Development of a quantitative, competitive polymerase chain reaction--enzyme-linked immunosorbent assay for the detection of Wuchereria bancrofti DNA

Development of a molecular xenomonitoring protocol to assess filariasis transmission

According to the World Health Organization, lymphatic filariasis (LF), a mosquito-borne neglected tropical disease (NTD), should be eliminated as a public health concern by the end of 2020. To this end, the goals of the Global Programme to Eliminate Lymphatic Filariasis (GPELF) include interrupting transmission through mass drug administration (MDA). After two decades, several countries have implemented MDA and are now ready to confirm whether transmission has been interrupted. The method for detecting the parasites in mosquito vectors known as xenomonitoring is a non-invasive tool for assessing the current transmission status of the filarial nematode Wuchereria bancrofti (which is responsible for 90% of cases) by their vectors. There are several methods available for detection of the worm in mosquito samples, such as dissection or polymerase chain reaction (PCR). However, most of these techniques still produce a considerable number of false-negative results. The present study describes a new duplex PCR protocol, which is an improvement on the traditional PCR methodology, enhanced by introducing the actin gene as an endogenous control gene. After adjusting the mosquito pool size, DNA extraction, and WbCx PCR duplex design, we achieved a reliable and sensitive molecular xenomonitoring protocol. This assay was able to eliminate 5% of false negative samples and detected less than one Wb larvae. This high sensitivity is particularly valuable after MDA, when prevalence declines. This new method could reduce the number of false-negative samples, which will enable us to improve our ability to generate accurate results and aid the monitoring strategies used by LF elimination programmes.

Loss of sensitivity of immunochromatographic test (ICT) for lymphatic filariasis diagnosis in low prevalence settings: consequence in the monitoring and evaluation procedures

BACKGROUND

Diagnostic tools for lymphatic filariasis (LF) elimination programs are useful in mapping the distribution of the disease, delineating areas where mass drug administrations (MDA) are required, and determining when to stop MDA. The prevalence and burden of LF have been drastically reduced following mass treatments, and the evaluation of the performance of circulating filarial antigen (CFA)-based assays was acknowledged to be of high interest in areas with low residual LF endemicity rates after multiple rounds of MDA. The objective of this study was therefore to evaluate the immunochromatographic test (ICT) sensitivity in low endemicity settings and, specifically, in individuals with low intensity of lymphatic filariasis infection.

METHODS

To perform this study, calibrated thick blood smears, ICT and Og4C3 enzyme-linked immunosorbent assay (ELISA) were carried out by night to identify Wuchereria bancrofti microfilarial and circulating filarial antigen carriers. A threshold determination assay regarding ICT and ELISA was performed using serial plasma dilutions from individuals with positive microfilarial counts.

RESULTS

All individuals harbouring microfilariae (positive blood films) were detected by ICT and ELISA, but among individuals positive for ELISA, only 35.7 % of them were detected using ICT (Chi square: 4.57; p-value = 0.03), indicating a moderate agreement between both tests (kappa statistics = 0.49). Threshold determination analyses showed that ELISA was still positive at the last plasma dilution with negative ICT result.

CONCLUSIONS

These findings suggest a loss of sensitivity for ICT in low endemicity settings, especially in people exhibiting low levels of circulating filarial antigen, raising serious concern regarding the monitoring and evaluation procedures in the framework of LF elimination program.

Diagnosis of brugian filariasis by loop-mediated isothermal amplification

In this study we developed and evaluated a Brugia Hha I repeat loop-mediated isothermal amplification (LAMP) assay for the rapid detection of Brugia genomic DNA. Amplification was detected using turbidity or fluorescence as readouts. Reactions generated a turbidity threshold value or a clear visual positive within 30 minutes using purified genomic DNA equivalent to one microfilaria. Similar results were obtained using DNA isolated from blood samples containing B. malayi microfilariae. Amplification was specific to B. malayi and B. timori, as no turbidity was observed using DNA from the related filarial parasites Wuchereria bancrofti, Onchocerca volvulus or Dirofilaria immitis, or from human or mosquito. Furthermore, the assay was most robust using a new strand-displacing DNA polymerase termed Bst 2.0 compared to wild-type Bst DNA polymerase, large fragment. The results indicate that the Brugia Hha I repeat LAMP assay is rapid, sensitive and Brugia-specific with the potential to be developed further as a field tool for diagnosis and mapping of brugian filariasis.

Brugian filariasis is a debilitating neglected tropical disease caused by infection with the filarial parasites Brugia malayi or Brugia timori. Adult worms live in the lymphatic system and produce large numbers of microfilariae that predominantly circulate in the blood at night. Bloodsucking mosquitoes spread the disease by ingesting microfilariae that develop into infective stage larvae in the insect. In rural areas, diagnosis still relies largely on microscopic examination of night blood and morphological assessment of stained microfilariae. Loop-mediated isothermal amplification (LAMP) is a technique that can amplify DNA with high specificity, sensitivity and rapidity under isothermal conditions. The operational simplicity, versatility and low-cost of the technique make it particularly appealing for use in diagnosis and geographical mapping of neglected tropical diseases. In the present study, we have developed and evaluated a Brugia Hha I repeat LAMP assay for the rapid detection of B. malayi and B. timori genomic DNA. The results indicate that the Brugia Hha I repeat LAMP diagnostic assay is sensitive and rapid, detecting a single microfilariae in blood within 30 minutes, and Brugia-specific. The test has the potential to be developed further as a field tool for use in the implementation and management of mass drug administration programs for brugian filariasis.

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.

Development of loop-mediated isothermal amplification method for detecting Wuchereria bancrofti DNA in human blood and vector mosquitoes

We have developed loop-mediated isothermal amplification (LAMP) method to detect Wuchereria bancrofti DNA. The sensitivity and specificity of LAMP method were equivalent to those of PCR method which detects SspI repeat sequence in W. bancrofti genomic DNA: both methods detected one thousandth of W. bancrofti DNA from one microfilaria (Mf), and did not cross-react with DNAs of Brugia malayi, B. pahangi, Dirofilaria immitis, human and Culex quinquefasciatus. We also examined the sensitivity of LAMP using the mimic samples of patient's blood or blood-fed mosquitoes containing one W. bancrofti Mf per sample. The LAMP method was able to detect W. bancrofti DNA in 1000 μl of blood or in a pool of 60 mosquitoes, indicating its usefulness in detecting/monitoring W. bancrofti infection in humans and vector mosquitoes in endemic areas.

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 role of monitoring mosquito infection in the Global Programme to Eliminate Lymphatic Filariasis

In addition to monitoring infection in the human host, there is also a need to assess larval infection in the vector mosquito population to evaluate the success of interventions for eliminating lymphatic filariasis transmission from endemic communities. Here, we review the current status of the available tools for quantifying vector infection and existing knowledge and evidence regarding potential infection thresholds for determining transmission interruption, to assess the potential for using vector infection monitoring as a tool for evaluating the success of filariasis treatment programmes.

Serum immune complexes as diagnostic and therapeutic markers in lymphatic filariasis

In the present study we examined the utility of measuring antigen-specific soluble immune complexes (ICs) for the differential diagnosis and therapeutic monitoring of Bancroftian filariasis. Antigen-specific ICs were detected in significantly elevated levels in 90% of lymphatic filarial subjects suffering from disease manifestations, and very low levels in 8-10% of microfilaria (MF) carriers. The high incidence of antigen-specific ICs in the circulation of filarial subjects with clinical manifestations in contrast to the negligible presence in MF carriers facilitates serological differentiation of these two important stages of lymphatic filarial infection. A sudden spurt in circulating levels of antigen-specific ICs was recorded in all MF carriers on diethylcarbamazine (DEC) therapy, and the magnitude of the increase correlated positively with the pretreatment levels of circulating MF. Thus, it is evident from the present study that circulating ICs (CICs) are potential serological markers for both the differential diagnosis of filarial infection and therapeutic monitoring of MF carriers.

Detection of Brugia parasite DNA in human blood by real-time PCR

Brugian filariasis (caused by the nematodes Brugia malayi and B. timori) is an important cause of disability in Southeast Asia. Improved diagnostic tests are needed for filariasis elimination programs (to identify areas of endemicity and to monitor progress) and for diagnosis of the disease in infected individuals. We have developed and evaluated two real-time PCR assays for detecting Brugia DNA in human blood and compared the results of these assays to those of "gold standard" assays. One assay uses a TaqMan probe (TaqM) to amplifiy a 320-bp "HhaI repeat" DNA sequence. The other assay uses a minor groove binding probe (MGB) and modified nucleotides in primers (Eclipse MGB) to amplify a 120-bp fragment of the HhaI repeat. This assay detects 22 copies of the target sequence, and it is more sensitive than the TaqM assay. Both assays were evaluated with human blood samples from two different areas of endemicity. The MGB assay was as sensitive as membrane filtration and microscopy for the detection of B. malayi infection in 57 blood samples recovered at night from patients in Sulawesi, Indonesia. The MGB assay also detected parasite DNA in 17 of 31 (55%) of microfilaria-negative day blood samples from these subjects. This test was more sensitive than the conventional and the TaqM PCRs (and was almost as sensitive as night blood membrane filtration) for the detection of infection in 52 blood samples recovered at night from individuals in an area of B. timori endemicity on Alor Island, Indonesia, where microfilaria-positive individuals had low densities after mass treatment. Thus, the Eclipse MGB real-time PCR assay is a sensitive means of detecting Brugia parasite DNA in human blood.

PCR and dissection as tools to monitor filarial infection of Aedes polynesiensis mosquitoes in French Polynesia

Background

Entomological methods may provide important tools for monitoring the transmission of filariasis in French Polynesia. In order to standardize our PCR method and refine our protocol to assess filarial infection levels in mosquitoes, we compared dissection of the vector, Aedes polynesiensis, with the poolscreening polymerase chain reaction (PS-PCR) assay.

Methods

(1) Mosquitoes were collected in human landing catches in five areas in Moorea island, French Polynesia. (2) A fraction of the captured mosquitoes was dissected for Wuchereria bancrofti larvae. (3) Laboratory-reared mosquitoes (uninfected as well as experimentally infected ones) were repeatedly tested to optimize a PS-PCR protocol (DNA extracts from 1–50 pooled mosquitoes were tested with an internal standardized system and primers specific for the Ssp1 repeat sequence. PCR products were analysed by gel electrophoresis). (4) Another fraction of the captured mosquitoes was assayed by PS-PCR according the optimized protocol.

Results

The prevalence of field-mosquito infection with W. bancrofti ranged from 1 to 8 % by dissection (L1–L3) and point estimates of infection prevalence, as assayed by PS-PCR, ranged from 0.4 to 3.7 %. There was a moderately strong correlation between larval infection rates as determined by dissection and PCR.

Discussion

Our results suggest that the PS-PCR assay is specific and highly sensitive for detecting parasite DNA. We obtained similar although not identical results with dissections of mosquitoes. PS-PCR appears to be adequate for testing large numbers of mosquitoes in the context of filariasis elimination programs. The role and advantages of using entomologic methods to monitor filariasis programs are discussed.

PCR and dissection as tools to monitor filarial infection of Aedes polynesiensis mosquitoes in French Polynesia

BACKGROUND

Entomological methods may provide important tools for monitoring the transmission of filariasis in French Polynesia. In order to standardize our PCR method and refine our protocol to assess filarial infection levels in mosquitoes, we compared dissection of the vector, Aedes polynesiensis, with the poolscreening polymerase chain reaction (PS-PCR) assay.

METHODS

(1) Mosquitoes were collected in human landing catches in five areas in Moorea island, French Polynesia. (2) A fraction of the captured mosquitoes was dissected for Wuchereria bancrofti larvae. (3) Laboratory-reared mosquitoes (uninfected as well as experimentally infected ones) were repeatedly tested to optimize a PS-PCR protocol (DNA extracts from 1-50 pooled mosquitoes were tested with an internal standardized system and primers specific for the Ssp1 repeat sequence. PCR products were analysed by gel electrophoresis). (4) Another fraction of the captured mosquitoes was assayed by PS-PCR according the optimized protocol.

RESULTS

The prevalence of field-mosquito infection with W. bancrofti ranged from 1 to 8 % by dissection (L1-L3) and point estimates of infection prevalence, as assayed by PS-PCR, ranged from 0.4 to 3.7 %. There was a moderately strong correlation between larval infection rates as determined by dissection and PCR.

DISCUSSION

Our results suggest that the PS-PCR assay is specific and highly sensitive for detecting parasite DNA. We obtained similar although not identical results with dissections of mosquitoes. PS-PCR appears to be adequate for testing large numbers of mosquitoes in the context of filariasis elimination programs. The role and advantages of using entomologic methods to monitor filariasis programs are discussed.

Estimation of the prevalence of lymphatic filariasis by a pool screen PCR assay using blood spots collected on filter paper

The prevalence of lymphatic filariasis was estimated by PCR-based pool screening of night blood collected from 865 individuals living in ten areas endemic for Wuchereria bancrofti, Brugia malayi or B. timori in Indonesia. A total of 232 microfilaraemics were identified by filtration of 1 ml of blood. The microfilaria (mf) prevalence ranged from 6% to 54%, and the mf density in microfilaraemics ranged from 1 mf/ml to 6028 mf/ml. PCR assays both for W. bancrofti or Brugia spp. detected a single mf present on a 30 microl dried filter paper blood spot. One hundred and seventy-eight pools of five blood spots in each pool (pool-5) were tested by PCR and 101 (57%) pools were positive. When pool size was increased to 10 spots per pool (pool-10), 65 (70%) of 93 pools were positive. Pearson's correlation and linear regression showed a strong correlation between filtration and pool screen PCR results for pool-10 (r=0.835) and pool-5 (r=0.695). Based on the determination coefficient (R), the results of pool-10 PCR (R=0.697) gave a better prediction compared with pool-5 PCR (R=0.483). This study suggests that pool screen PCR may be a useful tool for monitoring the Global Program to Eliminate Lymphatic Filariasis.

PCR and Mosquito dissection as tools to monitor filarial infection levels following mass treatment

BACKGROUND: Entomological methods may provide important tools for monitoring the progress of lymphatic filariasis elimination programs. In this study, we compared dissection of the vector, Culex quinquefasciatus, with the polymerase chain reaction (PCR) to assess filarial infection levels in mosquitoes in the context of a lymphatic filariasis elimination program in Leogane, Haiti. METHODS: Mosquitoes were collected using gravid traps located in 4 sentinel communities with Wuchereria bancrofti microfilaria prevalence that ranged from 0.8% to 15.9%. Captured mosquitoes were divided between dissection, to enumerate W. bancrofti larvae (L1, L2, L3) and desiccation for later analysis by PCR. PCR was conducted on DNA extracts from pooled mosquitoes (1-15 pooled females) utilizing a competitive PCR system with primers specific for the Ssp I repeat. PCR products were analyzed with a hybridization ELISA using probes specific for a control sequence and the Ssp I repeat. RESULTS: The prevalence of mosquito infection with W. bancrofti ranged from 0%-3.66% by dissection (L1-L3) and point estimates of infection prevalence, as assayed by PCR, ranged from 0.25% - 9.16%. Following mass treatment, W. bancrofti infection prevalence dropped significantly as determined by PCR and dissection in 2 of the 4 sentinel sites (Leogane and Barrier Jeudi, P = 0.04 and P = 0.005, respectively). Although transmission declined in the other two sites, larval recoveries were low and these changes were not statistically significant. DISCUSSION: Our results suggest that a single round of mass treatment can have an impact on transmission of lymphatic filariasis. The use of entomologic methods as a tool to monitor filariasis programs and the statistical limitations of mosquito trapping are discussed.

Diagnosis of human filariases (except onchocerciasis)

The traditional method of diagnosing filarial infections is to examine blood or skin samples for microfilariae and for many this is still the standard procedure. However, since the present global campaign to eliminate lymphatic filariasis new diagnostic tools have emerged like PCR, antigen detection using finger-prick blood taken during the day and ultrasound to visualize adult worms. The last two can be applied in endemic countries with limited resources and enable the detection of early infections. As well as their value in control schemes, the latter is particularly important for the individual since recent research has shown that damage is usually caused long before symptoms appear. The usefulness in different situations and the advantages and disadvantages of the various new tools for diagnosis of lymphatic filariasis are discussed. For loiasis, immunodiagnostic methods have not been very successful but repetitive DNA sequences in the Loa genome have been found to be species specific. Techniques based on them are particularly useful for diagnosing cases of occult infection without microfilaraemia. There have been no advances in the diagnosis of Mansonella perstans but both immunodiagnostic and PCR tests show promise in differentiating M. streptocerca, and the latter in differentiating M.ozzardi, from Onchocerca. In addition to the human filariae, the dog parasites Dirofilaria immitis and D. repens can also occur in humans but do not produce microfilariae in them. ELISAs and PCR probes have been devised and can usefully differentiate between pulmonary dirofilariasis and lung cancer.

Field application of PCR-based assays for monitoring Wuchereria bancrofti infection in Africa

Approximately 50 million people in Egypt and sub-Saharan Africa have bancroftian filariasis and together they represent about a third of all cases of lymphatic filariasis (LF) world-wide. Currently, the Global Programme to Eliminate Lymphatic Filariasis, which was launched by the World Health Organization (WHO) in 1998, is largely based on repeated annual cycles of mass drug administration (MDA) to endemic populations. Also, some countries, including Egypt, are taking steps to improve LF vector-control interventions, to break the transmission cycle more effectively than is achievable with MDA alone. New tools and strategies for monitoring and evaluating elimination campaigns are needed. The last 20 years have witnessed dramatic advances in the diagnosis of LF for epidemiological purposes. The recent introduction and development of molecular technologies have moved parasite-detection systems from traditional methods (that are labour-intensive, tedious and often impractical) to improved PCR-based assays that have considerable potential for field use. The present article highlights the strengths and limitations of the PCR-based assays when used to detect filarial infections in mosquitoes (particularly for the xenomonitoring of elimination campaigns).

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.

An internal control for the detection of Onchocerca volvulus DNA by PCR-ELISA and rapid detection of specific PCR products by DNA Detection Test Strips

We developed a polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA) for the detection of Onchocerca volvulus DNA. To standardize the PCR and to avoid false-negative results, an internal control DNA was co-amplified by the same set of primers. We differentiated the wild-type PCR product of the O-150 DNA sequence from the internal control by specific DNA probes. Detection of biotinylated PCR products by DNA probes was performed by ELISA to quantify the PCR product or by DNA Detection Test Strips as a rapid field technique. The methods were evaluated on skin biopsies from individuals living in an area endemic for O. volvulus in Uganda, but with low microfilaria densities because of ivermectin treatment. Microfilaria density was assessed by a single skin snip and a second skin snip was examined by PCR. Among 69 samples from microfilaria carriers, 47 (68%) were positive by ELISA and 55 (80%) by test strip detection of PCR products. When 39 samples of microfilaria-negative individuals from the same area were tested, 10 (27%) were positive by ELISA and 12 (31%) by test strips. None of the 19 samples obtained from persons living in an area not endemic for O. volvulus but endemic for Mansonella streptocerca was positive in either test. Although the ELISA is theoretically more sensitive than the test strips for the detection of PCR products, examination of field samples revealed that the test strip method had a higher operational sensitivity and was more convenient to perform. Thus, the DNA Detection Test Strips are a rapid and low-tech tool for identification of PCR products in laboratories of countries endemic for onchocerciasis.

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.

Quantification of PCR amplification products of Brugia HhaI repeat DNA using a semiautomated Q-PCR system

The sensitive, rapid and species-specific diagnosis of Brugia infections in humans or animal models is important in determining the level of parasitemia and the efficacy of chemotherapy or vaccinations. The HhaI family of highly repeated DNA sequences from Brugia have been useful in polymerase chain reaction (PCR)-based diagnosis of brugian filarial infections in blood samples and in mosquitoes. A PCR assay was developed using a biotinylated primer, a non-biotinylated primer and a species-specific chemiluminescent probe [tris[2,2'bipyridine] ruthenium (II) chelate, TBR] to detect PCR amplified Hhal family repeats. Individual blood samples from jirds infected with Brugia malayi or B. pahangi and with different levels of microfilaremia were tested in this assay. Known concentrations of Brugia DNA and DNA from the blood of uninfected control jirds were used as positive and negative controls, respectively. The PCR products generated by this method were analyzed using a semi-automated quantitative (Q)-PCR system. The levels of parasite DNA can be calculated from the luminosity units generated. Significant amounts of parasite DNA were detected in blood samples from infected jirds, and these values were correlated with the levels of microfilaremia. In contrast, reductions in circulating microfilaria following treatment with ivermectin correlated with low levels of measurable DNA. Using this system, we were also able to detect HhaI repeat DNA in the spleens of B. pahangi- infected jirds at 56 days post-infection when circulating microfilariae were not readily detectable. The results indicate that the species-specific Hhal Q-PCR detection and quantification method is rapid and sensitive, is useful in the detection of Brugia DNA in blood and other tissues and is suited for use in clinical settings because it does not require radioactive isotopes and gel-based protocols.

A stress-responsive glyoxalase I from the parasitic nematode Onchocerca volvulus

Glyoxal, methylglyoxal and other physiological alpha-oxoaldehydes are formed by the lipid peroxidation, glycation and degradation of glycolytic intermediates. They are detoxified enzymically by the glyoxalase system. To investigate the physiological function of glyoxalase I in parasitic organisms, the cDNA for glyoxalase I from the filarial nematode Onchocerca volvulus (designated Ov-GloI) has been cloned and characterized. The isolated cDNA contains an open reading frame of 579 bp encoding a protein with a calculated molecular mass of 21930 Da. Owing to the high degree of sequence identity (60%) with human glyoxalase I, for which the X-ray structure is available, it has been possible to build a three-dimensional model of Ov-GloI. The modelled core of Ov-GloI is conserved compared with the human glyoxalase I; however, there are critical differences in the residues lining the hydrophobic substrate-binding pocket of Ov-GloI. A 22 kDa protein was obtained by heterologous expression in Escherichia coli. A homogeneous enzyme preparation was obtained by affinity purification and functional characterization of the recombinant enzyme included the determination of kinetic constants for methylglyoxal and phenylglyoxal as well as inhibition studies. Gel filtration demonstrated a dimeric structure. To assess the role of Ov-GloI as a potential vaccine candidate or serodiagnostic tool, the serological reactivity of the recombinant Ov-GloI was analysed with sera from microfilaria carriers and specific IgG1 antibodies were detected. The effects of oxidative insult, namely plumbagin and xanthine/xanthine oxidase, on the gene transcript level of Ov-GloI were investigated. By using a semi-quantitative PCR ELISA it was shown that Ov-GloI is expressed at elevated levels under conditions of oxidative stress.

Identification of a stress-responsive Onchocerca volvulus glutathione S-transferase (Ov-GST-3) by RT-PCR differential display

The effects of oxidative insult on gene transcript levels in the filarial nematode Onchocerca volvulus were investigated using differential display RT-PCR. Oxidative stress was applied with the reagents paraquat, plumbagin and xanthine-xanthine oxidase. In all three cases, a cDNA fragment encoding a novel glutathione S-transferase (GST) resembling members of the theta-class was identified as upregulated (PQ29, PG112, XOD26). The subsequently isolated full-length cDNA harbors a 753-bp open reading frame encoding a GST with 268 amino acid residues and a predicted molecular mass of 31 kDa. This stress-responsive GST (Ov-GST-3) possesses only 14 and 21% sequence identity with the other O. volvulus GSTs (Ov-GST-1 and Ov-GST-2, respectively). Interestingly, Ov-GST-3 shares higher sequence identity with GSTs that are upregulated due to environmental stress. In order to confirm the specific upregulation of the Ov-GST-3 transcripts identified by differential display and to analyze the mRNA levels of the other Ov-GSTs (Ov-GST-1 and Ov-GST-2) under elevated stress conditions, a semi-quantitative polymerase chain reaction-enzyme-linked immunosorbent assay was performed. The Ov-GST-3 gene transcript level increased dramatically in response to xanthine-xanthine oxidase and to a lesser extent with paraquat and plumbagin. In contrast, Ov-GST-1 and Ov-GST-2 did not show any significant alterations in their steady-state mRNA levels in response to oxidative stress when examining the same mRNA samples. The present study clearly demonstrates that Ov-GST-3 is a critical enzyme in the defense against oxidative stress.

The filarial genome project: analysis of the nuclear, mitochondrial and endosymbiont genomes of Brugia malayi

The Filarial Genome Project (FGP) was initiated in 1994 under the auspices of the World Health Organisation. Brugia malayi was chosen as the model organism due to the availability of all life cycle stages for the construction of cDNA libraries. To date, over 20000 cDNA clones have been partially sequenced and submitted to the EST database (dbEST). These ESTs define approximately 7000 new Brugia genes. Analysis of the EST dataset provides useful information on the expression pattern of the most abundantly expressed Brugia genes. Some highly expressed genes have been identified that are expressed in all stages of the parasite's life cycle, while other highly expressed genes appear to be stage-specific. To elucidate the structure of the Brugia genome and to provide a basis for comparison to the Caenorhabditis elegans genome, the FGP is also constructing a physical map of the Brugia chromosomes and is sequencing genomic BAC clones. In addition to the nuclear genome, B. malayi possesses two other genomes: the mitochondrial genome and the genome of a bacterial endosymbiont. Eighty percent of the mitochondrial genome of B. malayi has been sequenced and is being compared to mitochondrial sequences of other nematodes. The bacterial endosymbiont genome found in B. malayi is closely related to the Wolbachia group of rickettsia-like bacteria that infects many insect species. A set of overlapping BAC clones is being assembled to cover the entire bacterial genome. Currently, half of the bacterial genome has been assembled into four contigs. A consortium has been established to sequence the entire genome of the Brugia endosymbiont. The sequence and mapping data provided by the FGP is being utilised by the nematode research community to develop a better understanding of the biology of filarial parasites and to identify new vaccine candidates and drug targets to aid the elimination of human filariasis.

The control of the human filariases

There is a growing momentum for the global control and elimination of the major human filariases as public health problems worldwide. The renewed optimism for undertaking this objective reflects the development of simple and potentially cost-effective strategies for mass drug delivery in onchocerciasis and the availability of new extremely effective drug combinations to treat infection, and new methods of morbidity control in lymphatic filariasis. It also reflects the development and current availability of very effective diagnostic, surveillance and control modeling tools for both parasites. Control programming will also be aided by our greater understanding of the biology of transmission, host immunity and disease pathogenesis.