BackgroundIn falciparum malaria sequestration of erythrocytes containing mature forms of Plasmodium falciparum in the microvasculature of vital organs is central to pathology, but quantitation of this hidden sequestered parasite load in vivo has not previously been possible. The peripheral blood parasite count measures only the circulating, relatively non-pathogenic parasite numbers. P. falciparum releases a specific histidine-rich protein (PfHRP2) into plasma. Quantitative measurement of plasma PfHRP2 concentrations may reflect the total parasite biomass in falciparum malaria.Methods and FindingsWe measured plasma concentrations of PfHRP2, using a quantitative antigen-capture enzyme-linked immunosorbent assay, in 337 adult patients with falciparum malaria of varying severity hospitalised on the Thai–Burmese border. Based on in vitro production rates, we constructed a model to link this measure to the total parasite burden in the patient. The estimated geometric mean parasite burden was 7 × 1011 (95% confidence interval [CI] 5.8 × 1011 to 8.5 × 1011) parasites per body, and was over six times higher in severe malaria (geometric mean 1.7 × 1012, 95% CI 1.3 × 1012 to 2.3 × 1012) than in patients hospitalised without signs of severity (geometric mean 2.8 × 1011, 95% CI 2.3 × 1011 to 3.5 × 1011; p < 0.001). Parasite burden was highest in patients who died (geometric mean 3.4 × 1012, 95% CI 1.9 × 1012 to 6.3 × 1012; p = 0.03). The calculated number of sequestered parasites increased with disease severity and was higher in patients with late developmental stages of P. falciparum present on peripheral blood smears. Comparing model and laboratory estimates of the time of sequestration suggested that admission to hospital with uncomplicated malaria often follows schizogony—but in severe malaria is unrelated to stage of parasite development.ConclusionPlasma PfHRP2 concentrations may be used to estimate the total body parasite biomass in acute falciparum malaria. Severe malaria results from extensive sequestration of parasitised erythrocytes.

Because of their sequestration in the microcirculation, the pathogenic late stages of Plasmodium falciparum are under-represented in peripheral blood samples from patients with falciparum malaria. Excreted products of the parasite might help to estimate this sequestered biomass. We quantified the stage-dependent production and release per parasite of P. falciparum histidine-rich protein 2 (PfHRP2) with the objective of measuring the sequestered biomass. A simple method to relate parasite stage to parasite age was developed to facilitate this. In four isolates of P. falciparum, the median (range) PfHRP2 content was 2.0fg (0.5-4.3fg) for a young ring stage infected erythrocyte, and 5.4fg (2.1-10.2fg) for the schizont stage. The amount of PfHRP2 in the parasitized erythrocyte increased most during development to the mature trophozoite stage. The median (range) amount of PfHRP2 secreted per parasite per entire erythrocytic cycle was 5.2fg (1.1-13.0fg). A median of 89% of the total PfHRP2 was excreted at the moment of schizont rupture. This assessment of the stage-dependent release of PfHRP2 is an essential prerequisite for future studies aimed at estimating the total patient parasite mass from the peripheral blood PfHRP2 concentration.

The history of phage therapy started with its first clinical application in 1919 and continues its development to this day. Phages continue to lack any market approval in Western medicine as a recognized drug, but are increasingly used as an experimental therapy for the compassionate treatment of patients experiencing antibiotic failure. The few formal experimental phage clinical trials that have been completed to date have produced inconclusive results on the efficacy of phage therapy, which contradicts the many successful treatment outcomes observed in historical accounts and recent individual case reports. It would therefore be wise to identify why such a discordance exists between trials and compassionate use in order to better develop future phage treatment and clinical applications. The multitude of observations reported over the years in the literature constitutes an invaluable experience, and we add to this by presenting a number of cases of patients treated compassionately with phages throughout the past decade with a focus on osteoarticular infections. Additionally, an abundance of scientific literature into phage-related areas is transforming our knowledge base, creating a greater understanding that should be applied for future clinical applications. Due to the increasing number of treatment failures anticipatedfrom the perspective of a possible post-antibiotic era, we believe that the introduction of bacteriophages into the therapeutic arsenal seems a scientifically sound and eminently practicable consideration today as a substitute or adjuvant to antibiotic therapy.

We describe the success of adjunctive bacteriophage therapy for refractory Pseudomonas aeruginosa urinary tract infection in the context of bilateral ureteric stents and bladder ulceration, after repeated failure of antibiotics alone. No bacteriophage-resistant bacteria arose, and the kinetics of bacteriophage and bacteria in urine suggest self-sustaining and self-limiting infection.

Increasing development of antimicrobial resistance is driving a resurgence in interest in phage therapy: the use of bacteriophages to treat bacterial infections. As the lytic action of bacteriophages is unaffected by the antibiotic resistance status of their bacterial target, it is thought that phage therapy may have considerable potential in the treatment of a wide range of topical and localized infections. As yet this interest has not extended to intravenous (IV) use, which is surprising given that the historical record shows that phages are likely to be safe and effective when delivered by this route. Starting almost 100 years ago, phages were administered intravenously in treatment of systemic infections including typhoid, and Staphylococcal bacteremia. There was extensive IV use of phages in the 1940s to treat typhoid, reportedly with outstanding efficacy and safety. The safety of IV phage administration is also underpinned by the detailed work of Ochs and colleagues in Seattle who have over four decades' experience with IV injection into human subjects of large doses of highly purified coliphage PhiX174. Though these subjects included a large number of immune-deficient children, no serious side effects were observed over this extended time period. The large and continuing global health problems of typhoid and Staphylococcus aureus are exacerbated by the increasing antibiotic resistance of these pathogens. We contend that these infections are excellent candidates for use of IV phage therapy.