Lymphatic filariasis (LF) is a mosquito-borne disease which with several other infectious diseases are collectively referred to as neglected tropical diseases (NTDs). LF iscaused by thread-like parasitic filarial worms belonging to the species Wuchereria bancrofti, Brugia malayi, and B.timori. W. bancrofti is the most widespread of the three causative organisms. The disease affects over 120 million people mainly in Asia, Africa, Pacific countries and territories, and north-west South America. About 1 billion people are at risk of LF in 81 countries. When a vector mosquito bites a person who has LF , it picks up microscopic worms known as microfilariae which circulate in the person's blood. W. bancrofti microfilarie are nocturnally periodic in many parts of the world and appear in peripheral blood only at night. Their presence at this time coincides with the peak biting of people by vector mosquitoes. Once inside the stomach of the mosquito, microfilariae penetrate the gastric wall and migrate to the insect’s thoracic muscles where they mature. They then migrate to the mosquito’s labium (non-biting lower lip) of the proboscis. A mosquito’s salivary glands play no direct role in transmission of LF, in contrast to malaria. The process from the time mosquitoes take an infected blood meal to the presence of the infective filarial worms in the labium takes 7-21 days. When the mosquito again sucks blood, the 1.2-1.6 mm long infective larvae break through the labium and creep onto a person’s skin. Infective larvae enter the human host skin through the bite wound and travel to the lymph vessels where they grow into adults. W. bancrofti is fairly poorly transmitted by mosquitoes. The infection becomes patent in people only after many infective bites.
Wuchereria bancrofti is the most widespread causative agent of lymphatic filariasis.
Adult larvae lodge in the human lymph nodes for 5-7 years, producing numerous microfilariae. The lymphatic system is the network of nodes and vessels that maintain the delicate fluid balance between the tissues and blood and is an essential component for the body's immune defence system. The filarial worms damage the lymphatic system leading to its improper functioning. Subsequently this results in accumulation of fluid and swelling of certain parts of the body including legs, arms, breasts and genitalia. Swelling of the legs is usually accompanied by a hardening and thickening of the skin, hence the use of the term ‘elephantiasis’ to describe such symptoms and lymphatic filariasis in general. There is considerable social and psychological stigma related to these manifestations of the disease. Sometimes people develop the symptoms years after being infected. Men for instance develop hydrocele or swelling of the scrotum after the death of an adult worm. The majority of people however remain asymptomatic for lymphatic filariasis in spite of their being carriers of microfilariae and having the capacity to infect others through vector mosquitoes.
Lymphatic filariasis commonly manifests as elephantiasis, as shown above.
Vectors of lymphatic filariasis:
Culex quinquefasciatus is the major vector of LF globally. Other vector species belong to the genera Anopheles, Aedes and Mansonia. Generally C. quinquefasciatus breeds in polluted water in urban environments. It’s commonly favoured larval sites include pit latrines, soakage pits and drainage canals. On the other hand, Aedes species generally breed in a variety of artificial and natural habitats in urban and rural areas. Suitable sites may include water containers in and around houses, discarded tyres, tree holes and leaf axils. In contrast to the relatively more urban C. quinquefasciatus and Aedes mosquitoes, many Anopheles species of medical importance generally breed in unpolluted and often sunlit pools. Such sites can range in size from foot and hoof prints to flooded rice paddies and vegetated pools at the edges of streams and swamps. Mansonia species breed in swampy areas where their larvae breath air through a siphon attached to roots or stems of certain plants such as water hyacinth. Larvae of most other mosquito species use their breathing siphons to take in air from the water surface. In many areas, therefore, Cx. quinquefasciatus is the major vector of LF in the relatively more-polluted urban and peri-urban settings while Anopheles may be more important in rural settings.
Lymphatic filariasis prevention:
Priority for control of LF at community and country level usually focuses on mass drug administration (MDA) recommended by the Global Lymphatic Filariasis Elimination Programme (GLFEP) launched in 2000. The commonly used drugs for MDA are diethyl carbamazine and ivermectin. Integration of vector control with MDA is however considered to be important as it can help to address key challenges such as those related to: non-compliance by infected individuals during MDA leading to low coverage with treatment; development of anti-filarial drug resistance; risk of re-establishment of transmission from infected migrant individuals moving into an area where LF had been previously controlled. It has been demonstrated in places such as India, Zanzibar and the Pacific region that total interruption of LF transmission is possible when intensified vector control is combined with efficient MDA.
Vector control techniques:
The choice of vector control interventions for LF needs to take into account the breeding habits and biting behavior of the different vector species. Moreover, control measures targeted at Anopheles vectors of malaria and Aedes vectors of dengue are likely to have an impact on transmission of LF by the same mosquito genera in areas where these diseases overlap in their distribution. The following are among the common vector control methods:
• Indoor residual spraying (IRS)
This is an effective method for the control of adult mosquitoes that usually bite indoors at night and rest on walls while they digest host-blood and develop their eggs. Mosquitoes that bite outdoors but enter houses and animal shelters to rest can also be targeted with IRS. This is the same technique generally used to control Anopheles vectors of malaria in Africa. In the case of LF, IRS would also impact on night-biting and indoor resting populations of C. quninquefasciatus, Aedes and Mansonia mosquitoes. While IRS is a well proven intervention against malaria vectors, it has in most cases not been evaluated on large scale against LF vectors as it is considered not to be cost-effective for this disease. IRS has successfully been used to control LF in the Pacific countries and territories often in integration with mass drug administration.
• Expanded polystyrene beads:
The introduction of expanded polystyrene beads into septic tanks and pit latrines can produce a drastic reduction in Culex mosquito populations. The polymer beads which are bought as tiny compact granules are first expanded by heating them using boiling water. The expanded beads when poured in enclosed breeding sites form a thin layer which floats on the water surface. This mechanically prevents gravid mosquitoes from laying eggs, or larvae and pupae from breathing. A notable demonstration on the use of polystyrene beads to successfully control populations of C. quinquefasciatus has in the past been reported in Zanzibar.
• Long lasting insecticide impregnated nets (LLINs):
These are currently the preferred form of insecticide treated nets (ITNs) being promoted
by WHO and Roll Back Malaria partners as a cost effective and sustainable method for
protection against malaria. LLINs are nets treated in the factory with an insecticide
incorporated into the net fabric. They are effective in killing mosquitoes trying to bite
people while sleeping under nets at night. LLINs can therefore be used against certain
Anopheles, Culex, Aedes and Mansonia species, although with varying degree of success
owing to mosquito behavior and insecticide resistance, among other factors.
• Environmental management and biological control
Environmental sanitation involving cleaning up of drains or larval control using bio-
larvicides such as Bacillus sphaericus can effectively be used to significantly reduce
populations of C. quinquefasciatus in urban and peri-urban areas. Removal of certain
aquatic vegetation from potential breeding sites of Mansonia species is also a feasible
option of reducing the vector in clearly defined settings.