A polymerase chain reaction assay for the detection of Wuchereria bancrofti in blood samples from French Polynesia

Selection and exploitation of prevalent, tandemly repeated genomic targets for improved real-time PCR-based detection of Wuchereria bancrofti and Plasmodium falciparum in mosquitoes

BACKGROUND

Optimization of polymerase chain reaction (PCR)-based diagnostics requires the careful selection of molecular targets that are both highly repetitive and pathogen-specific. Advances in both next-generation sequencing (NGS) technologies and bioinformatics-based analysis tools are facilitating this selection process, informing target choices and reducing labor. Once developed, such assays provide disease control and elimination programs with an additional set of tools capable of evaluating and monitoring intervention successes. The importance of such tools is heightened as intervention efforts approach their endpoints, as accurate and complete information is an essential component of the informed decision-making process. As global efforts for the control and elimination of both lymphatic filariasis and malaria continue to make significant gains, the benefits of diagnostics with improved analytical and clinical/field-based sensitivities and specificities will become increasingly apparent.

METHODOLOGY/PRINCIPAL FINDINGS

Coupling Illumina-based NGS with informatics approaches, we have successfully identified the tandemly repeated elements in both the Wuchereria bancrofti and Plasmodium falciparum genomes of putatively greatest copy number. Utilizing these sequences as quantitative real-time PCR (qPCR)-based targets, we have developed assays capable of exploiting the most abundant tandem repeats for both organisms. For the detection of P. falciparum, analysis and development resulted in an assay with improved analytical and field-based sensitivity vs. an established ribosomal sequence-targeting assay. Surprisingly, analysis of the W. bancrofti genome predicted a ribosomal sequence to be the genome's most abundant tandem repeat. While resulting cycle quantification values comparing a qPCR assay targeting this ribosomal sequence and a commonly targeted repetitive DNA sequence from the literature supported our finding that this ribosomal sequence was the most prevalent tandemly repeated target in the W. bancrofti genome, the resulting assay did not significantly improve detection sensitivity in conjunction with field sample testing.

CONCLUSIONS/SIGNIFICANCE

Examination of pathogen genomes facilitates the development of PCR-based diagnostics targeting the most abundant and specific genomic elements. While in some instances currently available tools may deliver equal or superior performance, systematic analysis of potential targets provides confidence that the selected assays represent the most advantageous options available and that informed assay selection is occurring in the context of a particular study's objectives.

Monitoring and evaluation of lymphatic filariasis interventions: an improved PCR-based pool screening method for high throughput Wuchereria bancrofti detection using dried blood spots

Background

Effective diagnostic tools are necessary to monitor and evaluate interruption of Lymphatic Filariasis (LF) transmission. Accurate detection of Wuchereria bancrofti (Wb) microfilaria (mf) is essential to measure the impact of community treatment programmes. PCR-based assays are specific, highly sensitive tools allowing the detection of Wuchereria bancrofti DNA in human blood samples. However, current protocols describing the pool screening approach, use samples of less than 60 μl of blood, which limits the sensitivity of the pool-screen PCR assay. The purpose of this study was to improve the pool-screen PCR protocol to enhance its sensitivity and usefulness for population scale studies.

Findings

DNA extractions were performed with the DNeasy kit, the PCR with the Wb LDR primers and the SYBR-Green dye. Improvements of our pool-screen real-time PCR (qPCR) assay allowed the detection of as little as one Wb microfilaria diluted in a pool of at least 12 blood samples of 60 μl each. Using this assay, mf burdens can be predicted using a standard curve derived from mf spiked dried blood samples. The sensitivity achieved is equivalent to the detection of a single LF positive individual carrying a mf burden as low as 18 mf/ml, in a pool of blood samples from at least 12 individuals.

Conclusions

Due to its sensitivity, rapidity and cost-effectiveness, we suggest this qPCR pool-screening assay could be used as a diagnostic tool for population- scale filariasis elimination monitoring and evaluation.

Inter and intra-specific diversity of parasites that cause lymphatic filariasis

Lymphatic filariasis is caused by three closely related nematode parasites: Wuchereria bancrofti, Brugia malayi and Brugia timori. These species have many ecological variants that differ in several aspects of their biology such as mosquito vector species, host range, periodicity, and morphology. Although the genome of B. malayi (the first genome sequenced from a parasitic nematode) has been available for more than five years, very little is known about genetic variability among the lymphatic dwelling filariae. The genetic diversity among these worms is not only interesting from a biological perspective, but it may have important practical implications for the Global Program to Eliminate Lymphatic Filariasis, as the parasites may respond differently to diagnostic tests and/or medical interventions. Therefore, better information on their genetic variability is urgently needed. With improved methods for nucleic acid extraction and recent advances in sequencing chemistry and instrumentation, this gap can be filled relatively inexpensively. Improved information on filarial genetic diversity may increase the chances of success for lymphatic filariasis elimination programs.

Molecular diagnosis of infections and resistance in veterinary and human parasites

Since 1977, >2000 research papers described attempts to detect, identify and/or quantify parasites, or disease organisms carried by ecto-parasites, using DNA-based tests and 148 reviews of the topic were published. Despite this, only a few DNA-based tests for parasitic diseases are routinely available, and most of these are optional tests used occasionally in disease diagnosis. Malaria, trypanosomiasis, toxoplasmosis, leishmaniasis and cryptosporidiosis diagnosis may be assisted by DNA-based testing in some countries, but there are very few cases where the detection of veterinary parasites is assisted by DNA-based tests. The diagnoses of some bacterial (e.g. lyme disease) and viral diseases (e.g. tick borne encephalitis) which are transmitted by ecto-parasites more commonly use DNA-based tests, and research developing tests for these species makes up almost 20% of the literature. Other important uses of DNA-based tests are for epidemiological and risk assessment, quality control for food and water, forensic diagnosis and in parasite biology research. Some DNA-based tests for water-borne parasites, including Cryptosporidium and Giardia, are used in routine checks of water treatment, but forensic and food-testing applications have not been adopted in routine practice. Biological research, including epidemiological research, makes the widest use of DNA-based diagnostics, delivering enhanced understanding of parasites and guidelines for managing parasitic diseases. Despite the limited uptake of DNA-based tests to date, there is little doubt that they offer great potential to not only detect, identify and quantify parasites, but also to provide further information important for the implementation of parasite control strategies. For example, variant sequences within species of parasites and other organisms can be differentiated by tests in a manner similar to genetic testing in medicine or livestock breeding. If an association between DNA sequence and phenotype has been demonstrated, then qualities such as drug resistance, strain divergence, virulence, and origin of isolates could be inferred by DNA-based tests. No such tests are in clinical or commercial use in parasitology and few tests are available for other organisms. Why have DNA-based tests not had a bigger impact in veterinary and human medicine? To explore this question, technological, biological, economic and sociological factors must be considered. Additionally, a realistic expectation of research progress is needed. DNA-based tests could enhance parasite management in many ways, but patience, persistence and dedication will be needed to achieve this goal.

Diagnosis of parasitic diseases: old and new approaches

Methods for the diagnosis of infectious diseases have stagnated in the last 20–30 years. Few major advances in clinical diagnostic testing have been made since the introduction of PCR, although new technologies are being investigated. Many tests that form the backbone of the “modern” microbiology laboratory are based on very old and labour-intensive technologies such as microscopy for malaria. Pressing needs include more rapid tests without sacrificing sensitivity, value-added tests, and point-of-care tests for both high- and low-resource settings. In recent years, research has been focused on alternative methods to improve the diagnosis of parasitic diseases. These include immunoassays, molecular-based approaches, and proteomics using mass spectrometry platforms technology. This review summarizes the progress in new approaches in parasite diagnosis and discusses some of the merits and disadvantages of these tests.

The development and evaluation of a single step multiplex PCR method for simultaneous detection of Brugia malayi and Wuchereria bancrofti

A single step novel multiplex polymerase chain reaction (PCR) has been developed for simultaneous detection of human filarial parasites, Brugia malayi and Wuchereria bancrofti, from blood samples and mosquitoes. The primers used were novel and have been tested with the parasite DNA amplifying 188bp (BM) and 129bp (WB) DNA fragments, specific to B. malayi and W. bancrofti, respectively, in a single reaction. The specificity of the PCR product was confirmed by DNA sequencing and slot blot hybridization assay. The test was found highly sensitive for both B. malayi and W. bancrofti by detecting the parasitaemia up to the level of one microfilaria per reaction. The assay was further evaluated on 98 blood samples and 144 mosquito samples collected from filarial endemic areas. The PCR was found to be more efficient in comparison to microscopy by detecting 8% and 5% more filarial parasites in field-collected blood and mosquito samples, respectively. This novel PCR that offers scope for simultaneous detection of both the parasites may be used as a diagnostic tool for the detection of filariasis in population and can be adopted for rapid surveillance and monitoring of mosquitoes for use in the effective control of filariasis.

[Diagnosing imported helminthiasis]

In recent years, there has been an increase in cases of imported helminthiasis in Spain because of two complementary causes: immigration and international travel. Although the prevalence of helminthiasis is high in the immigrant population, the risk of transmission to the Spanish population is low. In this review, we provide clues to aid in the diagnosis of the helminthiasis, highlighting the geographic characteristics, clinical findings and analytical results of the most frequent types. The low sensitivity of the classic parasitological diagnostic test, mainly in tissue helminthiasis, is described. In addition, the advantages and limitations of the common serological methods for detecting related circulating antigens and antibodies are presented. Certain molecular methods used in the diagnosis of imported helminthiasis and the best strategies for screening of this condition are discussed.

Random amplified polymorphic DNA (RAPD) for differentiation between Thai and Myanmar strains of Wuchereria bancrofti

BACKGROUND

Lymphatic filariasis (LF) is a mosquito-borne disease caused by mosquito-transmitted filarial nematodes, including Wuchereria bancrofti and Brugia malayi. The Lymphatic Filariasis Elimination Program in Thailand has reduced the prevalence of nocturnally subperiodic W. bancrofti (Thai strain), mainly transmitted by the Ochlerotatus (Aedes) niveus group in Thailand to 0.57/100,000 population. However, it is estimated that more than one million Myanmar migrants with high prevalence of bancroftian filariasis have settled in the large urban cities of Thailand. These infected migrants carry the nocturnally periodic W. bancrofti (Myanmar strain) which has Culex quinquefasciatus as the main mosquito vector. Although transmissions of the Myanmar strain of W. bancrofti by the Thai Cx. quinquefasciatus has never been reported, previous study showed that Cx. quinquefasciatus could nurture the Myanmar strain of W. bancrofti to the infective stage. Thus, the potential now exists for a re-emergence of bancroftian filariasis in Thailand. The present study was undertaken in an attempt to differentiate between the Thai and Myanmar strains of W. bancrofti.

METHODS

The microfilarial periodicity of Thai and the Myanmar strains of W. bancrofti were determined. Comparative morphology and morphometry of microfilariae and a study of random amplified polymorphic DNA (RAPD) was performed. The Nei's genetic distance was calculated, and a phylogenetic tree was constructed using the Unweighted Pair Group Method with Arithmetic mean (UPGMA).

RESULTS

The Thai strain of W. bancrofti was nocturnally subperiodic, and the Myanmar strain of W. bancrofti was nocturnally periodic. The body length, cephalic space length, and cephalic space width of the Thai strain of W. bancrofti were significantly larger than those of the Myanmar strain of W. bancrofti (p < 0.05). However, an overlapping mean of these parameters made it impractical for field application. RAPD-PCR profiles showed specific bands characteristic for the Myanmar strain of W. bancrofti. The phylogenetic tree indicated two genetically distinct clusters of the Thai and Myanmar strains of W. bancrofti.

DISCUSSION

This study was the first report on the genetic polymorphism of the Thai and Myanmar strains of W. bancrofti. Differentiation between the Thai and Myanmar strains of W. bancrofti could not rely on morphological criteria alone. However, RAPD profiles revealed a significant diversity between the two strains. The RAPD-PCR technique was suitable for differentiating Thai and Myanmar strains of W. bancrofti. The RAPD marker could be used for epidemiological assessment of the Myanmar strains of W. bancrofti in Thailand.

Detection of filarial parasites in domestic cats by PCR-RFLP of ITS1

Lymphatic filariasis has been targeted by the World Health Organization (WHO) to be eliminated by the year 2020. In addition to chemotherapy and vector control, the control of reservoir hosts is necessary for the control program to succeed. Malayan filariasis, caused by Brugia malayi, is endemic in the South of Thailand where domestic cats serve as the major reservoir host. However, in nature, domestic cats also carry B. pahangi, Dirofilaria immitis and D. repens infections and it is difficult to distinguish the different filarial species from each other just by morphology. To assess the burden of filarial parasites, we performed a study on domestic cats in an endemic area of malayan filariasis in the Prasang district, of Surat Thani, a province in Southern Thailand. Together with Giemsa staining and acid phosphatase activity studies, we performed PCR-RFLP analysis on the first internal transcribed spacer (ITS1) region of ribosomal DNA (rDNA). PCR-RFLP with Ase I could clearly differentiate between B. malayi, B. pahangi, D. immitis and D. repens. Out of the 52 cats studied, filarial parasites were identified in 5 (9.5%) cats, of which 4 (7.6%) were B. pahangi and 1 (1.9%) D. immitis. This PCR-RFLP technique detected two additional cats that were not detected by microscopy. The domestic cats are not an important host of B. malayi in this region. We could develop the PCR-RFLP assay test for differentiating filarial nematodes which can be applied to survey human, animal reservoir hosts and mosquito vectors in endemic areas.

Lymphatic filariasis in Ghana: establishing the potential for an urban cycle of transmission

Lymphatic filariasis is a significant public health and economic problem in many tropical and sub-tropical regions. Unplanned urbanization leading to a lack of proper sanitary conditions has resulted in an increase in the urban-based transmission of a number of vector-borne diseases, including lymphatic filariasis. It has been well established that lymphatic filariasis is endemic in rural areas of Ghana. The goal of this study was to determine if there is a potential of establishing urban transmission cycles in Ghana's major cities. We clinically and immunologically assessed 625 individuals from the three major urban areas (Bawku, Bolgatanga and Secondi/Takoradi), finding that the prevalence of infection with Wuchereria bancrofti ranged from 0 to 12.5%. The results of a polymerase chain reaction based analysis of mosquitoes collected from these areas suggested that there is a low but detectable prevalence of mosquitoes infected with W. bancrofti. We conclude that there may be a potential for an established urban transmission of lymphatic filariasis in Ghana.

Combined detection of Brugia malayi and Wuchereria bancrofti using single PCR

A single step PCR method has been developed for the combined detection of the human filarial parasites, Brugia malayi and Wuchereria bancrofti. Parasites' DNA were isolated from filaria positive blood samples that were collected from endemic areas. The primers used were Hha1 and Ssp I, which amplified the DNA fragments of 322 bp and 188 bp specific to B. malayi and W. bancrofti, respectively. The sensitivity of the assay was tested with blood and mosquito samples having one W. bancrofti in a pool of 10 B. malayi. The assay was further evaluated on field collected blood and mosquito samples. Use of this assay as a diagnostic tool for the detection of filariasis being the most promising aspect of this study, offers scope for detection of both the parasites even at low levels of infection.

Rapid detection of Wuchereria bancrofti in mosquitoes by LightCycler polymerase chain reaction and melting curve analysis

A LightCycler real-time polymerase chain reaction (PCR) assay was developed to detect Wuchereria bancrofti DNA in blood-fed mosquitoes. The assay is based on fluorescence melting curve analysis of the PCR product generated from a family of repeated DNA elements: the 182 bp SspI repeat, specific to the genus Wuchereria. According to the melting temperature, W. bancrofti infected-mosquitoes were differentiated from Brugia malayi-infected and non-infected mosquitoes as well as from genomic DNA of Dirofilaria immitis and human DNA. The method proved to be 100% sensitive in all W. bancrofti-infected mosquitoes. Melting curve analysis offers a rapid alternative for the specific detection of W. bancrofti in mosquitoes. It is very accurate and sensitive, allows a high throughput and can be performed on very small samples. The method therefore has great potential for application in epidemiological studies.

Analysis of Wuchereria bancrofti infections in a village community in northern Nigeria: increased prevalence in individuals infected with Onchocerca volvulus

Infections with Wuchereria bancrofti causing lymphatic filariasis still represent one of the major health problems in the tropics, with 120 million people infected and over 750 million exposed to this filarial parasite. We have studied lymphatic filariasis infections as part of a multi-parasite survey in a village community in the savannah of northern Nigeria. We analysed serum samples from 341 individuals aged 5-70 years, detecting a W. bancrofti circulating antigen using the commercially available ICT Filariasis card test. The prevalence of infections was 10% and clearly age-dependent, increasing from below 2% in children to over 20% in subjects older than 40 years. Measuring IgG4 antibodies against the recombinant W. bancrofti antigen SXP1 showed that 36% of all tested individuals had been at least exposed to the parasite. Antibody levels also increased very significantly with age. A further analysis measuring Onchocerca volvulus-specific IgG4 antibodies showed a very significant association between infections with O. volvulus and those with W. bancrofti. Our data show that infections with W. bancrofti in Nigeria are still a frequently occurring health problem, since they are more prevalent than previously reported, and that individuals with an O. volvulus infection are more often infected with W. bancrofti than expected statistically.

Development and standardization of a rapid, PCR-based method for the detection of Wuchereria bancrofti in mosquitoes, for xenomonitoring the human prevalence of bancroftian filariasis

PCR has recently been studied as a promising tool for monitoring the progress of efforts to eliminate lymphatic filariasis. PCR can be used to test concurrently at least 30 pools, with as many as 40 mosquitoes in each pool, for the presence of filarial larvae. The SspI PCR assay for the detection of Wuchereria bancrofti DNA in pools of mosquitoes has been used since 1994 in a variety of laboratories worldwide. During that time, the original assay has been modified in these different laboratories and no standardized assay currently exists. In an effort to standardize and improve the assay, a meeting was held on 15-16 November 2001, at Emory University in Atlanta, with representatives from most of the laboratories currently using the assay. The first round of testing was designed to test the four most promising methods for DNA extraction from pools of mosquitoes. Two of the four methods stood out as clearly the best and these will be now optimised and evaluated in two further rounds of testing.

Recent advances in molecular diagnostic techniques for human lymphatic filariasis and their use in epidemiological research

Diagnosis of lymphatic filariasis for epidemiological studies is an area of science that has been greatly modernized over the last two decades. Recent developments and the introduction of new technologies have led to the replacement of traditional diagnostic methods that were insensitive, tedious and often impractical with molecular biology techniques that have considerable potential for field use. This paper reviews some of these major new techniques and the applications of molecular diagnosis in the field of lymphatic filariasis.

Evaluation of ICT filariasis card test using whole capillary blood: comparison with Knott's concentration and counting chamber methods

An immunochromatographic card test (ICT) that uses fingerprick whole blood instead of serum for diagnosis of bancroftian filariasis has recently been developed. The card test was validated in the field in Kenya by comparing its sensitivity to the combined sensitivity of Knott's concentration and counting chamber methods. A total of 102 (14.6%) and 117 (16.7%) persons was found to be microfilaremic by Knott's concentration and counting chamber methods, respectively. The geometric mean intensities (GMI) were 74.6 microfilariae (mf)/ml and 256.5 mf/ml by Knott's concentration and counting chamber methods, respectively. All infected individuals detected by both Knott's concentration and counting chamber methods were also antigen positive by the ICT filariasis card test (100% sensitivity). Further, of 97 parasitologically amicrofilaremic persons, 24 (24.7%) were antigen positive by the ICT. The overall prevalence of antigenemia was 37.3%. Of 100 nonendemic area control persons, none was found to be filarial antigen positive (100% specificity). The results show that the new version of the ICT filariasis card test is a simple, sensitive, specific, and rapid test that is convenient in field settings.

Evaluation of PCR-based methods for the diagnosis of infection in bancroftian filariasis

The value of the polymerase chain reaction (PCR) in the diagnosis of Wuchereria bancrofti infection was evaluated in comparison to microscopical examination of night blood smears, Nuclepore filtration, serology and ultrasonography. No correlation was found between PCR-based deoxyribonucleic acid (DNA) probing and serology. We did not find any evidence of free filarial DNA in either blood plasma or chylocoele fluid. We conclude that the 2 PCR-based techniques evaluated are not more sensitive than Nuclepore filtration for detection of W. bancrofti microfilaraemia, need at least 1 intact microfilaria in the volume of blood used for DNA extraction, and were much inferior to ultrasonography for detection of amicrofilaraemic adult worm carriers.

Evaluation of the ICT whole blood antigen card test to detect infection due to nocturnally periodic Wuchereria bancrofti in South India

The commercially available ICT Card Test for bancroftian filariasis was evaluated for its sensitivity and specificity in detecting microfilaria carriers among 189 individuals each in filariasis-endemic and nonendemic areas in South India, and compared to both conventional night blood finger prick thick blood smear examination and venous blood membrane filtration. Though the specificity of the test was 100% in comparison to both, the sensitivity was 98.5% against the finger prick thick blood smear and 71.9 compared to the membrane filtration technique. Similarly, the positive predictive value was 100% against both techniques, but the negative predictive values were 99.5% against the finger prick thick blood smear and 88.3% compared to the membrane filtration technique. The test's lower sensitivity compared to the filtration technique requires further investigation.

Epidemiology and immunopathology of bancroftian filariasis

Human lymphatic filariasis affects 120 million people worldwide. Although the disease is considered to be potentially erradicable by the World Health Organization, comprehensive studies on epidemiological aspects as well as mechanisms of pathology development are still premature. The following review summarizes currently available data on these topics and ends by discussing the latest control strategies.

Sequence conservation of repeat 3 region of the gene coding for the 15 kDa polyprotein within human and simian Loa loa

The human and simian strains of Loa loa microfilariae are morphologically identical even though their periodicities vary. When using primate models (Mandrillus sphinx) of human loaisis for vaccination trials, the absence of any ongoing simian L. loa infection must be demonstrated. Nested primers derived from a human strain of L. loa (targeted on the repeat 3 region of the gene encoding the 15 kDa polyprotein; 15r3) amplified at 366 bp sequence from simian L. loa genomic DNA and blood lysates from mandrills infected with simian L. loa. This nested-PCR assay has been tested on 12 amicrofilaremic (AMF) mandrills (without filarial microfilariae) and was positive in four mandrills. The nested-PCR product derived from simian L. loa genomic DNA and from three of four AMF mandrills has been sequenced. No difference was observed between the four sequences, which, in addition, were 99.18% identical to the 15r3 of human L. loa. Therefore, the 15r3 sequence is conserved within human and simian L. loa. These results suggest that the four PCR-positive mandrills without circulating microfilariae had occult simian L. loa infections. The study demonstrates the ability of a nested-PCR assay to identify animals naturally infected with simian L. loa.

A sensitive polymerase chain reaction based assay for the detection of Setaria digitata: the causative organism of cerebrospinal nematodiasis in goats, sheep and horses

A sensitive PCR assay for the detection of Setaria digitata has been developed. Two oligonucleotide primers (17 nt) were designed from a previously cloned and characterized tandemly arranged repetitive sequence of Setaria digitata. Using these primers, it was possible to amplify small quantities (100 fg) of S. digitata genomic DNA. A simple procedure, using proteinase K and non-ionic detergent NP 40, was followed to process the host blood samples and mosquitoes harbouring L3 larvae. The sensitivity of the polymerase chain reaction based assay surpasses the microscopic detection and the previously reported oligonucleotide based chemiluminescent detection of microfilariae in infected host blood samples and L3 larvae in mosquitoes.

PCR-ELISA for the detection of Brugia malayi infection using finger-prick blood

A polymerase chain reaction assay based on the enzyme-linked immunosorbent assay (PCR-ELISA) has been developed to detect Brugia malayi infection in an area of low endemicity in Malaysia. Blood samples from 239 subjects were tested: 192 amicrofilaraemic individuals, 14 microfilaraemic persons and 3 chronic elephantiasis cases from endemic areas and 30 city-dwellers (non-endemic controls). PCR products were examined by ELISA and Southern hybridization. In the PCR-ELISA, digoxigenin-labelled PCR products were hybridized to a biotin-labelled probe. This was followed by incubation in streptavidin-coated microtitre wells and detection using anti-digoxigenin-peroxidase and ABTS [2,2'-azinobis(3-ethylbenzthiazoline-6-sulphonic acid)]. All microfilaraemic samples were positive by PCR-ELISA and Southern hybridization and all samples from non-endemic subjects and chronic elephantiasis patients were negative. The PCR-ELISA detected 12 times as many B. malayi infections as did thick blood film examination. Nineteen of the 194 samples from the endemic area gave positive results by both PCR-ELISA and Southern hybridization, and an additional 5 samples were positive by PCR-ELISA only. The PCR-ELISA was specific and sensitive, detected more infections, and was more reproducible than Southern hybridization.

Human occult loiasis: field evaluation of a nested polymerase chain reaction assay for the detection of occult infection

A nested polymerase chain reaction (nested PCR) assay, targeted on the repeat 3 region (15r3) of the gene coding for a Loa loa 15 kD polyprotein, was developed to detect L. loa infection. The assay has a sensitivity of 95% and is 100% specific with regard to sympatric filarial parasites: Mansonella perstans, Onchocerca volvulus and Wuchereria bancrofti. In this field study in a mixed filarial (L. loa and M. perstans) endemic region of Gabon, 157 L. loa amicrofilaraemic blood samples (AMF; diagnosed by leucoconcentration followed by standard microscopic examination) from the residents from four villages were screened by the 15r3-nested PCR assay. The assay detected 106 occult infected subjects among the 157 AMF individuals (68%), including 59 of 87 adults (68%) and 47 of 70 children (67%). In each village the prevalence of occult infection was, respectively, 38%, 52%, 79% and 80% for Moyabi, Djoutou, N'djokaye and Okoumbi. The annual transmission potential (ATP) of loiasis has been estimated to be 250 infective larvae (L 3) per man per year for Moyabi and Djoutou, 1800 for N'djokaye and 433000 L3/man/year for Okoumbi. This implies a correlation between occult infection of loiasis and the intensity of transmission. By contrast, the prevalence of L. loa microfilariae was 21% for Okoumbi, 22%, for N'djokaye and 19% for Djoutou and Moyabi. These results show that the prevalence of loiasis in this region of Gabon is higher than previously described by standard microscopic examination and that the application of this assay will be significant in the development of control strategies for loiasis.

Diagnosis of Wuchereria bancrofti infection by the polymerase chain reaction using urine and day blood samples from amicrofilaraemic patients

A sensitive and specific polymerase chain reaction (PCR) based on a highly repeated deoxyribonucleic acid (DNA) sequence (188 bp; SspI repeat) was tested for the detection of Wuchereria bancrofti DNA in blood and urine samples collected during the day from individuals in Coque, Recife, Brazil, an endemic area for W. bancrofti. All microfilaraemic individuals were also positive by PCR, irrespective of the samples used. The PCR system was capable of detecting W. bancrofti DNA in amicrofilaraemic individuals: c. 93% were positive by PCR when day blood samples were used and 59.7% when urine samples collected at 07:00 were used. Thus, nocturnally periodic W. bancrofti infection can be detected in blood samples collected during the day, which is convenient for large-scale screening. In addition, non-invasive urine collection provided suitable samples for PCR, which is clearly advantageous for preliminary mass diagnosis.

New tools for diagnosis and monitoring of bancroftian filariasis parasitism: The polynesian experience

Bancroftian filariasis is endemic in French Polynesia and control programs with diethylcarbamazine, started in the 1950s, led to a sharp reduction of the microfilaria prevalence. Consequently, the control program was interrupted in 1982. Ten years later, however, the incidence of the parasitism again reached pre-control levels (20-30% microfilaremia in some islands), indicating that the adult worms (for which no diagnostic tool was available) had persisted. Apart from research on chemotherapy strategies, the Institut Malardé has been actively involved in developing and evaluating more-powerful diagnostic tools than the unique detection of microfilariae by blood smear examination. These include: (1) the detection of adult worm circulating antigens in humans, and (2) the detection of Wuchereria bancrofti larvae in mosquitoes, using DNA probes. In this paper, Luc Nicolas reviews the available diagnostic tools to detect W. bancrofti and their implementation in epidemiological areas, based on the Polynesian experience.

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.

The ICT Filariasis Test: A rapid-format antigen test for diagnosis of bancroftian filariasis

Antigen testing is now recognized as the method of choice for detection of Wuchereria bancrofti infections. Unlike tests that detect microfilariae, antigen tests can be performed with blood collected during the day or night. However, existing enzyme-linked immunosorbent assay (ELISA) tests for filarial antigenemia are difficult to perform in the field, and this has limited their use in endemic countries. In this article, Gary Weil, Patrick Lammie and Niggi Weiss review their experience with a new rapid-format filarial antigen test. They found that the ICT card test was very easy to perform and that it was comparable with ELISA for the detection of filarial antigen in sera from people with microfilaremia. The introduction now of an antigen test suitable for use in the field is especially timely, in that it may facilitate implementation of new strategies proposed by the World Health Organization for control and elimination of lymphatic filariasis.