Gabriela Zollner

Bio: Gabriela Zollner is an academic researcher from Walter Reed Army Institute of Research. The author has contributed to research in topics: Plasmodium vivax & Population. The author has an hindex of 7, co-authored 12 publications receiving 401 citations. Previous affiliations of Gabriela Zollner include United States Department of the Army.

Comparison of PCR and microscopy for the detection of asymptomatic malaria in a Plasmodium falciparum/vivax endemic area in Thailand

Abstract: The main objective of this study was to compare the performance of nested PCR with expert microscopy as a means of detecting Plasmodium parasites during active malaria surveillance in western Thailand. The study was performed from May 2000 to April 2002 in the village of Kong Mong Tha, located in western Thailand. Plasmodium vivax (PV) and Plasmodium falciparum (PF) are the predominant parasite species in this village, followed by Plasmodium malariae (PM) and Plasmodium ovale (PO). Each month, fingerprick blood samples were taken from each participating individual and used to prepare thick and thin blood films and for PCR analysis. PCR was sensitive (96%) and specific (98%) for malaria at parasite densities ≥ 500/μl; however, only 18% (47/269) of P. falciparum- and 5% (20/390) of P. vivax-positive films had parasite densities this high. Performance of PCR decreased markedly at parasite densities <500/μl, with sensitivity of only 20% for P. falciparum and 24% for P. vivax at densities <100 parasites/μl. Although PCR performance appeared poor when compared to microscopy, data indicated that the discrepancy between the two methods resulted from poor performance of microscopy at low parasite densities rather than poor performance of PCR. These data are not unusual when the diagnostic method being evaluated is more sensitive than the reference method. PCR appears to be a useful method for detecting Plasmodium parasites during active malaria surveillance in Thailand.

Population dynamics of sporogony for Plasmodium vivax parasites from western Thailand developing within three species of colonized Anopheles mosquitoes.

Abstract: The population dynamics of Plasmodium sporogony within mosquitoes consists of an early phase where parasite abundance decreases during the transition from gametocyte to oocyst, an intermediate phase where parasite abundance remains static as oocysts, and a later phase where parasite abundance increases during the release of progeny sporozoites from oocysts. Sporogonic development is complete when sporozoites invade the mosquito salivary glands. The dynamics and efficiency of this developmental sequence were determined in laboratory strains of Anopheles dirus, Anopheles minimus and Anopheles sawadwongporni mosquitoes for Plasmodium vivax parasites circulating naturally in western Thailand. Mosquitoes were fed blood from 20 symptomatic Thai adults via membrane feeders. Absolute densities were estimated for macrogametocytes, round stages (= female gametes/zygotes), ookinetes, oocysts, haemolymph sporozoites and salivary gland sporozoites. From these census data, five aspects of population dynamics were analysed; 1) changes in life-stage prevalence during early sporogony, 2) kinetics of life-stage formation, 3) efficiency of life-stage transitions, 4) density relationships between successive life-stages, and 5) parasite aggregation patterns. There was no difference among the three mosquito species tested in total losses incurred by P. vivax populations during early sporogony. Averaged across all infections, parasite populations incurred a 68-fold loss in abundance, with losses of ca. 19-fold, 2-fold and 2-fold at the first (= gametogenesis/fertilization), second (= round stage transformation), and third (= ookinete migration) life-stage transitions, respectively. However, total losses varied widely among infections, ranging from 6-fold to over 2,000-fold loss. Losses during gametogenesis/fertilization accounted for most of this variability, indicating that gametocytes originating from some volunteers were more fertile than those from other volunteers. Although reasons for such variability were not determined, gametocyte fertility was not correlated with blood haematocrit, asexual parasitaemia, gametocyte density or gametocyte sex ratio. Round stages and ookinetes were present in mosquito midguts for up to 48 hours and development was asynchronous. Parasite losses during fertilization and round stage differentiation were more influenced by factors intrinsic to the parasite and/or factors in the blood, whereas ookinete losses were more strongly influenced by mosquito factors. Oocysts released sporozoites on days 12 to 14, but even by day 22 many oocysts were still present on the midgut. The per capita production was estimated to be approximately 500 sporozoites per oocyst and approximately 75% of the sporozoites released into the haemocoel successfully invaded the salivary glands. The major developmental bottleneck in early sporogony occurred during the transition from macrogametocyte to round stage. Sporozoite invasion into the salivary glands was very efficient. Information on the natural population dynamics of sporogony within malaria-endemic areas may benefit intervention strategies that target early sporogony (e.g., transmission blocking vaccines, transgenic mosquitoes).

Imidacloprid as a Potential Agent for the Systemic Control of Sand Flies

Abstract: Our goal was to study the effectiveness of the insecticide imidacloprid as a systemic control agent. First, to evaluate the blood-feeding effect, we fed adult female Phlebotomus papatasi with imidacloprid-treated rabbit blood and monitored blood-feeding success and survival. Second, to evaluate the feed-through effectiveness of this insecticide, we fed laboratory rats and sand rats with insecticide-treated food and evaluated the survival of sand fly larvae feeding on rodents' feces. In the blood-feeding experiment, 89.8% mortality was observed with the higher dose (5 mg/ml) and 81.3% with the lower dose (1 mg/ml). In the larvicide experiments, both sand fly species demonstrated a typical dose-response curve with the strongest lethal effect for the 250 ppm samples. Lutzomyia longipalpis larvae, however, were less sensitive. In all experiments, 1(st) instar larvae were more sensitive than the older stages. First instar P. papatasi larvae feeding on sand rat feces passed the larvicidal threshold of 90% mortality at doses higher than 50 ppm. In comparison, in older stages 90% mortality was obtained with a dose of only 250 ppm. Overall, results support the feasibility of imidacloprid as a systemic control agent that takes advantage of the tight ecological association between the reservoir host and the sand fly vector.

Comparison of three carbon dioxide sources on phlebotomine sand fly capture in Egypt

Abstract: Lighted Centers for Disease Control and Prevention (CDC) light traps were baited with carbon dioxide (CO2) produced from three different sources to compare the efficacy of each in collecting phlebotomine sand flies in Bahrif village, Aswan Governorate, Egypt. Treatments consisted of compressed CO2 gas released at a rate of 250 ml/min, 1.5 kg of dry ice (replaced daily) sublimating from an insulated plastic container, CO2 gas produced from a prototype FASTGAS (FG) CO2 generator system (APTIV Inc., Portland, OR), and a CDC light trap without a CO2 source. Carbon dioxide was released above each treatment trap's catch opening. Traps were placed in a 4 x 4 Latin square designed study with three replications completed after four consecutive nights in August 2007. During the study, 1,842 phlebotomine sand flies were collected from two genera and five species. Traps collected 1,739 (94.4%) Phlebotomus papatasi (Scopoli), 19 (1.0%) Phlebotomus sergenti, 64 (3.5%) Sergentomyia schwetzi, 16 (0.9%) Sergentomyia palestinensis, and four (0.2%) Sergentomyia tiberiadis. Overall treatment results were dry ice (541) > FG (504) > compressed gas (454) > no CO2 (343). Total catches of P. papatasi were not significantly different between treatments, although CO2-baited traps collected 23-34% more sand flies than the unbaited (control) trap. Results indicate that the traps baited with a prototype CO2 generator were as attractive as traps supplied with CO2 sources traditionally used in sand fly surveillance efforts. Field-deployable CO2 generators are particularly advantageous in remote areas where dry ice or compressed gas is difficult to obtain.

Update on Rapid Diagnostic Testing for Malaria

Abstract: To help mitigate the expanding global impact of malaria, with its associated increasing drug resistance, implementation of prompt and accurate diagnosis is needed. Malaria is diagnosed predominantly by using clinical criteria, with microscopy as the current gold standard for detecting parasitemia, even though it is clearly inadequate in many health care settings. Rapid diagnostic tests (RDTs) have been recognized as an ideal method for diagnosing infectious diseases, including malaria, in recent years. There have been a number of RDTs developed and evaluated widely for malaria diagnosis, but a number of issues related to these products have arisen. This review highlights RDTs, including challenges in assessing their performance, internationally available RDTs, their effectiveness in various health care settings, and the selection of RDTs for different health care systems.

Submicroscopic Infection in Plasmodium falciparum-Endemic Populations: A Systematic Review and Meta-Analysis

Abstract: Introduction. Light microscopy examination of blood slides is the main method of detecting malaria infection; however, it has limited sensitivity. Low-density infections are most likely to be missed, but they contribute to the infectious reservoir. Quantifying these submicroscopic infections is therefore key to understanding transmission dynamics and successfully reducing parasite transmission. Methods. We conducted a systematic review of endemic population surveys in which P. falciparum prevalence had been measured by both microscopy and a more-sensitive polymerase chain reaction (PCR)-based technique. The combined microscopy: PCR prevalence ratio was estimated by random-effects meta-analysis, and the effect of covariates was determined by meta-regression. Results. Seventy-two pairs of prevalence measurements were included in the study. The prevalence of infection measured by microscopy was, on average, 50.8% (95% confidence interval [CI], 45.2%-57.1%) of that measured by PCR. For gametocyte-specific detection, the microscopy prevalence was, on average, 8.7% (95% CI, 2.8%-26.6%) of the prevalence measured by PCR. A significantly higher percentage of total infections was detected by microscopy in areas of high, compared with low, transmission (74.5% when the prevalence determined by PCR was >75% versus 12.0% when the prevalence determined by PCR was <10%). Discussion. Microscopy can miss a substantial proportion of P. falciparum infections in surveys of endemic populations, especially in areas with low transmission of infection. The extent of the submicroscopic reservoir needs to be taken into account for effective surveillance and control.

Multilevel and Longitudinal Modeling Using Stata, 3rd Edition

Abstract: This text is a Stata-specific treatment of generalized linear mixed models, also known as multilevel or hierarchical models. These models are "mixed" in the sense that they allow fixed and random effects and are "generalized" in the sense that they are appropriate not only for continuous Gaussian responses but also for binary, count, and other types of limited dependent variables.

The international limits and population at risk of Plasmodium vivax transmission in 2009

Abstract: Background: A research priority for Plasmodium vivax malaria is to improve our understanding of the spatial distribution of risk and its relationship with the burden of P. vivax disease in human populations. The aim of the research outlined in this article is to provide a contemporary evidence-based map of the global spatial extent of P. vivax malaria, together with estimates of the human population at risk (PAR) of any level of transmission in 2009. Methodology: The most recent P. vivax case-reporting data that could be obtained for all malaria endemic countries were used to classify risk into three classes: malaria free, unstable (,0.1 case per 1,000 people per annum (p.a.)) and stable ($0.1 case per 1,000 p.a.) P. vivax malaria transmission. Risk areas were further constrained using temperature and aridity data based upon their relationship with parasite and vector bionomics. Medical intelligence was used to refine the spatial extent of risk in specific areas where transmission was reported to be absent (e.g., large urban areas and malaria-free islands). The PAR under each level of transmission was then derived by combining the categorical risk map with a high resolution population surface adjusted to 2009. The exclusion of large Duffy negative populations in Africa from the PAR totals was achieved using independent modelling of the gene frequency of this genetic trait. It was estimated that 2.85 billion people were exposed to some risk of P. vivax transmission in 2009, with 57.1% of them living in areas of unstable transmission. The vast majority (2.59 billion, 91.0%) were located in Central and South East (CSE) Asia, whilst the remainder were located in America (0.16 billion, 5.5%) and in the Africa+ region (0.10 billion, 3.5%). Despite evidence of ubiquitous risk of P. vivax infection in Africa, the very high prevalence of Duffy negativity throughout Central and West Africa reduced the PAR estimates substantially. Conclusions: After more than a century of development and control, P. vivax remains more widely distributed than P. falciparum and is a potential cause of morbidity and mortality amongst the 2.85 billion people living at risk of infection, the majority of whom are in the tropical belt of CSE Asia. The probability of infection is reduced massively across Africa by the frequency of the Duffy negative trait, but transmission does occur on the continent and is a concern for Duffy positive locals and travellers. The final map provides the spatial limits on which the endemicity of P. vivax transmission can be mapped to support future cartographic-based burden estimations.

The limits and intensity of Plasmodium falciparum transmission: implications for malaria control and elimination worldwide

Abstract: Background The efficient allocation of financial resources for malaria control using appropriate combinations of interventions requires accurate information on the geographic distribution of malaria risk. An evidence-based description of the global range of Plasmodium falciparum malaria and its endemicity has not been assembled in almost 40 y. This paper aims to define the global geographic distribution of P. falciparum malaria in 2007 and to provide a preliminary description of its transmission intensity within this range.