Filariasis – Causes, Transmission, Diagnosis and Prevention.

Definition

Filariasis is an infectious tropical disease caused by any one of several thread-like parasitic round worms. The two species of worms most often associated with this disease are Wuchereria bancrofti and Brugia malayi. The larval form of the parasite transmits the disease to humans by the bite of a mosquito.

 

Microscopic view of Wuchereria bancrofti

Depending on the area which the worms affect, filariasis is classified as-

• Lymphatic filariasis (Elephantiasis) – which affects the lymphatic system, including the lymph nodes.
• Subcutaneous filariasis – which affects the subcutaneous layer of the skin.
• Serous cavity filariasis – which affects the serous cavity of the abdomen.

Filariasis is not a life-threatening infection but it can cause lasting damage to the lymphatic system. The disease causes no symptoms in the initial stage. Therefore, most people initially are not aware that they have filariasis. Lymphedema with thickening of the skin and underlying tissues is the classical symptom of filariasis.

Lymphatic filariasis

The clinical course of lymphatic filariasis is broadly divided into the following:

• Asymptomatic microfilaremia – Patients with microfilaremia are generally asymptomatic, although those with heavy microfilarial loads may develop acute and chronic inflammatory granulomas secondary to splenic destruction; passage of cloudy, milk like urine may denote chyluria
• Acute phases of adenolymphangitis (ADL)
• Chronic, irreversible lymphedema

Lymphatic filariasis

Lymphatic filariasis symptoms predominantly result from the presence of adult worms residing in the lymphatics. They include the following:

• Fever
• Inguinal or axillary lymphadenopathy
• Testicular and/or inguinal pain
• Skin exfoliation
• Limb or genital swelling

The following acute syndromes have been described in filariasis:

• Acute ADL
• Filarial fever – Characterized by fever without associated adenitis
• Tropical pulmonary eosinophilia (TPE)

Transmission cycle of Filariae

• When a mosquito with infective stage larvae takes a blood meal, the parasites are deposited on the person’s skin from where they enter the body.
• The larvae migrate to the lymphatic vessels where they develop into adult worms. This process may take 6–12 months, affecting the dilation and functioning of the lymphatic vessels.
• The adult filariae live for several years in the human host, producing millions of microfilariae that circulate in the peripheral blood and are ingested by mosquitoes during blood-feeding.
• The larval forms further develop inside the mosquito before becoming infectious to humans.
• Thus, a cycle of transmission is established.

Life cycle of Filariae

Background of filariasis

Lymphatic Filariasis otherwise known as Elephantiasis is believed to have been around as early as 2000 B.C. A statue of Pharaoh Mentuhotep II depicts swollen limbs, a characteristic of elephantiasis. The first written account of lymphatic filariasis comes from the ancient Greek and Roman civilizations. In 1900, George Carmichael Low discovered microfilariae in the mouth of mosquitoes, and finally pinpointed the true mechanism of transmission. Due to this discovery, we now know that transmission is due to an infective bite from a mosquito.

One of the most break through discoveries was that made by Patrick Manson in 1877. Manson was the first to look for a host for microfilariae. In 1877, he was finally able to pinpoint the microfilariae in mosquitoes. This discovery was later applied to other tropical diseases such as malaria, and was the first discovery of an arthropod as a carrier.

In 1863, French surgeon Jean-Nicolas Demarquay became the first to record the observation of microfilariae in fluid extracted from an infected area. Three years later, Otto Henry Wucherer discovered microfilariae in urine in Brazil. However, the connection between these two discoveries was not made until Timothy Lewis noted the occurrence of microfilariae in both blood and urine. Lewis was also the first to make the association between these microfila.

Epidemiology of filariasis

1. bancrofti occurs in sub-Saharan Africa, Southeast Asia, the Indian subcontinent, many of the Pacific islands, and focal areas of Latin America and the Caribbean (including Haiti). B. malayi occurs mainly in China, India, Malaysia, the Philippines, Indonesia, and various Pacific islands. B. timori occurs on the Timor Island of Indonesia. Overall, approximately two-thirds of individuals infected with lymphatic filariasis are in Asia. The epidemiology of lymphatic filariasis is changing due to implementation of a global program of mass drug administration (MDA) to eliminate transmission. Not only has mapping of disease prevalence prior to MDA led to reclassification of some countries (Costa Rica, Suriname, Trinidad and Tobago) as no endemic, but some countries, including Togo, Vietnam, Cambodia, American Samoa, the Cook Islands, the Marshall Islands, Tonga, and Vanuatu, appear to have eliminated transmission entirely.

Causes of filariasis

Eight different thread-like nematodes cause filariasis. Most cases of filaria are caused by the parasite known as Wuchereria bancrofti.

• Lymphatic Filariasis (Elephantiasis) is caused by Wuchereria bancrofti, Brugia malayi, and Brugia timori.
• Subcutaneous Filariasis is caused by Loa loa (the eye worm), Mansonella streptocerca, and Onchocerca volvulus.
• Serous Cavity Filariasis is caused by the worms Mansonella perstans and Mansonella ozzardi.
• Most cases of filariasis are caused by the parasite known as Wuchereria bancrofti; The Culex, Aedes or Anopheles mosquitoes transmit the disease. Another parasite called Brugia malayi that causes filariasis is transmitted by the vector Mansonia and Anopheles mosquitoes.

Signs and symptoms

Manifestations can be protean and classified as:

Acute

• Feverwith chills and rigors
• Lymphedema with pain
• Lymphadenopathy (cervical, axillary, inguinal and generalised Acute Filarial Lymphangitis/Acute Dermatolymphangioadenitis)
• Chyluria
• Hematuria
• Inflammatory granuloma or abscesses
• Pain in testes
• Funiculitis
• Epididymoorchitis

Chronic

• Funiculitis
• Epididymoorchitis
• Hydrocele
• Breast edema

Occult

• Pulmonary eosinophilia, mono and polyarthritis
• Tenosynovitis
• Glomerulonephropathy
• Retroperitoneal lymphangitis (acute abdomen)
• central serous retinopathy
• iridocyclitis, recurrent scleritis and macular oedema
• endomyocardial fibrosis
• urticarial
• recurrent upper respiratory infections
• asthmatic bronchitis
• Any lymph node or any body part can be affected but commonly genital lymphatics are involved in males.

Asymptomatic

• Endemic normalsnegative for Mf but positive for antigens (pre-patency) and asymptomatic microfilaremic – is characterised by the presence of microfilaria in peripheral blood during night but without any overt clinical manifestations of filariasis with or without antigens – also known as Mf carriers

Diagnosis and Test

Laboratory tests can be divided into nonspecific and specific tests.

Specific tests

• Direct detection of microfilaria on blood smears
• Serologic tests
• DNA PCR and radiology

Nonspecific tests

• Eosinophilia
• High IgE levels and lymphoscintigraphy (that reveal dilated lymph channels or backflow even in the early stage of infection)

Direct methods

• Visualization of microfilaria (or the adult worm) – is made by microscopic examination of thick film of blood collected between 10:00 PM and 2:00 AM, with or without DEC provocation, stained by Geimsa or hematoxylineosin for the presence of microfilaria.
• Adult worm may be found in fluids drawn from swollen areas or serous collections.
• X-ray tests can show calcified adult worms in lymphatics.
• Ultrasonography can show the ’filarial dance’.1, 2, 7 Lymph node aspirate and chylus fluid may also yield microfilaria or worm.
• Direct diagnosis, though definitive, is difficult, because of timing inconvenience of blood collection, long pre-patency, low patency (<60%) and inadequate sensitivity.

Treatment and Medications

• The drug DEC (diethylcarbamazine) is the mainstay of therapy for filiariasis. The dose is 6 mg per kilogram of body weight given as a single dose daily. The total duration of treatment is for about three weeks.

Streamlined Dose
Age in Years	Dose of DEC	Number of Tablets
< 2	Nil	Nil
2–5	100 mg	1 tablet of 100 mg
6–14	200 mg	2 tablets of 100 mg each
15 & above	300 mg	3 tablets of 100 mg each

• Ivermectin is another drug that can be used. It is particularly useful against the parasites Bancrofti and
• Albendazole is anthelmintic drug which is shown to destroy the adult filarial worms when given in doses of 400mg twice daily for two weeks. The death of the adult worm induces severe scrotal reactions in bancroftian filariasis since this is the common site where they are lodged. Albendazole has no direct action against the microfilaria and does not immediately lower the microfilaria counts. But when given in single dose of 400 mg in combination with DEC or ivermectin, the destruction of microfilaria by these drugs becomes more pronounced.

Prevention and cure of entry lesions

• Entry lesions are common in patients with lymphoedema and are most frequently found between the toes and deep skin folds and around the toenails. Entry lesions, such as wounds, can also be found on the surface of the skin. Both fungi and bacteria can cause entry lesions. Fungal infections frequently damage the skin and create entry lesions, especially between the toes, and may cause itching. The entry lesions allow bacteria to enter the body through the skin and this can cause acute attacks. Fungi and bacteria can cause bad odour.
• Fungal infections are usually white or pink in colour and do not leak fluid. Bacterial infections may leak fluid that is thin and clear or thick and coloured.
• Antifungal and antibacterial creams can be used for local application.

Prevention of filariasis

Avoidance of mosquito bites through personal protection measures or community-level vector control is the best option to prevent lymphatic filariasis. Periodic examination of blood for infection and initiation of recommended treatment are also likely to prevent clinical manifestations.

Future perspectives

The recent availability of drugs to prevent transmission of the disease and simple, low cost treatment modalities which offer relief to person with evident disease, herald a brighter future in tackling this potentially eradicable disease. Mass administration of single annual doses of albendazole 400 mg along with DEC 6 mg/kg body weight is the recommended strategy to prevent transmission of filariasis for India. This has the added benefit of clearing the intestinal helminths in the community.