The World Health Organization (WHO) has updated its fact sheet on lymphatic filariasis.

Background Information:

• Wuchereria bancrofti, which is responsible for 90% of the cases
• Brugia malayi, which causes most of the remainder of the cases
• Brugia timori, which also causes the disease.

Adult worms nest in the lymphatic vessels and disrupt the normal function of the lymphatic system. The worms can live for approximately 6–8 years and, during their life time, produce millions of microfilariae (immature larvae) that circulate in the blood.

Mosquitoes are infected with microfilariae by ingesting blood when biting an infected host. Microfilariae mature into infective larvae within the mosquito. When infected mosquitoes bite people, mature parasite larvae are deposited on the skin from where they can enter the body. The larvae then migrate to the lymphatic vessels where they develop into adult worms, thus continuing a cycle of transmission.

Lymphatic filariasis is transmitted by different types of mosquitoes for example by the

• Culex mosquito, widespread across urban and semi-urban areas,
• Anopheles, mainly found in rural areas, and
• Aedes, mainly in endemic islands in the Pacific.

Key Messages:

886 million people in 52 countries worldwide remain threatened by lymphatic filariasis and require preventive chemotherapy to stop the spread of this parasitic infection.

In 2000 over 120 million people were infected, with about 40 million disfigured and incapacitated by the disease.

Lymphatic filariasis infection involves asymptomatic, acute, and chronic conditions.

The majority of infections are asymptomatic, showing no external signs of infection while contributing to transmission of the parasite. These asymptomatic infections still cause damage to the lymphatic system and the kidneys, and alter the body’s immune system.

When lymphatic filariasis develops into chronic conditions it leads to

• lymphoedema (tissue swelling) or
• elephantiasis (skin/tissue thickening) of limbs and
• hydrocele (scrotal swelling).

Involvement of breasts and genital organs is common. Such body deformities often lead to social stigma and sub-optimal mental health, loss of income-earning opportunities and increased medical expenses for patients and their caretakers. The socioeconomic burdens of isolation and poverty are immense.

Acute episodes of local inflammation involving skin, lymph nodes and lymphatic vessels often accompany chronic lymphoedema or elephantiasis. Some of these episodes are caused by the body’s immune response to the parasite. Most are the result of secondary bacterial skin infection where normal defences have been partially lost due to underlying lymphatic damage. These acute attacks are debilitating, may last for weeks and are the primary cause of lost wages among people suffering with lymphatic filariasis.

WHO launched its Global Programme to Eliminate Lymphatic Filariasis (GPELF) in 2000. In 2012, the WHO neglected tropical diseases roadmap reconfirmed the target date for achieving elimination by 2020.

WHO’s strategy is based on 2 key components:

• stopping the spread of infection through large-scale annual treatment of all eligible people in an area or region where infection is present; and
• alleviating the suffering caused by lymphatic filariasis through provision of the recommended basic package of care.

Large-scale treatment (preventive chemotherapy)

Elimination of lymphatic filariasis is possible by stopping the spread of the infection through preventive chemotherapy.

The WHO recommended preventive chemotherapy strategy for lymphatic filariasis elimination is mass drug administration (MDA), and involves administering an annual dose of medicines to the entire at-risk population. The medicines used have a limited effect on adult parasites but effectively reduce the density of microfilariae in the bloodstream and prevent the spread of parasites to mosquitoes.

The MDA regimen recommended depends on the co-endemicity of lymphatic filariasis with other filarial diseases. WHO recommends the following MDA regimens:

• albendazole (400 mg) alone twice per year for areas co-endemic with loiasis
• ivermectin (200 mcg/kg) with albendazole (400 mg) in countries with onchocerciasis
• diethylcarbamazine citrate (DEC) (6 mg/kg) and albendazole (400 mg) in countries without onchocerciasis

Recent evidence indicates that the combination of all three medicines can safely clear almost all microfilariae from the blood of infected people within a few weeks, as opposed to years using the routine two-medicine combination.

WHO now recommends the following MDA regimen in countries without onchocerciasis:

• ivermectin (200 mcg/kg) together with diethylcarbamazine citrate (DEC) (6 mg/kg) and albendazole (400 mg) in certain settings

The impact of MDA depends on the efficacy of the regimen and the coverage (proportion of total population ingesting the medicines). MDA with the two-medicine regimens have interrupted the transmission cycle when conducted annually for 4–6 years with effective coverage of the total population at risk.

Morbidity management

Morbidity management and disability prevention are vital for improving public health and are essential services that should be provided by the health care system to ensure sustainability.

Surgery can alleviate most cases of hydrocele. Clinical severity and progression of the disease, including acute inflammatory episodes, can be reduced and prevented with simple measures of hygiene, skin care, exercises, and elevation of affected limbs. People with lymphoedema must have access to continuing care throughout their lives, both to manage the disease and to prevent progression to more advanced stages.

WHO’s minimum package of care for Lymphatic filariasis:

• treatment for episodes of adenolymphangitis (ADL);
• guidance in applying simple measures to manage lymphoedema to prevent progression of disease and debilitating, inflammatory episodes of ADL;
• surgery for hydrocele;
• treatment of infected people with antifilarial medicines

Vector control

Depending on the parasite-vector species, measures such as

• insecticide-treated nets,
• indoor residual spraying or
• personal protection measures

may help protect people from infection.

The use of insecticide-treated nets in areas where Anopheles is the primary vector for filariasis enhances the impact on transmission during and after MDA. Historically, vector control has in select settings contributed to the elimination of lymphatic filariasis in the absence of large-scale preventive chemotherapy.

Useful Links:

Link to the updated fact sheet:

https://www.who.int/news-room/fact-sheets/detail/lymphatic-filariasis

Link to the US Centers for Disease Control (CDC) documentation on lymphatic filariasis:

https://www.cdc.gov/parasites/lymphaticfilariasis/