BRUGIA MALAYI FILARIASIS

Brugia malayi filariasis is caused by filarial worms which are known to produce swellings in affected body parts including the arms, genitals and legs. Filarial worms swim and migrate along the lymph nodes of these body parts and in the process produce grotesque swellings. Brugia malayi filariasis or lymphatic filariasis is a blood-borne protozoan disease that mainly affects the lymph nodes and lymphatic vessels of the body; and the disease is endemic in parts of the Pacific Island, Africa, Latin America and Asia.

Lymphatic filariasis is caused by a variety of filarial worms including Brugia malayi, Brugia timori and Wuchereria bancrofti. W. bancrofti is the causative agent of elephantiasis (bancroftian filariasis), an advanced lymphatic filariasis characterized by the coarse thickening, swelling, hardening and cracking of the legs and arms of affected individuals. Secondary bacterial and fungal infections usually ensue in the affected area, and this contributes to the tissue damage. Brugia species usually exist in three morphological forms as microfilariae, larva and as adult male and female worms. The microfilariae of Brugia species are sheathed and they contain nuclei.

Vector, reservoir and habitat of Brugia species

Mosquito species including those in the genus Aedes, Culex, Anopheles and Mansonia are usually the main insect vectors for W. bancrofti and Brugia species. Brugia species are reserved in their insect vectors as infective larva until they are passed on to their human hosts during a blood meal by the vectors. Other reservoirs of Brugia species include monkeys, and domestic animals such as dogs and cats.

Clinical signs and symptoms of Brugia species infection

The clinical symptoms of Brugia malayi filariasis varies depending on the endemicity of the infection, hosts immune response as well as the number of infecting filarial worms introduced into the body. Lymphatic filariasis including the bancroftian filariasis and those caused by Brugia species are presented with varying clinical episodes but the infection is mainly characterized by the blockage of lymphatic vessels in addition to other chronic and acute oedematous, thickened and fibrotic lesions at the affected body sites especially the arms and legs. The disease can be symptomatic or asymptomatic in nature; and some of the symptoms associated with it include recurrent fever, skin rashes, inflammation of lymphatic vessels and oedematous lesions at the legs or arms in severe cases.

Pathogenesis of Brugia infection

Brugia malayi filariasis is caused by Brugia species including B. timori and B. malayi. The parasite enters the body via the bite of the insect vector of the parasite especially mosquitoes in the Aedes, Culex or Anopheles genera. During blood meal, the mosquito vector leaves behind or deposits the infective larva of B. malayi on the skin of the bitten human host (Figure 1). The infective larva finally enters the host via the bite wound created during blood meal and especially when the host scratches the bite site. Infective larva penetrates the lymphatic vessels and lymph nodes where they develop into adult male and female worms.

Adult female worms produce microfilariae which enters the bloodstream. Mature worms of B. malayi can live inside the host’s tissues for many years (up to 15 years) if the immune system does not clear the parasite from the body. During another blood meal especially in the night time (since insect vectors of Brugia species and those of W. bancrofti are nocturnal in nature), the microfilariae of the parasite is taken up by the mosquito vector; and microfilariae develop into infective larva in the midgut of the insect vector from where they reach the mosquito’s proboscis until the next blood meal when transmission to susceptible human host occur (Figure 2). Elephantiasis or bancroftian filariasis is usually the end result of lymphatic filariasis in man especially filariasis caused by W. bancrofti.     

Figure 1: Life cycle of Brugia malayi. 1. During a blood meal, an infected mosquito introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. 2. They develop into adults that commonly reside in the lymphatics. The adult worms resemble those of Wuchereria bancrofti but are smaller. Female worms measure 43 to 55 mm in length by 130 to 170 μm in width, and male’s measure 13 to 23 mm in length by 70 to 80 μm in width. 3. Adult worms produce sheathed microfilariae (with nocturnal periodicity), measuring 177 to 230 μm in length and 5 to 7 μm in width. The microfilariae migrate into lymph and enter the blood stream reaching the peripheral blood. 4. A mosquito ingests the microfilariae during a blood meal. 5. After ingestion, the microfilariae lose their sheaths and work their way through the wall of the proventriculus and cardiac portion of the midgut to reach the thoracic muscles of the insect vector. 6. There the microfilariae develop into first-stage larvae. 7. The first-stage larvae subsequently develop into third-stage larvae. 7. The third-stage larvae migrate through the hemocoel to the mosquito’s proboscis until the next blood meal. CDC

Laboratory diagnosis of Brugia infection

The laboratory diagnosis of Brugia malayi filariasis is usually based on the identification of the microfilariae of Brugia species (Figure 2) in blood specimens of infected individuals by wet preparation, Giemsa staining technique and concentration technique. Microfilariae of Brugia species are usually present in large numbers in the blood of infected persons during the night hours. Thus blood samples for laboratory investigation of Brugia malayi filariasis should be collected during the night period for optimum result. Capillary or peripheral blood is often preferred to venous blood in detecting the microfilariae of the parasite because microfilaria is highest in numbers in capillary blood than in venous blood specimens since the parasite mainly affects the lymphatic vessels of the body.

Figure 2: Microfilaria of Brugia species in Giemsa stained thick blood smear. Microfilariae of Brugia malayi are sheathed; they contain nuclei, and they are tapered at the tail region. CDC

Treatment, control and prevention of Brugia species infection

Brugia malayi filariasis can be treated with diethylcarbamazine (DEC) which is cidal in action and kills filarial worms in blood. The control and prevention of lymphatic filariasis is usually dependent on the effective treatment of infected individuals as well as the use of insecticides and other vector control measures to eliminate the insect vectors of the parasite.

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