Oncocercosis ocular / Ocular onchocerciasis

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Onchocerciasis: the role of Wolbachia bacterial endosymbionts in parasite biology, disease pathogenesis, and treatment.

The discovery of Wolbachia intracellular bacteria within filarial nematodes, including Onchocerca volvulus, the causative agent of onchocerciasis or "river blindness," has delivered a paradigm shift in our understanding of the parasite's biology, to where we now know that the bacterial endosymbionts are essential for normal development of larvae and embryos and may support the long-term survival of adult worms. The apparent mutualistic dependency has also offered a novel approach to the treatment of onchocerciasis through the use of antibiotics to eliminate Wolbachia, delivering for the first time a treatment which has significant macrofilaricidal efficacy. Studies with other filarial nematode species have also highlighted a role for Wolbachia in transmission and infection of the mammalian host through a fascinating manipulation of mast cell-mediated vasodilation to enhance infectivity of vector-borne larvae. Wolbachia has also been identified as the principal driver of innate and adaptive Th1 inflammatory immunity, which can either contribute to disease pathogenesis or, with the Wolbachia-mediated recruitment of mast cells, enhance infectivity. The Wolbachia activation of innate inflammation also drives inflammatory adverse events in response to chemotherapy with either diethylcarbamazine (DEC) or ivermectin. In this review we summarize the experimental and field trial data which have uncovered the importance of Wolbachia symbiosis in onchocerciasis.

[Ocular onchocerciasis: a key role for Wolbachia]. / Okuläre Onchozerkose: Wolbachien haben eine Schlüsselrolle.

Onchocerciasis is caused by the parasitic worm Onchocerca volvulus, which releases millions of offspring (microfilariae). Microfilariae migrate through the skin and can enter the anterior or posterior regions of the eye. While alive, the microfilariae appear to cause little or no inflammation, being in the anterior chamber. However, when they die, either by natural attrition or after chemotherapy, the host response to degenerating worms can result in ocular inflammation (keratitis, uveitis, chorioretinitis, neuritis of the optic nerve) that causes progressive loss of vision and ultimately leads to blindness. With the use of a mouse model of corneal inflammation to study the pathogenesis of ocular onchocerciasis by injecting worm extracts directly into the corneal stroma, it was found that worms treated with the antibiotic doxycycline, which destroys Wolbachia, induced lower corneal stromal thickness and stromal haze (indicators of corneal oedema and opacity) and neutrophil infiltration compared with both untreated worms and worms that do not harbour Wolbachia. These data indicate that endosymbiotic Wolbachia bacteria in filarial parasites have a key role in the pathogenesis of river blindness. Worms recovered from patients treated for 6 weeks with doxycycline contained fewer Wolbachia bacteria and had abnormal embryogenesis, indicating a role for Wolbachia in the survival or fecundity of the worms. Antibiotic treatment may also reduce the severity of the inflammatory response in the cornea.

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.

Wolbachia-induced neutrophil activation in a mouse model of ocular onchocerciasis (river blindness).

Endosymbiotic Wolbachia bacteria are abundant in the filarial nematodes that cause onchocerciasis (river blindness), including the larvae (microfilariae) that migrate into the cornea. Using a mouse model of ocular onchocerciasis, we recently demonstrated that it is these endosymbiotic bacteria rather than the nematodes per se that induce neutrophil infiltration to the corneal stroma and loss of corneal clarity (Saint Andre et al., Science 295:1892-1895, 2002). To better understand the role of Wolbachia organisms in the pathogenesis of this disease, we examined the fate of these bacteria in the cornea by immunoelectron microscopy. Microfilariae harboring Wolbachia organisms were injected into mouse corneas, and bacteria were detected with antibody to Wolbachia surface protein. Within 18 h of injection, neutrophils completely surrounded the nematodes and were in close proximity to Wolbachia organisms. Wolbachia surface protein labeling was also prominent in neutrophil phagosomes, indicating neutrophil ingestion of Wolbachia organisms. Furthermore, the presence of numerous electron-dense granules around the phagosomes indicated that neutrophils were activated. To determine if Wolbachia organisms directly activate neutrophils, peritoneal neutrophils were incubated with either parasite extracts containing Wolbachia organisms, parasite extracts depleted of Wolbachia organisms (by antibiotic treatment of worms), or Wolbachia organisms isolated from filarial nematodes. After 18 h of incubation, we found that isolated Wolbachia organisms stimulated production of tumor necrosis factor alpha and CXC chemokines macrophage inflammatory protein 2 and KC by neutrophils in a dose-dependent manner. Similarly, these cytokines were induced by filarial extracts containing Wolbachia organisms but not by Wolbachia-depleted extracts. Taken together, these findings indicate that neutrophil activation is an important mechanism by which Wolbachia organisms contribute to the pathogenesis of ocular onchocerciasis.

Canine onchocercosis in Greece: report of further 20 cases and molecular characterization of the parasite and its Wolbachia endosymbiont.

Recently, sporadic cases of subconjunctival Onchocerca infection have been reported in dogs in Greece and Hungary. Herein we report further cases from Greece and the results of the molecular analysis of Onchocerca sp. removed from Greek dogs and its Wolbachia endosymbionts. Twenty dogs of various breeds, 1-11 years of age with subconjunctival onchocercosis (4 cases each in right or left eye, 12 cases in both eyes) were presented having similar manifestations. Periorbital swelling, exophthalmos, lacrimation, discharge, photophobia, conjunctival congestion, corneal edema, protrusion of the nictitating membrane, and subconjunctival granuloma or cyst formation were the most important clinical signs. After surgical excision of the periocular masses containing the worms, all animals recovered fully from onchocercosis. Based on the similarities of the clinical picture of the Greek and Hungarian cases, the similar morphology of the Greek and Hungarian isolates, and the identical sequences of the cytochrome oxidase gene of the filarial parasites and that of the 16S ribosomal RNA gene from their Wolbachia endosymbionts, the Onchocerca sp. isolated from dogs in Greece and Hungary appears to belong to the same species.

Immunopathogenesis of Onchocerca volvulus keratitis (river blindness): a novel role for endosymbiotic Wolbachia bacteria.

River blindness is thought to occur as a result of the host response to degenerating microfilariae in the eye. Utilizing a murine model of corneal inflammation (keratitis) to investigate the immune and inflammatory responses associated with river blindness, we recently demonstrated an important role for endotoxin-like products from endosymbiotic bacteria and for activation of Toll-like receptor 4 (TLR4). These observations have led to a new understanding of the pathogenesis of this disease

Immunopathogenesis of Onchocerca volvulus keratitis (river blindness): a novel role for TLR4 and endosymbiotic Wolbachia bacteria.

Infection with the parasitic nematode Onchocerca volvulus is associated with inflammation of the skin and cornea that can lead to blindness. Corneal damage is thought to occur as a result of the host inflammatory responses to degenerating microfilariae in the eye. We have utilized a murine model of corneal inflammation (keratitis) to investigate the immune and inflammatory responses associated with river blindness. Soluble extracts of O. volvulus, a filarial species that contains the endosymbiont bacteria Wolbachia or Acanthocheilonema viteae (a nematode not naturally infected with the bacteria) were injected into mouse corneas. Inflammatory responses and corneal changes were measured. We demonstrated a major role for endosymbiont Wolbachia bacteria and Toll-like receptor 4 (TLR4) in the pathogenesis of ocular onchocerciasis.

The role of endosymbiotic Wolbachia bacteria in the pathogenesis of river blindness.

Parasitic filarial nematodes infect more than 200 million individuals worldwide, causing debilitating inflammatory diseases such as river blindness and lymphatic filariasis. Using a murine model for river blindness in which soluble extracts of filarial nematodes were injected into the corneal stroma, we demonstrated that the predominant inflammatory response in the cornea was due to species of endosymbiotic Wolbachia bacteria. In addition, the inflammatory response induced by these bacteria was dependent on expression of functional Toll-like receptor 4 (TLR4) on host cells.