What are the current treatments for filarial diseases and their limitations?
Current treatments for onchocerciasis and lymphatic filariasis are based on repeated mass drug administration (MDA) of antiparasitic drugs through programmes directed by the WHO. WHO recommends MDA for onchocerciasis at least once yearly for 10-15 years, and for lymphatic filariasis once yearly for at least five years. The drugs used in MDA programmes are ivermectin for onchocerciasis; albendazole for lymphatic filariasis; and albendazole plus either ivermectin in areas where onchocerciasis is also endemic (i.e. African countries), or diethylcarbamazine (DEC) in areas where onchocerciasis is not co-endemic (i.e. non-African countries).
A bite from an infected insect allows filarial larvae to pass into the blood and migrate through the human body. These mature into adults that produce microfilariae, which the insect ingests during a blood meal, and the cycle goes on. MDA drugs can prevent this vector-borne transmission for several months by killing mainly the microfilariae, and inducing a temporary sterilization of adult worms. However, because adult worms (macrofilariae) continue to live in the body, they eventually produce new microfilariae, often before the next MDA, thus requiring repeated MDAs for several years to decades. Ivermectin treatment is safe and has been used widely in MDA programmes. However, the use of ivermectin in patients co-infected with high levels of Loa loa microfilaria in the blood can result in safety issues such as the occurrence of encephalopathy that can be fatal if not properly managed. Additionally, a suboptimal response to ivermectin in patients with onchocerciasis has been reported which may be a sign of resistance development. Furthermore, the morbidity associated with onchocerciasis and LF infection (itching, dermatitis, lymphedema, and blindness) are only partially improved or prevented and require repeated treatment with the current drugs.