Lymphatic filariasis is transmitted through mosquito bites.
The persons having circulating microfilariae are outwardly healthy but transmit the infection to others through mosquitoes.
The persons with chronic filarial swellings suffer severely from the disease but no longer transmit the infection.
In India, 99.4% of the cases are caused by the species – Wuchereria bancrofti whereas Brugia malayi is responsible for 0.6% of the problem.
In the adult stage, filarial worms live in the vessels of the lymphatic system. Lymphatic system is the network of lymph nodes and lymph vessels that maintains the fluid balance between the tissues and the blood which is an essential element of the body’s immune defense system.
Life Cycle of Filaria Parasite is the definitive host i.e. where the mature adult male and female parasites mate and produce microfilariae whereas the mosquito is the intermediate host. The adult parasites are usually found in the lymphatic system of man. They give birth to as many as 50,000 microfilariae per day, which find their way into blood circulation. The life span of microfilaria is not exactly known which preferably may survive up to a couple of months.
The parasite cycle in the mosquito begins when the microfilariae are picked up by the vector mosquitoes during their feeding on the infected person (microfilaria carrier). The microfilaria in mosquito develops into three stages and under optimum conditions of temperature and humidity;
the duration of the cycle in the mosquito (extensive incubation period) is about 10–14 days. When the infective mosquito feeds on other human host, the infective larvae are deposited at the site of mosquito bite from where the infective larvae get into lymphatic system. In the human host, the infective larvae develop into adult male and female worms. The adult worms survive for about 5–8 years or sometimes as long as 15 years or more.