an international initiative to coordinate genome research on the filarial nematode parasites of humans.
November 1995
Onchocerciasis is the world's second leading infectious cause of blindness, and is present in 34 countries of Africa, the Arabian peninsula and the Americas. As a public health problem the disease is most closely associated with Africa, where it constitutes a serious obstacle to socio-economic development. Onchocerciasis is often called "river blindness"; because of its most extreme manifestation and because the blackfly vector abounds in fertile riverside areas, which frequently remain uninhabited for fear of infection.
Some 120 million people worldwide are at risk of onchocerciasis, 96% of whom are in Africa. Of the 34 countries where the disease is endemic, 28 are in sub-Sahara Africa (plus Yemen) and six are in theAmericas. A total of 18 million people are infected with the disease and have dermal microfiliarae, of whom 99% are in Africa. Of those infected with the disease, 6 million suffer from severe itching or dermatitis and 270 000 are blind.
Onchocerciasis is caused by Onchocerca volvulus, a parasitic worm that lives for up to 14 years in the human body. Each adult female worm produces millions ofmicrofilariae (microscopic larvae) that migrate throughout the body and give rise to a variety of symptoms: serious visual impairment, including blindness; rashes, lesions, intense itching and depigmentation of the skin; lymphadenitis, which results in hanging groins and elephantiasis of the genitals; and general debilitation. Onchocerciasis manifestations begin to occur in persons one to three years after the injection of infective larvae.
Microfilariae produced in one person are carried to another by the
blackfly, which in Africa belongs to the Simulium damnosum
species complex. The blackfly lays its eggs in the water of
fast-flowing rivers. Adults emerge after 8-12 days and live for up to
four weeks, during which they can cover hundreds of kilometres in
flight.
After mating, the female blackfly seeks a bloodmeal and may ingest
microfilariae if the meal is taken from a person infected with
onchocerciasis. A few of these microfilariae may transform
into infective larvae within the blackfly, which are then injected
into the person from whom the next meal is taken and
subsequently develop into macrofilariae, thus completing the life
cycle of the parasite.
(OCP)
OCP was launched in 1974 in an area that originally encompassed
seven countries in West Africa. In 1986 the programme was extended to
include four additional countries,
bringing the current total of participating countries to 11 (see map,
attached). They are: Benin, Burkina Faso, C.te d'Ivoire, Ghana,
Guinea, Guinea-Bissau, Mali, Niger, Senegal, Sierra Leone and Togo.
The total operational area covers 1.23 million sq km and a combined
population of about 30 million people.
Since its inception, the programme has been jointly sponsored by WHO,
the WorldBank, the United Nations Development
Programme and the Food and Agriculture Organization and is supported
by a coalition of 22 donor countries and agencies. WHO acts as the
Executive Agency for the programme, while the World Bank is
responsible for mobilizing resources and administering the OCP Trust
Fund.
Methodology. The programme's principal method for controlling
onchocerciasis has been to break the cycle of transmission by
eliminating the blackfly. Larvae are destroyed by application of
selected insecticides through aerial spraying of breeding sites in
fast-flowing rivers. Once the cycle of river blindness has been
interrupted for 14 years the reservoir of adult worms dies out in the
human population, thus eliminating the source of the
disease. The parasite reservoir has now virtually died out in the
original 7-country operations area (map) and will be largely
eliminated in the remaining four countries by the year 2002.
To complement vector control activities, OCP distributes the
drug ivermectin free of charge to more than 2.2 million people in the
operations area. Supplies of the drug are donated to the OCP
programme by their manufacturer, Merck & Co. Ivermectin has
only limited impact on transmission, but kills the larval worms that
cause blindness and other onchocercal manifestations. It therefore
contributes substantially to overall control of the disease, and will
be used to eliminate any localized recrudescence that may occur after
OCP activities come to an end.
At the time of OCP's launch, more than 1 million people in West
Africa suffered from onchocerciasis, of whom 100 000 had serious eye
problems including 35 000 people who were blind. Today, the number of
infectedpeople within the original area of operations is practically
nil and vector control efforts have almost ceased.
Some 1.5 million people who were once infected with onchocerciasis no
longer have any trace of the disease. About 10 million children born
in the operational area since theprogramme's inception are now free
of any risk of contracting the disease.
By the turn of the century it is estimated the OCP will have
prevented almost 300 000 cases of blindness in the 11 countries
involved in the programme.
The successful vector control activities are opening up an estimated
25 million hectares of fertile riverine land for resettlement and
cultivation, land that was previously deserted for fear of
onchocerciasis. This land has the potential to feed an additional 17
million people annually through the use of indigenous technologies
and farming practices.
OCP is now nearing the end of its fourth 6-year funding cycle and is scheduled to come to an end by the year 2002. The estimated total cost for the programme will be US$550 million, or less than US$1 per year for each protected person.
The introduction of ivermectin in OCP has effectively and safely reduced the numbers of skin microfiliarae in infected people, in some circumstances to levels resulting in clinical improvement and decreased transmission of infection. This has led to definition of a new global strategy for controlling onchocerciasis based on yearly administration of single doses of ivermectin to affected populations. By the end of 1994, approximately 8 million individuals from hyper- and meso-endemic areas had received at least one treatment of ivermectin.
As Merck & Co. provide the drug free of charge, the main costs of onchocerciasis control programmes are in delivering the treatment, which are borne by the countries concerned and by nongovernmental development organizations (NGDOs). Several NGDOs currently work with health ministries in control programmes in 19 different countries under the supervision of a central co-ordinator at WHO.
In June 1994 the World Bank approved funding for a new initiative, the African Programme for Onchocerciasis Control (APOC),to be implemented in the remaining 16 countries where onchocerciasis exists and which were not covered by the original OCP. The Bank has estimated the total cost of APOC at US$120 million, to be dispensed over a projected period of 12 years. APOC's aim is to establish sustainable community-based ivermectin treatment activities that will eventually result in the elimination of onchocerciasis as a public health and socio- economic problem throughout Africa.
A programme to eliminate onchocerciasis as a public health problem in the Americas is being co-ordinated by the Pan- American Health Organization with the support of NGDOs and the Inter-American Development Bank.
For further information in Geneva, please contact
Health Communications and Public Relations, Tel. (4122) 791
3221
or
Division of Control of Tropical
Diseases, Tel. (4122) 791 3225.
In Burkina Faso:
Director, Onchocerciasis Control Programme, Tel. (226) 30 23 12, Fax (226) 30 21 47.
February 1996
In much of Asia, Africa and the Western Pacific, as well as in
certain regions of the Americas, lymphatic filariasis persists as a
major cause of clinical morbidity and as an impediment to
socioeconomic development.
This mosquito-borne disease is the world's second leading cause of
permanent and long- term disability and its prevalence is growing,
due in large part to rapid unplanned urbanization in many endemic
areas -- and in spite of the fact that safe, cost-effective methods
to control and eliminate the disease are available.
Lymphatic filariasis has been identified by the International Task
Force for Disease Eradication as one of the world's six "eradicable"
or "potentially eradicable" infectious diseases. This recognizes that
appropriate interventions can be effectively linked with pre-existing
national and local public health infrastructures to promote
chemotherapy programmes and vector control activities.
Three types of filarial parasites cause lymphatic filariasis:
Wuchereria bancrofti, Brugia malayi and Brugia timori, all of which
are transmitted to humans by mosquitos.
Adult worms, or macrofiliarae, settle into the lymphatic system and
mature over a period of 3-15 months. When fertilized, female adults
produce large numbers of larvae known as microfiliarae, which invade
the blood stream. Mosquitos can then ingest them with blood meals and
transmit them to other people, in whom they pass through a larval
sequence to become new adults.
The vast majority of microfiliarae remain in the body as immature
forms for six months to two years. They grow up to a third of a
millimetre in length -- moving, secreting, excreting and dying as
foreign bodies, and doing immense damage to the host in the process.
Adult macrofiliarae grow to several centimetres long, damaging the
lymphatic ducts.
The results of both conditions in combination are the signs and
symptoms of lymphatic filariasis disease. Elephantiasis causes one or
more limbs to become grossly swollen and covered with sores.
Hydrocoele is an equally grotesque enlargement of the male scrotum.
Infection with filarial parasites can also cause acute fevers,
inflammation of the lymphatic system, and the bronchial-asthmatic
condition known as tropical pulmonary eosinophilia (TPE).
A minimum of 120 million people in 73 endemic countries worldwide
are estimated to be infected with filarial parasites (map). The most
widespread parasite is W. bancrofti, which affects about 106 million
people in the tropical areas of Africa, India, South-East Asia, the
Pacific islands, and South and Central America. Of these, India has
by far the largest number of cases. The closely related Brugia malayi
and Brugia timori parasites, which are found only in South-East Asia,
affect some 12.5 million people.
There are two general strategies to reduce transmission of
filarial infection.
Treatment of the human population. Mass distribution programmes
should completely replace programmes based on selective treatment,
ie. treating only microfilaraemics who have been detected. The drug
ivermectin, which has proven highly successful in treating
onchocerciasis, is expected to be approved for use against lymphatic
filariasis as well. Presuming that to be the case, the recommended
regimens for mass treatment are as follow, either singly or
together:
1. An optimal single-dose regimen of ivermectin plus
diethylcarbamazine (DEC) results in 99% reduction of microfilaraemia
after one year. The combination regimen appears to be superior to
individual use of either drug.
2. Until ivermectin becomes available, single annual or semi-annual
mass distribution of DEC alone seems to be as effective as the old
standard 12-day course. It reduces microfilaraemia by 90% after one
year, with the added advantages of better compliance, fewer side
effects and decreased delivery costs. However, this regimen should
not be used in areas where onchocerciasis or loiasis is present.
3. DEC-fortified salt is cheap, effective and can be used safely in
pregnancy. Daily consumption over a period of 9-12 months is
generally well tolerated and results in a 99% reduction of
microfilaraemia or total elimination of lymphatic filariasis after
one year. This regimen requires the least programme management input
and can recover costs through consumer purchasing, but it is not
recommended in areas where there is co-existing onchocerciasis or
loiasis.
Vector control.In certain locales, reducing mosquito populations
has played an effective support role and can be an important factor
in achieving long-term interruption of transmission. Improved
techniques are now available for enhancing the effectiveness of
vector control, such as bednets and curtains impregnated with
insecticides, long-lasting indoor sprays, and community participation
in vector management.
Filariasis control should not be based on vector reduction alone, but
must use it only as a supplement for treatment programmes.
Use of these and other techniques has enabled the elimination of
lymphatic filariasis in Japan, Taiwan and South Korea. China is now
in the final stages of an exceptionally effective control programme,
having seen a dramatic reduction in infection levels during the past
decade.
Elimination Through Integrated Public Health Action
The basic tools of the WHO control strategy for lymphatic filariasis
only became available during the past two years, hence worldwide
implementation is still in its early stages. As recognized by the
International Task Force for Disease Eradication, the main advantage
of this strategy is that it can be easily integrated into
pre-existing control programmes aimed at other public health problems
like onchocerciasis and intestinal parasites.
For countries with existing programmes, reallocation of resources to
these new approaches can lead to greater population coverage and cost
effectiveness -- a particularly important factor given the low
priority often accorded filariasis by health authorities. Where there
are no programmes as yet, costs for filariasis control will be more
justifiable if it is integrated into other health care delivery
packages. Treatment with ivermectin, for example, will offer the
additional benefits of acting on other helminth and ecto-parasites as
well.
For further information, please contact the office of
WHO Geneva, Tel (4122) 791 2535 or 791 4858 (fax).