Onchocerciasis is the world’s second leading infectious cause of blindness. Rarely life-threatening, the disease causes chronic suffering and severe disability. In Africa, it constitutes a serious obstacle to socioeconomic development. It is often called river blindness because of its most extreme manifestation and because the blackflies that transmit the disease abound in riverside areas, where they breed in fast-flowing waters. Fertile riverine areas are frequently abandoned for fear of the disease.
Rarely life-threatening, lymphatic filariasis causes widespread and chronic suffering, disability, and social stigma. It can lead to grotesquely swollen limbs - a condition known as elephantiasis.
Disease information
Distribution
Endemic in over 80 countries in Africa, Asia, South and Central America and the Pacific Islands. More than 40% of all infected people live in India and one- third live in Africa.
Causative agent
Parasitic nematode worms of the family filariidae. Three species are of significance, Wuchereria bancrofti, Brugia malayi and Brugia timori.
Transmission
Via the bite of blood-feeding female mosquitoes which transmit immature larval forms of the parasitic worms from human to human.
W. bancrofti parasites are mainly transmitted by Culex quinquefasciatus mosquitoes and some species of Anopheles. Brugia parasites are mainly transmitted by Mansonia mosquitoes. In humans, adult worms can live for many years, producing large numbers of larval forms (known as microfilariae) which circulate in the lymphatics and blood where they can be ingested by blood-feeding mosquitoes, so completing the transmission cycle.
Symptoms
Infective larvae develop into adult worms (known as macrofilariae) in the afferent lymphatic vessels, causing severe distortion of the lymphatic system. Adult Wuchereria are often lodged in the lymphatics of the spermatic cord, causing scrotal damage and swelling. Elephantiasis - painful, disfiguring swelling of the limbs - is a classic sign of late-stage disease.
There are three basic disease stages:
1. Asymptomatic: patients have hidden damage to the lymphatic system and kidneys.
2. Acute: attacks of ‘filarial fever’ (pain and inflammation of lymph nodes and ducts, often accompanied by fever, nausea and vomiting) increase with severity of chronic disease.
3. Chronic: may cause elephantiasis and hydrocoele (swelling of the scrotum) in males or enlarged breasts in females.
Prevention and control
The global elimination strategy has two major components:
Mass administration of drugs to 'at-risk' populations with once-a-year, one-day treatment (to interrupt transmission)
Promotion of rigorous, simple hygiene techniques for lymphoedema (to alleviate and prevent suffering of affected individuals).
Lymphatic filariasis used to be treated with a 12-day treatment regimen using the drug diethylcarbamazine (DEC), but recent work has shown that a single dose of DEC is equally effective. Ivermectin has also been registered for treatment of filariasis, and albendazole was shown to have additional antifilarial effects.
The treatment strategy is now based on annual, single- dose, 2-drug regimens of ivermectin+albendazole in countries that are co-endemic for onchocerciasis, and of DEC+albendazole in all other countries.
Transmission can also be reduced by avoiding mosquito bites in endemic areas (e.g. through use of repellents, bednets, insecticides). Mosquito vectors often breed in polluted urban waters (such as blocked drains and sewers) so good sanitation and environmental management to minimize mosquito breeding places can play a major role in reducing the risk of the disease.
Treatment of onchocerciasis: treatment consists of an annual single dose of ivermectin. This drug has been donated for mass treatment 'for as long as necessary' by Merck & Co. Inc.
Treatment of lymphatic filariasis: treatment consists of an annual single-dose of ivermectin and/or diethylcarbamazine (DEC). Albendazole is also effective in conjunction with either ivermectin or DEC.
diethylcarbamazine (DEC) has been used for treatment of lymphatic filariasis for over 50 years. However, DEC can lead to severe side effects, especially where onchocerciasis or loiasis are also present. It is not, therefore, used in Africa.
ivermectin has been registered for use in lymphatic filariasis since July 1998. It is donated by Merck & Co. Inc.
ivermectin + DEC has been shown in lymphatic filariasis to be more effective in combination than treatment with either drug alone.
albendazole has been shown to be microfilaricidal and is probably also macrofilaricidal. Used to treat intestinal worm infections, albendazole is not yet registered for use in lymphatic filariasis. In 1998, SmithKline Beecham pledged to donate albendazole to WHO for 'use by governments and other collaborating organizations until lymphatic filariasis is eliminated from the world as a public health problem'.