Light depolarization measurements in malaria: A new job for an old friend

The Future in Sensing Technologies for Malaria Surveillance: A Review of Hemozoin-Based Diagnosis

Early and effective malaria diagnosis is vital to control the disease spread and to prevent the emergence of severe cases and death. Currently, malaria diagnosis relies on optical microscopy and immuno-rapid tests; however, these require a drop of blood, are time-consuming, or are not specific and sensitive enough for reliable detection of low-level parasitaemia. Thus, there is an urge for simpler, prompt, and accurate alternative diagnostic methods. Particularly, hemozoin has been increasingly recognized as an attractive biomarker for malaria detection. As the disease proliferates, parasites digest host hemoglobin, in the process releasing toxic haem that is detoxified into an insoluble crystal, the hemozoin, which accumulates along with infection progression. Given its magnetic, optical, and acoustic unique features, hemozoin has been explored for new label-free diagnostic methods. Thereby, herein, we review the hemozoin-based malaria detection methods and critically discuss their challenges and potential for the development of an ideal diagnostic device.

Efficient monitoring of the blood-stage infection in a malaria rodent model by the rotating-crystal magneto-optical method

Intense research efforts have been focused on the improvement of the efficiency and sensitivity of malaria diagnostics, especially in resource-limited settings for the detection of asymptomatic infections. Our recently developed magneto-optical (MO) method allows the accurate quantification of malaria pigment crystals (hemozoin) in blood by their magnetically induced rotation. First evaluations of the method using β-hematin crystals and in vitro P. falciparum cultures implied its potential for high-sensitivity malaria diagnosis. To further investigate this potential, here we study the performance of the method in monitoring the in vivo onset and progression of the blood-stage infection in a rodent malaria model. Our results show that the MO method can detect the first generation of intraerythrocytic P. berghei parasites 66–76 hours after sporozoite injection, demonstrating similar sensitivity to Giesma-stained light microscopy and exceeding that of flow cytometric techniques. Magneto-optical measurements performed during and after the treatment of P. berghei infections revealed that both the follow up under treatment and the detection of later reinfections are feasible with this new technique. The present study demonstrates that the MO method – besides being label and reagent-free, automated and rapid – has a high in vivo sensitivity and is ready for in-field evaluation.

Malaria diagnosis for malaria elimination

PURPOSE OF REVIEW

Limitations of blood smear microscopy contributed to failure of the 1950-1960s WHO Global Programme to Eliminate Malaria. All diagnostic methods encounter limits of detection (LOD) beyond which it will not be possible to identify infected individuals. When this occurs, it becomes difficult to continue evaluating progress of malaria elimination. The purpose of this review is to compare available diagnostic technologies, factors that underlie their LOD, and their potential roles related to the goal of elimination.

RECENT FINDINGS

Parasite-containing cells, parasite proteins, hemozoin, nucleic acids, and parasite-specific human antibodies are targets of diagnosis. Many studies report advantages of technologies to detect these diagnostic targets. Nucleic acid amplification tests and strategies for enriching capture of malaria diagnostic targets have consistently identified a parasite reservoir not detected by methods focused on the other biological targets. Exploiting magnetic properties of hemozoin may open new strategies for noninvasive malaria diagnosis.

SUMMARY

Microscopy and rapid diagnostic tests provide effective surveillance for malaria control. Strategies that detect a reservoir of submicroscopic infection must be developed and standardized to guide malaria elimination.

Hemozoin Detection for Human Malaria Diagnosis Investigated in Rodent Models: How Similar Is Similar?

New malaria diagnostic methods based on hemozoin (Hz) detection have been reported recently and were investigated in rodent models. These models are likely to produce unduly favorable results compared to the reality of Plasmodium falciparum malaria. Thus, for malaria diagnostics, results from rodent malaria experiments must be interpreted with caution.

Assessing anti-malarial drug effects ex vivo using the haemozoin detection assay

Background

In vitro sensitivity assays are crucial to detect and monitor drug resistance. Plasmodium falciparum has developed resistance to almost all anti-malarial drugs. Although different in vitro drug assays are available, some of their inherent characteristics limit their application, especially in the field. A recently developed approach based on the flow cytometric detection of haemozoin (Hz) allowed reagent-free monitoring of parasite maturation and detection of drug effects in culture-adapted parasites. In this study, the set-up, performance and usefulness of this novel assay were investigated under field conditions in Gabon.

Methods

An existing flow cytometer (Cyflow Blue) was modified on site to detect light depolarization caused by Hz. Blood from malaria patients was incubated for 72 hrs with increasing concentrations of chloroquine, artesunate and artemisinin. The percentage of depolarizing red blood cells (RBC) was used as maturation indicator and measured at 24, 48 and 72 hrs of incubation to determine parasite growth and drug effects.

Results

The flow cytometer was easily adapted on site to detect light depolarization caused by Hz. Analysis of ex vivo cultures of parasites, obtained from blood samples of malaria patients, showed four different growth profiles. In 39/46 samples, 50% inhibitory concentrations (IC50) were successfully determined. IC50 values for chloroquine were higher than 200 nM in 70% of the samples, indicating the presence of chloroquine-resistant parasites. For artesunate and artemisinin, IC50 values ranged from 0.9 to 60 nM and from 2.2 nM to 124 nM, respectively, indicating fully sensitive parasites.

Conclusion

Flow cytometric detection of Hz allowed the detection of drug effects in blood samples from malaria patients, without using additional reagents or complex protocols. Adjustment of the initial parasitaemia was not required, which greatly simplifies the protocol, although it may lead to different IC50 values. Further investigation of set-up conditions of the Hz assay, as well as future studies in various settings should be performed to further determine the usefulness of this assay as a tool for rapid resistance testing in malaria-endemic countries.