Lymphatic filariasis (LF) is a deforming and disabling infectious disease that causes elephantiasis of the extremities and genital deformity, especially hydroceles in men. Lymphatic filariasis is endemic in 72 countries around the world with 800 million people at risk for the infection. Since 2000, more than 890 million people have received antifilarial medications via mass drug administration (MDA) programs. These drugs cure infections and reduce transmission. While the 2020 elimination target set in 2000 was very successful for kick-starting Global Program for Elimination of Lymphatic Filariasis (GPELF), only about half of the job has been completed as of 2018, and the 2020 global elimination target in unlikely to be met. In 2017, 51 of the endemic countries still required MDA. Indeed, only 2 out of 35 LF-endemic countries in sub-Saharan Africa have stopped MDA and started post-MDA surveillance. Thus, despite significant progress, more time and better tools and strategies are needed to accelerate global elimination.
The parasite and the disease
LF is caused by threadlike nematode worms (species Wuchereria bancrofti, Brugia malayi and Brugia timori). Mosquitoes ingest a larval form of the parasite called microfilariae (approximately ¼ of a millimeter in length) when they feed on infected people. Microfilariae develop into third stage infective larvae (“L3”, about 1 mm long) in mosquitoes over a period of 2 weeks, and this stage initiates infections in humans when mosquitoes take their next blood meal. Thus, transmission requires a source of parasites in the human population and high biting rates of mosquitoes capable of transmitting the infection.
After transmission to humans, L3 develop over a period of 4 to 6 months into mature adult filarial worms that are several centimeters long. Adult worms live in lymphatic vessels and in lymph nodes. The adult female worms release microfilariae that move from the lymphatic system into the blood where they can be ingested by mosquitoes to complete the life cycle.
While many people with LF infections have no symptoms, damage to lymphatic vessels caused by filarial worms and inflammatory responses to the worms can lead to lymphedema with elephantiasis and to the acculumlation of fluid in the scrotum (known as hydrocele) of men who are infected. These conditions are disabling and associated with social stigma.