Use of fluorescence resonance energy transfer hybridization probes to evaluate quantitative real-time PCR for diagnosis of ocular toxoplasmosis

Serological and molecular rapid diagnostic tests for Toxoplasma infection in humans and animals

Infection by Toxoplasma gondii is prevalent worldwide. The parasite can infect a broad spectrum of vertebrate hosts, but infection of fetuses and immunocompromised patients is of particular concern. Easy-to-perform, robust, and highly sensitive and specific methods to detect Toxoplasma infection are important for the treatment and management of patients. Rapid diagnostic methods that do not sacrifice the accuracy of the assay and give reproducible results in a short time are highly desirable. In this context, rapid diagnostic tests (RDTs), especially with point-of-care (POC) features, are promising diagnostic methods in clinical microbiology laboratories, especially in areas with minimal laboratory facilities. More advanced methods using microfluidics and sensor technology will be the future trend. In this review, we discuss serological and molecular-based rapid diagnostic tests for detecting Toxoplasma infection in humans as well as animals.

Quantitative Real-Time PCR: Recent Advances

Quantitative real-time polymerase chain reaction is a technique for simultaneous amplification and product quantification of a target DNA as the process takes place in real time in a "closed-tube" system. Although this technique can provide an absolute quantification of the initial template copy number, quantification relative to a control sample or second sequence is typically adequate. The quantification process employs melting curve analysis and/or fluorescent detection systems and can provide amplification and genotyping in a relatively short time. Here we describe the properties and uses of various fluorescent detection systems used for quantification.

Molecular diagnosis of Toxoplasma gondii infection in Libya

Background

Toxoplasma gondii infections are prevalent in humans and animals throughout Libya. Current diagnosis is based on detection of Toxoplasma-specific IgM and IgG. In this study, we established and optimized a diagnostic PCR assay for molecular diagnosis of T. gondii in Libya.

Methods

From January to December, 2010, 177 blood and serum samples were collected from suspected patients. This includes: 140 women who have had spontaneous abortions, 26 HIV-positive patients, nine patients with leukemia and lymphoma, and two infants with ocular infection. Samples were screened for anti-Toxoplasma IgG and IgM antibodies before DNA extraction. The surface antigen gene 2 (SAG2) was targeted in a semi-nested PCR to amplify a 999 bp and a 614 bp fragment in the first and the second run respectively.

Results

A total of 54/140 (38.5 %) women who have had spontaneous abortions, 23/26 (88 %) HIV patients, 6/9 (66.6 %) of the leukaemia and lymphoma patients, and one child with ocular infection were seropositive for anti-Toxoplasma IgG and/or IgM. Genomic DNA was extracted from 38 selected seropositive samples. The PCR was sensitive enough to detect DNA concentration of 12 ng/μL. PCR analysis was performed for 38 selected seropositive patients (16 women who have had spontaneous abortions, 15 positive HIV patients, six leukaemia patients and one child with ocular infection). Our designed primers were successfully amplified in 22/38 (57.9 %) samples; 5/12 (35.7 %) from serum and 17/26 (65.8 %) from whole blood samples. All PCR positive samples were IgG-positive except two samples which were IgM and IgG & IgM-positive serum samples respectively. The semi-nested PCR confirmed five more samples. These included two leukaemia and two HIV-positive whole blood samples and one serum sample from an aborted woman.

Conclusion

The ability of PCR to diagnose active toxoplasmosis is needed in immunocompromised patients and congenital toxoplasmosis cases, especially when serological techniques fail. For the first time in Libya, we established and optimized semi-nested PCR of SAG2 gene. The developed PCR method was able to detect as little as 12 ng/μL of T. gondii DNA and was useful to diagnose the diseases in women who have had spontaneous abortions, HIV-positive patients, patients with leukemia and lymphoma, and infants with ocular infection.

The Toxoplasma MAG1 peptides induce sex-based humoral immune response in mice and distinguish active from chronic human infection

To distinguish active from inactive/chronic infection in Toxoplasma gondii-seropositive individuals, we have developed an enzyme-linked immunosorbent assay (ELISA) using specific peptides derived from Toxoplasma matrix antigen MAG1. We used this assay to measure matrix specific antibodies and pilot studies with infected mice established the validity of two peptides. The immune response against MAG1 occurs in about 12 days postinfection and displays a sex difference later on in mouse model, with males producing higher antibody titers than females. Serum samples from 22 patients with clinical toxoplasmosis and from 26 patients with serological evidence of past exposure to Toxoplasma (more than one year infection history) were analyzed. Both MAG1 peptides detected antibodies significant frequently and robustly from active stage than from the chronic stage of toxoplasmosis. The results indicate that both MAG1 peptides may be used as a tool to differentiate active from inactive infection. It also may be considered in the design of potential vaccines in humans.

Advances in diagnosis of protozoan diseases

Microscopy still remains the gold standard procedure for the diagnosis of many protozoan infections in animals, but the specific identification requires skilled and experienced personnel. Immunoassays, detecting antibodies or specific protozoan antigens, have been developed but often lack sensitivity and specificity due to close relationship between many protozoa. Recent research has focussed almost exclusively on molecular based techniques for the identification and quantification of parasite DNA in samples. Opinion differ on most appropriate targets to use and there are very few diagnostic kits available making comparison between laboratories difficult. Future research needs to focus on robust, cheap field diagnostic assays.

Multicenter comparative evaluation of five commercial methods for toxoplasma DNA extraction from amniotic fluid

Over the past few years, a number of new nucleic acid extraction methods and extraction platforms using chemistry combined with magnetic or silica particles have been developed, in combination with instruments to facilitate the extraction procedure. The objective of the present study was to investigate the suitability of these automated methods for the isolation of Toxoplasma gondii DNA from amniotic fluid (AF). Therefore, three automated procedures were compared to two commercialized manual extraction methods. The MagNA Pure Compact (Roche), BioRobot EZ1 (Qiagen), and easyMAG (bioMérieux) automated procedures were compared to two manual DNA extraction kits, the QIAamp DNA minikit (Qiagen) and the High Pure PCR template preparation kit (Roche). Evaluation was carried out with two specific Toxoplasma PCRs (targeting the 529-bp repeat element), inhibitor search PCRs, and human beta-globin PCRs. The samples each consisted of 4 ml of AF with or without a calibrated Toxoplasma gondii RH strain suspension (0, 1, 2.5, 5, and 25 tachyzoites/ml). All PCR assays were laboratory-developed real-time PCR assays, using either TaqMan or fluorescent resonance energy transfer probes. A total of 1,178 PCRs were performed, including 978 Toxoplasma PCRs. The automated and manual methods were similar in sensitivity for DNA extraction from T. gondii at the highest concentration (25 Toxoplasma gondii cells/ml). However, our results showed that the DNA extraction procedures led to variable efficacy in isolating low concentrations of tachyzoites in AF samples (<5 Toxoplasma gondii cells/ml), a difference that might have repercussions since low parasite concentrations in AF exist and can lead to congenital toxoplasmosis.

Incidence and diagnosis of active toxoplasma infection among liver transplant recipients in Western Turkey

Toxoplasmosis is a serious and potentially life-threatening disease in liver transplant recipients while they are immunosuppressed. We report the clinical and laboratory findings related to active toxoplasma infection associated with 40 immunosuppressed liver transplant procedures that took place over a 12-month period at a major transplant unit in Izmir, Turkey. Twenty-seven (67.5%) of the 40 transplant recipients were found to be seropositive for toxoplasma infection and therefore at risk of reactivated infection. From the serological status of the donors, which was ascertained in 38 of 40 cases, we identified 3 (7.9%) of 38 transplants to be from a seropositive donor to a seronegative recipient. In 10 (26.3%) of 38 transplants, both the donor and recipient were seronegative, and this excluded toxoplasma as a risk. A comparison of real-time polymerase chain reaction (PCR) and nested PCR was undertaken in combination with a range of serological assays (the Sabin-Feldman dye test, enzyme immunoassay immunoglobulin M, and immunosorbent agglutination assay immunoglobulin M). Ethylene diamine tetraacetic acid blood samples from 3 of the 30 recipients at risk from toxoplasma were found positive by PCR, but only 1 of these was found positive in both assays. Among the 3 PCR-positive patients, immunoglobulin M and immunoglobulin G antibody levels increased in only 1 patient. Correlations between symptoms, laboratory findings, and clinical management (use of anti-toxoplasma therapy) are presented. Our findings suggest that toxoplasma presents a significant risk to our liver transplant population and that PCR is a helpful addition in identifying active infections and hence in informing clinical management decisions.

A prospective study of diagnosis of Toxoplasma gondii infection after bone marrow transplantation

Active infection with Toxoplasma gondii in immunocompromised transplant recipients can lead to toxoplasmosis, which may have a rapid disease course and in some cases be fatal. It is of paramount importance to diagnose toxoplasmosis at an early stage, and to initiate specific treatment to improve the outcome. Polymerase chain reaction (PCR) is today the primary diagnostic tool to diagnose toxoplasmosis in immunocompromised patients. Timely diagnosis may, however, be difficult if toxoplasmosis is at first asymptomatic. To investigate the magnitude of toxoplasmosis after bone marrow transplantation (BMT), we conducted a screening study by PCR where 21 autologous and 12 allogeneic BMT recipients were included. Peripheral blood samples were taken one week prior to BMT; thereafter, blood samples were drawn weekly for the first 6 months, and monthly up to one year after BMT. The samples were analyzed by conventional PCR and real-time PCR. T. gondii DNA was detected in peripheral blood from one patient 5 days post allogeneic BMT. There were no clinical signs of toxoplasmosis. Medical records were reviewed and showed a previously undiagnosed eye infection in another allogeneic BMT recipient. These two patients were seropositive for T. gondii. We concluded that monitoring for T. gondii DNA in peripheral blood samples using PCR might be a valuable method for identifying toxoplasma-seropositive stem cell transplant recipients.

Comparison of immunoblotting, calculation of the Goldmann-Witmer coefficient, and real-time PCR using aqueous humor samples for diagnosis of ocular toxoplasmosis

We compared three biological methods for the diagnosis of ocular toxoplasmosis (OT). Paired aqueous humor and serum samples from 34 patients with OT and from 76 patients with other ocular disorders were analyzed by three methods: immunoblotting or Western blotting (WB), the calculation of the Goldmann-Witmer coefficient (GWC), and PCR. WB and GWC each revealed the intraocular production of specific anti-Toxoplasma immunoglobulin G in 81% of samples (30 of 37). PCR detected toxoplasmic DNA in 38% of samples (13 of 34). Nine of the 13 PCR-positive patients were immunocompetent. Combining the techniques significantly improved the diagnostic sensitivity, to 92% for the GWC-WB combination, 90% for the WB-PCR combination, and 93% for the GWC-PCR combination. The combination of all three techniques improved the sensitivity to 97%.

Evaluation of a real-time PCR assay based on the repetitive B1 gene for the detection of Toxoplasma gondii in human peripheral blood

In this paper, we examined the diagnostic value of a real-time polymerase chain reaction (PCR) using fluorescence resonance energy transfer (TaqMan assay) with a new set of primers and probe targeting the B1 gene to reproducibly detect and quantify Toxoplasma gondii in human blood. A total of 183 buffy coat samples from patients serologically classified as recent toxoplasmosis (immunoglobulin M (IgM)+, n = 35) or chronic infection (IgM- and immunoglobulin G (IgG)+, n = 110), and seronegative individuals (n = 38) was investigated. Of the IgM seropositive patients, 17:35 (48.6%) presented parasitaemia, whereas 3.6% positivity was achieved in those individuals that theoretically corresponded to chronic infection (4:110). In the seronegative group, the assay provided 7.9% (3/38) of positive results. Interestingly, one of them was confirmed as positive in a conventional PCR targeting the Toxoplasma B1 gene after hybridization with an internal probe. Real-time PCR was able to accurately quantify the parasite load when concentrations of T. gondii DNA are low, revealing a parasite burden ranged from 9.92 x 10(-3) to 8.73 x 10(-1) tachyzoites genome per milliliter of blood. The chance of an IgM+ patient to present parasitemia detected by the TaqMan procedure was 19.02 times greater than in IgM- individuals (P < 0.05). It was observed a positive association between the optical density values of the IgM serological tests and the number of circulating parasites in the acute patients (P < 0.0001). The specificity of the molecular test was 95.3% when calculated using IgM+ patients as disease group and IgM- as nondisease group. The low sensitivity observed in the IgM seropositive group (48.6%) could be due to the use of buffy coat as clinical material for DNA extraction. An amplification control based on the human beta-actin gene was used in parallel to monitor PCR inhibition and to control for DNA integrity.

[Real-time PCR in the diagnosis of toxoplasmosis: the way to standardisation?]

Severity of toxoplasmosis is highly correlated to the immune status of the infected individual. Foetus and immunocompromised patient are mostly at risk to develop life threatening forms. In this situation, serological diagnosis gives poor information. DNA detection using polymerase-chain-reaction technology (PCR) has significantly improved the management of this disease. Even so, the growing number of conventional PCR assays has finally led to variable performance results. Real-Time PCR (RT-PCR) in toxoplasmosis has been developed since 2000. This new technology can improve standardisation. Moreover, quantification of parasitic load in samples becomes possible. This review describes the main RT-PCR procedures actually under use and the studies comparing different target genes. The effective benefit of quantification is also discussed. Reducing number of procedures and more systematic external quality control should be considered, in order to improve reliability in PCR results, which has undoubtedly become a major tool in toxoplasmosis diagnosis.

Real-time PCR targeting a 529-bp repeat element for diagnosis of toxoplasmosis

Sensitive and rapid detection of infection with Toxoplasma gondii in transplanted immunocompromised patients is crucial for a good prognosis. Two DNA fragments are used currently for detecting T. gondii infection by PCR, i.e., the B1 gene and a 529-bp repeat element that exists in 200-300 copies/genome. This study investigated whether targeting the 529-bp repeat element gives better sensitivity and accuracy than can be obtained when targeting the B1 gene (35 copies) when concentrations of T. gondii DNA are low. The results demonstrated that detection of the 529-bp repeat element increased diagnostic sensitivity and accuracy. Addition of an internal amplification control did not affect the PCR performance and was useful in order to monitor PCR inhibition by non-specific DNA in the LightCycler instrument. The real-time PCR was used successfully in a clinical context to monitor parasitaemia in the blood of a transplant recipient suffering from toxoplasmosis.

Comparison between two amplification sets for molecular diagnosis of toxoplasmosis by real-time PCR

PCR is now commonly applied to the diagnosis of toxoplasmosis. Although several methods are available, comparative studies are few, making it difficult to compare the performance of each technique. We compared the sensitivities of two real-time PCR assays through a prospective study on fetuses, neonates, and immunocompromised patients and on the ocular diagnosis of toxoplasmosis. The first system targeted the widely used B1 gene (GenBank accession number AF179871) while the second (RE) targeted a more recently described sequence repeated roughly 200 to 300 times (GenBank accession number AF146527). We demonstrated that molecular diagnosis requires the duplication of PCR assays, especially with the B1 system, as only one PCR was positive in 33.3% of cases. Our study showed that the RE target was more sensitive for all biological samples (amniotic fluid, placenta, aqueous humor, whole blood, and cerebrospinal and bronchoalveolar fluids) and significantly improved the performance of the diagnosis of toxoplasmosis. Taking into consideration all clinical samples, the mean gain in the crossing point value was 4.2 +/- 1.7 cycles and was even more significant for amniotic fluid (5.8 +/- 1.7 cycles).

Advances in real-time PCR: application to clinical laboratory diagnostics

The polymerase chain reaction (PCR) has become one of the most important tools in molecular diagnostics, providing exquisite sensitivity and specificity for detection of nucleic acid targets. Real-time monitoring of PCR has simplified and accelerated PCR laboratory procedures and has increased information obtained from specimens including routine quantification and differentiation of amplification products. Clinical diagnostic applications and uses of real-time PCR are growing exponentially, real-time PCR is rapidly replacing traditional PCR, and new diagnostic uses likely will emerge. This review analyzes the scope of present and potential future clinical diagnostic applications of this powerful technique. Critical discussions focus on basic concepts, variations, data analysis, instrument platforms, signal detection formats, sample collection, assay design, and execution of real-time PCR.

Molecular epidemiology: A multidisciplinary approach to understanding parasitic zoonoses

Sound application of molecular epidemiological principles requires working knowledge of both molecular biological and epidemiological methods. Molecular tools have become an increasingly important part of studying the epidemiology of infectious agents. Molecular tools have allowed the aetiological agent within a population to be diagnosed with a greater degree of efficiency and accuracy than conventional diagnostic tools. They have increased the understanding of the pathogenicity, virulence, and host-parasite relationships of the aetiological agent, provided information on the genetic structure and taxonomy of the parasite and allowed the zoonotic potential of previously unidentified agents to be determined. This review describes the concept of epidemiology and proper study design, describes the array of currently available molecular biological tools and provides examples of studies that have integrated both disciplines to successfully unravel zoonotic relationships that would otherwise be impossible utilising conventional diagnostic tools. The current limitations of applying these tools, including cautions that need to be addressed during their application are also discussed.

Nucleic acid amplification-based techniques for pathogen detection and identification

Nucleic acid amplification techniques have revolutionised diagnostic and research industries. Current technologies that allow the detection of amplification in real-time are fast becoming industry standards, particularly in a diagnostic context. In this review, we describe and explore the application of numerous real-time detection chemistries and amplification techniques for pathogen detection and identification, including the polymerase chain reaction, nucleic acid sequence-based amplification, strand displacement amplification and the ligase chain reaction. The emergence of newer technologies, such as lab-on-a-chip devices and photo-cleavable linkers, is also discussed.

Emerging technologies for the detection and genetic characterization of protozoan parasites

The development and adaptation of new technologies for the genetic characterization and identification of parasites continue to accelerate, providing an increasing number of research and analytical tools. We review emerging technologies that have applications in this area, including real-time PCR and microarrays, and discuss the fundamental principles of some of these technologies and how they are applied to characterize parasites. We give special consideration to the application of genetic data to biological questions, where selection of the most appropriate technique depends on the biological question posed by the investigator.

Advances in real-time PCR: application to clinical laboratory diagnostics

The polymerase chain reaction (PCR) has become one of the most important tools in molecular diagnostics, providing exquisite sensitivity and specificity for detection of nucleic acid targets. Real-time monitoring of PCR has simplified and accelerated PCR laboratory procedures and has increased information obtained from specimens including routine quantification and differentiation of amplification products. Clinical diagnostic applications and uses of real-time PCR are growing exponentially, real-time PCR is rapidly replacing traditional PCR, and new diagnostic uses likely will emerge. This review analyzes the scope of present and potential future clinical diagnostic applications of this powerful technique. Critical discussions focus on basic concepts, variations, data analysis, instrument platforms, signal detection formats, sample collection, assay design, and execution of real-time PCR.