A Sub-Microscopic Gametocyte Reservoir Can Sustain Malaria Transmission

Novel diagnostic tools, including PCR and high field gradient magnetic fractionation (HFGMF), have improved detection of asexual Plasmodium falciparum parasites and especially infectious gametocytes in human blood. These techniques indicate a significant number of people carry gametocyte densities that fall below the conventional threshold of detection achieved by standard light microscopy (LM).To determine how low-level gametocytemia may affect transmission in present large-scale efforts for P. falciparum control in endemic areas, we developed a refinement of the classical Ross-Macdonald model of malaria transmission by introducing multiple infective compartments to model the potential impact of highly prevalent, low gametocytaemic reservoirs in the population. Models were calibrated using field-based data and several numerical experiments were conducted to assess the effect of high and low gametocytemia on P. falciparum transmission and control. Special consideration was given to the impact of long-lasting insecticide-treated bed nets (LLIN), presently considered the most efficient way to prevent transmission, and particularly LLIN coverage similar to goals targeted by the Roll Back Malaria and Global Fund malaria control campaigns. Our analyses indicate that models which include only moderate-to-high gametocytemia (detectable by LM) predict finite eradication times after LLIN introduction. Models that include a low gametocytemia reservoir (requiring PCR or HFGMF detection) predict much more stable, persistent transmission. Our modeled outcomes result in significantly different estimates for the level and duration of control needed to achieve malaria elimination if submicroscopic gametocytes are included.It will be very important to complement current methods of surveillance with enhanced diagnostic techniques to detect asexual parasites and gametocytes to more accurately plan, monitor and guide malaria control programs aimed at eliminating malaria

A simple method for defining malaria seasonality

BACKGROUND: There is currently no standard way of defining malaria seasonality, resulting in a wide range of definitions reported in the literature. Malaria cases show seasonal peaks in most endemic settings, and the choice and timing for optimal malaria control may vary by seasonality. A simple approach is presented to describe the seasonality of malaria, to aid localized policymaking and targeting of interventions. METHODS: A series of systematic literature reviews were undertaken to identify studies reporting on monthly data for full calendar years on clinical malaria, hospital admission with malaria and entomological inoculation rates (EIR). Sites were defined as having 'marked seasonality' if 75% or more of all episodes occurred in six or less months of the year. A 'concentrated period of malaria' was defined as the six consecutive months with the highest cumulative proportion of cases. A sensitivity analysis was performed based on a variety of cut-offs. RESULTS: Monthly data for full calendar years on clinical malaria, all hospital admissions with malaria, and entomological inoculation rates were available for 13, 18, and 11 sites respectively. Most sites showed year-round transmission with seasonal peaks for both clinical malaria and hospital admissions with malaria, with a few sites fitting the definition of 'marked seasonality'. For these sites, consistent results were observed when more than one outcome or more than one calendar year was available from the same site. The use of monthly EIR data was found to be of limited value when looking at seasonal variations of malaria transmission, particularly at low and medium intensity levels. CONCLUSION: The proposed definition discriminated well between studies with 'marked seasonality' and those with less seasonality. However, a poor fit was observed in sites with two seasonal peaks. Further work is needed to explore the applicability of this definition on a wide-scale, using routine health information system data where possible, to aid appropriate targeting of interventions

Social determinants associated with Giardia duodenalis infection in southern Côte d’Ivoire

The purpose of this investigation was to analyze the association between different social determinants of health at the local scale and Giardia duodenalis infection in diverse settlements in southern Côte d'Ivoire. Stool samples from 306 individuals aged 1-16 years were collected from six rural villages and a small town. Five variables were categorized to classify the increasing risk levels of acquiring intestinal parasites. Giardia prevalences (%) and odds ratios (ORs) were evaluated. Correlation and regression analyses were conducted to determine the correlation coefficients and to propose predictive models based on social determinants to forecast the risk of giardiasis. The overall observed prevalence of Giardia was 21.6 %. When the analysis was conducted at the local level, the percentage of infected people varied from a minimum of 12.7 up to 36.4 %. A significant association (p < 0.001) was found between the selected social determinants and G. duodenalis prevalence in the different localities. Correlation and regression analyses allowed us to describe two predictive models to estimate the OR of Giardia transmission. This study helps to clarify the possible impact of different social determinants of health on the risk of giardiasis at the local scale. Both predictive models could be suitable in order to assess the likelihood of the transmission of intestinal parasitic infection in developing countries