G.J.A. van Gemert

Infectivity of cultured Plasmodium falciparum gametocytes to mosquitoes.

Various factors that may influence routine and high levels of mosquito infection with cultured Plasmodium falciparum gametocytes are considered in this paper. One of the most important is the choice of an appropriate isolate, with facilities for cryopreservation and a good technique for initiation of cultures. The use of automated culture systems with strict adherence to detail and routine has eliminated much of the variability. The quality of the serum used for the culture of gametocytes and inclusion in the feed material for mosquitoes is of the highest importance. Blood collection for culture purposes must preferably involve alcohol as an antiseptic for cleaning donor skin or suitable receptacles. Mosquito blood meals should not include plasma with citrate phosphate dextrose or sera collected in microtainer tubes or from volunteers on proguanil-chloroquine prophylaxis. Sera of individuals on chloroquine alone do not influence transmission. Haematocrits of from 5 to 10% permit the culture of equally infective gametocytes. It was impossible to predict the outcome of an infection in mosquitoes based on the number of female gametocytes or gametes. Within any experiment, the oocyst load initially increased, followed by a decline with progressively lower numbers of gametocytes accompanied by a progressive increase in the efficiency of transmission. Some of the variability of mosquito infection within an experiment was due to individual differences in the speed of blood digestion of the mosquitoes. A new membrane feeder is described with three different sizes to accommodate a variety of goals.(ABSTRACT TRUNCATED AT 250 WORDS)

Feeding behaviour and sporozoite ejection by infected Anopheles stephensi.

Anopheles stephensi mosquitoes infected with Plasmodium falciparum sporozoites were allowed to feed individually through fresh whole thickness mouse skin. More sporozoites were ejected into the skin in clusters than into the blood. Deposition of sporozoites in the blood was an infrequent occurrence and always coincided with ejection of these stages into the skin--perhaps a spill-over effect. The number of probes before feeding (median 4.5) was not correlated with the sporozoite inoculum (median 8), nor was the number of sporozoites in the glands (median 14,500). However, the number of sporozoite clusters in the skin (median 1) was positively correlated with the inoculum size. The median value of the sporozoite inoculum was 22, when only those mosquitoes that ejected sporozoites were included. When feeding was interrupted and recommended on a new membrane, sporozoite ejection occurred with equal frequency on both occasions. Sporozoites disappeared from the site of bites in living mice within 2 h of feeding. The epidemiological significance of these observations is discussed.

The Spiroindolone Drug Candidate NITD609 Potently Inhibits Gametocytogenesis and Blocks Plasmodium falciparum Transmission to Anopheles Mosquito Vector

ABSTRACT The global malaria agenda has undergone a reorientation from control of clinical cases to entirely eradicating malaria. For that purpose, a key objective is blocking transmission of malaria parasites from humans to mosquito vectors. The new antimalarial drug candidate NITD609 was evaluated for its transmission-reducing potential and compared to a few established antimalarials (lumefantrine, artemether, primaquine), using a suite of in vitro assays. By the use of a microscopic readout, NITD609 was found to inhibit the early and late development of Plasmodium falciparum gametocytes in vitro in a dose-dependent fashion over a range of 5 to 500 nM. In addition, using the standard membrane feeding assay, NITD609 was also found to be a very effective drug in reducing transmission to the Anopheles stephensi mosquito vector. Collectively, our data suggest a strong transmission-reducing effect of NITD609 acting against different P. falciparum transmission stages.

Plasmodium falciparum ookinetes migrate intercellularly through Anopheles stephensi midgut epithelium.

The migration ofPlasmodium falciparum andP. berghei ookinetes through the midgut epithelium inAnopheles stephensi was studied by transmission electron microscopy. With ruthenium red (RR) staining, the results of previous studies were confirmed:P. falciparum ookinetes take an intercellular route through the midgut epithelium. In the same mosquito species, the rodent parasiteP. berghei appeared to take an intracellular position, as previously suggested by other authors. The intra- or intercellular ookinete migration ofP. berghei orP. falciparum, respectively, can perhaps be related to the higher mortality ofP. berghei-infected mosquitoes within the first 2 days of infection. Evidence is presented that oocyst capsule formation begins as early as during the migration of the ookinete. After localization between the epithelial cells and the midgut basal lamina, the rapidly expanding oocyst stretches the overlying layer of the latter at the haemocoelic surface while a new basal lamina is generated between the oocyst and epithelial cell.

Measurement by membrane feeding of reduction in Plasmodium falciparum transmission induced by endemic sera.

The standard laboratory test for reduction in malaria transmission is based on the measurement of oocyst numbers in mosquitoes fed on blood meals containing test and control sera. Interpretation of the results, however, is often hampered by the large variation in numbers of infected mosquitoes and oocysts. The objective of this study was to compare 3 measures for the assessment of transmission reduction (so-called R values) and to define the experimental criteria that allow interpretation of the results. To determine variability in R values of control sera, a replicate experiment was performed with 10 non-endemic sera of Dutch blood donors. Furthermore, 2 measures for calculation of transmission reduction were compared in a triplicate experiment using Plasmodium falciparum, Anopheles gambiae and malaria endemic sera. Calculations using the geometric mean of Williams are currently used to identify blocking and non-blocking sera. However, calculations using log-transformed data could distinguish more gradual levels of transmission reduction activity by endemic sera--i.e. blocking, reducing and non-blocking activity. Grading of transmission reduction activity is important for epidemiological studies on transmission immunity and for validation of future transmission-blocking vaccines.

A Scalable Assessment of Plasmodium falciparum Transmission in the Standard Membrane-Feeding Assay, Using Transgenic Parasites Expressing Green Fluorescent Protein–Luciferase

BACKGROUND The development of drugs and vaccines to reduce malaria transmission is an important part of eradication plans. The transmission-reducing activity (TRA) of these agents is currently determined in the standard membrane-feeding assay (SMFA), based on subjective microscopy-based readouts and with limitations in upscaling and throughput. METHODS Using a Plasmodium falciparum strain expressing the firefly luciferase protein, we present a luminescence-based approach to SMFA evaluation that eliminates the requirement for mosquito dissections in favor of a simple approach in which whole mosquitoes are homogenized and examined directly for luciferase activity. RESULTS Analysis of 6860 Anopheles stephensi mosquitoes across 68 experimental feeds shows that the luminescence assay was as sensitive as microscopy for infection detection. The mean luminescence intensity of individual and pooled mosquitoes accurately quantifies mean oocyst intensity and generates comparable TRA estimates. The luminescence assay presented here could increase SMFA throughput so that 10-30 experimental feeds could be evaluated in a single 96-well plate. CONCLUSIONS This new method of assessing Plasmodium infection and transmission intensity could expedite the screening of novel drug compounds, vaccine candidates, and sera from malaria-exposed individuals for TRA. Luminescence-based estimates of oocyst intensity in individual mosquitoes should be interpreted with caution.

Sporozoite load of mosquitoes infected with Plasmodium falciparum

In the laboratory, mosquitoes given a second blood meal 5-11 d after an infective one have more sporozoites in their salivary glands than do those given a single infective blood meal only. The presence of specific anti-sporozoite antibody in the second blood meal does not reduce the number of sporozoites in salivary glands. On the contrary, the presence of the raised immunoglobulin levels--even non-specific ones--may result in higher gland infections. Oocyst maturation is extremely asynchronous in mosquitoes given a single blood meal, the maturation time being 10-22 d or more. The explanation for the increased density of sporozoites in salivary glands in mosquitoes having a second blood meal may be acceleration of oocyst maturation. Multiple blood meals are a normal event for infectious mosquitoes in nature, and therefore have no special epidemiological significance. However, in the laboratory a second blood meal could be a simple procedure for increasing the efficiency of sporozoite production.

A combination of new screening assays for prioritization of transmission-blocking antimalarials reveals distinct dynamics of marketed and experimental drugs

OBJECTIVES The development of drugs to reduce malaria transmission is an important part of malaria eradication plans. We set out to develop and validate a combination of new screening assays for prioritization of transmission-blocking molecules. METHODS We developed high-throughput assays for screening compounds against gametocytes, the parasite stages responsible for onward transmission to mosquitoes. An existing gametocyte parasitic lactate dehydrogenase (pLDH) assay was adapted for use in 384-well plates, and a novel homogeneous immunoassay to monitor the functional transition of female gametocytes into gametes was developed. A collection of 48 marketed and experimental antimalarials was screened and subsequently tested for impact on sporogony in Anopheles mosquitoes, to directly quantify the transmission-blocking properties of antimalarials in relation to their effects on gametocyte pLDH activity or gametogenesis. RESULTS AND CONCLUSIONS The novel screening assays revealed distinct stage-specific kinetics and dynamics of drug effects. Peroxides showed the most potent transmission-blocking effects, with an intermediate speed of action and IC50 values that were 20-40-fold higher than the IC50s against the asexual stages causing clinical malaria. Finally, the novel synthetic peroxide OZ439 appeared to be a promising drug candidate as it exerted gametocytocidal and transmission-blocking effects at clinically relevant concentrations.

A semi-automated luminescence based standard membrane feeding assay identifies novel small molecules that inhibit transmission of malaria parasites by mosquitoes

Current first-line treatments for uncomplicated falciparum malaria rapidly clear the asexual stages of the parasite, but do not fully prevent parasite transmission by mosquitoes. The standard membrane feeding assay (SMFA) is the biological gold standard assessment of transmission reducing activity (TRA), but its throughput is limited by the need to determine mosquito infection status by dissection and microscopy. Here we present a novel dissection-free luminescence based SMFA format using a transgenic Plasmodium falciparum reporter parasite without resistance to known antimalarials and therefore unrestricted in its utility in compound screening. Analyses of sixty-five compounds from the Medicines for Malaria Venture validation and malaria boxes identified 37 compounds with high levels of TRA (>80%); different assay modes allowed discrimination between gametocytocidal and downstream modes of action. Comparison of SMFA data to published assay formats for predicting parasite infectivity indicated that individual in vitro screens show substantial numbers of false negatives. These results highlight the importance of the SMFA in the screening pipeline for transmission reducing compounds and present a rapid and objective method. In addition we present sixteen diverse chemical scaffolds from the malaria box that may serve as a starting point for further discovery and development of malaria transmission blocking drugs.

The Relative Contribution of Symptomatic and Asymptomatic Plasmodium vivax and Plasmodium falciparum Infections to the Infectious Reservoir in a Low-Endemic Setting in Ethiopia

Background The majority of Plasmodium vivax and Plasmodium falciparum infections in low-endemic settings are asymptomatic. The relative contribution to the infectious reservoir of these infections compared to clinical malaria cases is currently unknown. Methods We assessed infectivity of passively recruited symptomatic malaria patients (n = 41) and community-recruited asymptomatic individuals with microscopy-detected (n = 41) and polymerase chain reaction (PCR)-detected infections (n = 82) using membrane feeding assays with Anopheles arabiensis mosquitoes in Adama, Ethiopia. Malaria incidence and prevalence data were used to estimate the contributions of these populations to the infectious reservoir. Results Overall, 34.9% (29/83) of P. vivax- and 15.1% (8/53) P. falciparum-infected individuals infected ≥1 mosquitoes. Mosquito infection rates were strongly correlated with asexual parasite density for P. vivax (ρ = 0.63; P < .001) but not for P. falciparum (ρ = 0.06; P = .770). Plasmodium vivax symptomatic infections were more infectious to mosquitoes (infecting 46.5% of mosquitoes, 307/660) compared to asymptomatic microscopy-detected (infecting 12.0% of mosquitoes, 80/667; P = .005) and PCR-detected infections (infecting 0.8% of mosquitoes, 6/744; P < .001). Adjusting for population prevalence, symptomatic, asymptomatic microscopy-detected, and PCR-detected infections were responsible for 8.0%, 76.2%, and 15.8% of the infectious reservoir for P. vivax, respectively. For P. falciparum, mosquito infections were sparser and also predominantly from asymptomatic infections. Conclusions In this low-endemic setting aiming for malaria elimination, asymptomatic infections were highly prevalent and responsible for the majority of onward mosquito infections. The early identification and treatment of asymptomatic infections might accelerate elimination efforts.