The time taken by the parasite to complete its development in the hosts is called the incubation period. The time taken by the parasite to complete its development in the definite host (from the time of entry of the infective larvae to the presence of microfilariae) is called the Intrinsic incubation period (Biological incubation) and the time taken by the parasite (from the ingestion of microfilariae to the infective stage) for its development in the mosquito is called Extrinsic incubation period.
It is not easy to differntiate the developing stages of the different filarial infection in the mosquitoes. The main point of distinction are that the larvae of animal filaria develop in the Malphigian tubules and only infective forms are found in other parts of the mosquito. The larvae of human filarial infection (W.bancrofti, B. Malayi) develops in the thoraeic muscles of the mosquito. Their infective larvae can be differntiated infective larvae of human filaria have 2 or 3 caudal lobes where as those of the animal have a single lobe only.
Life Cycle of Filaria Parasite Man is the definitive host i.e. where the adult male and female mature 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.
Reservoir of Infection The reservoir or source of infection is a person with circulating microfilariae in peripheral blood. The microfilaria carriers are usually without any recognizable symptoms of illness. The individuals with advanced disease often turn out to be negative for microfilariae.
Man is the natural host. All ages are susceptible to infection. In endemic areas, filarial infection has been recorded even in infants aged 6 months, but the infection has been found to rise with age up to 20–30 years of age and not consistently thereafter. Filaria disease appears in a small percentage of infected individuals, commonly in the age group over 10 years. Urbanization, industrialization, migration of people and specific habits are some of the social factors associated with filariasis.
The time interval between the introduction of infective larvae and the first appearance of detectable microfilariae is known as ‘pre–patent’ period. It usually varies from 1 to 1½ years. The transmission of filariasis can only be interrupted either through effective control of vector species of mosquitoes or Mass Drug Administration of DEC, a drug effective against the circulating microfilariae in the blood of infected persons. It is usually effective in killing mf and probably some of the adult worms and reduce the parasite load in the human reservoir.
The objective of the Mass Drug Administration (MDA) is to interrupt the transmission by reducing the microfilaria density in the community. However, efforts should be made to reduce the breeding of the mosquitoes by adopting various vector control measures.