Human filariases are important tropical diseases world-wide.
Certain types of zoonotic filariases may also be transmitted to humans in climate zones cooler than the tropics, for example in Southern Europe. The common feature for all filarial infections is the mode of transmission which occurs through the bite of an infected insect.
As the larvae enter the human body they develop into adult worms and live, according to the type, in a variety of tissues and cause different types of diseases.
Lymphatic filariasis is mainly caused by two tropical filarial worms, i.e. Brugia malayi and Wuchereria bancrofti. A symptomatic infection usually only affects those residing in endemic regions for prolonged periods of time, normally for several years.
The adult worms lodge in the lymphatic system, and symptoms emerge as the lymph circulation is disturbed by parasite-induced fibrosis of the lymph nodes or channels.
The first symptoms consist of recurrent febrile lymphangitis and lymphadenitis, usually involving the limbs.
After several years, the obstructed lymphatic channels become symptomatic, and the patient will develop chronic lymph node enlargement. A typical symptom is a chronic swelling of a limb. The patient may also develop, for example, giant scrotal swelling. As lymphoedema becomes chronic, changes in tissue structures become irreversible; this condition is known as elephantiasis.
Eosinophilia is common. In chronic infection, x-rays of the infected areas may show calcifications.
Lymphatic filariasis is diagnosed by identifying microfilariae (minute larvae) from a blood sample collected at night. If a patient with eosinophilia is suspected to have helminthiasis but there are no clear features suggesting any specific infectious agent, the primary investigation is serological screening of helminth antibodies also revealing antibodies for filarial worms.
Pharmacotherapy is used to treat lymphatic filariasis. In elephantiasis, surgery may also be considered.
Onchocerciasis (river blindness)
Onchocerciasis is caused by the filarial worm Onchocerca volvulus. The disease is encountered mainly in sub-Sahara Africa but may also occasionally occur in other regions. The prevalence of onchocerciasis-induced blindness has decreased thanks to a control program organised by the WHO.
The infection is acquired through bites by blackflies of the genus Simulium. Repeated bites are needed for the development of a clinical disease.
After human infection, the larvae mature into adult worms and form subcutaneous nodules where they mate and produce huge amounts of microfilariae which are released to the subcutaneous tissues.
Symptoms include pruritus and subcutaneous nodules. If the infection becomes chronic, the patient may gradually become blind due to the inflammatory reaction to the microfilariae that have migrated to the eye.
A specific diagnosis is based on an immediate microscopic examination of thin slices of skin (skin snips).
Pharmacotherapy is used to treat onchocerciasis. The destruction of microfilariae is fairly easy with drug therapy (ivermectin), but the adult worms are very treatment resistant. However, annual microfilaricide treatment is effective against blindness.
Loiasis
Loiasis is caused by Loa Loa, which is particularly prevalent in sub-Sahara Africa. In loiasis, a type of filariasis, adult worms migrate continually throughout the subcutaneous tissues of humans whilst producing microfilariae.
Typical symptoms include pruritus and the transient observation of an adult worm as it crosses the conjunctiva of the human eye.
Diagnosis is based on the detection of microfilariae from a blood sample collected at midday.