-time polymerase chain reaction (HVUSqPCR: limit of detection 22 parasites/mL). All villagers older than 6 months were invited to participate. RESULTS: A census before the surveys identified 7355 residents in the study villages. Parasite prevalence was 224/5008 (4 %) by RDT, 229/5111 (5 %) by microscopy, and 988/4975 (20 %) when assessed by HVUSqPCR. Of these 164 (3 %) were infected with P. falciparum, 357 (7 %) with Plasmodium vivax, 56 (1 %) with a mixed infection, and 411 (8 %) had parasite densities that were too low for species identification. A history of fever, male sex, and age of 15 years or older were independently associated with parasitaemia in a multivariate regression model stratified by site. CONCLUSION: Light microscopy and RDTs identified only a quarter of all parasitaemic participants. The asymptomatic Plasmodium reservoir is considerable, even in low transmission settings. Novel strategies are needed to eliminate this previously under recognized reservoir of malaria transmission.

Elimination of Plasmodium falciparum in an area of multi-drug resistance.

BACKGROUND: Resistance to the artemisinin derivatives in Plasmodium falciparum has emerged in Cambodia and is now spreading throughout South-East Asia. The rapid elimination of P. falciparum seems to be the only viable option to avoid a public health disaster but this is difficult because even in low transmission settings many residents have asymptomatic parasitaemias. METHODS: In response to a large number of malaria cases reported in three remote villages on the Thai-Myanmar border where malaria is endemic and the disease is seasonal, surveys were conducted using an ultra-sensitive qPCR assay (LOD 22 parasites per mL). In one of the villages where it was feasible, mass anti-malarial drug administration was proposed to the population as a potential solution, and this was adopted. RESULTS: In the three villages 204/356 (57.3 %), 212/385 (55.1 %) and 195/286 (68.2 %) of the resident populations were positive by qPCR (approximately one-third P. falciparum and two-thirds P. vivax). Of those positive for P. falciparum 62 % carried single point mutations in the P. falciparum kelch protein (a marker of artemisinin resistance). In one of the villages 217 of 674 inhabitants received at least one dose of dihydroartemisinin-piperaquine chemoprevention in June 2012, 155 (71.4 %) received two consecutive months, and 98 (45.2 %) received three treatment doses. The chemoprevention was generally well tolerated. The sub-microscopic reservoir of P. falciparum malaria was eliminated during the six-month follow-up period (prevalence fell from 7 to 0 %); P. vivax malaria persisted (prevalence fell from 35 to 8 %). From June to October 2012 (rainy season) the number of clinical episodes of P. falciparum was six times lower (46), than during the same period in the previous year (290). CONCLUSION: Mass drug administration with dihydroartemisinin-piperaquine may be an effective strategy to eliminate P. falciparum rapidly where multi-drug resistance is present.

Uncomplicated Plasmodium vivax malaria in pregnancy associated with mortality from acute respiratory distress syndrome.

The association between severe malaria and Plasmodium vivax species is contentious. On the Thai-Myanmar border, all pregnant women are followed systematically with active weekly malaria screening. Over a 27-year period of providing antenatal care, 48,983 have been prospectively followed until pregnancy outcome (miscarriage or delivery) and 4,298 women have had P. vivax detected at least once. Reported here is the first known P. vivax-associated death amongst these women. The initial patient presentation was of uncomplicated P. vivax (0.5% parasitaemia) in a term, multigravida woman who responded rapidly to oral artesunate and mefloquine treatment, clearing her blood stage parasites within 48 hours. The patient appeared well, was ambulatory and due to be discharged but became unwell with acute respiratory distress syndrome (ARDS) requiring ventilation three days (67 hours) into treatment. Despite induction and delivery of a stillborn foetus, ventilatory requirements increased and the patient died on day 7. The patient had a low body mass index. Sensitive detection with nested PCR confirmed only the presence of P. vivax species and concomitant infections such as tuberculosis and human immunodeficiency virus (HIV) were also ruled out. The contemporaneous treatment of acute uncomplicated P. vivax and the onset of ARDS on day 3 in this patient implies a possible but unconfirmed association with death in this patient. Assuming this death was caused by P. vivax, the risk of ARDS-related maternal mortality in this setting did not differ significantly between Plasmodium falciparum and P. vivax (0.24 per 1,000 (1/4,158) versus 0.23 per 1,000 (1/4,298), contrary to the increased risk of maternal mortality from P. falciparum compared to P. vivax, 2.89 per 1,000 (12/4,158) versus 0.23 per 1,000 (1/4,298), P = 0.003.