Filariasis by Dr. Susan MacDonald

Filariasis refers to a group of vector-borne infections caused by microscopic, threadlike worms called roundworms or nematodes. The 3 main types are:

Lymphatic Filariasis (Elephantiasis) due to Wuchereria bancrofti or Brugia malayi.

Mansonella ozzardi, Mansonella streptocerca, and Mansonella perstans are other known agents. Each of these diseases has a life cycle involving a vector, immature larvae called microfilariae (mf), mature adult forms, and the human host. Depending on the specific agent and period of exposure, illness may range from minimal effects to severe, debilitating disease. Newly exposed individuals may have more severe acute symptoms than residents in the endemic areas.

There is an asymptomatic incubation period of at least 6 months and up to 6 years. As this may lead to difficulty in diagnosis, some have suggested routine post-travel serologic screening for those individuals with >1 year exposure in an endemic area.

Risk is low with an increased risk in the long term traveller, missionaries, field scientists, and volunteers, as disease usually requires repeated exposure to the particular infected vector over months to years. As the adult worm cannot multiply in the human host, disease manifestations will depend on the frequency of bites as well as the worm burden.

In Onchocerciasis, if a traveller or resident lives for more than 3 months in a black fly habitat, then there appears to be a greater risk for infection.

The major risk of Filariasis remains the morbidity due to eye, skin, subcutaneous, and lymphatic involvement rather than a fatal outcome.

Lymphatic Filariasis is found in the tropics and subtropics of India, China, Indonesia, Southeast Asia, Africa, South America, and the Pacific.

endemic

uncertain

non-endemic

Loa Loa is endemic to the rain forests of Central and West Africa, including Democratic Republic of Congo, Angola, Gabon, Central African Republic, Cameroon, Nigeria, Chad, Sudan, Benin, and Guinea. A few cases have been reported elsewhere eg. Uganda, Malawi, Zambia, and Ethiopia.

Onchocerciasis is found in 37 countries, in Central Africa, northern South America, Central America and the Arabian peninsula. It is especially associated with fast flowing rivers and their surroundings where Simulium breeds.

Lymphatic Filariasis is transmitted to man by the bite of infected Culex, Anopheles, and Aedes mosquitoes. The infective larvae mature to adults, males and females, in lymphatic vessels and nodes where female worms produce microfilariae which then circulate in the bloodstream. A mosquito bite of an infected human host then completes the cycle. In Bancroftian filariasis, the time from infection to appearance of microfilariae is 6-12 months while in Brugian filariasis it may be only 3 ½ months. Microfilariae generally circulate in the bloodstream at night, but can be daytime in some regions. This is referred to as the periodicity. Adult worms live 4-6 years.

Loa Loa is transmitted by the bite of an infected tabanid fly, genus Chrysops. Adult forms migrate through subcutaneous tissues resulting in intermittent swellings called Calabar swellings, and also migrate beneath the eye conjunctiva (Eye worm). Microfilariae circulate in the bloodstream during the day. Adult worms live 4-12 years.

Onchocerciasis is transmitted by the bite of an infected black fly, genus Simulium. Adult worms are found in subcutaneous nodules. Microfilariae migrate to subcutaneous tissues, skin, and eyes. It takes approximately 12 months for maturation to the adult form and subsequent microfilariae production. Adults live 10-15 years.

Lymphatic filariasis is often asymptomatic. Its major symptoms are due to blockage of the lymphatics by adult worms, with recurrent episodes of fever, chills, tender lymph nodes, and malaise for 3-15 days at a time, leading to retrograde adenolymphangitis.The groin and genitalia in males are most frequently affected and may suppurate and scar. If there are recurrent acute episodes, obstructive lymphatic disease leads to hydrocoele, lymphedema, elephantiasis of the arms, legs, genitalia, or breasts, and chyluria (rupture of renal lymphatics into the urinary collecting system).

Travellers generally present with acute attacks of lymphangitis, but only if they have been exposed to intense biting over a prolonged period of time. It is unusual for a traveller to develop the chronic forms of illness. It appears, however, that newly exposed individuals who do become infected suffer more rapid progression to chronic or irreversible disease than natives of the endemic area, resulting in lymphedema and elephantiasis within 6-12 months of exposure. Disease can actually be significantly reduced by leaving the endemic area.

A condition called Tropical Pulmonary Eosinophilia results from a marked hypersensitivity reaction to microfilariae in the lungs, leading to pulmonary infiltrates and eosinophilia. It is most common in young adult males who are long term residents and presents as a nocturnal dry cough, wheezing and fatigue with rales on examination.

Loa Loa has varying symptoms, from asymptomatic to severe infection with encephalitis, cardiomyopathy, and kidney failure. Visitors to endemic areas tend to have more allergic symptoms than endemic individuals, with itch, subcutaneous (Calabar) swellings, and urticaria. Calabar swellings (due to a migrating adult worm) occur especially on the face and limbs and present as itch or pain followed by areas of edema. Subconjunctival worms are more common in endemic individuals.

Onchocerciasis has symptoms related mainly to the skin, eye, and lymph tissue. Adult worms become encapsulated in fibrous nodules while the microfilariae inhabit and subsequently damage the skin and eye, leading to loss of skin elasticity and early aging as well as blindness. Skin changes range from papules to lichenification to depigmentation and finally, to “hanging groin” (a pouch of swollen lymph tissue). Eye damage includes corneal injury (sclerosing keratitis), uveitis, and chorioretinitis. Nodules may be palpable and lymph nodes may be enlarged. Itching is a common complaint, especially in expatriates or non-immune visitors with a brief exposure. Non-immunes rarely present with eye lesions or nodules.

The traveller should follow standard mosquito precautions with usage of insect repellant (DEET), long sleeves, screens, bed nets, and avoidance of areas and time of day of maximal biting.

DEC (diethylcarbamazine) at a dose of 300 mg. weekly has been effective in preventing Loiasis in long term travellers to endemic areas.

In endemic communities, mass treatment with medication on a regular basis and control of the vector by aerial spraying with larvicides are helpful preventive measures.

Definitive diagnosis depends on finding the microfilariae in blood or skin snips or demonstrating the adult worms by ultrasound or biopsy. Serology by ELISA is helpful in individuals from non endemic areas who are seronegative initially. Other lab studies such as eosinophilia and elevated serum IgE are also useful.

Parasitologic diagnosis: In Lymphatic filariasis and Loa Loa, examination of a Giemsa thick stained blood film, with concentration techniques if needed, can reveal specific worm morphology to differentiate between the various agents. Detection of the microfilariae is difficult in non-immunes (travellers or expatriates) as few microfilariae are present in the blood stream. Loa Loa is best isolated during daytime hours and W. bancrofti and B. malayi during nocturnal and occasionally daytime hours (depending on the periodicity).

Onchocerciasis requires skin snips for isolation of the microfilariae. A corneoscleral punch biopsy removes a tiny piece of skin which is then placed into normal saline on a slide or in the well of a specialized plate and examined under the microscope for the emergence of microfilariae. Subcutaneous nodules can also be biopsied for adult worms. A slit lamp exam may reveal intraocular microfilariae. An infrequently performed test is the Mazzotti test, in which 25-50 mg. of DEC is given to an individual with a low level of worms and intense itching develops if Onchocerca is present. As this can result in severe symptoms and eye damage in heavily infected individuals, care must be taken with this test.

Serology: Tests for filaria-specific IgG are based on antigens derived from B. malayi or the dog heartworm, Dirofilaria immitis, but cannot distinguish between the different filaria, when the infection occurred, or whether it has been successfully treated. This is generally an ELISA test, and is most helpful for the traveller or expatriate with brief exposure.

Circulating Antigen: This test measures W.Bancrofti or B. malayi circulating antigen by ELISA or by a rapid card test. Daytime blood samples can be used.

PCR: Highly specific and sensitive PCR assays can detect low levels of infection in Onchocerciasis in non-residents who may not have detectable parasites in skin snips.

Conservative measures such as elevation of swollen limbs, washing inflamed lymphatic areas with soap and water, and application of antibiotic creams can be very helpful for chronic lymphatic disease.

Medication such as DEC (diethylcarbamazine) is used for both Lymphatic filariasis and Loa Loa but is contraindicated in Onchocerciasis, where Ivermectin is the drug of choice. In non-residents, repeated doses of medication are usually required due to the different effects on the adult and immature worms.

Nodulectomy and aspiration of varicocoele can also be attempted where appropriate.