Diagnosis of Plasmodium vivax malaria using a specific deoxyribonucleic acid probe

Use of PCR in the field

Polymerase chain reaction (PCR) represents a powerful new technology with a variety of field applications. While most of these are still experimental, implementation of PCR-based detection of Onchocerca volvulus in black flies, and subspecific differentiation strongly suggest that potential problems can be overcome. Because of high sensitivity and specificity, PCR provides in some cases the only means to address central parasitological questions, and may well become the 'gold standard' by which other diagnostic techniques are measured. In spite of these advantages, routine implementation of PCR,at present,requires transportation of samples to a central facility for processing, and personnel whose technical competence is high. In addition, reagents are expensive. Robert Barker here weighs up these considerations with regard to the potential utility of PCR assays for some applications.

Devices for rapid diagnosis of Malaria: evaluation of prototype assays that detect Plasmodium falciparum histidine-rich protein 2 and a Plasmodium vivax-specific antigen

The ParaSight F test was developed as a pioneer industry effort in the large-scale, process-controlled production of a device for the rapid diagnosis of malaria. This device performed well in field settings but was limited to the detection of a single malaria species, Plasmodium falciparum. The ParaSight F+V assay advanced upon the ParaSight F test format by incorporating a monoclonal antibody directed against a proprietary Plasmodium vivax-specific antigen, in addition to the antibody directed against P. falciparum histidine-rich protein 2, which was used in the ParaSight F assay. The modified assay was developed to add the capability to detect P. falciparum and P. vivax in a single-test-strip format. The present study evaluated three distinct ParaSight F+V prototypes with samples from symptomatic patients in regions of Thailand and Peru where malaria is endemic. Over a 2-year enrollment period (1998 and 1999), a total of 4,894 patients consented to participation in the study. Compared with the results for duplicate microscopic examinations of Giemsa-stained blood smears as the reference diagnostic standard, each successive prototype showed substantial improvement in performance. The final ParaSight F+V prototype, evaluated in 1999, had an overall sensitivity for detection of asexual P. falciparum parasites of 98%. The sensitivity of the device was 100% for P. falciparum densities of >500 parasites/ micro l, with a sensitivity of 83% for parasite densities of </=500/ micro l. The specificity for the exclusion of P. falciparum was 93%. For P. vivax, the overall sensitivity was 87% for the final 1999 prototype. The sensitivities calculated for different levels of P. vivax parasitemia were 99% for parasite densities of >5,000/ micro l, 92% for parasite densities of 1,001 to 5,000/ micro l, 94% for parasite densities of 501 to 1,000/ micro l, and 55% for parasite densities of 1 to 500/ micro l. The specificity for the exclusion of P. vivax was 87%. The areas under the receiver operating characteristic curves for the diagnostic performance of the assay for the detection of P. falciparum and P. vivax were 0.8907 and 0.8522, respectively. These findings indicate that assays for rapid diagnosis have the potential to enhance diagnostic capabilities in those instances in which skilled microscopy is not readily available.

Alternative polymerase chain reaction method to identify Plasmodium species in human blood samples: the semi-nested multiplex malaria PCR (SnM-PCR)

A simplified protocol for the identification of Plasmodium species by semi-nested multiplex polymerase chain reaction (SnM-PCR) in human blood samples is compared with microscopical examination of thin and thick blood films in 2 field trials in Côte d'Ivoire and Cameroon. Also, dried blood spots or liquid blood collected from Dutch soldiers returning from Goma, Zaire (n = 141), Angola (n = 40), and from Marechaussee (Dutch border police) returning from various parts of the world (n = 161) were examined, together with miscellaneous other material obtained from laboratories and hospitals. The method is based on features of the small subunit nuclear ribosomal ribonucleic acid (RNA) gene (ssrDNA), a multicopy gene which possesses both highly conserved domains and domains characteristic for each of the 4 human malaria parasites. The first reaction of the SnM-PCR includes a universal reverse primer with 2 forward primers specific for Plasmodium and mammals, respectively. The mammalian-specific primer was included as a positive control to distinguish uninfected cases from simple PCR failures. The second PCR reaction includes a Plasmodium-specific forward primer plus species-specific reverse primers for P. vivax, P. ovale, P. falciparum and P. malariae. The technique worked better with samples collected in the field as dried blood spots on filter paper and heparinized blood rather than with frozen pelleted blood; it was more sensitive and more specific than the standard microscopical examination.

Molecular methods for diagnosis and epidemiological studies of parasitic infections

Direct microscopy is widely used for the diagnosis of parasitic infections although it often requires an experienced microscopist for accurate diagnosis, is labour intensive and not very sensitive. In order to overcome some of these shortcomings, molecular or nucleic acid-based diagnostic methods for parasitic infections have been developed over the past 12 years. The parasites which have been studied with these techniques include the human Plasmodia, Leishmania, the trypanosomes, Toxoplasma gondii, Entamoeba histolytica, Giardia, Trichomonas vaginalis, Cryptosporidium parvum, Taenia, Echinococcus, Brugia malayi, Wuchereria bancrofti, Loa loa and Onchocerca volvulus. Early methods, which involved hybridisation of specific probes (radiolabelled and non-radiolabelled) to target deoxyribonucleic acid (DNA), have been replaced by more sensitive polymerase chain reaction (PCR)-based assays. Other methods, such as PCR-hybridisation assays, PCR-restriction fragment length polymorphism (PCR-RFLP) assays and random amplified polymorphic DNA (RAPD) analysis have also proved valuable for epidemiological studies of parasites. The general principles and development of DNA-based methods for diagnosis and epidemiological studies will be described, with particular reference to malaria. These methods will probably not replace current methods for routine diagnosis of parasitic infections in developing countries where parasitic diseases are endemic, due to high costs. However, they will be extremely useful for genotyping parasite strains and vectors, and for accurate parasite detection in both humans and vectors during epidemiological studies.

Demonstration by the polymerase chain reaction of mixed Plasmodium falciparum and P. vivax infections undetected by conventional microscopy

Mixed malaria infections (Plasmodium falciparum and P. vivax) are suspected to occur at a greater frequency than is detected by conventional light microscopy. To determine this frequency we carried out a prospective 'blinded' comparison of diagnosis by conventional light microscopy and enzymatic amplification of the circumsporozoite gene extracted from dried spotted blood samples. Patients were previously healthy, active duty Thai soldiers assigned to a malaria risk area presenting with malaria. Microscopy (oil immersion objective at 1000 x magnification) involved examination of Giemsa-stained thick and thin blood films by an experienced microscopist. Whole blood samples (25 microliters) dried on filter paper were used for species-specific parasite deoxyribonucleic acid (DNA) amplification by the polymerase chain reaction (PCR) and hybridization with radiolabelled P. falciparum and P. vivax probes. Of 137 consecutive cases of malaria studied, 9% (3/32) of microscopically diagnosed P. falciparum infections and 5% (5/104) of microscopically diagnosed P. vivax infections were found to be mixed by the PCR/DNA probe systems, while 1 case was diagnosed as mixed by both microscopy and PCR. The possibility that malaria patients may have undetected mixed infections should be kept in mind because of the specific therapy required both for P. falciparum and for radical cure of P. vivax.

Imported malaria in Australia

OBJECTIVE

To obtain information on imported malaria in Australia.

DESIGN AND SETTING

Patients with malaria diagnosed by microscopy by laboratories in southeast Queensland and northern New South Wales between October 1987 and December 1988 were included in this study. The chi 2 test was used to analyse data.

PATIENTS

Blood films, details of prophylactic regimens and clinical history were received from 146 patients.

MAIN OUTCOME MEASURES

This study was to determine the percentage of patients infected with each malaria species, where they contracted their infections, whether they had taken appropriate prophylactic agents and to determine the relationship between parasitaemia and previous exposure, symptoms and prophylaxis.

RESULTS

One hundred patients (68.5%) had Plasmodium vivax malaria and 33 patients (22.6%) had Plasmodium falciparum malaria, with 3 (2.1%) infected with both of these malaria species. Ten cases (6.8%) were presumptively diagnosed as infections but diagnosis by microscopy was uncertain. Seventy-four per cent of the P. vivax infections and 85% of the P. falciparum infections were contracted in Papua New Guinea. Patients presented with a range of blood parasite concentrations that did not correlate with their symptoms, previous infection with malaria (P greater than 0.25) or the regular taking of prophylactic drugs (P greater than 0.1). Only 41% of the patients took any prophylactic drugs; the figure for temporary visitors to malarious areas was 54%. Only 11.6% of patients were taking the recommended drugs for the malarious areas they visited or were resident in.

CONCLUSIONS

Constant monitoring of malarial infections is needed to ensure that appropriate advice is given to the traveller. Our study supports the dogma that malaria should be suspected in all patients who have been to malarious areas, regardless of previous malarious attacks, prophylaxis, treatment or the time which has elapsed since leaving the area.