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Lymphatic Filariasis

Etiology


Lymphatic filariasis (LF) is a parasitic infection caused by worms (nematodes) which can lead to changes in the lymphatic system and trigger chronic lymphedema, abnormal enlargement of parts of the body (elephantiasis), pain, severe disability, stigma, and social exclusion. In the Americas, Wuchereria bancrofti is the only worm species transmitted by the Culex mosquito (mainly of the species quinquefasciatus) which are the most common vectors. Adult worms in the human host are whitish or pinkish, thin (1.5-2.5 mm), and can measure up to 1 meter in length.

Only four countries are endemic to lymphatic filariasis in the American continent: Brazil (State of Pernambuco), Dominican Republic, Guyana and Haiti. It is estimated that up to 13.4 million individuals are at risk of infection. Costa Rica, Suriname and Trinidad and Tobago were eliminated from the list of endemic countries by PAHO/WHO in 2011.

Life cycle of the parasite


In order to complete its life cycle, W. bancrofti requires a definitive host (the man, as there is no significant animal reservoir) and a vector (Culex quinquefasciatus is the main vector responsible for transmission of lymphatic filariasis in the Americas). Lymphatic filariasis is transmitted when the third-stage larvae (L3) of the worm are deposited on the skin by an infected mosquito vector at the time that the mosquito is feeding. The larvae then penetrate the skin at the bite, migrate to the lymphatic system and in approximately a year mature into adult male and female worms. Adult females release larvae (called microfilariae) that eventually migrate into the bloodstream and can reach a maximum concentration between 10pm and 2am (nocturnal periodicity). Ingestion of circulating microfilariae by another mosquito vector and their development into L3 larvae closes the life-cycle of the parasite and sustains transmission in endemic areas.

Signs and symptoms


Lymphatic filariasis is characterized by a chronic course, which is aggravated by highly-symptomatic acute episodes. The prepatent period (interval between the entry of the L3 larvae and the appearance of detectable microfilaraemia) can last several months. The presence of microfilariae in the blood may in fact be asymptomatic, while clinical manifestations, if present, can arise from a few months to a year after infection. 

Clinical manifestations are usually classified into acute and chronic form. Acute symptoms include recurrent attacks of fever with painful inflammation and swelling of the lymph nodes and ducts. Involvement of the genital tract is common, and orchi-epididymitis (inflammation of the testis and the epididymus) and funiculitis (inflammation of the spermatic cord) are typical findings in male patients--- The affected lymph ducts appear dilated and with thickened walls. These “acute attacks” last a few days and can be attributed to a combination of factors: presence of living adult worms, death of worms possibly complicated by bacterial infection, the immune system response to filarial or microfilarial antigens, etc.   

Chronic manifestations stem from repeated episodes of inflammation that can lead to a progressive obstruction of the lymphatic ducts with accumulation of fluid in the interstitial tissues, resulting in lymphedema, a condition that most often affects the arms and the legs. In male patients, the obstruction of the spermatic lymphatic duct leads to accumulation of fluid in the scrotal sac (hydrocele). The increasing fibrosis and sclerosis of the subcutaneous tissues can accelerate the progression of lymphedema to elephantiasis (thickening and hardening of skin fold in nodule formation and epidermal pigmentation changes), which is associated with excessively enlarged extremities and disability. Stigma and social exclusion are frequent among such patients.
 
Diagnosis 


A suggestive clinical picture of the disease and a high eosinophil count may help the diagnosis. The standard technique used is the direct identification of microfilariae in thick blood smears. The blood sample must be collected near the time of maximum concentration of microfilariae in order to increase the sensitivity of the test. There is the availability of tests based on immunochromatographic card-based tests (ICT), which detect filarial antigens in blood collected after a finger prick, these are independent of the concentration of microfilariae and prove to be sensitive and specific. The ICT test can be used for mapping, monitoring, and evaluation of the transmission of lymphatic filariasis through community-level studies.
 
Treatment, prevention and control

A combination of diethylcarbamazine (DEC) 6 mg/kg and albendazole (ALB) 400 mg single-administration, is the treatment of choice for lymphatic filariasis in the Americas. The combination of DEC + ALB significantly reduce the microfilariae for up to a year. It has also shown to have some effect on filaricidal adult worms. The drug combination is associated with mild and temporary adverse reactions but is generally considered very safe.
 
The significant effect of DEC+ALB on microfilarial load is the reason why the current recommended strategy by WHO for elimination of lymphatic filariasis is the annual mass distribution (mass drug administration, MDA) of these two drugs to all  individuals living in an area where lymphatic filariasis is transmitted (microfilaremia transmitted is equal to or greater than 1%). At least five annual rounds are considered necessary to interrupt transmission, which can be accelerated by the implementation of vector control measures directed against the mosquitoes. 

In individuals who have already developed chronic manifestations such as lymphedema and elephantiasis, management of the affected limbs is necessary in order to prevent any further deterioration of health. This includes elevating and washing the limb, and treating skin infections, all measures that can usually be performed by the patients themselves. Surgery is needed in case of hydrocele. 
 
PAHO/WHO’s response 

  • Lymphatic filariasis is a disease considered "potentially eradicable"

  • In 1997 the World Health Assembly passed Resolution WHA50.29 on the elimination of lymphatic filariasis as a public health problem. 

  • In October of 2009, PAHO passed Resolution CD49.R19 on the elimination of neglected infectious diseases and other poverty-related Infections, urging Member States to take the necessary steps to eliminate LF as a public health problem by 2015. 

  • In May 2013, the World Health Assembly adopted the Resolution WHA 66.12, and reaffirmed the goals of 2020 for 17 neglected tropical diseases, including Lymphatic Filariasis, by 2020. 

  • In June 2013, the Organization of the Americas States (AG/RES.2810(XLIII-O/13) ratified the Resolution CD49.R19 and 23 countries approved the resolution. 

  • PAHO.WHO collaborates with endemic countries to obtain donated medicines and other supplies to achieve interruption and elimination of transmission. PAHO/WHO coordinates these actions through the Secretariat of the Regional Program Group Review (RPRG) for the elimination of LF, and helps endemic countries prepare to obtain verification of elimination of the disease. 
Last Updated on Wednesday, 16 April 2014 14:44

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