Filariasis

It is also known as Bancroftian filariasis and elephantiasis. Various other names are also used for aspects of this group of diseases including river blindness.

Filariasis is a group of diseases that affect humans and animals. The agent is a nematode parasite of the order Filariidae, commonly called filariae. They are usually classified according to the final habitat of the adult worms in the human host.

• The cutaneous group includes Loa loa, Onchocerca volvulus, and Mansonella streptocerca.
• The lymphatic group includes Wuchereria bancrofti, Brugia malayi, and Brugia timori.
• The body cavity group includes Mansonella perstans, Mansonella streptocerca and Mansonella ozzardi.

The cutaneous and lymphatic groups are the most important.

There are hundreds of filarial parasites, but only 8 species cause infections in humans. A few other species can cause incomplete infection but they are unable to complete the life cycle in the human.

The life cycle, in common with all nematodes, has 5 developmental or larval stages in a vertebral host and an arthropod intermediate host and vector.

• Adult female worms produce thousands of first-stage larvae or microfilariae that are ingested by a feeding insect vector.
• The arthropod vectors are mosquitoes or flies. They may have a circadian rhythm in which they feed and this correlates with a circadian rhythm of the microfilariae in the circulation.
• The highest concentration of microfilariae usually occurs at the time of day when the local vector is most active in feeding.
• Microfilariae undergo 2 stages of development in the insect.
• Larvae at the third stage are then inoculated back into the vertebral host during feeding and the final 2 stages of development follow.

More than 120 million people in the world are estimated to be affected by filariasis, with 40 million seriously affected according to the World Health Organisation. It is found throughout the tropics and subtropics. At least 21 million people are infected with O volvulus in equatorial Africa and Central and South America. Approximately 3 million people in Central Africa are infected with L loa. In 1997, the World Health Organization started a programme to eradicate lymphatic filariasis from the world.
Both sexes are affected equally. The rate of infection increases throughout childhood and adolescence although it may be many years before the clinical features are seen.

The features of filariasis vary between species. They vary in terms of the part of the body affected and if the condition is acute or chronic.

Lymphatic filariasis

This is caused by Wuchereria bancrofti, Brugia malayi, and Brugia timori. It is spread by mosquitoes of the genera Aedes, Anopheles, Culex, and Mansonia. The symptoms are predominantly the result of adult worms in the lymphatics.

• The microfilia in the circulation usually produce no symptoms, although heavy infection may induce acute and chronic inflammatory granulomas and destruction of the spleen.
• Symptoms include fever, inguinal or axillary lymphadenopathy, testicular pain, inguinal pain, skin exfoliation, and swelling of limbs or genitals.
• Repeated episodes of inflammation and lymphoedema damages lymphatics to cause chronic swelling with elephantiasis of the legs, arms, scrotum, vulva, and breasts.
• Hydrocoele is common with chronic W bancrofti infection but is rare with B malayi and B timori infection.
• Passage of cloudy urine suggests chyluria.

Tropical pulmonary eosinophilia

This is a form of occult filariasis. Presenting symptoms include:

• Paroxysmal dry cough
• Scattered wheezes and crackles are heard in both lungs.
• Dyspnoea
• Anorexia
• Malaise
• Weight loss.
• Lymphadenopathy and hepatomegaly may be found.

Onchocerciasis

Onchocerca are in the cutaneous group. The intermediate host is the Simulium species of blackflies. The disease is variously known as hanging groins, leopard skin, river blindness, or sowda.
Symptoms result from the presence of microfilariae in the skin and include

• The classical triad of infection is dermatitis, skin nodules that are onchocercomas and ocular lesions.
• Skin nodules tend to occur over bony prominences and include
  • Oedema
  • Pruritus
  • Erythema
  • Papules
  • Altered pigmentation
  • Lichenification
• Lymphadenitis
• Blindness.

Eye lesions are dependent upon the duration and severity of infection and are caused by an abnormal immune response to microfilariae. The common eye findings are punctate keratitis, pannus formation, corneal fibrosis, iridocyclitis, glaucoma, choroiditis, and optic atrophy.

Loaiasis

Mango flies or deerflies of Chrysops transmit loaiasis. The symptoms of Loa loa infection are usually:

• Subcutaneous swellings on the extremities
• Localized pain
• Pruritus
• Urticaria.

The diagnostic feature of disease is a Calabar swelling. This is a large transient area of localized non-erythematous subcutaneous oedema, usually around the joints. It is much more common amongst ex-patriates with the disease than the local population,¹ suggesting a different immune response.

Rare features include arthritis, breast calcification, meningoencephalopathy, endomyocardial fibrosis, peripheral neuropathy, pleural effusions, and retinopathy.

Mansonella Species

M ozzardi, M perstans, and M streptocerca are the body cavity infections and are usually asymptomatic.
If symptoms are present, they may include:

• Fever
• Pruritus
• Skin lumps
• Lymphadenitis
• Abdominal pain.

Blood

The usual means of detecting the parasite is by examination of peripheral blood. Most species, and all those that produce lymphatic involvement, may be detected by this method. It may be necessary to take the blood at a time when the circadian rhythm gives a high count. Another technique is to give a small dose of the drug DEC to precipitate them into the circulation. In Loa loa disease, the parasites may be absent from the blood except at an advanced stage.

Skin Biopsy

Multiple skin snip specimens from various body sites can be used to detect O volvulus and M streptocerca. For African onchocerciasis, the recommended sites are the gluteus and calf. For American onchocerciasis, the scapula and deltoid skin is preferred.
Lymph nodes and skin nodes may be biopsied and adults worms may be found.

Slit Lamp

Microfilariae of O volvulus may be detected in the cornea or anterior chamber of the eye using slit-lamp examination.

Immunological Tests

Circulating filarial antigen may be detected using in the commercially available kits to test venous blood. This can be used in diagnosis and to monitor treatment. Antibody tests are also available.
Eosinophilia is marked in all forms of filarial infection. Serum IgE and IgG4 is elevated with active disease.

Urinalysis

If lymphatic filariasis is suspected, urine should be examined macroscopically for chyluria and then concentrated to examine microscopically for microfilariae. Chyluria results from obstruction of lymphatic drainage.

Imaging

In tropical pulmonary eosinophilia, there are diffuse pulmonary infiltrates on chest x-ray.
Obstruction of the inguinal and scrotal lymphatics can be demonstrated and monitored by ultrasound.

Mazzotti Test

This may be used to diagnose cutaneous filariasis when skin snip results are negative. A small does of DEC, usually 50 to 100mg, is given and an intense pruritus follows within hours. Steroids may be necessary to control this response. It can produce a severe response in heavy infection with a severe systemic reaction. A DEC patch test that causes a localized skin reaction may be used instead.

Control of the disease appears to be aimed at treatment of individuals and treatment of populations² rather than vector control.

Non-Drug

Bed rest, limb elevation, and compression bandages are the traditional management of chronic lymphoedema.

Drugs

• The most commonly used drug to treat the condition is diethyl-carbamazine, usually abbreviated to DEC. It is not licensed for use in the UK but can be used on a named patient basis. The dose for both adults and children is 6mg/kg, 4 times daily for 2 or 3 weeks.
• A lower dose may be used initially in heavy infection for fear of adverse reactions to the disintegrating parasites and steroid cover may be given for this reason.
• Other drugs may be used alone or in combination with DEC.
• Invermectin may be used for W bancrofti alone or in combination with DEC. It is highly effective but adverse reactions need supervision.³ A single-dose ivermectin (150-200 micrograms/kg) with or without albendazole (400 mg) appears to be effective to treat Wuchereria bancrofti infection.⁴
• Suramin is currently the only drug for onchocerciasis that is effective against adult worms but use is limited by toxicity.
• Mebendazole and its analogue flubendazole may be used. Albendazole is another possibility.

Because of the risk of adverse effects, the patient should be observed for a while after initiating treatment.

Community Treatment

The elimination of lymphatic filariasis from a community requires delivery of annual drug treatments to at least 80% of the eligible members of endemic populations for at least 5 years. For various reasons, this is not being achieved in much of sub-Sahara Africa.⁵

• Filarial diseases are rarely fatal, but those affected tend to suffer poor health, they have more time off work and are less productive.⁶
• The WHO has identified lymphatic filariasis as the second leading cause of permanent and long-term disability in the world after leprosy.
• The morbidity of human filariasis mainly results from the host reaction to microfilariae or developing adult worms in different areas of the body.

This is based on avoidance of bites by vectors when in endemic areas. The article on malaria discusses avoidance of mosquito bites.