Lymphatic filariasis

Epidemiology

Cause

©GAELF. Many species of mosquitoes are responsible for the transmission of lymphatic filariasis

Lymphatic filariasis (LF) is caused by infection with threadlike worms called nematodes of the family Filarioidea: 90% of infections are caused by Wuchereria bancrofti and the remainder by Brugia spp. . Humans are the exclusive host of infection with W. bancrofti. Although certain strains of B. malayi can also infect some animal species (felines and monkeys), the life cycle in these animals is perceived as epidemiologically distinct from that in humans.

Transmission

Adult male and female worms lodge in the lymphatics. Fecund females release larvae (microfilaria) which periodically circulate in the blood. Microfilaria circulating in the blood can be ingested by feeding mosquito vectors. Microfilaria must mature in the vector before becoming infective. The mosquitoes can then spread infective larvae to new hosts when feeding.

The major vectors of W. bancrofti are mosquitoes of the genus Culex (in urban and semi-urban areas), Anopheles (in rural areas of Africa and elsewhere) and Aedes (in islands of the Pacific).

The parasites of B. malayi are transmitted by various mosquito species of the genus Mansonia; in some areas, anopheline mosquitoes are responsible for transmitting infection. Brugian parasites are confined to areas of east and south Asia, notably India, Indonesia, Malaysia and Thailand.

Transmission in a community is influenced by the number of infected persons (prevalence), the density of microfilaria in the blood of infected persons, the density of vector mosquitoes, characteristics of the vector that affect development of infective larvae and frequency of human-vector contact.

Elimination strategy

Elimination of lymphatic filariasis is possible by interrupting the transmission cycle. Providing treatment on a large-scale to entire communities where the infection is present can stop the spread of infection. This strategy of preventive chemotherapy, called mass drug administration (MDA), involves a combined dose of 2 medicines given annually to an entire at-risk population in the following way: albendazole (400 mg) together with ivermectin (150–200 mcg/kg) or with diethylcarbamazine citrate (DEC) (6 mg/kg). These medicines have a limited effect on adult parasites but effectively reduce microfilariae from the bloodstream and prevent the spread of microfilaria to mosquitoes. MDA with albendazole (400 mg) alone should be given preferably twice per year to stop the spread of LF in areas where Loa loa is present.

Adult worms can remain viable for years. Therefore it is necessary to deliver several rounds of MDA. At least five rounds are recommended to reduce infections in the community to levels below a threshold at which mosquitoes are unable to continue spreading the parasites from person to person and new infections are prevented.

Vector control is supplemental to the core strategy of MDA and can enhance elimination efforts by reducing the mosquito density and preventing human-mosquito contact. Malaria control interventions such as residual spraying and sleeping under long-lasting insecticidal nets have collateral benefits in reducing transmission of LF.

Geographical distribution

An estimated 120 million people in tropical and subtropical areas of the world are infected with lymphatic filariasis; of these, almost 25 million men have genital disease (most commonly hydrocoele) and almost 15 million, mostly women, have lymphoedema or elephantiasis of the leg. A recent estimation of the impact of MDA during the past 13 years suggests >96.71 million cases were prevented or cured, yet as many as 36 million cases of hydrocoele and lymphoedema remain. Of the total population requiring preventative chemotherapy, 57% live in the South-East Asia Region (9 countries) and 37% live in the African Region (35 countries).

As one of the leading causes of global disability, LF accounts for at least 2.8 million DALYs; this does not include significant co-morbidity of mental illness commonly experienced by patients and their caregivers.