Onchocerca volvulus, Wolbachia and river blindness.

Chronic infection with filarial nematodes results in development of a suppressive response to an immense parasite burden, thereby limiting pathological and clinical manifestations. However, pro-inflammatory responses to dead and degenerating Onchocerca volvulus worms and release of endosymbiotic Wolbachia bacteria result in corneal opacification, sacrification and visual impairment. This review discusses host and parasite factors implicated in maintaining this balance of pro- an anti-inflammatory responses, and will focus on adaptive and innate immunity to filarial antigens and endosymbiotic Wolbachia bacteria.

Important experimental parameters for determining infection rates in arthropod vectors using pool screening approaches.

Measuring transmission of a vector-borne infection is essential to understanding infection dynamics. When infection prevalence in the vector population is low, transmission is often measured by pool screening (also referred to as group testing). Several investigators have developed statistical methods to recover infection prevalence estimates from pool screen data. These are based on models that contain certain assumptions, and a pool screening approach must be designed to take these into account if accurate estimates of infection prevalence are to be obtained. Here we describe these assumptions and discuss appropriate sampling protocols. The sources of error inherent in pool screening are described, and we show that, under most conditions in which one would want to use group testing, most of the error results from sampling and not the pooling process. Issues involved in developing a sampling protocol, including the total number of insects to be screened and optimal pool size, are explored. The meaning of confidence intervals associated with prevalence estimates and the appropriate interpretation of these intervals are discussed.

Wolbachia endosymbiont levels in severe and mild strains of Onchocerca volvulus.

Epidemiological, clinical and genetic data have all suggested that the filarial parasite Onchocerca volvulus, the causative agent of onchocerciasis (or river blindness) exists as two strains in West Africa. The severe strain induces severe ocular disease in a large proportion of the infected population, while the mild strain induces little ocular disease. Although DNA probes based upon a non-coding repeat sequence family can distinguish the two strains, the underlying basis for this difference in pathogenicity is not understood. Recently, several studies have implicated products produced by the Wolbachia endosymbiotic bacterium of O. volvulus in the pathogenesis of onchocerciasis. This suggested the hypothesis that differences in the Wolbachia endosymbiont population might be responsible for the pathogenic differences noted in the two strains. To test this hypothesis, quantitative PCR assays were used to measure the amount of Wolbachia DNA per nuclear genome in a collection of well characterized samples of mild and severe strain O. volvulus. The median ratio of Wolbachia DNA to nuclear DNA was significantly greater in severe strain parasites than in mild strain parasites. These data support the hypothesis that the pathogenic differences seen in severe and mild strain O. volvulus may be a function of their relative Wolbachia burden and provide additional support to the hypothesis that Wolbachia products may play a central role in the pathogenesis of ocular onchocerciasis.

Identification of an epitope of a recombinant Onchocerca volvulus protein that induces corneal pathology.

Ocular onchocerciasis results from immune recognition of parasite proteins released into the eye by degenerating microfilariae. Previous studies have shown that pathology similar to human ocular onchocerciasis can be induced in sensitized mice by intracorneal injection with Onchocerca volvulus antigens. In the current study, we used this murine model to map the segments of O. volvulus protein disulfide isomerase (OvPDI) associated with the development of corneal pathology. Subclones of OvPDI were constructed encompassing one or more predicted T cell epitopes. Keratitis was induced in BALB/c mice after subcutaneous immunizations with OvPDI, followed by intracorneal challenge of OvPDI constructs. Truncated OvPDI proteins containing amino acids 450-481 of OvPDI were found to induce keratitis, whereas constructs that did not include this region did not induce corneal pathology. Consistent with this observation, two peptides derived from the 450-481 region stimulated T cell proliferation to a greater degree than control carrier protein. DNA sequence analysis of cDNAs encoding OvPDI from blinding and non-blinding strains of O. volvulus indicated no differences in the primary amino acid sequence of the 450-481 domain. Immunization of animals with OvPDI induced antibodies recognizing a 55 kDa host protein, identical to the predicted molecular weight of the mouse PDI homologue. Together, these data implicate specific antigenic epitopes of OvPDI in the development of O. volvulus mediated corneal pathology.

Corneal changes of uncertain etiology in mesoendemic onchocercal communities of Northern Nigeria.

BACKGROUND: We report an unusual type of corneal change in some communities mesoendemic for onchocerciasis in Kaduna State of Northern Nigeria. METHODS: Thirty-four villages with an overall average microfilarial skin snip positivity rate of 71% and a total population of about 10,000 were included in a controlled trial of ivermectin for onchocerciasis. Detailed slit lamp examination of 806 persons within this population revealed at least 27 individuals with these lesions. There were 11 women and 16 men, aged between 5 and 70 years with a mean age of 45 years and a SD of 15 years. Drawings and photographs of 22 of these individuals were available for assessment. RESULTS: The lesions were peripheral, silvery white, and of two main types: the first group's lesions were flaky, refractile, and crystalline in appearance; the second group's lesions were more cylindrical in outline and were crisscrossed, giving a lattice-like pattern. There were indeterminate forms in between these polar topographies. Of the 25 subjects who had skin snips performed, 23 were microfilaria-positive. There was no accompanying inflammation and the posterior segments showed no evidence of crystalline deposits or retinitis pigmentosa, as is found in Bietti's dystrophy. The lesions coexisted with typical sclerosing keratitis in six individuals, and changes noted in the posterior pole of ten individuals were typical of onchocercal chorioretinitis. The evidence for consanguinity was not compelling. Differences between and similarities to Bietti's and Schnyder's crystalline corneal dystrophy, which are known to be composed of cholesterol crystals, are discussed. The lesions are also compared with lattice dystrophy known to be composed of amyloid. These lesions may be related to onchocercal infection. CONCLUSIONS: We describe some unusual peripheral corneal changes in individuals living within areas of Northern Nigeria mesoendemic for onchocerciasis. These changes take the form of flaky crystals or lattice within the anterior stroma and are common in middle-aged individuals. This may be related to onchocercal infection.

Onchocerciasis (river blindness).

A 37-year-old African man presented for excision of a dermal nodule after a diagnosis of ocular onchocerciasis (river blindness). A nodule from the patient's left buttock contained several adult filarial worms, and results from adjacent skin biopsy specimens revealed numerous dermal microfilariae. The patient was admitted to the hospital and treated with one dose of ivermectin. Recommendations were made for ivermectin treatments every 6 months for up to 10 years. The history, clinical presentation, diagnosis, and treatment of onchocerciasis are discussed.

Humoral and cell-mediated immune response against human retinal antigens in relation to ocular onchocerciasis.

Autoimmune mechanisms are thought to be involved in the pathogenesis of the chorioretinal changes in ocular onchocerciasis. The humoral autoimmune response was determined by measuring serum levels of autoantibodies, directed against human S-antigen and interphotoreceptor retinoid binding protein (IRBP) using an enzyme immunoassay. The cell-mediated immune response to these antigens and a crude retinal extract was investigated by means of a two-step migration inhibition factor assay. Patients with onchocerciasis (n = 50) were subdivided into three groups: 1. without ocular involvement (n = 10), 2. with ocular onchocerciasis limited to the anterior segment (n = 19), 3. with onchocercal chorioretinopathy (n = 21). A group of endemic controls from Sierra Leone, West Africa were also studied. The cellular immune response to Concanavalin A was measured to assess the general capacity of lymphocytes to respond to a mitogen. High levels of anti-human S-antigen and IRBP antibodies were detected in patients with onchocerciasis and endemic controls. The levels of both anti-human S-antigen and IRBP antibodies were significantly higher in onchocerciasis patients compared to endemic controls (Mann-Whitney ranksum test; p less than 0.001 respectively 0.002). No relationship could be demonstrated between the anti-retinal antibody level and the occurrence of chorioretinitis in ocular onchocerciasis. The occurrence of the anti-retinal antibodies as a result of crossreactivity of anti-retinal antibodies with parasitic antigens or of induction of polyclonal B-cell activation due to parasitic infection is discussed, since high antibody levels were also found in patients with Bancroftian filariasis from Papua New Guinea and Surinam. The migration inhibition factor assay, in which the cell-mediated immune response to human S-antigen, IRBP and retinal extract was tested, showed that four out of 50 (8%) patients with onchocerciasis and four out of 25 (16%) endemic controls reacted with at least one retinal antigen. From the patients with onchocercal chorioretinopathy two out of 21 (10%) showed a positive cellular response. The general mitogen response tested with Con A was positive in all these individuals. In conclusion, circulating antibodies against human S-antigen or human IRBP are thus nor specific for onchocerciasis and in themselves not sufficient to cause chorioretinopathy in onchocerciasis, although their pathogenic role in an ongoing chorioretinitis cannot be excluded. Furthermore a role for a cell-mediated anti-retinal autoimmune mechanism in the pathogenesis of chorioretinitis in onchocerciasis as studied with human S-antigen, IRBP or crude retinal extract could not be demonstrated.

CD4(+) depletion selectively inhibits eosinophil recruitment to the cornea and abrogates Onchocerca volvulus keratitis (River blindness).

Previous studies demonstrated that in the murine model of Onchocerca volvulus keratitis, neutrophils and eosinophils are recruited into the cornea in a biphasic manner in response to intrastromal injection. To determine if CD4(+) T cells regulate migration of neutrophils and eosinophils into the cornea, CD4(+) cells were depleted using monoclonal antibody GK1.5 before intrastromal injection of parasite antigens. Depletion of CD4(+) cells abrogated corneal opacification at later but not early stages of disease. Consistent with this observation, CD4 depletion significantly impaired recruitment of eosinophils to the cornea but had no effect on neutrophils. These data indicate that CD4(+) T cells mediate sustained O. volvulus keratitis by regulating eosinophil recruitment to the cornea.

Risk factors for optic nerve disease in communities mesoendemic for savannah onchocerciasis, Kaduna State, Nigeria.

Ophthalmic examinations on 6831 individuals aged 5 years or more, living in 34 guinea savannah communities mesoendemic for onchocerciasis, in Kaduna State, Nigeria, revealed a relatively high prevalence (9%) of optic nerve disease (OND). Further investigations were performed to determine what proportion of this burden of OND might be due to onchocercal infection. Information on history of cerebro-spinal meningitis (CSM), past use of diethylcarbamazine (DEC) and chloroquine, consumption of cassava and locally produced alcohol was collected for all individuals by questioning. In addition, a nested case-control study of 81 cases of OND and 136 age and sex-matched controls was performed to investigate whether syphilis or a variety of other neurological disorders were responsible for a substantial proportion of cases of OND. Our data suggest that in this population, onchocercal infection is the single most important cause of OND and may account for 50% of all cases. Some 13% of cases were associated with signs suggestive of glaucoma. DEC use might be responsible for up to 30% of all OND. We found no evidence to suggest that any of the following are important causes of OND in the communities studied: CSM, syphilis, neurological syndromes such as polyneuropathy or other generalized neurological disease, consumption of raw cassava, consumption of locally prepared alcohol.

Oncocercosis / Oncocercosis

La oncocercosis es una parasitosis producida por una filaria humana, denominada oncocerca volvulus, transmitida por mosquitos hembras del género simulium la cual afecta sólo en las áreas rurales. En México existen tres focos perfectamente delimitados: Soconusco y los Altos en Chiapas México, y en la Sierra de Ixtlán Oaxaca, México. En 1925 se confirma la existencia de oncocercosis en Chiapas y en 1926 en Oaxaca. A partir de 1930 la lucha se intensificó en las áreas afectadas: deshierbe en corrientes de agua, uso de hexaclorociclohexano, en 1942 se inauguró el Centro Médico de Investigaciones sobre Oncocercosis, tiempo más adelante se utilizó como larvicida el D.D.T. y se sostuvo e incrementó la desnodulización. Enumera también las políticas y programas de salud que se tomaron de 1931 a 1984. Toda la población de las áreas afectadas está en peligro de enfermar; las infecciones son frecuentes y no existe inmunidad, la presencia es mayor en jovenes y adultos. El primer censo fue en 1936, registrando 25,000 enfermos oncocercosos en Chiapas y 5,000 en Oaxaca; en el recuento de 1942 las cifras se elevaron a 50,000 en Chiapas y 6,000 en Oaxaca. Para 1970 se calculó un número de 100,000 enfermos en Chiapas. En 1990 se plantean las siguientes estrategias: estratificación epidemiológica, uso de ivermectina en el tratamiento de enfermos, se continúa con el uso de hetrazán, coordinación técnica y operativa con el programa de Guatemala, apoyo de organismos oficiales, impulsar la participación comunitaria, extirpar nódulos a todos los enfermos y prevenir lesiones oculares

Oncocercosis / Oncocercosis

La oncocercosis es una parasitosis producida por una filaria humana, denominada oncocerca volvulus, transmitida por mosquitos hembras del género simulium la cual afecta sólo en las áreas rurales. En México existen tres focos perfectamente delimitados: Soconusco y los Altos en Chiapas México, y en la Sierra de Ixtlán Oaxaca, México. En 1925 se confirma la existencia de oncocercosis en Chiapas y en 1926 en Oaxaca. A partir de 1930 la lucha se intensificó en las áreas afectadas: deshierbe en corrientes de agua, uso de hexaclorociclohexano, en 1942 se inauguró el Centro Médico de Investigaciones sobre Oncocercosis, tiempo más adelante se utilizó como larvicida el D.D.T. y se sostuvo e incrementó la desnodulización. Enumera también las políticas y programas de salud que se tomaron de 1931 a 1984. Toda la población de las áreas afectadas está en peligro de enfermar; las infecciones son frecuentes y no existe inmunidad, la presencia es mayor en jovenes y adultos. El primer censo fue en 1936, registrando 25,000 enfermos oncocercosos en Chiapas y 5,000 en Oaxaca; en el recuento de 1942 las cifras se elevaron a 50,000 en Chiapas y 6,000 en Oaxaca. Para 1970 se calculó un número de 100,000 enfermos en Chiapas. En 1990 se plantean las siguientes estrategias: estratificación epidemiológica, uso de ivermectina en el tratamiento de enfermos, se continúa con el uso de hetrazán, coordinación técnica y operativa con el programa de Guatemala, apoyo de organismos oficiales, impulsar la participación comunitaria, extirpar nódulos a todos los enfermos y prevenir lesiones oculares