Filariasis - Wuchereria bancrofti

• Filarial (filum (L)=thread) worms are slender thread like transmitted by the bite of blood-sucking insects.
• Filarial worms reside in the subcutaneous tissues, lymphatic system or body cavities of humans.
• Adult male worms are smaller, female worms are ovoviviparous, give birth to microfilarial larvae
• Microfilariae released by the female worm are seen in the peripheral blood or subcutaneous tissues
• Depending on the presence of largest number of microfilariae in blood, filarial worms can exhibit nocturnal, diurnal or no periodicity at all
• Lifecycle in 2 hosts- definitive host-man, intermediate host-blood sucking arthropods
• Eight species of filarial worms infect man, infection termed filariasis
• Traditionally, it refers to lymphatic filariasis caused by Wuchereria bancrofti or Brugia species

Wuchereria bancrofti

• Genus named after Wucherer, a Brazilian physician, who reported microfilaria in 1868.
• Manson in 1878 identified in China, Culex mosquito as the vector.
• Filariasis known from ancient times- if left untreated, the infectivity can expand into chronic disease called Lymphatic filariasis, it also causes asthmatic disease.
• Elephantiasis is a classic sign of late-stage disease. Also called Malabar leg
• No vaccine is commercially available, but various antifilarial medications reported to cure

Epidemiology

• W. bancrofti is distributed widely in the tropics and subtropics of sub-Saharan Africa, South East Asia, India and the Pacific islands.
• In India, the endemic areas are along the sea coast and along the banks of the rivers.

 Morphology

• W. bancrofti is a dioecious worm with sexual dimorphism.
• Adults are whitish, translucent, thread-like worms with smooth cuticle and tapering ends.
• The body is fragile, making removing it difficult from tissues.
• Males and females can be distinguished by structure and size of their tail tips.

• The male worm is smaller (40 mm x 0.1mm) wide, and shows a ventrally curved tail. The tip of the tail has 15 pairs of minute caudal papillae, which serve as sensory organs. 
• Contrary to above, the female is larger (60 x 0.25 mm). Its tail steadily tapers and is rounded at the tip. No extra sensory structures are seen. Its vulva is present towards the anterior side of the worm.
• Adult males and females are mainly coiled together in the abdominal and inguinal lymphatics and in the testicular tissues, are hard to separate.
• The adult worms live for many years. 

Microfilaria

    The juveniles or embryos or first-stage larvae are referred to as microfilariae. They are present in the circulation. They are released encased in the elongated egg shell, which persists as a sheath. The sheath is delicate and close fitting but  projects at the anterior and posterior ends of the microfilaria. It can actively move forward and backward within the sheath. 

This sheath, along with the area in which the worms reside, makes identification  in humans easier.

The microfilaria has a colourless, translucent body with a blunt head and pointed tail. It is a miniature adult, and is 250-300 μm long and 6-10 μm wide. When stained with Leishman or other Romanowsky stains, structural details can be made out.                          

Along the central axis of microfilaria, a column of granules can be seen, which are called somatic cells or nuclei. At the head end is a clear space devoid of granules, called the cephalic space. 

In the anterior half of the microfilaria is an oblique area devoid of granules, called the nerve ring. Approximately midway along the length of body is the anterior V-spot which represents the rudimentary excretory system. The posterior V-spot (tail spot) represents the cloaca or anal pore. The genital cells (G-cells) are situated anterior to the anal pore, in M. bancrofti, the tail tip is devoid of nuclei.

Microfilaria do not multiply or undergo any further development in human body. If they are not taken up by the female mosquito vector, they die. Lifespan is about 2-3 months. A microfilarial density of at least 15 per drop of blood is necessary for infecting mosquitoes.

 Periodicity

The microfilariae circulate in the blood stream. In most Asian countries, including India, they show a nocturnal periodicity in peripheral circulation of human. It is seen in large numbers in peripheral blood only at night, between 10 pm and 4 am. This correlates with the night biting habit of the vector (culex mosquitoes). During the day, they are present in the deep veins, and during the night, they migrate to the peripheral circulation.

The cause of this periodicity remains unknown, but the advantages of the microfilariae being in the peripheral blood during these hours may ensure the vector, the night time mosquito, will have a higher chance of transmitting them elsewhere. Physiological changes also are associated with sleeping, such as lowered body temperature, oxygen tension, and adrenal activity, and an increased carbon dioxide tension, among other physical alterations, any of which could be the signals for the rhythmic behavior of microfilarial parasites.

If the hosts sleep by day and are awake at night, their periodicity is reversed.

Life cycle

W. bancrofti carries out its lifecycle in two hosts. Humans serve as the definitive host and mosquitos as the intermediate host. The adult parasites reside in the lymphatics of the human host. They are found mostly in the afferent lymphatic channels of the lymph glands in the lower part of the body.

Definitive host - Humans serve         Intermediate host- mosquitoes

Infective form -Actively motile third stage filarifom larvae

Mode of transmission- Bite of infected vector/ mosquito carrying filariform larvae

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. They develop into adults that commonly reside in the lymphatics. Adults produce microfilariae, which are sheathed and have nocturnal periodicity, except the South Pacific microfilariae which have the absence of marked periodicity. The microfilariae migrate into lymph and blood channels moving actively through lymph and blood. A mosquito ingests the microfilariae during a blood meal. After ingestion, the microfilariae lose their sheaths and they migrate to the thoracic muscles. There the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae. The third-stage infective larvae migrate through the hemocoel to the mosquito’s prosbocis and can infect another human when the mosquito takes a blood meal.

     The life cycle of Wuchereria bancrofti commences, when a male and a female worm, mate within lymphatic vessels of an infected human being. The female discharges thousands of microfilariae (prelarval eggs) into the blood flow. When the host is aware and awake, the microfilariae are likely to stay in deep blood vessels. During the sleep they reach the peripheral/marginal blood vessels. This enables them to get picked up by the night biting mosquito.

In Mosquito:  In the stomach of the mosquito, ex-sheathing occurs, they lose their sheaths, penetrate the stomach wall and migrate to the thoracic muscles where further development occurs. First stage larvae, sausage shaped with a spiky tail and second stage larvae develop then into the elongated actively motile third stage filariform larva which is the infective form. It travels through the hemocoel to the mosquito's proboscis. 

There is no multiplication of microfilaria in the mosquito and one microfilaria develops into one infective larva only. 

The time for development of microfilaria in the mosquito from the bite till the development of infective third stage filariform larva is the extrinsic incubation period. Its duration varies with environmental factors such as temperature and humidity, as well as with the vector species and is around 10-20 days, under optimal conditions.

In Humans: When the mosquito bites another person, the larvae are injected into the skin of another human. Large number of infective mosquito bites are required to ensure transmission- as many as 15,000 infective bites per person. Many larvae fail to penetrate the skin itself and many are destroyed by immunological and defense mechanisms.

 After penetrating the skin, they travel to the lymph vessels and in abdominal or inguinal lymph nodes, they mature into fully adult forms within six months. Fully developed females can live up to seven years. The gravid female releases large numbers of microfilariae, as many as 50,000 per day, which pass through thoracic duct and pulmonary capillaries to enter into peripheric circulation. They are ingested with the blood meal by mosquito and the cycle is repeated.

 Prepatent period

The period from the entry of the infective third stage larvae into the human host till the first appearance of microfilaria in circulation is the biological incubation period or the prepatent period. This is usually about 8-12 months.

Clinical incubation period

The period from the entry of the infective third stage larvae into the human host till the first appearance of earliest clinical manifestation is called the clinical incubation period. This is variable, but usually about 8-16 months.

Pathogenesis

Infection caused by W. bancrofti is termed Wuchereriasis or Bancroftian filariasis. It can present as

Ø  Classical filariasis

Ø  Occult filariasis

Classical filariasis

Lymphatic filariasis, considered globally as a neglected tropical disease (NTD), is a parasitic disease caused by microscopic, thread-like worms which live in the human lymphatic system. The lymphatic system maintains the body’s fluid balance and fights infections. Blockage of lymph vessels and lymph nodes by adult worms causes filariasis. Blockage could be due to mechanical factors or allergic inflammatory reaction to worm antigens and secretions. The affected lymph nodes and vessels are infiltrated with macrophages, eosinophils, lymphocytes and plasma cells. The worms inside lymph nodes can cause granuloma formation, with subsequent scarring and calcification. Recurrent secondary bacterial infections cause further damage.

Clinical manifestations

Wuchereria bancrofti infection is usually asymptomatic but with microscopic hematuria or proteinuria, dilated lymphatics and in men, scrotal infection.

ADL, acute adenolymphangitis, is characterised by high fever, lymphatic inflammation (lymphangitis, lymphadenitis) and transient local edema. High grade, sudden onset fever associated with rigors lasts for 2-3 days.  Lymphangitis (inflamed lymph vessels) and lymphadenitis (inflammation of lymph nodes) occur followed by lymphedema, which starts as swelling around the ankle, spreading to the back of foot and leg, arms, breasts, scrotum, or other parts of the body. Lymphoangiovarix which is the dilation of lymph vessels is also seen.

In men, swelling of the scrotum, called hydrocele is seen. Here, accumulation of fluid occurs due to obstruction of lymph vessels. Rupture of lymph vessels leading to the release of lymph results in lymphorrhagia.

The dysfunction of lymphatic vessels leading to inflammation and decreased current of the lymph fluid result in skin and lymph system infections. Affected limbs become grossly swollen; the skin may become thick and secondary infections are frequent due to lymphatic dysfunction. 

Over time, the disease causes thickening and hardening of the skin, a condition called elephantiasis which can be lethal and incurable. Commonly seen in leg, it is characterised by edema, thickened skin, cracks, fissures with secondary bacterial and fungal infections.

Occult filariasis

It occurs as a result of hypersensitivity reaction to microfilarial antigens, not directly due to lymphatic involvement. Microfilaria are absent in blood since they are destroyed by tissues.

Clinical manifestations

Massive eosinophilia, hepatosplenomegaly, pulmonary symptoms like dry nocturnal cough, dyspnea and asthmatic wheezing. Classical features of lymphatic filariasis absent. Arthritis, glomerulonephritis, thrombophlebitis etc. may be seen.

Occult filariasis also called Meyers Kouwenaar syndrome.

Tropical pulmonary eosinophilia

Occult filarial infection might also cause pulmonary tropical eosinophilia, which is typically present in patients living in Asia. Pulmonary tropical eosinophilia syndrome can cause: shortness of breath, cough, wheezing, chest pain, hyper eosinophilia, splenomegaly, and bloody sputum. There might be high levels of antifilarial antibodies and IgE (Immunoglobulin E). Children and young adults commonly affected in areas of endemic filariasis.

 Diagnosis 

The standard method for diagnosing active infection is the identification of microfilariae in a blood smear by microscopic examination. The microfilariae that cause lymphatic filariasis circulate in the blood at night (called nocturnal periodicity). Blood collection should be done at night to coincide with the appearance of the microfilariae, and a thick smear should be made and stained with Giemsa or hematoxylin and eosin. For increased sensitivity, concentration techniques can be used.

Adult filarial worms can be seen in sections of biopsied lymph nodes, but not routinely employed in diagnosis.

Intradermal injections of filarial antigens induce an immediate hypersensitivity reaction.  Not of diagnostic importance, because of high false positive and negative reactions.

Imaging techniques such as high frequency ultrasonography (USG) of scrotum and breast along with Doppler imaging may result in identification of motile adult worm (filaria dance sign) within the lymphatics.

X-ray used to detect dead and calcified worms. In Tropical pulmonary eosinophilia, chest x-ray helpful in detecting worms.

Serologic techniques provide an alternative to microscopic detection of microfilariae for the diagnosis of lymphatic filariasis. Patients with active filarial infection typically have elevated levels of antifilarial IgG4 in the blood that the human body develops to battle in opposition to antigens excreted by adult female Wuchereria bancrofti worms can be detected using routine assays like complement fixation, indirect haemagglutination (IHT), indirect fluorescent antibody (IFA) etc

Molecular diagnosis by polymerase chain reaction (PCR) is achievable, too.

Because lymphedema may develop many years after infection, lab tests are most likely to be negative with these patients.

Indirect evidences include eosinophilia or elevated serum IgE levels.

Treatment

Diethylcarbamazine (DEC) is the drug of choice. The drug kills the microfilariae in the bloodstream and occasionally adult worms in the lymphatic vessels. It has few side effects which include: fever, headache, dizziness, nausea and joint-muscle pain.

DEC should only be consumed, if Wuchereria bancrofti has been recognized. This is, since many people with lymphedema are not contaminated with parasites.

DEC can depreciate Onchocerciasis (an eye disease caused by Onchocerca volvulus) and can cause encephalopathy (brain disease) and demise in people who are contaminated with Loa-loa.

Another drug, ivermectin, can also be used, however it kills microfilariae only.

Tetracyclines inhibit endosymbiotic bacteria that are essential for the fertility of the worm, thus have an effect in the treatment of filariasis.

Supportive treatment

Antifilarial drugs may not cure chronic conditions. Other measures like elevation of affected limb, use of elastic bandage and local foot care are beneficial to reduce some symptoms of elephantiasis.

The swollen skin is susceptible to bacterial infections because immune defenses cannot work correctly due to the impaired stream of fluids. Therefore, the skin must be kept hygienic.

Surgery is required for hydrocele.  

Prophylaxis

To avert new infections

1)     Eradication of vector mosquitoes-elimination of breeding places, use of antilarval chemicals, using mosquito net and mosquito repellents

2)     Detection and treatment of carriers- use of DEC