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.

The reliability of anterior segment lesions as indicators of onchocercal eye disease in Guatemala

World Health Organization certification criteria for onchocerciasis elimination use anterior segment eye lesion prevalence as an indicator of mass ivermectin treatment program success. Lesions either contain visible microfilaria (noninflammatory punctate keratitis [PK] or microfilariae in anterior chamber [MFAC]), or microfilaria obscured by inflammation (inflammatory PK). To assess the utility of these disease indicators, two experienced ophthalmologists independently examined persons from endemic (N = 325) and nonendemic (N = 348) Guatemalan communities. Thirty-six (11.1%) and nine (2.6%) persons from endemic and nonendemic areas respectively had lesions found by either ophthalmologist (prevalence ratio = 4.3, 95% CI 2.1-8.8, P < 0.001). All lesions in nonendemic areas were inflammatory PK in whom no persons were seropositive for onchocerciasis. Overall, observer agreement was moderate (Kappa = 0.49), and most (61%) discordance occurred with inflammatory PK lesions. Our findings suggest that inflammatory punctate keratitis is neither a specific nor a reliable indicator of onchocercal eye disease. Future prevalence surveys should rely upon noninflammatory lesions as disease indicators.

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.

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.

IL-4 and IL-13 regulation of ICAM-1 expression and eosinophil recruitment in Onchocerca volvulus keratitis

PURPOSE

The presence of eosinophilic granulocytes in ocular tissue is a hallmark of the host response to environmental and parasite allergens. Using a mouse model of Onchocerca volvulus-mediated keratitis (river blindness), the present study examined the role of the cytokines interleukin (IL)-4 and IL-13 in regulating recruitment of eosinophils to the cornea through expression of intercellular cell adhesion molecule (ICAM)-1.

METHODS

C57BL/6 mice received an intrastromal injection of recombinant IL-4 and IL-13 (rIL-4 and IL-13) or were immunized by subcutaneous injection prior to receiving an intrastromal injection of a soluble O. volvulus extract. Expression of ICAM-1 and recruitment of eosinophils to the cornea were monitored by immunohistochemistry.

RESULTS

Expression of ICAM-1 was elevated after injection of rIL-4 or IL-13 together with recombinant tumor necrosis factor (rTNF)-alpha. Conversely, expression of ICAM-1 in O. volvulus-mediated keratitis was significantly reduced after subconjunctival injection of a monoclonal antibody (mAb) to IL-4 or IL-13. In addition, combined in vivo neutralization of IL-4 and IL-13 inhibited recruitment of eosinophils, but not of neutrophils, to the corneal stroma.

CONCLUSIONS

These findings demonstrate that expression of ICAM-1 and recruitment of eosinophils to the cornea are tightly regulated by IL-4 and IL-13, and indicate that these cytokines are a potential target for immune intervention in ocular allergy and parasitic infections of the eye.

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.

Onchocerca volvulus keratitis (river blindness) is exacerbated in BALB/c IL-4 gene knockout mice

To determine the outcome of Onchocerca volvulus keratitis in IL-4(-/-) BALB/c mice, animals were immunized subcutaneously and injected into the corneal stroma with soluble O. volvulus antigens. IL-4(-/-) BALB/c mice had a deviated cellular response, with decreased serum IgE and IgG1 and elevated IgG2a compared with control BALB/c mice. In marked contrast to control BALB/c, C57BL/6, and IL-4(-/-) C57BL/6 mice, IL-4(-/-) BALB/c mice developed severe corneal opacification and neovascularization that was associated with a pronounced neutrophil infiltrate to the corneal stroma. STAT-6(-/-) BALB/c mice had the same phenotype as IL-4(-/-) BALB/c mice, and complement depletion had no effect on the severity of O. volvulus keratitis in these mice. These findings indicate that on a BALB/c background, IL-4 has a critical role in regulating neutrophil recruitment to the cornea and development of O. volvulus keratitis.

Antigenic mimicry: Onchocerca volvulus antigen-specific T cells and ocular inflammation

PURPOSE

Molecular mimicry has been suggested to play a role in the development of ocular onchocerciasis. The Onchocerca volvulus antigen Ov39 is cross-reactive with the retinal antigen hr44 and induces ocular inflammation in rats after immunization. This study was undertaken to determine whether Ov39-derived T-cell lines, which proliferate in response to stimulation with hr44, can transfer disease to naive Lewis rats.

METHODS

Two separately derived IL-2-dependent CD4(+) T-cell lines, LKOV39 1.8 and LKOV39 4.5, specific to Ov39 were transferred to naïve Lewis rats. A T-cell line specific to the peripheral nerve protein P2 served as a positive control for transfer of disease. Ocular tissues were analyzed by immunohistology, and sera were tested for the presence of antibodies to hr44.

RESULTS

Transfer of both T-cell lines caused inflammation of the limbus, iris, and choroid. In addition, LKOV39 1.8, which produced slightly more inflammation, induced activation of retinal microglia. LKOV39 4.5 induced a dose-dependent influx of CD8(+) cells into the limbus and the uvea. Sera from rats that received the T-cell lines had no significant antibody responses to hr44.

CONCLUSIONS

These findings indicate that CD4(+) cell lines specific to the antigen Ov39 can induce ocular inflammation in naïve rats and suggest that recruitment of CD8(+) T cells may play a regulatory role. The inflammation is milder than that produced by immunization. The absence of antibody responses to hr44 in the animals receiving the T-cell lines may indicate a role for antibody in the development of ocular onchocerciasis.

Subconjunctival zoonotic Onchocerca in an Albanian man

A case of subconjunctival infection with a zoonotic species of Onchocerca is described, in a 16-year-old Albanian man who had immigrated to Greece. This is the first report of human infection with Onchocerca in this tissue location and only the eighth report of zoonotic Onchocerca in man.

River blindness: a role for parasite retinoid-binding proteins in the generation of pathology?

A new family of fatty acid- and retinoid-binding proteins has recently been identified in nematodes. These are apparently nematode specific and have very different structures and binding characteristics to their mammalian counterparts. Retinoids have important roles in vision, tissue differentiation and repair, and can profoundly affect collagen synthesis. Binding proteins released by a parasite might therefore play a part in the generation of the skin and eye pathology seen in river blindness. They might also be involved in the formation of the subcutaneous nodules induced by this parasite.

Distinct roles for PECAM-1, ICAM-1, and VCAM-1 in recruitment of neutrophils and eosinophils to the cornea in ocular onchocerciasis (river blindness)

Infiltration of granulocytes into the transparent mammalian cornea can result in loss of corneal clarity and severe visual impairment. Since the cornea is an avascular tissue, recruitment of granulocytes such as neutrophils and eosinophils into the corneal stroma is initiated from peripheral (limbal) vessels. To determine the role of vascular adhesion molecules in this process, expression of platelet endothelial cell adhesion molecule 1 (PECAM-1), ICAM-1, and VCAM-1 on limbal vessels was determined in a murine model of ocular onchocerciasis in which Ags from the parasitic worm Onchocerca volvulus are injected into the corneal stroma. Expression of each of these molecules was elevated after injection of parasite Ags; however, PECAM-1 and ICAM-1 expression remained elevated from 12 h after injection until 7 days, whereas VCAM-1 expression was more transient, with peak expression at 72 h. Subconjunctival injection of Ab to PECAM-1 significantly inhibited neutrophil recruitment to the cornea compared with eyes injected with control Ab (p = 0.012). Consistent with this finding, corneal opacification was significantly diminished (p < 0.0001). There was no significant reduction in eosinophils. Conversely, subconjunctival injection of Ab to ICAM-1 did not impair neutrophil recruitment, but significantly inhibited eosinophil recruitment (p = 0.0032). Injection of Ab to VCAM-1 did not significantly inhibit infiltration of either cell type to the cornea. Taken together, these results demonstrate important regulatory roles for PECAM-1 and ICAM-1 in recruitment of neutrophils and eosinophils, respectively, to the cornea, and may indicate a selective approach to immune intervention.

A dominant role for Fc gamma receptors in antibody-dependent corneal inflammation

Although production of specific Ab is a critical element of host defense, the presence of Ab in tissues leads to formation of immune complexes, which can trigger a type III Arthus reaction. Our studies on a mouse model of river blindness showed that Ab production is essential for recruitment of neutrophils and eosinophils to the cornea and for development of corneal opacification. In the current study, we determined the relative contribution of complement and FcgammaR interactions in triggering immune complex-mediated corneal disease. FcgammaR(-/-) mice, C3(-/-) mice, and immunocompetent control (B6/129Sj) mice were immunized s.c. and injected intrastromally with Onchocerca volvulus Ags. Slit lamp examination showed that control mice, C3(-/-) mice, and control mice injected with cobra venom factor developed pronounced corneal opacification, whereas corneas of FcgammaR(-/-) mice remained completely clear. Furthermore, recruitment of neutrophils and eosinophils to the corneal stroma was significantly impaired in FcgammaR(-/-) mice, but not in C3(-/-) mice or cobra venom factor-treated mice. We therefore conclude that FcgammaR-mediated cell activation, rather than complement activation, is the dominant pathway of immune complex disease in the cornea. These findings demonstrate a novel role for FcgammaR interactions in mediating ocular inflammation.

Canine ocular onchocercosis in Hungary

An adult male mongrel dog that had spent its entire life in Hungary, was found to have infection with filaroid nematodes of the genus Onchocerca. The gravid male and female parasites were embedded in bean-sized granulomatous masses on the conjunctiva and the sclera of both eyes. The cuticle of females consisted of two separated layers in longitudinal sections, the external layer bearing ridges and the internal layer showing striations. The ridges were marked, rounded in shape, and the ratio of body diameter to the distance between ridges varied between 7:1 and 10:1. At midbody of the worms, two striations could be seen between each pair of ridges: one under every ridge and one between neighbouring ridges. Numerous exceptionally small (96.4 microm x 6.4 microm) microfilariae were seen in the uteri of females and the surrounding tissues and isolated from skin biopsy materials. The morphology and location of the parasite and histopathological lesions of the Hungarian case were similar to that described in dogs in the United States. This case is the first documented ocular Onchocerca infection in dogs outside the western United States. Thus, onchocercosis should be considered in the differential diagnosis of ocular and periocular nodules in dogs also in Europe.

CXC chemokine receptor 2 but not C-C chemokine receptor 1 expression is essential for neutrophil recruitment to the cornea in helminth-mediated keratitis (river blindness)

Infiltration of neutrophils and eosinophils into the mammalian cornea can result in loss of corneal clarity and severe visual impairment. To identify mediators of granulocyte recruitment to the corneal stroma, we determined the relative contribution of chemokine receptors CXC chemokine receptor (CXCR)-2 (IL-8R homologue) and CCR1 using a murine model of ocular onchocerciasis (river blindness) in which neutrophils and eosinophils migrate from peripheral vessels to the central cornea. CXCR2(-/-) and CCR1(-/-) mice were immunized s.c. and injected into the corneal stroma with Ags from the parasitic helminth Onchocerca volvulus. We found that production of macrophage-inflammatory protein (MIP)-2, KC, and MIP-1 alpha was localized to the corneal stroma, rather than to the epithelium, which was consistent with the location of neutrophils in the cornea. CCR1 deficiency did not inhibit neutrophil or eosinophil infiltration to the cornea or development of corneal opacification. In marked contrast, neutrophil recruitment to the corneas of CXCR2(-/-) mice was significantly impaired (p < 0.0001 compared with control, BALB/c mice) with only occasional neutrophils detected in the central cornea. Furthermore, CXCR2(-/-) mice developed only mild corneal opacification compared with BALB/c mice. These differences were not due to impaired KC and MIP-2 production in the corneal stroma of CXCR2(-/-) mice, which was similar to BALB/c mice. Furthermore, although MIP-1 alpha production was lower in CXCR2(-/-) mice than BALB/c mice, eosinophil recruitment to the cornea was not impaired. These observations demonstrate the critical role for CXCR2 expression in neutrophil infiltration to the cornea and may indicate a target for immune intervention in neutrophil-mediated corneal inflammation.

Immune mechanisms in Onchocerca volvulus-mediated corneal disease (river blindness)

Infection with the parasitic nematode Onchocerca volvulus can lead to severe visual impairment and blindness. In an effort to characterize the molecular basis for the inflammatory response in the cornea, we have developed a murine model for O. volvulus-mediated keratitis in which parasite antigens are injected into the corneal stroma of sensitized mice. This model reproduces the two main clinical features of human disease, corneal opacification and neovascularization. Histological analysis of corneas from these mice reveals a biphasic recruitment of neutrophils and eosinophils to the central cornea, along with a small, but persistent number of CD3+ cells. In this review, we present evidence that production of antigen-specific T cell and antibody responses are essential for development of O. volvulus keratitis, and we propose a sequence of molecular and cellular events that lead to migration of inflammatory cells to the cornea and to loss of corneal clarity.

Impaired eosinophil recruitment to the cornea in P-selectin-deficient mice in Onchocerca volvulus keratitis (River blindness)

PURPOSE

A murine model of helminth-induced keratitis (river blindness) that is characterized by a biphasic recruitment of neutrophils (days 1-3) and eosinophils (days 3+) to the cornea has been developed. The purpose of this study was to determine the relative contribution of P- and E-selectin in recruitment of these inflammatory cells from limbal vessels to the corneal stroma.

METHODS

P- and E-selectin gene knockout (-/-) mice were immunized with antigens extracted from the parasitic helminth Onchocerca volvulus. One week after the last immunization, parasite antigens were injected directly into the corneal stroma. Mice were killed on days 1 and 3 postchallenge, and eyes were immunostained with either anti-eosinophil major basic protein (MBP) or with anti-neutrophil Ab. The number of cells in the cornea was determined by direct counting.

RESULTS

Recruitment of eosinophils to the cornea was significantly impaired in P-selectin(-/-) mice (63.9% fewer eosinophils on day 1 [P: = 0.0015], and 61% fewer on day 3 [P: < 0.0001]) compared with control C57BL/6 mice. In contrast, P-selectin deficiency had no effect on neutrophil recruitment to the cornea. There was no inhibition of eosinophil and neutrophil migration to the corneas of E-selectin(-/-) mice, indicating that there is no direct role for this adhesion molecule in helminth-induced keratitis.

CONCLUSIONS

The present study demonstrates that P-selectin is an important mediator of eosinophil recruitment to the cornea. P-selectin interactions may therefore be potential targets for immunotherapy in eosinophil-mediated ocular inflammation.

Angiogenic activity of Onchocerca volvulus recombinant proteins similar to vespid venom antigen 5

Although the mechanisms underlying the host inflammatory response in ocular onchocerciasis have been examined, the role of particular parasite proteins in this process remains largely unexplored. Recently, it was found that one of the most abundant expressed sequence tags in Onchocerca volvulus infective larvae encoded a protein with similarities to a component of vespid venom. This clone was designated O. volvulus Activation associated Secreted Protein -1 (Ov-asp-1). We report the characterization of three members of a family of proteins, designated the Ov-ASP family, of which Ov-ASP-1 is a member. Sequence based and phylogenetic analyses suggest that these proteins form a filarial specific protein family related to both the vespid venom antigen 5 and the vertebrate CRISP/Tpx family of proteins. The three members of the Ov-ASP family exhibit distinct patterns of expression in the life cycle of O. volvulus. Genomic Southern blot analyses indicate that several genes encoding sequences related to the Ov-asp family are present in the genome of O. volvulus. Recombinant proteins expressed from full length cDNAs encoding two members of the Ov-asp family were found to induce an angiogenic response after injection into corneas of naive mice, and vessel formation was associated with only minor inflammatory cell infiltration. These data suggest that Ov-ASP proteins may directly induce an angiogenic response and may therefore contribute to corneal neovascularization in onchocercal keratitis.

An essential role for antibody in neutrophil and eosinophil recruitment to the cornea: B cell-deficient (microMT) mice fail to develop Th2-dependent, helminth-mediated keratitis

Invasion of the corneal stroma by neutrophils and eosinophils and subsequent degranulation disrupts corneal clarity and can result in permanent loss of vision. In the current study, we used a model of helminth-induced inflammation to demonstrate a novel role for Ab in mediating recruitment of these inflammatory cells to the central cornea. C57BL/6 and B cell-deficient (microMT) mice were immunized s. c. and injected intrastromally with Ags from the parasitic helminth Onchocerca volvulus (which causes river blindness). C57BL/6 mice developed pronounced corneal opacification, which was associated with an Ag-specific IL-5 response and peripheral eosinophilia, temporal recruitment of neutrophils and eosinophils from the limbal vessels to the peripheral cornea and subsequent migration to the central cornea. In contrast, the corneas of microMT mice failed to develop keratitis after intrastromal injection of parasite Ags unless Ags were injected with immune sera. Eosinophils were recruited from the limbal vessels to the peripheral cornea in microMT mice, but failed to migrate to the central cornea, whereas neutrophil recruitment was impaired at both stages. With the exception of IL-5, T cell responses and peripheral eosinophils were not significantly different between C57BL/6 and microMT mice. Taken together, these findings not only demonstrate that Ab is required for the development of keratitis, but also show that recruitment of neutrophils to the cornea is Ab-dependent, whereas eosinophil migration is only partially dependent upon Ab interactions.

Zoonotic Onchocerca (Nematoda:Filarioidea) in the cornea of a Colorado resident

OBJECTIVE

A female patient, resident in the state of Colorado, presented with iritis of the right eye. Slit-lamp examination showed the presence of a thin, threadlike worm entwined in the cornea. The patient was taken to surgery for removal of the parasite.

DESIGN

A case report.

INTERVENTION

A 3-mm-long supertemporal incision was made in the cornea and further dissected until the worm could be grasped and removed by gentle traction.

RESULTS

The worm, a filarial nematode, was identified as a member of the genus Onchocerca, most likely Onchocerca cervicalis, a natural parasite of horses. The patient had an uneventful recovery, and 1 week after surgery, her visual acuity, intraocular pressure, and corneal edema were all resolving.

CONCLUSION

In the United States and elsewhere, most cases of zoonotic filarial infection involving the eye are caused by Dirofilaria or Dipetalonema-like worms. However, the current case was caused by a species of Onchocerca. This is the first case of zoonotic Onchocerca from the eye to be reported, only the second case of zoonotic Onchocerca in the United States, and the seventh case worldwide. The worm was removed surgically, and the patient had an uneventful recovery.

The role of eosinophils and neutrophils in helminth-induced keratitis

PURPOSE

Intrastromal injection of mice with antigens from the parasitic helminth that causes river blindness (Onchocerca volvulus) induces eosinophil recruitment to the corneal stroma at the time of maximum corneal opacification and neovascularization. The present study was conducted to examine the role of eosinophils and neutrophils in onchocercal keratitis in control C57Bl/6 mice and in interleukin-5 gene knockout (IL-5(-/-)) mice.

METHODS

C57Bl/6 and IL-5(-/-) mice were immunized subcutaneously and injected intrastromally with soluble O. volvulus antigens. Mice were killed at various times thereafter. Development of keratitis was assessed by slit lamp examination, and inflammatory cells in the cornea were identified by immunohistochemistry.

RESULTS

A biphasic recruitment of inflammatory cells was observed in C57Bl/6 mice; neutrophils predominated during the first 72 hours after intrastromal injection and subsequently declined, whereas eosinophil recruitment increased as time elapsed and comprised the majority (90%) of cells in the cornea by day 7. In contrast, neutrophils were the predominant inflammatory cells in IL-5(-/-) mice at early and late time points and were associated with extensive stromal damage and corneal opacification and neovascularization. Eosinophils were not detected in these mice at any time.

CONCLUSIONS

In the absence of eosinophils, neutrophils can mediate keratitis induced by helminth antigens. Together with the early neutrophilic infiltrate in control animals, these observations indicate that neutrophils have an important role in onchocercal keratitis.

Onchocercal eye disease and the impact of Mectizan treatment

Onchocerciasis continues to be a major cause of blindness, particularly in those sub-Saharan African countries which are outside the area of West Africa monitored by the Onchocerciasis Control Programme (OCP). Onchocercal ocular disease and blindness develop as a result of long exposure to onchocercal infection. Until 1987, suramin and diethylcarbamazine were the only drugs available for the treatment of onchocerciasis and they could not be used for community therapy because of their toxicity and the dosage schedules required. The registration of Mectizan (ivermectin, MSD) for treatment of human onchocerciasis in 1987, and the donation of this drug by Merck & Co. for as long as it is needed, provided a new opportunity for the safe treatment and control of the disease. The data available on the impact of repeated doses of Mectizan on ocular onchocercal disease indicate a significant reduction of ocular microfilarial loads and regression of early lesions of the anterior segment, including iridocyclitis and sclerosing keratitis. Such improvements are seen more rapidly when Mectizan is used than when onchocerciasis is limited by vector control alone. Mectizan treatment also has a beneficial effect on onchocercal optic-nerve disease and visual-field loss. Long-term maintenance of Mectizan therapy should lead to a reduction in the prevalence of blindness in endemic communities.

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.

Pleomorphism of stromal eosinophils in murine experimental onchocercal keratitis

Onchocercal keratitis (river blindness) is one of the leading worldwide causes of blindness. Light microscopic analysis of human specimens and corneal tissue from experimental models has implicated the eosinophil as an important cell in the inflammatory response. Our previous studies in experimental murine onchocercal keratitis have demonstrated that the inflammatory infiltrate is composed primarily of eosinophils displaying ring shaped or bilobed nuclei. However, a number of cells were not characterizable by light microscopy, presumably due to mechanical distortion. To more fully characterize the inflammatory cell infiltrate, we examined corneal specimens by transmission electron microscopy. In addition to typical eosinophils with bilobed and ring shaped nuclei, this approach revealed cells with variable nuclear morphology and cell shape which contained the dense cored granules characteristic of eosinophils. Hence, the degree of pleomorphism of eosinophils is broader than appreciated and underscores the importance of this cell in experimental murine onchocercal keratitis.

Immunopathology of onchocerciasis: a role for eosinophils in onchocercal dermatitis and keratitis

This review has sought to demonstrate that eosinophils have an important role in the immunopathology of onchocercal dermatitis and keratitis. The most compelling evidence is the consistent presence of eosinophils and eosinophil granule proteins at the site of tissue damage, either after parasite death or direct injection of parasite antigens. A more definitive role for eosinophils in onchocercal keratitis will be determined using IL-5 gene knockout mice (an animal model of onchocercal dermatitis has yet to be established). Identification of chemoattractants and adhesion molecules necessary for eosinophil recruitment will indicate possible approaches to immune intervention.

Forest onchocerciasis in Cameroon: its distribution and implications for selection of communities for control programmes

The prevalence of infection and disease due to Onchocerca volvulus in regions at different altitudes and distances from the nearest breeding site for the Simulium vectors were studied in a forested area of Cameroon. A total of 1,785 subjects in 14 villages underwent clinical and parasitological evaluation. According to WHO endemicity criteria, villages 267 and 720 m above the sea level (asl) were hyperendemic, whereas lower villages, at 55 n asl, were meso-endemic. Altitude was positively correlated and distance from vector breeding site negatively correlated with several clinical indicators of onchocerciasis: prevalence of infection, microfilarial density, onchocercomata, and blindness. Location and altitude of his or her village and the subject's duration of residence in the village, age and sex were all found to be independent predictors of infection by logistic regression. The observed pattern of forest onchocerciasis reflects an interplay of multiple factors rather than a simple relationship between endemicity and distance from the nearest river. In order to select communities most affected by onchocerciasis for control programmes, two indicators, the prevalence of blindness and the prevalence of onchocercomata, might be used.

Impact of annual dosing with ivermectin on progression of onchocercal visual field loss

Reported are the results of a randomized, double-masked, placebo-controlled trial of annual ivermectin dosing in 34 rural communities, Kaduna State, northern Nigeria, where guinea savanna onchocerciasis is mesoendemic. A total of 939 individuals underwent Friedmann field analysis at the first examination and saw at least 19 spots in at least one eye. Of these, 636 (68%) completed a subsequent Friedmann field analysis 2-3 years later. The adjusted incidence rate ratio for the ivermectin group versus the placebo group was 0.64 (95% confidence interval (CI): 0.42-0.98). There was some evidence that the impact of ivermectin was greatest among those who had received one dose of ivermectin. The majority of the deteriorations occurred in eyes that gave evidence of optic atrophy at the first examination. An analysis restricted to individuals with optic atrophy at baseline indicated a reduction of 45% in the incidence of visual field deterioration in the ivermectin group (95% CI: 8-67%). Previous findings have shown that ivermectin has an impact on the incidence of optic atrophy. Our results indicate, for the first time, that ivermectin has a substantial impact on the progression of visual field loss among those with pre-existing optic atrophy.

Onchocerca volvulus-mediated keratitis: cytokine production by IL-4-deficient mice

Corneal inflammation similar to human onchocercal keratitis can be induced in mice by subcutaneous immunization of a soluble extract of Onchocerca volvulus (OvAg) followed by direct injection of OvAg into the corneal stroma. Previous studies have shown that corneal pathology is associated with increased systemic and corneal Th2 cytokine expression and that IL-4 gene knockout (IL-4-/-) mice develop less severe or no O. volvulus-mediated keratitis. The current study examined the contribution of Th2 cytokines to the diminished OvAg-induced corneal immunopathology observed in IL-4-/- mice. IL-4-/- mice (129Sv x C57B1/6), wild-type F2 littermates (IL-4+/+), and C57B1/6 mice were sensitized by repeated subcutaneous immunization with OvAg. Ten days after the final immunization, mice were sacrificed, spleens were removed, and cells were incubated with OvAg. Cells from immunocompetent C57B1/6 and IL-4+/+ mice produced IL-4 and IL-5, but no IFN-gamma, whereas cells from IL-4-/- mice had elevated IFN-gamma and no IL-4. Interestingly, cells from these animals produced levels of IL-5 protein equivalent to those of C57B1/6 and IL-4+/+ mice. To determine cytokine production in corneas during the onset of onchocercal keratitis, OvAg-immunized mice were injected intracorneally with OvAg, and cytokine gene expression in the cornea was determined by RT-PCR. Temporal analysis of cytokine gene expression in corneas of immunocompetent mice showed that the Th2-associated cytokines IL-4, IL-5, IL-10, and IL-13 were produced within 1 day of intrastromal injection, with sustained elevations for 10 days. Maximal IFN-gamma mRNA levels were not detected until Day 10. This was in contrast to IL-4-/- mice in which IFN-gamma appeared at Day 1 and remained elevated for at least 10 days. Moreover, in corneas from IL-4-/- mice, all Th2 cytokines with the exception of IL-4 were up-regulated and expressed with kinetics similar to that of IL-4+/+ littermates. Histologically, corneas from IL-4-/- mice were less edematous and contained fewer eosinophils and other inflammatory cells than those from immunocompetent controls. As there was no difference in peripheral eosinophil levels, these data indicate that the diminished severity of onchocercal keratitis in IL-4-/- mice is not due to failure to develop systemic or local Th2 cytokine responses or to produce eosinophils, but that IL-4 may be involved in recruitment of eosinophils and other inflammatory cells into the corneal stroma.

Interleukin 4 and T helper type 2 cells are required for development of experimental onchocercal keratitis (river blindness)

Inflammation of the corneal stroma (stromal keratitis) is a serious complication of infection with the nematode parasite Onchocerca volvulus. Because stromal keratitis is believed to be immunologically mediated in humans, we used a murine model to examine the role of T cells and T helper cell cytokines in the immunopathogenesis of these eye lesions. BALB/c mice immunized subcutaneously and injected intrastromally with soluble O. volvulus antigens (OvAg) developed pronounced corneal opacification and neovascularization. The corneal stroma was edematous and contained numerous eosinophils and mononuclear cells. Stromal keratitis in immunized mice was determined to be T cell dependent based on the following observations: (a) T cell-deficient nude mice immunized and injected intrastromally with OvAg fail to develop corneal pathology; and (b) adoptive transfer of spleen cells from OvAg-immunized BALB/c mice to naive nude mice before intrastromal injection of OvAg results in development of keratitis. OvAg-stimulated lymph node and spleen cell cytokine production was dependent on CD4 cells and included interleukin (IL)-4 and IL-5, but not interferon gamma, indicating a predominant T helper type 2 cell-like response. Inflamed corneas from immunized BALB/c mice and from reconstituted nude mice had greatly elevated CD4 and IL-4 gene expression compared with interferon gamma. Mice in which the IL-4 gene was disrupted failed to develop corneal disease, demonstrating that IL-4 is essential in the immunopathogenesis of O. volvulus-mediated stromal keratitis.

Onchocerciasis in two dogs

Although widely reported in cattle, horses, and other ungulates, onchocerciasis has only recently been reported in 1 dog. We report 2 additional dogs with onchocerciasis involving the palpebral conjunctiva, third palpebra, and sclera. Both dogs were evaluated because of ocular or periocular masses. Histologically, viable adult parasites were surrounded by minimal fibrosis and few macrophages. Pyogranulomas were often centered around degenerating or mineralized Onchocerca organisms. Onchocerciasis should be considered in the differential diagnosis of ocular or periocular nodules in dogs, particularly dogs from western states.

Immunologic cross-reactivity in the pathogenesis of ocular onchocerciasis

PURPOSE

Onchocerca volvulus, a filarial worm, is a major cause of infectious blindness and inflammatory eye disease. An autoimmune cause for ocular onchocerciasis has been suggested since the identification of a recombinant antigen of O. volvulus that shows immunologic cross-reactivity with a host ocular component of 44,000 M(r). The aim of this study was to establish the distribution of the cross-reactive antigens in both host tissues and the parasite, and to determine if significant autoantibody responses to the host antigen could be detected in infected persons.

METHODS

The tissue and organ distribution of the 44,000 M(r) antigen was determined by immunocytochemistry and Western blot analysis. Human autoantibody responses to the ocular antigen were demonstrated by Western blot analysis using sera collected from persons with onchocerciasis, with and without posterior segment pathology, Bancroftian filariasis, and Europeans with no filarial infection.

RESULTS

The tissue distribution of the 44,000 M(r) antigen correlates with the sites of pathology in onchocerciasis and antibody reactivity against this antigen could be detected in all persons with onchocerciasis and posterior segment pathology. The antigen is also recognized by sera from persons with Bancroftian filariasis, but not from normal persons.

CONCLUSIONS

A role is proposed for immunologic cross-reactivity in the pathogenesis of onchocerciasis and it is suggested that intraocular presentation of the cross-reactive parasite antigen by microfilariae is essential for the development of disease.

Immune-mediated Onchocerca volvulus sclerosing keratitis in the mouse

Sclerosing keratitis is the major cause of blindness due to onchocerciasis; its pathogenesis is poorly understood. We have previously reported an immune-mediated model of experimental interstitial keratitis in guinea pigs following intrastromal challenge with soluble antigens from Onchocerca volvulus. This model system is ideal for evaluation of pathogenicity of multiple purified antigen preparations; however, reagents necessary for detailed immunologic analysis of the inflammatory cellular infiltrate are not yet available for guinea pigs. Because of the ready availability of these reagents for mice, a mouse model has been developed. The inflammatory response observed in this model was analogous to that seen in human onchocercal sclerosing keratitis as well as in the guinea pig model of onchocercal sclerosing keratitis. Granulocytes were present in the acute inflammatory response, whereas the chronic response showed lymphocytes, plasma cells, and histiocytes. Neovascularization and scarring of the corneal stroma was also observed. This model will be helpful in examining the mechanisms of immunopathogenesis and the contribution of the host genetic background to the disease.

A study of onchocerciasis with severe skin and eye lesions in a hyperendemic zone in the forest of southwestern Cameroon: clinical, parasitologic, and entomologic findings

Prior to the initiation of an onchocerciasis control program based on the mass administration of ivermectin in the rain forest of southwestern Cameroon, a preliminary baseline study of the area was conducted. The results of this study showed that onchocerciasis was hyperendemic in the area. Skin symptoms and signs were observed including pruritus (67.4% of the population examined), onchocerca nodules (51.6%), skin depigmentation (18.5%), and hanging groins (5.7%). Except for pruritus, the prevalence of these symptoms increased with age. Of the eyes examined, 44.9% had microfilariae in the anterior chamber, 33.5% had choroidoretinitis, 28.0% had punctate keratitis, 8.3% had papillary abnormalities, and 3.6% had sclerosing keratitis. Vision in 10.5% of the eyes examined was classified as blind or very poor (visual acuity = 0-0.10), in 15.7% as poor (visual acuity = 0.11-0.39), and in 73.8% as good (visual acuity = 0.4-1.00). Unlike previous reports that have linked serious ocular damage mainly to savanna onchocerciasis, the present study showed that forest onchocerciasis also caused significant ocular pathology, including blindness. Parasitologically, positive skin snips were recorded for 92.7% of the persons examined, with both sexes being equally infected. The parasite load, expressed as the geometric mean number of microfilariae per skin snip, was 53.6, and was much higher in males than in females. The flv vector, Simulium squamosum, had a high infection rate of 7.5% infective females in Bakumba and 6.8% infective females in Ngbandi, the two fly-catching points. The transmission potential was 266 infective larvae per person per month in Bakumba and 189 in Ngbandi.

Ocular onchocerciasis and the intensity of infection in the community. IV. The degraded forest of Sierra Leone

To assess the pattern of onchocercal ocular disease and blindness in south Sierra Leone, ophthalmological surveys were carried out in 13 highly infected villages located in various river basins. The most important finding was the blinding potential of onchocerciasis in the degraded forest area where the prevalence of onchocercal blindness reached levels of up to 6%. This is remarkable since previous studies have claimed onchocerciasis in the forest to cause little blindness. Ocular onchocerciasis undoubtedly constitutes a problem of public health importance in south Sierra Leone. The rates of onchocercal ocular disease and blindness, however, were significantly lower than those found in savanna villages with similar levels of endemicity. The community pattern of ocular onchocerciasis was not significantly different from the classical pattern in the forest but this could be explained by the low endemicity levels in the forest villages studied. It is therefore not possible to deduce from this study whether the pattern of ocular onchocerciasis in south Sierra Leone is of the forest type, or a pattern on its own.

Onchocerciasis in a California dog

A 15-year-old dog that had lived all of its life on ranches and at the Wildlife Waystation on the western edge of the San Gabriel mountains near Los Angeles, California, developed an extensive granulomatous lesion involving the right eye and associated tissues requiring removal of both the eye and the lesion. Microscopic examinations of the tissues revealed the presence of living and dead gravid female worms and male worms belonging to the genus Onchocerca. Unsheathed microfilariae presumed to be Onchocerca species were found in the skin as well. Because Onchocerca species are not natural parasites of dogs, it is presumed that this infection was acquired accidentally from bovine, equine, or other animal host sharing the environment. This appears to be the first published record of patent onchocerciasis in a dog.

Experimental ocular onchocerciasis in cynomolgus monkeys. IV. Chorioretinitis elicited by Onchocerca volvulus microfilariae

Onchocerciasis is a major cause of blindness worldwide, and much of the blindness is caused by onchocercal chorioretinitis. In an experimental animal model for ocular onchocerciasis, intravitreal injections of 10,000 live Onchocerca volvulus microfilariae isolated from infected humans into the eyes of cynomolgus monkeys (Macaca fascicularis) resulted in patchy, progressive loss of retinal pigment with pigment clumping. Areas of pigment loss were less extensive in animals that had been sensitized with microfilariae. Intravitreal injections of dead O. volvulus microfilariae resulted in mild vitritis with relatively less clinical change noted in the retina and choroid. Histopathologic examination revealed thinning and loss of outer retinal layers with pigment migration into the retina, and inflammation was more pronounced in eyes that received live microfilariae. Clinical changes appeared in eyes receiving live microfilariae before the development of significant antibody or cell-mediated immune responses. O. volvulus microfilariae appear to be more suitable than O. lienalis microfilariae in producing lesions which resemble human onchocerciasis in the primate model.

Ocular manifestations of onchocerciasis in a rain forest area of west Africa

The epidemiology and natural history of onchocerciasis and its ocular complications in rain forest areas are poorly understood. The present study was conducted on a rubber plantation in a hyperendemic area in the rain forest of Liberia, West Africa, where 800 persons were examined. The prevalence of infection was 84% overall 29% had intraocular microfilariae, and 2.4% were blind in one or both eyes. Onchocerciasis was the cause of all binocular blindness and one-third of all visual impairment. Over half of the visual impairment caused by onchocerciasis was due to posterior segment diseases. Chorioretinal changes were present in 75% of people, and included intraretinal pigment clumping in 52% and retinal pigment epithelium atrophy in 32%. Atrophy of the retinal pigment epithelium was associated with increasing age and severity of infection. Intraretinal pigment was strongly associated with anterior uveitis. There was a strong correlation between uveitis and the inflammatory chorioretinal sequelae: retinitis, intraretinal pigment, subretinal fibrosis, and optic neuropathy. These findings indicate that considerable visual impairment associated with rain forest onchocerciasis is common and is due largely to chorioretinal disease.

Longitudinal study of lesions of the posterior segment in onchocerciasis

Onchocerciasis is a major cause of blindness, and much of the blindness due to onchocerciasis is caused by chorioretinitis. Little is known about the progression of lesions in the posterior segment in either untreated or treated disease. The authors studied the progression of onchocercal chorioretinitis in 57 patients from 1 to 3 years. Changes were documented from detailed ocular examinations, fundus photographs, and fluorescein angiograms, and included live intraretinal microfilariae, intraretinal hemorrhages, cotton-wool opacities, intraretinal pigment, white and shiny intraretinal deposits, retinal pigment epithelial window defects, and atrophy. Depigmentation at the edge of chorioretinal scarring progressed at a rate of up to 200 microns per year. Ivermectin or mebendazole treatment did not appear to alter the progress of depigmentation at the edge of chorioretinal scars. These observations suggest that onchocercal chorioretinitis is associated with early changes in the retina and retinal pigment epithelium, and that disease in the posterior segment may progress rapidly.

Ocular involvement in patients with onchocerciasis after repeated treatment with ivermectin

We assessed ocular changes after therapy at six and 12 months with ivermectin (150 micrograms/kg of body weight) in a 12-month prospective study of 29 patients with ocular onchocerciasis and 15 patients with onchocerciasis without ocular involvement. The patients lived in a hyperendemic area in Sierra Leone, West Africa, where no vector control was instituted. Five months after initial treatment, the microfilarial load in skin and eyes had decreased significantly (P less than .0000), but 28 of 44 (63%) patients had positive skin-snip test results and nine of 29 (31%) patients with ocular involvement had active ocular disease. Twelve months after initial treatment, 15 of 41 (37%) patients had positive skin-snip test results and eight of 26 (31%) showed active ocular involvement. All patients with persistent ocular disease after therapy showed evidence of active onchocerciasis at that time, which suggests that a dose of ivermectin at six-month intervals is not sufficient for intensely infested patients with severe ocular disease. We developed an ocular involvement score to evaluate the patient's total ocular status and observed a significant relation between the pretreatment severity of ocular involvement and the persistence of active ocular disease after treatment with ivermectin.

Immunopathology of ocular onchocerciasis. I. Inflammatory cells infiltrating the anterior segment

Ocular tissue (conjunctiva and iris) was obtained from 12 adult African men with active ocular onchocerciasis and from nine age-matched persons from the same endemic region but without onchocercal infection. These tissues were examined immunohistologically and two major findings were noted. First, mild-to-moderate chronic inflammatory cellular infiltration was present in the conjunctiva of the onchocerciasis patients. T lymphocytes (CD3+) were the major inflammatory cells, and the T suppressor/cytotoxic (CD8+) subset was significantly increased in the ocular onchocerciasis patients (P less than 0.03). Second, in the onchocerciasis patients, non-lymphoid cells of the conjunctiva and iris, such as vascular endothelium, pericytes and fibroblasts were in an activated state, as shown by increased expression of Class II MHC antigens (P less than 0.02, conjunctiva; P less than 0.05, iris). These concomitant findings of lymphocyte infiltration and resident cell activation indicate a dynamic state of localized host responsiveness presumably to the microfilarial parasites and their products in the anterior segments of the eyes of patients with ocular onchocerciasis.