Real-Time Polymerase Chain Reaction Method for the Detection of Onchocerca volvulus in Post-Elimination Surveillance of Onchocerciasis in Ecuador

Onchocerciasis has been declared eliminated in Ecuador and surveillance measures are of great interest. In this study, we examined the infectivity rates of Simulium exiguum by Onchocerca volvulus in previously hyperendemic areas in Esmeraldas province of Ecuador. These areas had previously undergone mass administration of ivermectin, which led to the interruption of transmission in 2009 and the certification of elimination in 2014. The study included three communities in Río Cayapas and one in Río Canandé, and a total of 2,950 adult S. exiguum were collected in 2018. We used quantitative polymerase chain reaction with O. volvulus O-150 plasmid control DNA to analyze 59 pools. Our findings revealed that the infectivity rates were zero, indicating that the transmission of O. volvulus remained suspended in the area.

Optimized strategy for real-time qPCR detection of Onchocerca volvulus DNA in pooled Simulium sp. blackfly vectors

BACKGROUND

Onchocerca volvulus is a filarial parasite that is a major cause of dermatitis and blindness in endemic regions primarily in sub-Saharan Africa. Widespread efforts to control the disease caused by O. volvulus infection (onchocerciasis) began in 1974 and in recent years, following successful elimination of transmission in much of the Americas, the focus of efforts in Africa has moved from control to the more challenging goal of elimination of transmission in all endemic countries. Mass drug administration (MDA) with ivermectin has reached more than 150 million people and elimination of transmission has been confirmed in four South American countries, with at least two African countries having now stopped MDA as they approach verification of elimination. It is essential that accurate data for active transmission are used to assist in making the critical decision to stop MDA, since missing low levels of transmission and infection can lead to continued spread or recrudescence of the disease.

METHODOLOGY/PRINCIPAL FINDINGS

Current World Health Organization guidelines for MDA stopping decisions and post-treatment surveillance include screening pools of the Simulium blackfly vector for the presence of O. volvulus larvae using a PCR-ELISA-based molecular technique. In this study, we address the potential of an updated, practical, standardized molecular diagnostic tool with increased sensitivity and species-specificity by comparing several candidate qPCR assays. When paired with heat-stable reagents, a qPCR assay with a mitochondrial DNA target (OvND5) was found to be more sensitive and species-specific than an O150 qPCR, which targets a non-protein coding repetitive DNA sequence. The OvND5 assay detected 19/20 pools of 100 blackfly heads spiked with a single L3, compared to 16/20 for the O150 qPCR assay.

CONCLUSIONS/SIGNIFICANCE

Given the improved sensitivity, species-specificity and resistance to PCR inhibitors, we identified OvND5 as the optimal target for field sample detection. All reagents for this assay can be shipped at room temperature with no loss of activity. The qPCR protocol we propose is also simpler, faster, and more cost-effective than the current end-point molecular assays.

First report of filarial nematodes in the genus Onchocerca infecting black flies (Diptera: Simuliidae) in Iran

Black flies are blood-sucking insects of public health importance, and they are effective vectors of pathogens and parasites, such as filarial nematodes of the genus Onchocerca. Our previous surveys have shown that individuals of Simulium turgaicum are annoying pests of humans and livestock in the Aras River Basin of Iran. In the present study, adult black flies of S. turgaicum were trapped from different ecotopes of five villages in Khoda-Afarin County, Iran. By using a sensitive nested PCR assay and targeting the nuclear 18S rDNA-ITS1 marker, filarial infections were found in 38 (1.89%) of 2005 black flies. Homology exploration of 360 bp of the sequences indicated that the filarial worms are members of the family Onchocercidae, with maximum alignment scores of 93-95%. Phylogenetic analysis showed that two Iranian Onchocerca isolates were clustered in the O. fasciata-O. volvulus lineage and were well separated from other filarial nematodes. Both the entomological evidence (empty abdomen of the specimens) and climatologic data (adequate accumulated degree days for development) suggest that the filarial DNA was probably that of infective larvae of vertebrates. This is the first report of an infection by Onchocerca species in S. turgaicum and the first record of onchocercids in black flies in Iran; however, more research is required to demonstrate transmission of these filarial worms by black flies in nature.

LupiQuant: A real-time PCR based assay for determining host-to-parasite DNA ratios of Onchocerca lupi and host Canis lupus from onchocercosis samples

Onchocerca lupi is a filarial nematode that causes ocular onchocercosis in canines globally including North America and areas of Europe, North Africa, and the Middle East. Reported incidence of this parasite in canines has continued to steadily escalate since the early 21st century and was more recently documented in humans. Whole genome sequencing (WGS) of this parasite can provide insight into gene content, provide novel surveillance targets, and elucidate the origin and range expansion. However, past attempts of whole genome sequencing of other Onchocerca species reported a substantial portion of their data unusable due to the variable over-abundance of host DNA in samples. Here, we have developed a method to determine the host-to-parasite DNA ratio using a quantitative PCR (qPCR) approach that relies on two standard plasmids each of which contains a single copy gene specific to the parasite genus Onchocerca (major body wall myosin gene, myosin) or a single copy gene specific to the canine host (polycystin-1 precursor, pkd1). These plasmid standards were used to determine the copy number of the myosin and pkd1 genes within a sample to calculate the ratio of parasite and host DNA. Furthermore, whole genome sequence (WGS) data for three O. lupi isolates were consistent with our host-to-parasite DNA ratio results. Our study demonstrates, despite unified DNA extraction methods, variable quantities of host DNA within any one sample which will likely affect downstream WGS applications. Our quantification assay of host-to-parasite genome copy number provides a robust and accurate method of assessing canine host DNA load in an O. lupi specimen that will allow informed sample selection for WGS. This study has also provided the first whole genome draft sequence for this species. This approach is also useful for future focused WGS studies of other parasites.

Geostatistical modelling enables efficient safety assessment for mass drug administration with ivermectin in Loa loa endemic areas through a combined antibody and LoaScope testing strategy for elimination of onchocerciasis

The elimination of onchocerciasis through community-based Mass Drug Administration (MDA) of ivermectin (Mectizan) is hampered by co-endemicity of Loa loa, as individuals who are highly co-infected with Loa loa parasites can suffer serious and occasionally fatal neurological reactions from the drug. The test-and-not-treat strategy of testing all individuals participating in MDA has some operational constraints including the cost and limited availability of LoaScope diagnostic tools. As a result, a Loa loa Antibody (Ab) Rapid Test was developed to offer a complementary way of determining the prevalence of loiasis. We develop a joint geostatistical modelling framework for the analysis of Ab and Loascope data to delineate whether an area is safe for MDA. Our results support the use of a two-stage strategy, in which Ab testing is used to identify areas that, with acceptably high probability, are safe or unsafe for MDA, followed by Loascope testing in areas whose safety status is uncertain. This work therefore contributes to the global effort towards the elimination of onchocerciasis as a public health problem by potentially reducing the time and cost required to establish whether an area is safe for MDA.

Considering the serious adverse events that occur in individuals with high intensity of Loa loa parasite when being treated with ivermectin during the MDA program for the elimination of onchocerciasis, there is need for a comprehensive, safe and cost-effective strategy to delineate village or communities that are safe for MDA. In this study, we propose a hybrid strategy that uses information from the Loa antibody rapid test and the LoaScope diagnostic test to delineate whether an area is safe for MDA. We developed a joint geostatistical modelling framework that exploits both the association between antibody and LoaScope responses at community-level and the spatial correlation of the Loa loa prevalence surface to determine if the risk of observing individuals with high-intensity infections in a village or community is sufficiently low. Our results support the use of a two-stage strategy in which antibody test is used first as a screening tool and only those communities for which safety are in doubt are followed up with confirmatory LoaScope testing.

First description of Onchocerca flexuosa infections in Danish red deer (Cervus elaphus)

The prevalence of Onchocerca infection in wild cervids from Denmark was studied in 119 fallow deer (Dama dama), 123 red deer (Cervus elaphus), 51 roe deer (Capreolus capreolus) and eight sika deer (Cervus Nippon) shot during the hunting season from October 2017 to January 2018 from 18 geographical locations across Denmark. The carcasses were macroscopic checked for nodules, and skin samples were examined for microfilaria. All roe deer, fallow deer and sika deer were negative for Onchocerca, while 30.9% red deer were positive for either microfilaria, nodules or both. Significantly more adult red deer (50.8%; 37.6-62.4; p < 0.0001) were infected with Onchocerca than juveniles <1 year (7.8%; 2.1-18.5), while there was an insignificant (p = 0.075) difference in prevalence observed between males (17.4%; 7.8-31.4) and females (41.7%; 30.2-53.9). Onchocerca-positive red deer were observed from 91.7% (11/12) of the sampled geographical locations. Species identification was done on adult worms from nodules taken from the lumbar region of 20 red deer of different geographical origin by sequencing the mitochondrial 12S, 16S and nad5 gene fragments. The sequences matched with previously published sequences for Onchocerca flexuosa. The high prevalence of Onchocerca infection caused by O. flexuosa in red deer in Denmark shows that Denmark has favourable vector conditions and a suitable environment for the maintenance of the parasite. To our knowledge, this is the first systematic study of Onchocerca in wild-living cervids in Denmark.

Diversity of nematodes infecting the human-biting black fly species, Simulium nigrogilvum (Diptera: Simuliidae) in central Thailand

Black flies (Diptera: Simuliidae) are known as vectors of disease agents in humans and livestock, with some species being vectors of Onchocerca volvulus, the filarial nematode that is the causative agent of human onchocerciasis. Nematode infections in adult female black flies have been reported from some areas in northern and western Thailand, but not from other regions of Thailand. In this study, wild-caught adult female black flies from the central region of Thailand were examined for infections with nematodes. Collections of adult females were carried out at Khlong Lan district, Kamphaeng Phet province, central Thailand. A molecular approach, based on the mitochondrial (cox1, 12S rRNA) and nuclear (18S rRNA) genes, was used to identify the species of nematodes recovered from the specimens collected. A total of 911 wild-caught adult black flies were collected. Simulium nigrogilvum was the most abundant species (n = 708), followed by S. doipuiense complex (n = 179), S. chamlongi (n = 11), S. umphangense (n = 10), S. chumpornense (n = 1), S. multistriatum species-group (n = 1), and S. maewongense (n = 1). Nematode infections were detected in nine specimens of S. nigrogilvum, of which two were positive for filarial worms (one worm each, infection rate 0.28%) and seven were positive for non-filarial nematodes (11 worms in total, infection rate 0.99%). The two filarial nematodes (third-stage larvae) were identified molecularly as Onchocerca species type I, while the 11 non-filarial nematodes were classified into ascaridoid (n = 2), tylenchid (n = 6) and mermithid (n = 3) nematodes. The results of this study demonstrated that adult female S. nigrogilvum were parasitized with diverse nematodes (filarial and non-filarial). Detection of the infective larvae of Onchocerca sp. type I in S. nigrogilvum confirms that occurrence of zoonotic onchocerciasis is highly possible in Thailand. Additional in-depth investigation of the morphology, life cycle and host-parasite relationship of nematodes that parasitized this black fly host is still needed.

Coyotes as Reservoirs for Onchocerca lupi, United States, 2015-2018

The Onchocerca lupi nematode infects dogs, cats, and humans, but whether it can be spread by coyotes has been unknown. We conducted surveillance for O. lupi nematode infection in coyotes in the southwestern United States. We identified multiple coyote populations in Arizona and New Mexico as probable reservoirs for this species.

The Mbam drainage system and onchocerciasis transmission post ivermectin mass drug administration (MDA) campaign, Cameroon

Background

The impact of large scale Mass Drug Adminstration (MDA) of ivermectin on active onchocerciasis transmission by Simulium damnosum, which transmits the parasite O. volvulus is of great importance for onchocerciasis control programmes. We investigated in the Mbam river system area, the impact of MDA of ivermectin on entomological indices and also verify if there are river system factors that could have favoured the transmission of onchocerciasis in this area and contribute to the persistence of disease. We compared three independent techniques to detect Onchocerca larvae in blackflies and also analyzed the river system within 9 months post-MDA of ivermectin.

Method

Simulium flies were captured before and after 1, 3, 6 and 9months of ivermectin-MDA. The biting rate was determined and 41% of the flies dissected while the rest were grouped into pools of 100 flies for DNA extraction. The extracted DNA was then subjected to O-150 LAMP and real-time PCR for the detection of infection by Onchocerca species using pool screening. The river system was analysed and the water discharge compared between rainy and dry seasons.

Principal findings

We used human landing collection method (previously called human bait) to collect 22,274 adult female Simulium flies from Mbam River System. Of this number, 9,134 were dissected while 129 pools constituted for molecular screening. Overall biting and parous rates of 1113 flies/man/day and 24.7%, respectively, were observed. All diagnostic techniques detected similar rates of O. volvulus infection (P = 0.9252) and infectivity (P = 0.4825) at all monitoring time points. Onchocerca ochengi larvae were only detected in 2 of the 129 pools. Analysis of the river drainage revealed two hydroelectric dams constructed on the tributaries of the Mbam river were the key contributing factor to the high-water discharge during both rainy and dry seasons.

Conclusion

Results from fly dissection (Microscopy), real-time PCR and LAMP revealed the same trends pre- and post-MDA. The infection rate with animal Onchocerca sp was exceptionally low. The dense river system generate important breeding sites that govern the abundance of Simulium during both dry and rainy seasons.

The presence of parasite strains that respond sub-optimally to an approved drug, favourable breeding sites for the vector and infected individual in an area, will surely provide conditions for continuous and persistent transmission of a disease despite a long-term control intervention. We investigated the impact of ivermectin on entomological indices within 9 months following a large-scale MDA in the Mbam river drainage. The river system factors that could have favoured abundance vector breeding and contribute to the persistence of disease transmission were also examined within the study period. We observed vector abundance and high entomological indices throughout the study period following ivermectin MDA. We also observed high water discharge along the main river of the drainage basin in both the rainy and dry seasons and this is due to the presence of two dams constructed upstream at Bamendjing and Mape to regularize the course of river Sanaga in view of generating hydroelectric power at Edea. Factors favouring continuous and persistent disease transmission are present in this drainage basin despite over 20 years of annual IVM-MDA. There is need for alternative control strategy in order to accelerate the fight against onchocerciasis in the area.

Human case of Onchocerca dewittei japonica infection in Fukushima, Northeastern Honshu, Japan

A 73-year-old man living in Kawamata-machi, Fukushima Prefecture, Northeastern Honshu, Japan, visited a hospital with complaints of a subcutaneous swelling that had developed on the back of his left hand. The nodule was surgically removed from the vagina fibrosa tendinis of his left forefinger. Based on the histopathological characteristics, the causative agent of this nodule was identified as a female Onchocerca dewittei japonica (Spirurida: Onchocercidae). The species identification was confirmed by cox1 gene sequencing of the worm tissues from paraffin-embedded sections of the nodule. Although 11 cases of zoonotic onchocercosis have previously been recorded in Kyushu and Western Honshu, Japan, the present findings represent the first human case of infection with O. dewittei japonica in Northeastern Honshu, Japan.

Morphological and molecular characterization of Onchocerca fasciata (Nematoda, Onchocercidae) from dromedary camels (Camelus dromedarius) in Iran

Skin nodules of Onchocerca fasciata Railliet and Henry, 1910 (Spirurida, Onchocercidae) are a common finding in dromedary camels, though with a minimal clinical impact. There is little information about the morphology, molecular make-up and pathological impact of this parasite. Onchocerca fasciata nodules (1.3-2.1 cm in diameter and 509-841 mg in weight) were detected on the neck region in 31.5% of dromedary camels examined in Kerman province, southeastern Iran. Of 38 isolated nodules, only 23 (60.5%) contained viable worms. Measurement and morphological analyses were performed on isolated female worms by light microscopy. The identification of O. fasciata specimens was confirmed by sequence analysis of two mitochondrial genes (12S rDNA and cox1), which showed 0.4% divergence from available O. fasciata sequences. In addition, a phylogeny of filarial nematodes was constructed, based on these two mitochondrial genes and five nuclear genes (18S rDNA, 28S rDNA, MyoHC, rbp1, hsp70); this indicated that O. fasciata belongs to clade ONC3 of Onchocercidae, with representatives of the genera Onchocerca and Dirofilaria. Within the genus Onchocerca, O. fasciata is grouped with bovine parasitic species and the human parasitic Onchocerca volvulus, which suggests an impact of domestication on the radiation of the genus. Data provided here on the distribution and morphology of O. fasciata contribute to the molecular identification and phylogenetic position of the species.

Onchocerca lupi Nematodes in Dogs Exported from the United States into Canada

The Onchocerca lupi nematode is an emerging helminth capable of infecting pets and humans. We detected this parasite in 2 dogs that were imported into Canada from the southwestern United States, a region to which this nematode is endemic. We discuss risk for establishment of O. lupi in Canada.

Canine Infections with Onchocerca lupi Nematodes, United States, 2011-2014

Infections with Onchocerca lupi nematodes are diagnosed sporadically in the United States. We report 8 cases of canine onchocercosis in Minnesota, New Mexico, Colorado, and Florida. Identification of 1 cytochrome c oxidase subunit 1 gene haplotype identical to 1 of 5 from Europe suggests recent introduction of this nematode into the United States.

Isolation of Onchocerca lupi in Dogs and Black Flies, California, USA

We implicated the black fly as a vector for this filarial zoonotic parasitic infection.

In southern California, ocular infections caused by Onchocerca lupi were diagnosed in 3 dogs (1 in 2006, 2 in 2012). The infectious agent was confirmed through morphologic analysis of fixed parasites in tissues and by PCR and sequencing of amplicons derived from 2 mitochondrially encoded genes and 1 nuclear-encoded gene. A nested PCR based on the sequence of the cytochrome oxidase subunit 1 gene of the parasite was developed and used to screen Simulium black flies collected from southern California for O. lupi DNA. Six (2.8%; 95% CI 0.6%–5.0%) of 213 black flies contained O. lupi DNA. Partial mitochondrial16S rRNA gene sequences from the infected flies matched sequences derived from black fly larvae cytotaxonomically identified as Simulium tribulatum. These data implicate S. tribulatum flies as a putative vector for O. lupi in southern California.

Onchocerciasis caused by Onchocerca lupi: an emerging zoonotic infection. Systematic review

Globalization has increased circulation of people, their food, livestock and pets in the world, and changes in the environment, climate and human behaviour have led to the rapid expansion of emerging infections throughout the world. One of the reasons of a new pathogen affecting humans is the passage from an animal to a human being. Onchocerca (O.) lupi, a filarial worm first described in a wolf in 1967, is an emerging pathogen which has been incriminated as the etiological agent for 205 canine, 2 feline and 18 human infections in Europe, Tunisia, Turkey, Iran and the USA. Most frequent findings in animals and humans are monolateral or asymmetrical variably painful subconjunctival swellings and nodules containing immature or mature worms affecting the eye and/or adjacent tissues accompanied by conjunctival hyperemia. Occasionally, subcutaneous nodules and masses affecting the spinal cord have been observed in humans. Diagnosis of O. lupi is achieved by microscopy of excised adult female worms which exhibit a particular cuticular structure and molecular analysis. Treatment consists in worm removal accompanied by antihelminthic, antibiotic and anti-inflammatory therapy.

Onchocerciasis transmission in Ghana: persistence under different control strategies and the role of the simuliid vectors

Background

The World Health Organization (WHO) aims at eliminating onchocerciasis by 2020 in selected African countries. Current control focuses on community-directed treatment with ivermectin (CDTI). In Ghana, persistent transmission has been reported despite long-term control. We present spatial and temporal patterns of onchocerciasis transmission in relation to ivermectin treatment history.

Methodology/Principal Findings

Host-seeking and ovipositing blackflies were collected from seven villages in four regions of Ghana with 3–24 years of CDTI at the time of sampling. A total of 16,443 flies was analysed for infection; 5,812 (35.3%) were dissected for parity (26.9% parous). Heads and thoraces of 12,196 flies were dissected for Onchocerca spp. and DNA from 11,122 abdomens was amplified using Onchocerca primers. A total of 463 larvae (0.03 larvae/fly) from 97 (0.6%) infected and 62 (0.4%) infective flies was recorded; 258 abdomens (2.3%) were positive for Onchocerca DNA. Infections (all were O. volvulus) were more likely to be detected in ovipositing flies. Transmission occurred, mostly in the wet season, at Gyankobaa and Bosomase, with transmission potentials of, respectively, 86 and 422 L3/person/month after 3 and 6 years of CDTI. The numbers of L3/1,000 parous flies at these villages were over 100 times the WHO threshold of one L3/1,000 for transmission control. Vector species influenced transmission parameters. At Asubende, the number of L3/1,000 ovipositing flies (1.4, 95% CI = 0–4) also just exceeded the threshold despite extensive vector control and 24 years of ivermectin distribution, but there were no infective larvae in host-seeking flies.

Conclusions/Significance

Despite repeated ivermectin treatment, evidence of O. volvulus transmission was documented in all seven villages and above the WHO threshold in two. Vector species influences transmission through biting and parous rates and vector competence, and should be included in transmission models. Oviposition traps could augment vector collector methods for monitoring and surveillance.

The World Health Organization (WHO) aims at eliminating onchocerciasis by 2020 in selected African countries. The success of elimination using ivermectin treatment alone will depend on several interacting factors including baseline endemicity, treatment coverage and vector species mix. In Ghana, transmission persists despite prolonged control. We investigated entomological determinants of this persistence. Blackflies were collected from seven villages with 3–24 years of ivermectin treatment. A total of 12,196 flies was dissected, with 463 larvae (all Onchocerca volvulus) in 97 infected and 62 infective flies. Transmission indices in the wet season, at Gyankobaa and Bosomase, amounted to, respectively, 86 and 422 infective larvae/person/month after 3 and 6 years of ivermectin treatment. Infection levels at these villages were over 100 times the WHO threshold of one L3/1,000 parous flies. At Asubende, an infective fly was caught among ovipositing flies in nearby breeding sites, indicating that infection was just over the WHO threshold despite extensive ivermectin and vector control. Spatial and seasonal vector species composition influences the magnitude of transmission indices through variations in biting and parous rates, and vectorial competence and capacity, and should be reflected in transmission models. Oviposition traps could enhance vector collection for transmission monitoring and surveillance.

Zoonotic Onchocerca lupi infection in dogs, Greece and Portugal, 2011-2012

Onchocerca lupi infection is reported primarily in symptomatic dogs. We aimed to determine the infection in dogs from areas of Greece and Portugal with reported cases. Of 107 dogs, 9 (8%) were skin snip–positive for the parasite. DNA sequences of parasites in specimens from distinct dog populations differed genetically from thoses in GenBank.

The species delimitation problem in the Simulium damnosum complex, blackfly vectors of onchocerciasis

The Simulium damnosum Theobald complex (Diptera: Simuliidae) comprises 57 cytoforms grouped into six subcomplexes. Previous phylogenetic studies using gene sequences have not completely resolved the evolutionary relationships of the cytoforms. The present study investigated the systematics of the complex using a phylogeographic approach. The differentiation between eastern and western forms observed in the phylogenetic studies is confirmed in the estimated haplotype networks. However, haplotypes tend to group in geographical clades and not according to cytoforms. Spatial analyses of the molecular variance also resulted in optimal groupings of sequences that did not correspond to cytoform boundaries. Moreover, Mantel tests showed significant correlations, although not strong, between genetic and geographical distances. This suggests an isolation-by-distance model of differentiation. Furthermore, there are instances in which genetic differentiation between cytoforms is low and not significant. These results indicate a lack of clear genetic differentiation between the cytoforms, which may be explained either by a separation of the taxa recent enough to allow the accumulation of few genetic differences or by recombination between the genomes of the cytoforms, which may be the result of hybridization with introgression or of non-independent evolutionary lineages. The results also emphasize the need for further sampling and for the use of more variable markers in order to clarify the evolutionary history of the group.

Molecular genetic comparison of Onchocerca sp. infecting dogs in Europe with other spirurid nematodes including Onchocerca lienalis

In the past 15 years, subconjunctival onchocercosis has been reported from 63 dogs in south-western United States (Arizona, California, Utah) and Southern and Central Europe (Germany, Greece, Hungary, Portugal, Switzerland). To reveal the taxonomic status of the parasite responsible for these infections, fragments of the mitochondrial cytochrome oxidase subunit I (COI) and NADH dehydrogenase subunit 5 (ND5) genes of three European strains of canine Onchocerca sp. and the 16S ribosomal RNA (16S rRNA) gene of their Wolbachia endosymbionts were sequenced and compared to the homologous sequences of other spirurid nematodes. The evolutionary divergence between COI and ND5 gene sequences of Greek, Hungarian and Portuguese strains of canine Onchocerca sp. were similar in magnitude to that seen within Thelazia callipaeda or Onchocerca lienalis. The evolutionary divergence between the sequences of canine Onchocerca sp. and other Onchocerca spp. including O. lienalis were similar or higher in magnitude to that seen between other Onchocerca spp. The results of the current and earlier phylogenetic analyses indicate that canine Onchocerca sp. separated from other Onchocerca spp. early in the evolution. Based on the similar clinical pictures, the identical morphology of nematodes and the sequence analyses of COI and ND5 genes of the worms and 16S rRNA gene of their wolbachiae, the Onchocerca worms isolated from European dogs appear to belong to the same species. The results support the earlier biological and morphological arguments that a distinct species, most likely O. lupi originally described from the subconjunctival tissues of a Caucasian wolf is responsible for canine ocular onchocercosis in Europe.

Molecular phylogeny of the filaria genus Onchocerca with special emphasis on Afrotropical human and bovine parasites

Filarial parasites of the genus Onchocerca are found in a broad spectrum of ungulate hosts. One species, O. volvulus, is a human parasite that can cause severe disease (onchocerciasis or 'river blindness'). The phylogenetic relationships and the bionomics of many of the nearly 30 known species remain dubious. Here, the phylogeny of 11 species representing most major lineages of the genus is investigated by analysing DNA sequences from three mitochondrial genes (ND5, 12S and 16S rRNA) and portions of the intergenic spacer of the nuclear 5s rRNA. Special emphasis is given to a clade containing a yet unassigned specimen from Uganda (O. sp. 'Siisa'), which appears to be intermediate between O. volvulus and O. ochengi. While the latter can be differentiated by the O-150 tandem repeat commonly used for molecular diagnostics, O. volvulus and O. sp.'Siisa' cannot be differentiated by this marker. In addition, a worm specimen from an African bushbuck appears to be closely related to the bovine O. dukei and represents the basal taxon of the human/bovine clade. At the base of the genus, our data suggest O. flexuosa (red deer), O. ramachandrini (warthog) and O. armillata (cow) to be the representatives of ancient lineages. The results provide better insight into the evolution and zoogeography of Onchocerca. They also have epidemiological and taxonomic implications by providing a framework for more accurate molecular diagnosis of filarial larvae in vectors.

Molecular systematics of five Onchocerca species (Nematoda: Filarioidea) including the human parasite, O. volvulus, suggest sympatric speciation

The genus Onchocerca (Nematoda: Filarioidea) consists of parasites of ungulate mammals with the exception of O. volvulus, which is a human parasite. The relationship between O. volvulus, O. ochengi and O. gibsoni remains unresolved. Based on morphology of the microfilariae and infective larvae, vector transmission and geographical distribution, O. ochengi and O. volvulus have been placed as sister species. Nevertheless, the cuticle morphology and chromosomal data (O. volvulus and O. gibsoni have n=4 while O. ochengi is n=5) suggest that O. gibsoni could be more closely related to O. volvulus than O. ochengi. Sequences from the 12S rRNA, 16S rRNA and ND5 mitochondrial genes have been used to reconstruct the phylogeny of five Onchocerca species including O. volvulus. Analyses with maximum likelihood and maximum parsimony showed that O. ochengi is the sister species of O. volvulus, in accordance with the classification based on morphology and geographical location. The separate specific status of the species O. gutturosa and O. lienalis was supported, although their phylogenetic relationship was not well resolved. The analyses indicated that the basal species was O. gibsoni, a South-East Asian and Australasian species, but this result was not statistically significant. The possible involvement of sympatric speciation in the evolution of this group of parasites is discussed.

Molecular phylogenetic analysis of Onchocerca lupi and its Wolbachia endosymbiont

The morphology of Onchocerca lupi, responsible for canine ocular onchocercosis, is unique within the genus. Earlier analyses of the 5S ribosomal RNA gene spacer region sequence of the parasite and the 16S ribosomal RNA gene sequence of its Wolbachia endosymbiotic bacteria (Rickettsiales) supported the morphological and biological arguments that O. lupi is a distinct species. However, the exact phylogenetic position of O. lupi and its endosymbiont could not be unambiguously determined. Herein we report analyses based on the mitochondrial cytochrome oxidase I (COI) gene of the filarial species and the Wolbachia surface protein (wsp) and the bacterial cell-cycle ftsZ genes of their wolbachiae. Our results indicate that O. lupi separated from other Onchocerca spp. early in evolution. This is in line with the previous morphological analysis demonstrating that O. lupi is an atypical Onchocerca species showing both primitive and evolved characters. The phylogenetic trees generated for the COI sequences of filariae and the wsp and ftsZ sequences of their wolbachiae were congruent with each other, which supports the hypothesis that nematodes and their Wolbachia endobacteria share a long co-evolutionary history.

Electron microscopic and molecular identification of Wolbachia endosymbionts from Onchocerca lupi: implications for therapy

It was recently demonstrated that Wolbachia intracellular bacteria (alpha 2 proteobacteria, Rickettsiales) living in filarial nematodes are obligatory symbionts of their hosts. Herein, we report the electron microscopic and 16S ribosomal DNA-based (16S rDNA) identification of the endobacteria harboring in Onchocerca lupi. The worm nodules containing the nematodes were removed from three Hungarian dogs naturally infected with O. lupi. Wolbachia-like endobacteria were detected by electron microscopy in the lateral chords of both adult worms and microfilariae. The endosymbionts in O. lupi resemble in location, size, and morphology the wolbachiae found in other filariae. The presence of wolbachiae in O. lupi was also confirmed by PCR amplification of the 16S rDNA of the bacteria. The 16S rDNA-based phylogenetic analysis revealed that the endosymbionts of O. lupi infecting dogs belong to the supergroup C of Wolbachia pipientis and are not identical with those of other Onchocerca spp. sequenced so far. Since intermittent treatment with oxytetracycline has adulticid and microfilaricid activity by depletion of Wolbachia endobacteria, this antibiotic treatment regimen may offer an alternative of ivermectin or diethylcarbamazine in the suppression of postoperative microfilaridermia in Onchocerca-infected dogs and may prevent relapse.

[Human onchocerciasis and "sowda" in the Republic of Yemen]

The geophysics of the north Yemen, associating a north-south directed mountainous fish bone (rising in more of 2,000 meters), to numerous rivers or "wadis" is convenient to the development of simulium shelters, main vectors for cutaneous filariasis to Onchocerca sp. Following several missions of bio-clinical and epidemiological evaluations in neighbouring villages of wadis, it has been possible to study different clinical aspects: one reminding the classical african onchocerciasis with generalized and diffused dermatitis, and, on an other hand, a hyperreactive dermatitis on one side of the body and associated with a collateral lymphatic ganglion. This disease is well known for local populations as "aswad" meaning "black" or "sowda". Clinically whatever the studied focus, coexists the two types of onchodermatitis (uni or bilateral). Yhe sowda patients are proportionally less numerous than those touched by the generalized type. Frequent eye lesions of the West African onchocerciasis are not found in sowda cases. In classical optical microscopy, microfilaria is morphologically indifferenciable between sowda and onchocerciasis clinical aspects. Skin snips were carried out on patients of both groups. Identification of microfilaria by molecular biology through the study of the DNA genome was done out of 5 skin snips. Microfilaria was kept dry between laminas and the DNA extracted from rehydrated microfilaria. DNA was intensified with specific primers of Onchocerca type (O150PCR). This phase was followed by hybridisation of amplification products by PCR to specific stains: OVS-2 for Onchocerca volvulus species, OCH for Onchocerca ochengi, PFS1 and PSS1-BT respectively for the forest strain and the savannah strain of Onchocerca volvulus as described previously. We can distinguish 2 kinds of answers based on the clinical origin of the snip-tests: the first one concern 3 patients with numerous dermal microfilariae but without any clinical sowda and corresponding to microfilaria O. volvulus type but different from the forest or savannah strains found in sub-Saharan Africa. The second one corresponds to 2 patients with less than 5 microfilaria in their snip-test. They show the typical clinical picture of sowda. They are identified as microfilaria type Onchocerca but they do not belong to species volvulus, or to species ochengi. It seems quite probable that the clinical picture of sowda be the result of developing onchocerciasis of animal origin and not identified as to day. The ivermectin, therapeutic of choice for African onchocerciasis in annual unique cure seems less effective in the coverage of sowda. In that case rehearsal of cures every 3 months would be necessary for mass campaigns to limit the transmission of this filariasis.

Sensitive and specific serodiagnosis of onchocerciasis with recombinant hybrid proteins

Onchocerciasis remains a major health hazard in many tropical countries. However, the existing tools for diagnosis of the disease have limitations, particularly regarding the detection of low level or early infections. To design an optimized reagent, we exploited the high antibody reactivity of patient sera against the Onchocerca volvulus proteins Ov20 and Ov33, which have been described as highly sensitive and specific immunodiagnostic reagents for producing hybrid proteins. The construct OvH2 was composed of Ov20 fused to Ov33, while OvH3 consisted of the C-terminus of Ov20 linked to Ov33. When these constructs were tested with sera from patients with onchocerciasis and control sera, OvH2 showed a sensitivity of 98.5% and a specificity of 97.7% and OvH3 showed a sensitivity of 98.5% and a specificity of 95.35%. All non-responders were from Ecuador. These results surpass those of existing single recombinant antigens, suggesting that our hybrid proteins combined the sensitivity of the two parent proteins. Tests based on OvH2 should prove suitable for monitoring onchocerciasis control programs and individual diagnosis.

Morphologic and genetic characterization of Onchocerca lupi infecting dogs

In the past decades, sporadic cases of ocular Onchocerca infection have been reported in canids in US and Europe. The present study was undertaken to provide a detailed description of the morphologic characteristics of adults and microfilariae and to characterize the 5S ribosomal rRNA gene (5S rDNA) spacer sequences of Onchocerca lupi causing canine onchocercosis. The morphology of O. lupi is unique within the genus, and morphology based cluster analysis indicates that O. lupi is not closely related to the members of domestic cattle or horse clades occurring in North America and Europe. Similarly, the signature of the 5S rDNA spacer sequences of O. lupi does not resemble any other Onchocerca 5S rDNA spacer sequences including those of the members of domestic cattle or horse clades. The adult and microfilarial morphology and sequence signature supports the biological arguments that a distinct species, O. lupi and not O. lienalis, is responsible for canine ocular onchocercosis.

Influence of intercodon and base frequencies on codon usage in filarial parasites

Base frequency, codon usage, and intercodon identity were analyzed in five filarial parasite species representing five Onchocercidae genera. Wucheria bancrofti, Brugia malayi, Onchocerca volvulus, Acanthocheilonema viteae, and Dirofilaria immitis gene sequences were downloaded from NCBI, and analysis was performed using locally designed computer programs and other freely available applications. A clear sequence bias was observed among the nematode species examined. At the nucleotide level, AT basepairs were present in gene sequences at higher frequencies than GC. In addition, codons ending in A or T were used proportionately more than those with G or C in the third-codon position. In addition, the amino acids used most often corresponded to codons ending in AT basepairs. Intercodon base proportion was biased in that A was found most often at N4, second only to T in certain specific cases. Since all of these sequence biases were observed in a relatively consistent fashion among all of the organisms studied, we conclude that sequence bias is a genetic characteristic, which is associated with multiple filarial genera.

Onchocerca ochengi infections in cattle as a model for human onchocerciasis: recent developments

The bovine parasite Onchocerca ochengi is a nodule-dwelling filarial nematode, closely related to O. volvulus, the causal agent of human River Blindness, and, sharing with it, the same vector. This brief review, based on a presentation at the BSP Autumn Symposium 1999, describes recent work supported by the WHO Drug Development Research Macrofil programme and the Edna McConnell Clark Foundation vaccine development programme, to research the chemotherapy and immunology of onchocerciasis utilising this model system, with experimental infections in Liverpool and field infections in northern Cameroon. In a series of chemotherapeutic trials involving 10 compounds in 20 treatment regimes, the comparability of drug efficacy against O. ochengi with that described against O. volvulus has been demonstrated. Repeated, long-term treatment with oxytetracycline has been shown to be macrofilaricidal and the effect is hypothesized to be related to action on Wolbachia endobacteria, abundant in O. ochengi. Avermectins/milbemycins are not macrofilaricidal (even in high and repeated long-term treatments) but induce sustained abrogation of embryogenesis. In prospective, field exposure experiments with naive calves, prophylactic treatments with ivermectin and moxidectin prevented the development of adult worm infection, raising the possibility that drug-attenuated larval challenge infections may induce immunity. Putatively immune adult cattle exist in endemically exposed populations, and these have been shown to be significantly less susceptible to challenge than age-matched naive controls, whereas radically drug-cured, previously patently-infected cattle were not. Experimental infections with O. ochengi have revealed the kinetics of the immune response in relation to parasite development and demonstrate analogous responses to those reported in O. volvulus infection in humans and chimpanzees. In an immunization experiment with irradiated L3 larvae, cattle were significantly protected against experimental challenge--the first such demonstration of the experimental induction of immunity in a natural Onchocerca host-parasite system. Taken collectively, these studies not only demonstrate the similarity between the host-parasite relationships of O. ochengi in cattle and O. volvulus in humans, but promise to advance options for the control of human onchocerciasis.

Detection of the filarial parasite Mansonella streptocerca in skin biopsies by a nested polymerase chain reaction-based assay

To differentiate the skin-dwelling filariae Mansonella streptocerca and Onchocerca volvulus, a nested polymerase chain reaction (PCR) assay was developed from small amounts of parasite material present in skin biopsies. One nonspecific and one specific pair of primers were used to amplify the 5S rDNA spacer region of M. streptocerca. Biopsies with different microfilaria densities obtained from 104 Ugandans living in an area endemic for M. streptocerca were tested using both the nested PCR assay and standard parasitologic assessment of microfilariae. All 82 samples from microfilaria carriers were positive when tested using the nested PCR assay. In addition, M. streptocerca DNA could be detected in 16 samples thought to be microfilaria negative. Furthermore, six days following ivermectin treatment, M. streptocerca DNA was found in 12 of 14 microfilaria-negative biopsies. Control skin samples from patients infected with O. volvulus were all negative in the nested PCR assay. This assay improves the diagnosis of M. streptocerca and will facilitate further epidemiologic studies.

Onchocerca ochengi: morphological identification of the L3 in wild Simulium damnosum s.l., verified by DNA probes

In order to assess the prevalence of the cattle filaria Onchocerca ochengi in onchocerciasis vectors (Simulium damnosum s.l.) in North Cameroon, we searched for a means to morphologically identify its developing larvae, which closely resemble those of O. volvulus. To this end microfilariae of the 2 Onchocerca species were isolated from slaughter cattle in Ngaoundéré and injected into neonate Simulium species. Whereas the early developmental stages (sausage stage, L2 and pre-infective larva) were indistinguishable, the infective larvae (L3) of O. ochengi were longer (median: 740 microns), more slender (diameter = 19.3 microns = 2.6% of body length) and had a relatively shorter tail (4.9% of body length) than those of O. volvulus (680 microns, 20.5 microns, 3.0% and 5.8% respectively). The tail of O. ochengi L3 was thick and rounded, whereas it was slightly tapering in O. volvulus L3. O. ochengi L3 produced by feeding flies on infected cattle in a different area in North Cameroon (Sora Mboum) showed the same features as intrathoracically produced O. ochengi L3 from Ngaoundéré, but were even longer (785 microns). On the basis of the differences in length, relative diameter, length of the tail and shape of the tail, a simple key for the separation of O. volvulus and O. ochengi L3 was elaborated, and 248 L3 found in wild S. damnosum s.l. were separated into 'O. ochengi' (160 L3) and 'O. volvulus' (88 L3) following this key. Sequential dot blot hybridization of each of the 248 larvae with a DNA probe which reacts with O. ochengi and O. volvulus but not with other Onchocerca species (pOo5/1) and with an O. volvulus-specific DNA probe (pOv12) revealed that the morphological identification had been correct in 86-91% of the cases. Only a small proportion (6-9%) of the dot blots did not react with either probe. Since this proportion was equal in experiments using experimentally produced L3 and in experiments using wild L3, the nonhybridization was certainly due to a loss of L3 during washing of the filters and not due to the presence of other unknown L3 species resembling O. volvulus and O. ochengi. Our study shows that in Cameroon it is possible to identify O. volvulus and O. ochengi infective larvae during routine fly dissections by morphology alone.

Characteristics of nucleotide sequences flanking the trans-spliced leader SL1 exon in Dirofilaria immitis, Brugia malayi, and Brugia pahangi

Nucleotide sequences surrounding the trans-spliced leader SL1 exon in the 5S rRNA gene spacer regions of Dirofilaria immitis, Brugia malayi, and B. pahangi were determined after PCR amplification, aligned with the genus Onchocerca for comparison, and used for the prediction of secondary structures. The nucleotide sequence of this region in B. pahangi was first shown in the present study. Hypothetical secondary structures of the spacer region suggested that the SL1 transcript is capable to form a stable stem-loop structure which may render transposition of the SL1 sequence to mRNA molecules. A homologous sequence to Sm-binding site was assigned on a bulge loop. No significant difference was observed in adult worms of D. immitis irrespective of sex or location. No difference was apparent between the two species in genus Brugia.

cDNAs from Onchocerca sp. encoding members of the MRS3/MRS4 class of mitochondrial solute carriers

cDNA clones from the parasitic nematodes Onchocerca volvulus and Onchocerca gibsoni encode homologs of the yeast proteins MRS3 and MRS4. Together with an uncharacterised ORF on chromosome III of Caenorhabditis elegans, these constitute a new class of proteins belonging to the mitochondrial solute carrier protein superfamily. So far, five other members of this protein family have been identified in C. elegans, but levels of identity between these and the Onchocerca proteins were considerably lower. Consideration of cysteine content and overall charge implies that the natural substrates of the nematode proteins are small ions.

Characterization of a novel non-muscle myosin-related protein from Onchocerca gibsoni

A cDNA library was constructed in lambda gt11 using poly(A)+ mRNA from early larvae of Onchocerca gibsoni. Screening of the library using serum from a single onchocerciasis patient yielded several strongly immunoreactive clones, one of which (OGK2) was found to encode a novel myosin-related protein. cDNA clone OGK2 contained an insert of 2017 bp, consisting of continuous open reading frame in frame with the vector, hence this clone encodes 671 amino acid residues of a larger protein. A fragment (619 nt) of the OGK2 cDNA was subcloned into the expression vector pGEX-1N to generate a glutathione S-transferase fusion protein. Polyclonal antiserum raised to this fusion protein strongly recognised an O. gibsoni protein of approximately 220 kDa. Immunolocalization studies indicated that this protein was associated predominantly with the hypodermis and a number of other specific membrane layers in the adult parasite. Myosin-related proteins are frequently immunodominant parasite antigens and in a number of studies have been shown to confer a degree of protective immunity against the corresponding parasite. Evaluation of the protective potential of the OGK2 protein, therefore, appears to be warranted.

Organization and structure of an Onchocerca beta-tubulin gene

In filarial worms, as in other eukaryotes, microtubules are essential multifunctional components. The major protein of microtubules is tubulin, a heterodimer of two distinct polypeptides, alpha and beta. Tubulin is particularly important in helminthic parasites as a target for anthelminthic benzimidazoles, which bind to it and inhibit microtubule assembly. Two genomic Onchocerca gibsoni libraries were constructed in lambda NM1149 (EcoRI and HindIII). Three clones accounted for the entire gene: one from the EcoRI library (using a Plasmodium falciparum probe) containing the central part of the gene, and two from the HindIII library (using as probes PCR amplified fragments from the ends of the EcoRI clone) which, respectively, contained the 5'- and 3'-ends of the gene. The sequencing procedure for the EcoRI clone relied on the construction of a double-digested DraI/HindIII shotgun library. A number of recombinants were sequenced and aligned with each other for comparison. The sequencing of the overlapping 5'- and 3'-end clones was done by using a series of oligonucleotides. The sequence of the O. gibsoni beta-tubulin gene was completely determined, revealing an exceptionally complex structure as compared to the known beta-tubulin genes: 5,797 base pairs containing 12 exons and 11 introns. The deduced polypeptide is 444 amino acids long, and its sequence is highly conserved. The position of some introns appear to demarcate functional domains within the protein.

DNA probe-based classification of Simulium damnosum s. l.-borne and human-derived filarial parasites in the onchocerciasis control program area

The development of polymerase chain reaction-based methods using strain- and species-specific DNA probes for Onchocerca volvulus has permitted classification of individual parasites from every stage of the parasite's life cycle. This technology has been applied on a large scale basis by Onchocerciasis Control Program (OCP) in West Africa. The primary objective of the OCP in using the DNA probes was to obtain accurate estimates of the annual transmission potential of the blinding strain of O. volvulus. The DNA probe classification of larvae collected throughout the OCP area demonstrated that larvae of less pathogenic strains of O. volvulus and other filarial parasites carried by Simulium damnosum s.l. have resulted in a significant overestimation of the annual transmission potential for blinding onchocerciasis. This effect is particularly pronounced along the southern border of the OCP, where the blinding and less pathogenic strains of O. volvulus coexist, and in the north of the control area, where animal parasites, particularly O. ochengi, may even predominate. A second objective of the OCP in applying the DNA probe technology was to determine the distribution of blinding and less pathogenic O. volvulus in infected individuals along the southern border of the control area. Results obtained from these studies have generally confirmed the distribution pattern established by previous epidemiologic studies. In addition, DNA probe classifications have demonstrated that in areas where the blinding and less pathogenic strains of O. volvulus coexist, a single individual may simultaneously be infected with both strains of the parasite.

Genomic fingerprinting of Onchocerca species using random amplified polymorphic DNA

A method based on amplification of genomic DNA by decamer primers of random nucleotide sequence was used to obtain DNA fingerprints from different species of the genus Onchocerca. Each of the 20 primers tested allowed a clear distinction between the different species of the genus on an agarose gel. The technique offers the potential to construct species or strain specific probes and oligonucleotides PCR primers from the species specific fragments. A combination of these primers or others could be useful as population markers.

Identification and characterisation of a novel repetitive antigen from Onchocerca spp

A novel repetitive antigen from the cattle parasite Onchocerca gibsoni was shown to be recognised by sera from humans infected with Onchocerca volvulus, Wuchereria bancroftii or Brugia malayi. The O. gibsoni protein was produced in a recombinant form, and antibodies raised to this protein used to screen cDNA libraries for O. volvulus. A series of clones were isolated which encoded repetitive regions very similar to those in O. gibsoni, but interspersed between these were longer repeating units which we have not so far found in O. gibsoni. The repetitive antigen was shown to be of high molecular weight and present only in the insoluble (membrane) fraction of O. gibsoni microfilariae. Immunofluorescence techniques demonstrated that the antigen was associated both with muscle and with specific membrane layers, including a peripheral layer which corresponds to either the outer hypodermis or an inner region of the cuticle in adult female O. gibsoni. In many respects, the proteins encoded by the O. gibsoni and O. volvulus cDNA clones resembled repetitive antigens from several distantly related eukaryotic parasites, and a possible common role in immune evasion is discussed.

Molecular variation in Onchocerca spp

Over the past two decades there has been an upsurge of interest in defining morphological, immunological, biochemical, biological and genetic differences between species of Onchocerca to provide solutions to practical problems associated with finding models and epidemiological tools to assist with control of human onchocerciasis. The information gathered has confirmed the close relationship between species of Onchocerca and provided highly sensitive and specific probes to distinguish species and even strains of the same species. It has also identified pathways, especially using sequences from common DNA repeat units, that may lead to a better understanding of the progression of divergence of species of this genus than has previously been possible.

Immunological cross-reaction between an Onchocerca paramyosin-like molecule and a microfilaria surface antigen

A monoclonal antibody (2A5B9), previously shown to be reactive with a 14 kD surface associated antigen of Onchocerca microfilariae, was found to recognise a 92 kD molecule present in an adult worm extract. The antibody was used to select cDNA clones with a coding capacity larger than 14 kD, from a lambda gt11 library of O. volvulus. Nucleotide sequencing of the cDNA of one such clone revealed extensive homology to the myosin (unc-54) and paramyosin (unc-15) genes of Caenorhabditis elegans, similarly to myosin and paramyosin genes of Onchocerca volvulus, Brugia malayi, Dirofilaria immitis and Schistosoma mansoni. The immunological implications of antigenic cross-reactivity between a surface molecule and paramyosin, a known protective antigen, are discussed.

Molecular cloning and characterization of onchocystatin, a cysteine proteinase inhibitor of Onchocerca volvulus

A cDNA clone designated OV7 encodes a polypeptide that corresponds to a highly antigenic Onchocerca volvulus protein. OV7 has significant amino acid sequence homology to the cystatin superfamily of cysteine proteinase inhibitors. In this report we establish that the OV7 recombinant protein is active as a cysteine proteinase inhibitor, and we have named it onchocystatin. It contains a cystatin-like domain that inhibits the activity of cysteine proteinases at physiological concentrations. Recombinant glutathione S-transferase-OV7 (GST-OV7, 1 microM) and maltose-binding protein-OV7 (MBP-OV7, 4 microM) fusion polypeptides inhibit 50% of the enzymatic activity of the bovine cysteine proteinase cathepsin B. Neither fusion polypeptide inhibits serine or metalloproteinases activity. The Ki for GST-OV7 fusion polypeptide is 170 nM for cathepsin B and 70 pM or 25 nM for cysteine proteinases purified from a protozoan parasite Entamoeba histolytica or the free living nematode Caenorhabditis elegans, respectively. The 5' end of the OV7 clone was isolated by polymerase chain reaction and sequenced, thus extending the previous cDNA clone to 736 base pairs. This represents the complete coding sequence of the mature onchocystatin (130 amino acids). A hydrophobic leader sequence of 32 amino acids was found, indicating a possible extracellular function of the onchocerca cysteine proteinase inhibitor.

Nucleotide sequence of a cDNA from Onchocerca gibsoni encoding a novel repetitive antigen

mRNA from uterine microfilariae of the cattle parasite Onchocerca gibsoni was used for the construction of cDNA libraries. A cDNA clone encoding an antigen recognized by serum from human individuals infected with O. volvulus was found to contain five copies of an 87 bp unit. These 87 bp units were present in the genome in high copy number as long tandem arrays. These are the first cDNA sequence data obtained directly from larvae of any Onchocerca species.

Onchocerca volvulus DNA probe classification correlates with epidemiologic patterns of blindness

Onchocerciasis, or river blindness, results from infection with Onchocerca volvulus. The parasite is endemic to West Africa, in both rain forest and savanna bioclimes. Several lines of evidence suggest that different strains of the parasite exist in the rain forest and savanna. Furthermore, epidemiologic evidence indicates that ocular onchocerciasis is most severe in savanna regions. This has led to the hypothesis that there is a strain association with ocular pathology. To test this hypothesis, parasites from villages in which severe and mild onchocerciasis were endemic were classified with two strain-specific DNA probes. A strong correlation (P less than .001) was found between disease severity and probe recognition, supporting the hypothesis that pathogenicity is strain related. The results suggest that pFS-1 and pSS-1BT may be used to predict the pathogenic potential of parasite populations throughout much of West Africa.

PCR: how to kill unwanted DNA

Avoidance of contamination in the PCR laboratory requires the use of strict precautions. Among these, chemical decontamination of surfaces and equipment is desirable to prevent inadvertent contamination of samples by the gloved hand and by pipettors. We have investigated the use of sodium hypochloride (Clorox), in comparison to concentrated HCl, for PCR sterilization. Ten percent Clorox was found to eliminate all ethidium bromide-stainable DNA and to prevent PCR amplification of a 600-bp DNA segment within one minute of template treatment. RNA was similarly destroyed. By contrast, even 2.0 N HCl did not destroy DNA detectable by PCR within five minutes. Because of its high efficacy, low cost and relatively low corrosiveness, we recommend the use of ten percent Clorox as a decontaminant for elimination of DNA templates in the PCR laboratory.

Comparison of the body wall myosin heavy chain sequences from Onchocerca volvulus and Brugia malayi

The complete coding sequence of Onchocerca volvulus myosin heavy chain has been determined from a series of overlapping cDNAs. The protein sequences from the 2 filarids, one responsible for subcutaneous filariasis, the other for lymphatic filariasis, show 92% identity, and are 1957 amino acids long. Each protein sequence is also equally related, with 75% identity, to MHC-B, the protein encoded by the unc-54 gene of the free-living nematode C.elegans. Such analysis is useful in phylogenetic studies among nematodes, as well as in structure-function relationships among myosin isolates.

Identification and characterization of an Onchocerca volvulus cDNA clone encoding a microfilarial surface-associated antigen

The identification and characterization of a recombinant cDNA clone (OV103) expressing a microfilarial surface-associated antigen of Onchocerca volvulus is described. OV103 was identified and isolated from a lambda gt11 cDNA expression library derived from adult O. volvulus mRNA using a chimpanzee antiserum, taken 2 years after infection with third-stage larvae of O. volvulus. The cDNA clone encodes a 12.5-kDa protein that corresponds to a 15-kDa parasite protein present in microfilariae and adult female worms. The antigen encoded by this clone is located in the basal layer of the cuticle and the hypodermis of the female adult worm, and on the surface of microfilariae. OV103 fusion polypeptide is recognized only by some sera from onchocerciasis infected subjects (57%), but more significantly (89%) by sera from individuals that have low levels of patent infection. In addition, the antibody response to this protein developed before appearance of microfilariae in the skin of chimpanzees that had developed non-patent or low level patent infections, while the antibody response in chimpanzees with high levels of microfilariae appeared later at the time of appearance of microfilariae. Preliminary experiments indicated that affinity purified antibodies directed against OV103 fusion polypeptide mediated killing of nodular microfilariae in vitro in the presence of normal peripheral blood granulocytes.

Histochemical localization of gene expression in Onchocerca volvulus: in situ DNA histohybridization and immunocytochemistry

We report here the development of in situ hybridization and immunohistochemistry protocols which permit the histological identification of gene expression of a cloned antigen of Onchocerca volvulus, OI5, in the parasite. Skin nodules containing female adult worms were fixed in a modified Carnoy's fixative and embedded in paraffin. Histological staining of tissue sections revealed uniformly excellent morphology and RNA preservation. To localize mRNA by in situ hybridization, tissue sections were incubated with biotin-labeled pOI5, the plasmid containing the genomic sequence of the antigen, and hybridization signals were histochemically visualized using a streptavidin-enzyme conjugate and chromogenic substrates. The protein antigen was localized immunohistochemically by incubating the sections with specific antibodies prepared against a recombinant fusion protein containing the OI5 sequence (OI3), and visualized via a secondary antibody-biotin-enzyme conjugate procedure. The results reported here showed distinct localization of the OI5 mRNA and OI3 antigen in specific cellular and tissue regions of the adult parasite, and in microfilariae located within the uteri and in the surrounding host tissue. The specificity and high sensitivity of these histological detection methods should be generally applicable for the characterization of gene expression in the filarial parasite, particularly the insect-borne, infective filarial larvae, which are severely limited in quantity.

Onchocerca volvulus: application of the polymerase chain reaction to identification and strain differentiation of the parasite

Previous studies have demonstrated that the genome of Onchocerca volvulus contains a variable tandemly repeated DNA sequence family with a unit length of 150 bp. The variability of the 150-bp family has been exploited to develop O. volvulus strain and species specific DNA probes. Application of these DNA probes to the study of the epidemiologically most significant life cycle stages of the parasite has been confounded by several obstacles. These include the relative insensitivity of some of the DNA probes and the difficulty in releasing genomic DNA from infective larvae and skin microfilariae in a form that may be directly detected by hybridization to the probes. DNA sequence comparison of 18 known examples of the 150-bp repeat has been used to develop two populations of degenerate oligonucleotides. These oligonucleotides have been shown to support the amplification of the 150-bp repeat family from Onchocerca DNA, using the polymerase chain reaction. Both strain and species specific members of the repeat family are faithfully amplified, allowing characterization of a parasite on the basis of hybridization of the PCR amplification products to the previously developed DNA probes. This method is shown to be applicable to all diagnostically important forms of the parasite, including adults, infective larvae, and skin microfilariae. In addition, the method is capable of detecting O. volvulus infective larvae directly in extracts of blackfly vectors.