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 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.
This article will explore lymphatic filariasis, see separate articles on Body Cavity Filariasis and Cutaneous Filariasis.
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.
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- More than 120 million people in the world are estimated to be affected by lymphatic filariasis, with 40 million seriously affected according to the World Health Organisation.
- It is found throughout the tropics and subtropics.
- In 1997, the World Health Organisation 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.
Lymphatic filariasis is caused by Wuchereria bancrofti, Brugia malayi, and Brugia timori. It is spread by mosquitoes of the genera Aedes, Anopheles, Culex, and Mansonia. 90% of lymphatic filariasis is caused by W. bancrofti and the rest is mostly caused by B. malayi. B. malayi does not affect genitalia lymphatics.
The symptoms are predominantly the result of adult worms in the lymphatics. There are three broad types of clinical scenarios:
- Asymptomatic infection
- Acute infection
- Chronic infection
- This is seen commonly in areas where the disease is endemic.
- Patients have no symptoms but microfilaria can be detected in peripheral blood smears.
- These patients will already have irreversible lymphatic changes highlighting the importance of their detection and treatment.
This includes acute adeno-lymphangitis (ADL) and acute filarial lymphangitis (AFL)
This is the commonest acute presentation.
It is characterised by:
- Fever and painful lymphadenopathy in the groin and axillae.
- Affected areas are painful, tender, red and swollen - usually result of superimposed bacterial infection.
- Occur several times in a year and more so in rainy seasons. The reason for this is that in the rainy season the moisture between toes increases leading to fungal infections which damage the skin allowing worms to invade.
- This is why each episode of ADL enhances the development of lymphoedema.
Acute filarial lymphangitis
- Rare compared with ADL
- Caused by dying adult worms (either spontaneously or with treatment)
- Characterised by small tender nodules at the site of worms dying which may either be along the involved lymphatic or in the scrotum
- Tender and enlarged lymphatics may be seen
- There is no fever or secondary infections
- This is manifested by lymphoedema, elephantiasis and lesions of the genito-urinary system.
- Lymphoedema is the commonest and may progress to elephantiasis.
- The lower limbs are most commonly involved - but the upper limbs, genitalia and breast in females may also be involved.
- Frequent episodes of ADL lead to the progression of lymphoedema.
- Hydrocele is commonly seen in chronic infection.
- Chylocoele, chyluria and chylous ascites occur rarely.
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
- Weight loss
- Lymphadenopathy and hepatomegaly may be found
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.
Circulating filarial antigen may be detected using 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. Polymerase chain reaction tests are also available which have a high specificity and sensitivity.
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.
Obstruction of the inguinal and scrotal lymphatics can be demonstrated and monitored by ultrasound. More recently ultrasonography has been used to detect adult worms in male scrotal lymphatics and lymphoscintigraphy has been used to detect lymphatic changes.
It is important to appreciate that some of the newer and more sophisticated investigation methods may not be available in areas where the disease is endemic. Thus use of peripheral blood smears and immunological tests is the predominant detection method used.
Control of the disease appears to be aimed at treatment of individuals and treatment of populations rather than vector control.
- Bed rest, limb elevation, and compression bandages are the traditional management of lymphoedema.
- Once infection has occurred a "foot care programme" is paramount to break the cycle of infection and worsening lymphoedema. This involves washing of the affected area (including webs of the toes and deep folds), wiping area dry, clipping and cleaning nails, avoiding injuries or infections and applying antifungal substances.
- Prevention of repeated ADLs is also important and may require long term antibiotic therapy eg oral penicillin or long acting parenteral penicillin. Unfortunately, a lot of these simple measures are not achievable in areas where the disease is endemic due to economic and other political factors.
- The most commonly used drug to treat the condition is diethyl-carbamazine, usually abbreviated to DEC. It will kill both adult worms and microfilariae. It is not licensed for use in the UK but can be used on a named patient basis.
- Rapid destruction of worms can be associated with the development of fever, headache, myalgia, sore throat lasting 1-2 days. Thus lower doses may be used initially in heavy infection and steroid cover may be given for this reason.
- DEC should be given for a longer duration in Tropical Eosinophilia Syndrome (treatment for 1 month is usual).
- Other drugs may be used alone or in combination with DEC.
- Ivermectin may be used for W. bancrofti alone or in combination with DEC. It is highly effective but adverse reactions need supervision.
- Mebendazole and its analogue flubendazole may be used. Albendazole is another possibility. A single-dose ivermectin with or without albendazole appears to be effective to treat W. bancrofti infection. The global filariasis elimination programme advises this combination. There is a serious risk of adverse effects, so patients should be observed for a while after initiating 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-Saharan Africa.
Complications and prognosis
- Filarial diseases are rarely fatal, but those affected tend to suffer poor health, they have more time off work and are less productive.
- 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.
Further reading & references
- Lymphatic Filariasis, World Health Organization
- Palumbo E; Filariasis: diagnosis, treatment and prevention. Acta Biomed. 2008 Aug;79(2):106-9.
- Ottesen EA, Duke BO, Karam M, et al; Strategies and tools for the control/elimination of lymphatic filariasis. Bull World Health Organ. 1997;75(6):491-503.
- De Sole G, Remme J, Awadzi K, et al; Adverse reactions after large-scale treatment of onchocerciasis with ivermectin: combined results from eight community trials. Bull World Health Organ. 1989;67(6):707-19.
- Dunyo SK, Nkrumah FK, Simonsen PE; Single-dose treatment of Wuchereria bancrofti infections with ivermectin and albendazole alone or in combination: evaluation of the potential for control at 12 months after treatment. Trans R Soc Trop Med Hyg. 2000 Jul-Aug;94(4):437-43.
- Reddy M, Gill SS, Kalkar SR, et al; Oral drug therapy for multiple neglected tropical diseases: a systematic review. JAMA. 2007 Oct 24;298(16):1911-24.
- Gyapong M, Gyapong JO, Owusu-Banahene G; Community-directed treatment: the way forward to eliminating lymphatic filariasis as a public-health problem in Ghana. Ann Trop Med Parasitol. 2001 Jan;95(1):77-86.
- Babu BV, Swain BK, Rath K; Impact of chronic lymphatic filariasis on quantity and quality of productive work among weavers in an endemic village from India. Trop Med Int Health. 2006 May;11(5):712-7.
|Original Author: Dr Gurvinder Rull||Current Version: Dr Gurvinder Rull|
|Last Checked: 20/12/2010||Document ID: 3000 Version: 23||© EMIS|
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