Onchocerciasis in West Africa after 2002: a challenge to take up

Impact of ivermectin and vector control on onchocerciasis transmission in Togo: Assessing the empirical evidence on trends in infection and entomological indicators

BACKGROUND

The World Health Organization's 2021-2030 Road Map for Neglected Tropical Diseases boosted global commitment towards the elimination of onchocerciasis, but the timeline to elimination will vary strongly between countries in Africa. To assess progress towards elimination in the Republic of Togo, we reviewed the history of control and time trends in infection.

METHODOLOGY/PRINCIPAL FINDINGS

We collated all available programmatic, entomological, and epidemiological data since the initiation of the Onchocerciasis Control Programme (OCP) in Togo through different data sources. We then visualised data trends over time, to assess the impact of interventions on infection and transmission levels. Vector control was initiated by OCP from 1977 (northern and central parts of Togo) or 1988 (southern regions) up to 2002 (most areas) or 2007 ("special intervention zones" [SIZ], parts of Northern and Central Togo). Between 1988 and 1991, Togo initiated ivermectin mass drug administration (MDA) in eligible communities. The impact of vector control was high in most river basins, resulting in low annual biting rates and annual transmission potential declining to very low levels; the impact was lower in river basins designated as SIZ. Repeated, longitudinal ivermectin mass treatments have overall strongly reduced onchocerciasis transmission in Togo. Epidemiological surveys performed between 2014 and 2017 showed that the prevalence of skin microfilariae (mf) and anti-OV16 IgG4 antibodies had declined to low levels in several districts of the Centrale, Plateaux, and Maritime region. Yet, relatively high mf prevalences (between 5.0% and 32.7%) were still found in other districts during the same period, particularly along the Kéran, Mô and Ôti river basins (SIZ areas).

CONCLUSIONS/SIGNIFICANCE

Trends in infection prevalence and intensity show that onchocerciasis levels have dropped greatly over time in most areas. This demonstrates the large impact of long-term and wide-scale interventions, and suggest that several districts of Togo are approaching elimination.

Successful and currently ongoing parasite eradication programs

The eradication of parasitic diseases is not a new concept. The most successful programs of parasite eradication have occurred with species of veterinary importance. The first such program, the eradication of Texas Cattle Fever from the United States, is one of the great success stories of disease eradication. The American screwworm eradication program is ongoing and is serving as a guiding impetus for many of the ongoing or proposed vector eradication schemes around the world. The success of these programs prompted similar successful operations in human health. Although they once led the way, veterinary parasitologists have taken second place in eradication planning. The only three parasitic diseases of veterinary importance that have been targets of recent eradication programs are Hypoderma species in Great Britain and Europe, Cochliomyia hominivorax after its introduction into Libya from the Americas, and Echinococcus granulosus in Tasmania, Australia. There is also work on the eradication of the tick, Amblyomma variegatum, from the Caribbean Islands. Some animal diseases are targeted under the auspices of the human eradication programs, most notably the eradication of the tsetse fly from parts or all of Africa. This paper reviews some of the past or ongoing successful eradication programs and presents a brief summary of the history of the programs, the methods used or planned, and potential controversies surrounding their success and implementation.

Impact of ivermectin on illness and disability associated with onchocerciasis

The Onchocerciasis Control Program (OCP), one of the most successful vertical disease control programs in the history of public health, came to an end in 2003 with devolvement of responsibilities for control program activities passed to the countries affected. Fortunately, 15 years ago the Mectizan Distribution Program (MDP) was founded to provide a complementary approach to controlling the disabling consequences of this parasitic infection. With over 250 million doses of ivermectin distributed over the past 15 years, the MDP is well on its way to both solidifying the progress made by the OCP and extending program reach well beyond the boundaries of the OCP. Through the extensive clinical testing protocols implemented in a variety of countries in Aftica, ivermectin has been proven to be a safe and highly effective treatment for onchocerciasis. Regular distribution to populations living in endemic areas has demonstrated significant reductions in blinding ocular complications, transmission, and disability caused by onchocercal skin disease. As yet undocumented, are the likely significant impact regular population dosing with ivermectin has on intestinal helminth infections, lymphatic filariasis, and human scabies infection. While there are significant barriers to continued program success, focussed attention on expanding and improving community-directed ivermectin distribution is likely to lead to further progress against this resilient infection.

Parasites and the eye

Systemic parasitic infections can result in intraocular inflammation. They are a major cause of blindness in many parts of the world. Infection usually results from bloodborne transmission of the organism to the eye or ocular adnexal structures. Diagnosis is often made from clinical patterns of disease in the eye, with laboratory investigations and imaging to support the diagnosis. Various local and systemic therapies are available, but may have limited success in preserving vision.