Filariasis: Introduction, Classification, Life Cycle, Clinical Features, Lab Diagnosis, Treatment and Control
Introduction of Filariasis
Filariasis is an infectious disease caused by nematodes of the Filarioidea type e.g. Wuchereria bancrofti, Brugia malayi, Oncocerca volvulus, and Loa loa. These are spread by blood-feeding insects such as black flies and mosquitoes. The filarial worms reside in the subcutaneous tissues, lymphatic system, or body cavities of humans,
Classification of filarial worms based on location in the body
The females of parasites are viviparously giving birth to larvae so-called microfilaria. This parasite includes four genera and species are parasitic to humans. These filarial nematodes are as follows: 1. Wuchereria bancrofti 2. Brugia malayi 3. Oncocerca volvulus and 4. Loa loa but mainly two genera i.e. Wuchereria and Brugia are encountered in peripheral blood.
Central and West Africa
Lymphatics and lymph nodes
It has two stages adult worm and larva (microfilaria).
The adults are whitish, translucent, thread-like worms with smooth cuticles and tapering ends. The female is larger (70–100 × 0.25 mm) than the male (25–40 × 0.1 mm). The posterior end of the female worm is straight, while that of the male is curved vertically and contains 2 spicules of unequal length. Males and females remain coiled together usually in the abdominal and inguinal lymphatics and in the testicular tissues. The female worm is viviparous and directly liberates sheathed microfilariae into the lymph.
Lifespan: 10 to 15 years
The microfilaria has a colorless, translucent body with a blunt head, and pointed tail It measures 250–300 µm in length and 6–10 µm in thickness It is covered by a hyaline sheath, within which it can actively move forwards and backward Along the central axis of the microfilaria, a column of granules can be seen, which are called somatic cells or nuclei. The granules are absent at certain specific locations—a feature that helps in the identification of the species. Microfilariae do not multiply or undergo any further development in the human body. Their lifespan is believed to be about 2–3 months. It is estimated that a micro filarial density of at least 15 per drop of blood is necessary for infecting mosquitoes
The microfilariae circulate in the bloodstream. They show a nocturnal periodicity in peripheral circulation; being seen in large numbers in peripheral blood only at night (between 10 pm and 4 am). This correlates with the night-biting habit of the vector mosquito.
Definitive host: Man
Intermediate host: Female mosquito, of different species, acts as vectors in different geographic areas. The vector in most other parts of Asia is Culex quinquefasciatus (C. fatigans).
Demonstration of Microfilaria: Microfilaria can be demonstrated in blood, chylous urine, exudate of lymph varix, and hydrocele fluid. Peripheral blood is the specimen of choice. The method has the advantage that the species of the infecting filaria can be identified from the morphology of the microfilaria seen.
Unstained Film: Examination under the low-power microscope shows the actively motile microfilariae lashing the blood cells around. The timing of blood collection is critical and should be based on the periodicity of the microfilariae.
Concentration Techniques: When the microfilaria density is low, concentration techniques are used: Knott’s concentration technique
DEC provocation test
Biopsy: Adult filarial worms can be seen in sections of biopsied lymph nodes, but this is not employed in routine diagnosis
Skin Test: Intradermal injection of filarial antigens induces an immediate hypersensitivity reaction. But the diagnostic value of the skin test is very limited due to the high rate of false-positive and negative reactions.
Serodiagnosis: Complement fixation, indirect hemagglutination (IHA), indirect fluorescent antibody (IFA), immunodiffusion, and immune enzyme tests have been described. But the tests are not sufficiently sensitive or specific to be used either for individual diagnosis or surveys. Demonstration of Circulating Antigen Highly sensitive and specific test for detection of specific circulating filarial antigen (CFA)The Trop bio test is a semiquantitative ELISA for detection of CFA in serum or plasma specimen. Immunochromatographic filariasis card test (ICT) is a new and rapid filarial antigen test that detects soluble W. bancrofti antigens using a monoclonal antibody (ADIZ) in the serum of infected humans. Both assays have sensitivities of 93–100% and specificities approaching 100%.
Molecular Diagnostic Technique: Polymerase chain reaction (PCR) can detect filarial DNA from a patient’s blood, only when circulating microfilaria are present in peripheral blood but not in chronic carrier state usually the test provides sensitivities that are up to 10 fold greater than parasitic detection by direct examination and is 100% specific.
Eosinophilia (5–15%) is a common finding in filariasis.
Elevated serum IgE levels can also be seen.
Treatment of Filariasis
Diethylcarbamazine (DEC): effective against adult and microfilariae. Ivermectin: destroy microfilariae but not adults.
Drainage and filling:
urban chemical and biological larvicides
Indoor residual insecticide
Outdoor residual insecticide spraying
impregnated materials: nets, curtains, clothing
Merkell and Voge’s medical parasitology 9th edition.
Parasitology: 12th edition By K. D. Chatterjee
District laboratory practice in Tropical countries –Part-I. By Monica Chesbrough.