Population biology of human onchocerciasis

Human onchocerciasis (river blindness) is the filarial infection caused by Onchocerca volvulus and transmitted among people through the bites of the Simulium vector. Some 86 million people around the world are at risk of acquiring the nematode, with 18 million people infected and 600,000 visually impaired, half of them partially or totally blind. 99% of cases occur in tropical Africa; scattered foci exist in Latin America. Until recently control programmes, in operation since 1975, have consisted of antivectorial measures. With the introduction of ivermectin in 1988, safe and effective chemotherapy is now available. With the original Onchocerciasis Control Programme of West Africa coming to an end, both the new African Programme for Onchocerciasis Control and the Onchocerciasis Elimination Programme for the Americas, rely heavily on ivermectin self-sustained mass delivery. In consequence, the need for understanding the processes regulating parasite abundance in human and simuliid populations is of utmost importance. We present a simple mathematical framework built around recent analyses of exposure- and density-dependent processes operating, respectively, within the human and vector hosts. An expression for the basic reproductive ratio, R0, is derived and related to the minimum vector density required for parasite persistence in localities of West Africa in general and northern Cameroon in particular. Model outputs suggest that constraints acting against parasite establishment in both humans and vectors are necessary to reproduce field observations, but those in humans may not fully protect against reinfection. Analyses of host age-profiles of infection prevalence, intensity, and aggregation for increasing levels of endemicity and intensity of transmission in the Vina valley of northern Cameroon are in agreement with these results and discussed in light of novel work on onchocerciasis immunology.

A method to determine the coverage of ivermectin distribution in onchocerciasis-control programmes

Onchocerciasis is a human filarial infection responsible for an estimated 750,000 blind and severely visually disabled people. In some African countries, this disease represents the main cause of blindness, with considerable socio-economic impact on the affected communities. Since the introduction of ivermectin as a microfilaricidal agent, there is hope that visual loss from onchocerciasis can be eliminated through community-based ivermectin-distribution programmes. The African Programme for Onchocerciasis Control (APOC) has now been initiated, by the World Health Organization and World Bank, to distribute ivermectin in 19 African countries where onchocerciasis is endemic. Estimates of ivermectin coverage in distribution programmes for onchocerciasis control have so far been based on the number of treatments given, in any one year, to a target population. There is a need for a standardized method to evaluate, measure and monitor coverage over time. In April 1996, a cluster random-sampling method was used to calculate coverage with ivermectin in an urban population of 30,000 people in the Central African Republic. The method was adapted from that used to determine coverage by the WHO Expanded Programme on Immunization. It proved to be inexpensive and easy to perform, requiring only local equipment and personnel. The method used and results obtained in Bossangoa and the potential use of the technique in other distribution programmes are discussed.

[Human onchocerciasis in Africa]

Before the 1980s, the only available method for control of onchocerciasis was elimination of blackfly vector populations. This strategy was used with considerable success in the Onchocerciasis Control Programme in West Africa (OCP). The discovery of ivermectin, the first effective drug suitable for mass treatment of onchocerciasis, has revived international interest not only in fundamental research but also in development of new strategies to control onchocerciasis in the countries outside the OCP area. This report gives an overview of current parasitological, clinical, epidemiological and diagnostic data about onchocerciasis. Although little is known about the early development of Onchocerca volvulus in the human host, significant insight has been gained into the population dynamics of the parasite. The pathogenesis of cutaneous and ocular manifestations in onchocerciasis is now better understood. Epidemiological studies are under way to evaluate the extent of systemic manifestations. Recently developed diagnostic methods are more sensitive than conventional parasitological techniques. A new method for rapid assessment of endemic level has provided a detailed picture of the distribution of onchocerciasis. Species- and strain-specific DNA probes have been developed for identification of parasites in West Africa. New methods for quantifying disability allow evaluation of the socio-economic impact of the cutaneous and ocular complications of onchocerciasis.

The Onchocerciasis Elimination Program for the Americas: a history of partnership

The decision in 1987 by the pharmaceutical firm Merck & Co. to provide Mectizan (ivermectin) free of charge to river blindness control programs has challenged the international public health community to find effective ways to distribute the drug to rural populations most affected by onchocerciasis. In the Americas, PAHO responded to that challenge by calling for the elimination of all morbidity from onchocerciasis from the Region by the year 2007 through mass distribution of ivermectin. Since 1991, a multinational, multiagency partnership (consisting of PAHO, the endemic countries, nongovernmental development organizations, the Centers for Disease Control and Prevention in Atlanta, Georgia, as well as academic institutions and funding agencies) has developed the political, financial, and technical support needed to move toward the realization of that goal. This partnership is embodied in the Onchocerciasis Elimination Program for the Americas (OEPA), which is supported by the River Blindness Foundation (RBF) and now by the Carter Center. OEPA was conceived as a means of maintaining a regional initiative to eliminate what is otherwise a low priority disease. Since its inception in 1993, the OEPA has provided more than US$ 2 million in financial, managerial, and technical assistance to stimulate and/or support programs in Brazil, Colombia, Ecuador, Guatemala, Mexico, and Venezuela, so as to take full advantage of the Merck donation. Now halfway into a five-year, US$ 4 million grant provided through the Inter-American Development Bank, the OEPA's capacity to support the regional initiative is assured through 1999.

The burden of Onchocerca volvulus in Sudan

Onchocerciasis has been reported in Sudan since 1908 and now prevails in three endemic regions known as the southern, northern and eastern foci. The southern focus is the largest, with nodule and blindness rates exceeding 80% and 12%, respectively, in certain villages. Onchocercal infection in this region causes only a mild skin reaction although microfilarial loads in the skin are high. In contrast, those with onchocerciasis in the northern focus, located between the fourth and fifth Nile cataracts, present with limited but severe skin reactions, low nodule rates (16%), low microfilarial loads in the skin and no ocular involvement. The characteristics of patients from the eastern focus, close to the border with Ethiopian border are similar to those in the north, although most onchocercal skin disease in this area comprises the severe localized pruritus known as sowda.

Patterns of epidemiology and control of onchocerciasis in west Africa

This paper summarizes the work of the Onchocerciasis Control Programme (OCP) in West Africa, a programme which over a 22 year history has reduced the public health problems of blinding onchocerciasis in eleven countries of West Africa through vector control and, more recently, ivermectin distribution. The paper emphasizes the different approaches to control the programme has developed in the different parts of the programme area which have been determined by the epidemiology of the disease (savanna/forest form), the migratory characteristics of the vectors, intensity of the disease before commencement of treatment, the combined impact of vector control and ivermectin and the likelihood of infiltration of infective blackflies from outside the programme area. The programme has constantly monitored the impact of operations on the trends in prevalence, incidence, annual transmission potential, ocular morbidity and species of fly populations, and as a result, has identified areas where special interventions are required until the programme comes to an end in 2002. The paper illustrates the changes in intensity of infection as measured by community microfilarial load and annual transmission potential over the duration of the programme control activities. The paper also defines and justifies the control strategies in different areas and identifies areas for special interventions.

Onchocerciasis in Ecuador: evolution of chorioretinopathy after amocarzine treatment

AIMS

To investigate the impact of the macrofilaricidal drug, amocarzine, on the evolution of chorioretinopathy in onchocerciasis.

METHODS

A prospective uncontrolled cohort study was performed using subjects infected with Onchocerca volvulus in a hyperendemic onchocerciasis focus in Esmeraldas Province in Ecuador. Study subjects were recruited into four cohorts in which ophthalmic and parasitological data were collected for 2, 3, 4, and 5 years respectively.

RESULTS

Complete ophthalmic follow up was obtained for 294 individuals in the four cohorts. The incidence of retinal pigment epithelial atrophy tended to remain constant between cohorts while that of chorioretinal scarring with a greater observation period. The incidence rate of cases with new or extending chorioretinal lesions was greater with an increasing period of follow up. An association was seen between the cumulative microfilarial loads in the skin and the development of new chorioretinal lesions (p < 0.05). No relation was noted between cumulative microfilarial loads and the progression of existing disease.

CONCLUSION

Amocarzine therapy did not prevent the natural evolution of chorioretinal disease. It was suggested that ocular microfilariae were necessary for the induction of chorioretinopathy in previously unaffected eyes and that extension of existing disease might also be related to the presence of ocular microfilariae or to other immunological mechanisms.

Establishing a reliable visual function test and applying it to screening optic nerve disease in onchocercal communities

The computer Controlled Video Perimetry (CCVP) is a computer screening test for detecting visual function loss caused by onchocerciasis, glaucoma, etc. Installed on portable computers, the CCVP has been shown to be high acceptability in field community investigation. However, it is regarded to be difficult in obtaining reliable results from portable computer screening tests because of human behavioural variants and the lack of standard testing environment. In this paper, we propose an architecture for implementing a more reliable CCVP system. In particular, a self-organising neural network is applied to manage measurement noise caused by behavioural factors. A control unit is introduced to manage the overall behaviour of the system. The integrated test system has been used to screen optic nerve disease in onchocercal communities of rural Nigeria and the experimental results obtained from a large number of test records are very encouraging: reliable results from volatile test environments may be obtained using the proposed method.

[The use of ivermectin in the control of onchocerciasis]

Onchocerciasis is an infection with the nematode Onchocerca volvulus. The main clinical symptoms are caused by the microfilariae. They include ocular lesions leading to blindness. Onchocerciasis is widely distributed in Africa from the Sahara to the southern tip, and is also found in some areas of South and Central America. Ivermectin was shown to be an effective treatment in the early 1980's, and is safe and better tolerated than diethylcarbamazine. We report the results of ivermectin treatment of onchocerciasis, and various features of the control obtained by large-scale ivermectin treatment programs. In large-scale programs, ivermectin (150 micrograms/kg) is administered once a year. This dose paralyses the microfilariae, such that they are carried away by the lymph to the lymph nodes where they are destroyed. This dose thereby reduces the load of microfilaria by 90%. The effects of a dose of ivermectin last about two or three years, and the lesions in the anterior segment of the eye can be cured or substantially reduced. Regular treatment prevents severe lesions of the posterior segment of the eye. The effects of repeated treatment on lesions of the retina are currently under investigation. Frequent doses of ivermectin prevent the development of embryo parasites in the females, and reduces the number of adults by attrition. Large-scale treatment programs reduce the transmission of the parasite by its vectors. There are several problems impeding large-scale treatment programs. Choosing patients for priority treatment requires expensive and sometimes aggressive methods of diagnosis. Thus new techniques for the identification of communities in which onchocerciasis is a serious public health problem are required. The choice of strategies for distribution, to optimize the cost, benefit ratio and feasibility, remain controversial. Wide distribution by mobile teams is effective, but expensive. Active distribution by trained community distributors is a cheaper potential alternative. Clinic-based or passive distribution requires the population to present to be able to obtain ivermectin. Thus, although cheap, this approach is generally poorly effective. A further complication is the clearly defined criteria on which these methods should be evaluated.

Criterion for inclusion in onchocerciasis control programmes based on ivermectin distribution

Data on onchocercal lymphatic and skin lesions from 45 communities located in the West African savanna were analysed to determine if the criterion for inclusion in ivermectin mass treatment based on risk of onchocercal blindness needs to be modified. Only 16 cases of elephantiasis were reported among 10,108 people examined. Other lymphatic lesions were almost exclusively found in people older than 30 years living in villages at risk of onchocercal blindness. Permanent skin lesions also affected older people and were 3 times more frequent in villages at risk of onchocercal blindness. Papular rash and pruritus affected younger people. No evidence was, therefore, found to change the criterion based on risk of onchocercal blindness presently in use in savanna areas.

DNA probe-based classification of Simulium damnosum s. l.-borne and human-derived filarial parasites in the onchocerciasis control program area

The development of polymerase chain reaction-based methods using strain- and species-specific DNA probes for Onchocerca volvulus has permitted classification of individual parasites from every stage of the parasite's life cycle. This technology has been applied on a large scale basis by Onchocerciasis Control Program (OCP) in West Africa. The primary objective of the OCP in using the DNA probes was to obtain accurate estimates of the annual transmission potential of the blinding strain of O. volvulus. The DNA probe classification of larvae collected throughout the OCP area demonstrated that larvae of less pathogenic strains of O. volvulus and other filarial parasites carried by Simulium damnosum s.l. have resulted in a significant overestimation of the annual transmission potential for blinding onchocerciasis. This effect is particularly pronounced along the southern border of the OCP, where the blinding and less pathogenic strains of O. volvulus coexist, and in the north of the control area, where animal parasites, particularly O. ochengi, may even predominate. A second objective of the OCP in applying the DNA probe technology was to determine the distribution of blinding and less pathogenic O. volvulus in infected individuals along the southern border of the control area. Results obtained from these studies have generally confirmed the distribution pattern established by previous epidemiologic studies. In addition, DNA probe classifications have demonstrated that in areas where the blinding and less pathogenic strains of O. volvulus coexist, a single individual may simultaneously be infected with both strains of the parasite.

Onchocerciasis and other eye problems in developing countries: a challenge for optometrists

BACKGROUND

Onchocerciasis, also known as River Blindness, affects about 18 million people around the world, resulting in severe visual impairment or blindness for approximately 2 million.

METHODS

The disease is transmitted through the bite of a tiny black fly, which breeds in fast moving rivers and streams. The fly injects immature forms of the parasite worm, Onchocerca volvulus, whose microfilariae migrate to superficial tissues, and may invade any part of the eye. In the eye, living microfilariae may be found in any ocular structure, however, sclerosing keratitis, a severe corneal involvement is the major cause of blindness from the disease.

RESULTS

Substantial efforts are currently underway to control the disease in Latin America and equatorial Africa, now that an effective, nontoxic medication, ivermectin, is available.

CONCLUSIONS

Optometrists are helping to solve the logistic challenges for treatment of this disease, as most onchocerciasis endemic areas are remote with difficult access.

Blindness in the tropics

Blindness is a major problem in most developing countries. It occurs at ten times the rate seen in the developed countries and in over 80% of cases is either preventable or curable. The four main causes are cataract, trachoma, onchocerciasis and xerophthalmia. Cataract, which is responsible for half the blindness, can be effectively cured with modern cataract surgery. Trachomatous blindness follows frequent episodes of reinfection, which can be prevented by simple hygienic measures. Onchocerciasis can be halted by the simple administration of a once-a-year dose of ivermectin, a drug which is currently being provided at no cost to all those with this infection. Vitamin A deficiency, which causes xerophthalmia, can be prevented by vitamin A distribution programs or dietary education. The challenge is to deliver these interventions effectively in the areas of need.

Present challenges in the global prevention of blindness

Cataract is responsible for 50% of world blindness, with at present an estimated backlog of 13.5 million cases in need of surgery. Low-cost cataract surgery must be made more available in developing countries, making use of alternative approaches for outpatient surgery and optimal management of available resources. Trachoma control needs to be targeted at the worst affected areas in endemic countries, with more emphasis on behavioural, educational and community aspects of the disease. Vitamin A deficiency and xerophthalmia control are becoming matters of maternal and child health care, with early intervention during infancy in view of the mortality issue. There are good prospects for the prevention of blindness from onchocerciasis, through the availability of ivermectin, but large-scale distribution schemes are still needed in most of the African countries concerned. The early detection and management of open-angle glaucoma still poses a major problem in developing countries, and further development of appropriate technology is needed in this field. Another area where more efforts are needed is ocular trauma, which is commonly the cause of unilateral loss of vision. General preventive measures must be enforced and better training provided to health personnel to deal competently with such cases, in order to prevent late complications. Diabetes, finally, is on the increase in many developing countries, giving rise to problems in dealing effectively with the ensuing retinopathy.(ABSTRACT TRUNCATED AT 250 WORDS)

[Control of onchocerciasis with ivermectin: results of a mass campaign in northern Cameroon]

A mass chemotherapy campaign to treat onchocerciasis with ivermectin was carried out in northern Cameroon, in the North Vina valley, a savanna area that is part of the extended Vina-Pendé-Longone focus. More than 20,000 people aged five years and over were treated at least once and more than 30,000 treatments were distributed. One to four treatments were given at intervals of six months or one year. Clinical and parasitological examinations of representative samples of the treated population and full ophthalmological examinations of males aged 15 to 35 years were carried out before each treatment to assess the efficacy of the different protocols of treatment. Secondary effects were recorded daily during the week after administration of ivermectin in the whole population treated. The evolution of arterial blood pressure was monitored over seven days in about 300 adults living in the hyperendemic region. The impact of mass treatment on natural transmission of the disease was evaluated by daily capture and dissection of blackflies during the month before and the two months after treatment. All levels of onchocerciasis endemicity were represented in the study area (maximum community microfilarial load (CMFL), 300 microfilariae per biopsy). Six months and one year after the first treatment, loads were down by more than 90% and more than 60% respectively.(ABSTRACT TRUNCATED AT 250 WORDS)