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
Man 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.

Transmission of Filariasis
Factors favoring the Spread of the Disease
Filariasis is seen mainly in developing countries. Lymphatic filariasis is often associated with urbanization, industrialization, illiteracy, poverty and poor sanitation. Migration of people favored the spread of filariasis. The movement of people from one place to another has led to the extension of filariasis into areas where filariasis was not so prevalent. It largely explains the presence of filariasis in the urban areas of developing countries.

Climate is an important factor in the epidemiology of filariasis. Regions which are damp and moist and have stagnant water all year round afford a good breeding ground for the mosquitoes. It influences the breeding of mosquitoes, their longevity and also determines the development of the parasite in the insect vector.