Ivermectin does not reduce the burden of itching in an onchocerciasis endemic community

Onchocerciasis drug development: from preclinical models to humans

Twenty diseases are recognized as neglected tropical diseases (NTDs) by World Health Assembly resolutions, including human filarial diseases. The end of NTDs is embedded within the Sustainable Development Goals for 2030, under target 3.3. Onchocerciasis afflicts approximately 20.9 million people worldwide with > 90% of those infected residing in Africa. Control programs have made tremendous efforts in the management of onchocerciasis by mass drug administration and aerial larviciding; however, disease elimination is not yet achieved. In the new WHO roadmap, it is recognized that new drugs or drug regimens that kill or permanently sterilize adult filarial worms would significantly improve elimination timelines and accelerate the achievement of the program goal of disease elimination. Drug development is, however, handicapped by high attrition rates, and many promising molecules fail in preclinical development or in subsequent toxicological, safety and efficacy testing; thus, research and development (R&D) costs are, in aggregate, very high. Drug discovery and development for NTDs is largely driven by unmet medical needs put forward by the global health community; the area is underfunded and since no high return on investment is possible, there is no dedicated drug development pipeline for human filariasis. Repurposing existing drugs is one approach to filling the drug development pipeline for human filariasis. The high cost and slow pace of discovery and development of new drugs has led to the repurposing of “old” drugs, as this is more cost-effective and allows development timelines to be shortened. However, even if a drug is marketed for a human or veterinary indication, the safety margin and dosing regimen will need to be re-evaluated to determine the risk in humans. Drug repurposing is a promising approach to enlarging the pool of active molecules in the drug development pipeline. Another consideration when providing new treatment options is the use of combinations, which is not addressed in this review. We here summarize recent advances in the late preclinical or early clinical stage in the search for a potent macrofilaricide, including drugs against the nematode and against its endosymbiont, Wolbachia pipientis.

Modelling the impact of ivermectin on River Blindness and its burden of morbidity and mortality in African Savannah: EpiOncho projections

Background

The African Programme for Onchocerciasis Control (APOC) has refocused its goals on the elimination of infection where possible, seemingly achievable by 15–17 years of annual mass distribution of ivermectin in some African foci. Previously, APOC had focused on the elimination of onchocerciasis as a public health problem. Timeframes have been set by the World Health Organization, the London Declaration on Neglected Tropical Diseases and the World Bank to achieve these goals by 2020–2025.

Methods

A novel mathematical model of the dynamics of onchocercal disease is presented which links documented associations between Onchocerca volvulus infection and the prevalence and incidence of morbidity and mortality to model outputs from our host age- and sex-structured onchocerciasis transmission framework (EpiOncho). The model is calibrated for African savannah settings, and used to assess the impact of long-term annual mass administration of ivermectin on infection and ocular and skin disease and to explore how this depends on epidemiological and programmatic variables.

Results

Current onchocerciasis disease projections, which do not account for excess mortality of sighted individuals with heavy microfilarial loads, underestimate disease burden. Long-term annual ivermectin treatment is highly effective at reducing both the morbidity and mortality associated with onchocerciasis, and this result is not greatly influenced by treatment coverage and compliance. By contrast, impact on microfilarial prevalence and intensity is highly dependent on baseline endemicity, treatment coverage and systematic non-compliance.

Conclusions

The goals of eliminating morbidity and infection with ivermectin alone are distinctly influenced by epidemiological and programmatic factors. Whilst the former goal is most certainly achievable, reaching the latter will strongly depend on initial endemicity (the higher the endemicity, the greater the magnitude of inter-treatment transmission), advising caution when generalising the applicability of successful elimination outcomes to other areas. The proportion of systematic non-compliers will become far more influential in terms of overall success in achieving elimination goals.

Onchocerciasis--river blindness

Onchocerciasis results from infestation by the nematode Onchocerca volvulus and is characterized by troublesome itching, skin lesions, and eye manifestations. Although partially controlled by international mass prevention programs, onchocerciasis remains a major health hazard and is endemic in Africa, Arabia, and the Americas. Onchocerciasis is spread by bites from infested black flies, which transmit larvae that subsequently develop into adult filariae. Skin symptoms are commonly nonspecific and include severe pruritus, acute and chronic dermatitis, vitiligo-like hypopigmentation, and atrophy. Onchocercal ocular disease covers a large spectrum of manifestations, which in severe cases, may lead to blindness. Diagnosis is usually made by direct visualization of the larvae emerging from superficial skin biopsies, "skin snips." In some cases, the microfilariae can also be directly observed at the slit lamp when migrating into the anterior chamber of the eye. Ivermectin is, at present, the drug of choice for skin and ocular manifestations. Recent research using a chemotherapeutic approach that targets filarial Wolbachia symbionts in the treatment and control of onchocerciasis, however, suggests that 100 mg/d of doxycycline for 6 weeks might be effective in reducing the filarial load and preventing ocular symptoms.

[Onchocerciasis]

Onchocerciasis is an infestation caused by the nematode, Onchocerca volvulus, and characterized by eye manifestations, skin lesions and troublesome itching. Although partially controlled by international mass treatment programs, onchocerciasis remains a major health hazard in endemic areas in Africa, Arabia, and the Americas. Onchocerciasis is spread by bites from infested blackflies which transmit larvae that subsequently develop into adult filariae. Skin findings are commonly non-specific, and include severe pruritus, acute and chronic dermatitis, vitiligo-like hypopigmentation and atrophy. Onchocercal ocular disease has a large spectrum of manifestations and may even lead to blindness. Diagnosis is usually made by direct visualization of the larvae emerging from superficial skin biopsies, "skin snips". In some cases, the microfilariae can also be directly observed with a slit lamp when they migrate into the anterior chamber of the eye. Ivermectin is highly microfilaricidal, and is the current drug of choice for both skin and ocular manifestations.

Development of reactive onchocercal skin lesions during a placebo-controlled trial with ivermectin among persons without lesions at baseline

Clinical trials of the effects of ivermectin on onchocercal skin disease have documented reduction in itching, but a less than clear benefit on reactive skin lesions. It has been suggested that one of the positive effects might be the prevention of new lesions. A study among a rural adult farming population in southwestern Nigeria provided ivermectin in three treatment groups and a placebo to community members who were examined and treated at 3-monthly intervals over a 15-month period. Among the 1206 people recruited for the study, 627 (52%) had no lesions at baseline examination. Atotal of 291 participants without baseline lesions attended all five follow-up examinations, and only their results were analysed. Members of all four groups developed new lesions, but those receiving ivermectin had a consistently lower proportion of lesions than the placebo group. This difference reached statistical significance at the 5% level in three of the five periods and was below the 10% level at the other two periods. These findings are suggestive of an inhibiting effect of ivermectin among those without lesions at the beginning of a community treatment programme, and justify community treatment as a way of limiting morbidity and social stigma associated with these lesions.

The effects of ivermectin on onchocercal skin disease and severe itching: results of a multicentre trial

OBJECTIVE

To determine the effects of ivermectin in annual, 3-monthly and 6-monthly doses on onchocercal skin p6isease (OSD) and severe itching.

METHOD

A multicentre, double-blind placebo controlled trial was conducted among 4072 residents of rural communities in Ghana, Nigeria and Uganda. Baseline clinical examination categorized reactive skin lesions as acute papular onchodermatitis, chronic papular onchodermatitis and lichenified onchodermatitis. Presence and severity of itching was determined by open-ended and probing questions. Clinical examination and interview took place at baseline and each of 5 subsequent 3-monthly follow-up visits.

RESULTS

While prevalence and severity of reactive lesions decreased for all 4 arms, those receiving ivermectin maintained a greater decrease in prevalence and severity over time. The difference between ivermectin and placebo groups was significant for prevalence at 9 months and for severity at 3 months. Differences between placebo and ivermectin groups were much more pronounced for itching. From 6 months onward, the prevalence of severe itching was reduced by 40-50% among those receiving ivermectin compared to the trend in the placebo group.

CONCLUSION

This is an important effect on disease burden as severe itching is for the affected people the most troubling complication of onchocerciasis. The difference among regimens was not significant, and the recommended regimen of annual treatment for the control of ocular onchocerciasis appears also the most appropriate for onchocerciasis control in areas where the skin manifestations predominate. The final determination of the effect on skin lesions requires a longer period of study.

Clinical and parasitological responses after up to 6.5 years of ivermectin treatment for onchocerciasis

There are plans to use mass treatment with ivermectin to clear all Africa of the worst ocular and cutaneous effects of onchocerciasis. However, there remains uncertainty about the most suitable treatment regimen and the likely effects of ivermectin on onchocercal skin disease. We have followed 948 subjects for over 6 years in a double-blind, randomized, controlled study of ivermectin for onchocerciasis in a hyperendemic focus in Sierra Leone. Using an intention-to-treat analysis we found a microfilarial prevalence of 16% 6 months after up to 4 annual doses of ivermectin, and 13% prevalence in the group receiving up to 10 doses of ivermectin at 6-monthly intervals. Microfilarial loads were well suppressed in both groups, but repopulation data suggest that adult female worms are still alive and fecund, strongly underlining the need to continue treatment. A clear effect of ivermectin was demonstrated on itching, with about one-third of cases being alleviated. Significant reductions in the prevalence of serious hyperkeratosis, and possibly dyspigmentation (leopard skin), were noted, but not for any other onchocercal skin lesion. Six-monthly and annual treatment regimens with ivermectin were equally effective in terms of dermatological and parasitological impact.

The effect of ivermectin treatment on the antibody response to antigens of Onchocerca volvulus

The effect of the microfilaricidal drug ivermectin on the antibody response to a detergent extract of adult Onchocerca volvulus (OvAg) and a number of specific recombinant peptides was examined. Three of the peptides were combined in a serodiagnostic 'cocktail' and the effect of ivermectin on the diagnostic performance of this assay was assessed. Immunoglobulin (Ig) G1 serum levels in response to OvAg significantly decreased following ivermectin treatment. The antibody response to only one recombinant peptide (OvMBP29) was significantly affected, with IgG levels decreasing following treatment. Levels of total IgE increased following treatment. No correlation was observed between initial antibody level (or change in antibody level) and any adverse reaction to treatment. The serodiagnostic 'cocktail' was 100% sensitive before and after the use of ivermectin. A serodiagnostic assay using specific recombinant peptides can be used to evaluate infection in the absence of dermal microfilariae in areas where ivermectin is used.