Filariasis

Lymphatic filariasis, onchocerciasis and loiasis are the three common forms of filariasis in humans.

The majority of lymphatic filariasis is caused by two species of filarial nematode (roundworm), Wuchereria bancrofti and Brugia malayi. Infection is transmitted via the bite of an infected mosquito (Culex spp). Adult worms settle in the lymphatics where they cause mechanical obstruction and scarring. Chronic infection results in the characteristic lymphoedema with lichenification of the overlying skin. The microfilarae (juvenile form) are detectable in the peripheral blood at night time.

Onchocerciasis is an infection caused by the filiarial nematode Onchocerca volvulus. The worm is transmitted via the bite of Simulum spp (blackflies) which are found near fast flowing bodies of freshwater. Adult worms settle in subcutaneous tissue causing nodular swellings as well as other skin rashes. Microfilarae migrate around the body, including to the eye where they cause inflammation, which can lead to blindness. Onchocerciasis is the second most common infectious cause of blindness worldwide and is also termed River Blindness. Microfilarae are not detectable in peripheral blood but can be visualised in skin snips.

Loiasis is an infection caused by the filarial nematode Loa loa. The worm is transmitted via the bite of an infected deer fly (Chrysops spp). Often infection is asymptomatic, however loiasis may manifest with soft tissue swellings (Calabar swellings) and occasionally with the presence of a visible adult worm migrating across the subconjunctiva. Microfilarae are detectable in peripheral blood during
the middle of the day.

The syndromes produced by the various species of filarial worms are usually associated with eosinophilia. A patient with an eosinophilia who has lived in, or visited, a filaria- endemic area might reasonably be tested for filariasis.

Detection of Filariasis (except Onchocerca volvulus) by microscopy:

• Sample type: 20 millilitres of anti-coagulated blood (citrate tube) are required so that the microfilariae can be detected by filtration. Day blood (for Loa loa) should be taken between 12pm (noon) and 2pm local time and night blood (for Wuchereria bancrofti or Brugia malayi) at 12am (midnight). Samples should be kept at room temperature until processed.

Correct blood collection times for diagnosis of human filariasis

With the exception of Onchocerca volvulus, a definitive diagnosis of filariasis is usually made by the demonstration of microfilariae in the peripheral blood.

Detection of Onchocerca volvulus by microscopy

• Sample type: Onchocerca volvulus is diagnosed by demonstration of microfilariae in skin snips. Please contact the department before sample is taken for information about sample transport and to let the laboratory know the sample when the sample will be arriving.

Detection of filariasis by serology

• Sample type: A minimum of 0.5ml of serum is required (can be transported at room temperature).

A filaria ELISA, using Brugia pahangi as antigen is used as a ‘generic’ screening test. A negative result does not exclude the diagnosis and this is especially so with onchocerciasis.

The filaria ELISA is a non-specific screening test that is positive in many types of filariasis and cross reacts in cases of strongyloidiasis. It is most useful in the diagnosis of TPE (Tropical Pulmonary Eosinophilia) where high antifilarial antibody levels are required to make the diagnosis. Positive results are reported at Levels 1 to 9. Levels 1 and 2 are regarded as weak positives; Levels 5 and over are strong positives.

Reactive symptomatic cases with moderate eosinophilia tend to give high level positives. Non-reactive cases, which may be asymptomatic though microfilariae are present, give low levels of positivity and may be negative. Known causes of false positive results are Strongyloides, Hookworm (about 50% of cases) and occasionally Ascaris infection. We are unable to determine the species of Filaria infections using our ELISA test. This may be done if microfilariae are seen in a blood film, or by staining the microfilariae obtained by filtration.